Search for neutral MSSM Higgs bosons at LEP

Abstract

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by
the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined
for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no
significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper
bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number
of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large
exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses
of neutral Higgs bosons.

Full text

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Eur. Phys. J. C 47, 547–587 (2006)
Digital Object Identifier (DOI) 10.1140/epjc/s2006-02569-7
THE EUROPEAN
PHYSICAL JOURNAL C
Search for neutral MSSM Higgs bosons at LEP
The LEP Collaborations ALEPH, DELPHI, L3 and OPAL
The LEP Working Group for Higgs Boson Searchesa
The ALEPH Collaboration
S. Schael1, R. Barate2, R. Bruneli` ere2, I. De Bonis2, D. Decamp2, C. Goy2, S. J´ ez´ equel2, J.-P. Lees2, F. Martin2,
E. Merle2, M.-N. Minard2, B. Pietrzyk2, B. Trocm´ e2, S. Bravo3, M.P. Casado3, M. Chmeissani3, J.M. Crespo3,
E. Fernandez3, M. Fernandez-Bosman3, L. Garrido3,44, M. Martinez3, A. Pacheco3, H. Ruiz3, A. Colaleo4,
D. Creanza4, N. De Filippis4, M. de Palma4, G. Iaselli4, G. Maggi4, M. Maggi4, S. Nuzzo4, A. Ranieri4, G. Raso4,49,
F. Ruggieri4, G. Selvaggi4, L. Silvestris4, P. Tempesta4, A. Tricomi4,38, G. Zito4, X. Huang5, J. Lin5, Q. Ouyang5,
T. Wang5, Y. Xie5, R. Xu5, S. Xue5, J. Zhang5, L. Zhang5, W. Zhao5, D. Abbaneo6, T. Barklow6,50,
O. Buchm¨ uller6,50, M. Cattaneo6, B. Clerbaux6,48, H. Drevermann6, R.W. Forty6, M. Frank6, F. Gianotti6,
J.B. Hansen6, J. Harvey6, D.E. Hutchcroft6,52, P. Janot6, B. Jost6, M. Kado6,37, P. Mato6, A. Moutoussi6,
F. Ranjard6, L. Rolandi6, D. Schlatter6, F. Teubert6, A. Valassi6, I. Videau6, F. Badaud7, S. Dessagne7,
A. Falvard7,46, D. Fayolle7, P. Gay7, J. Jousset7, B. Michel7, S. Monteil7, D. Pallin7, J.M. Pascolo7, P. Perret7,
J.D. Hansen8, J.R. Hansen8, P.H. Hansen8, A.C. Kraan8, B.S. Nilsson8, A. Kyriakis9, C. Markou9, E. Simopoulou9,
A. Vayaki9, K. Zachariadou9, A. Blondel10,42, J.-C. Brient10, F. Machefert10, A. Roug´ e10, H. Videau10, V. Ciulli11,
E. Focardi11, G. Parrini11, A. Antonelli12, M. Antonelli12, G. Bencivenni12, F. Bossi12, G. Capon12, F. Cerutti12,
V. Chiarella12, G. Mannocchi12, P. Laurelli12, G. Mannocchi12,40, G.P. Murtas12, L. Passalacqua12, J. Kennedy13,
J.G. Lynch13, P. Negus13, V. O’Shea13, A.S. Thompson13, S. Wasserbaech14, R. Cavanaugh15,39,
S. Dhamotharan15,47, C. Geweniger15, P. Hanke15, V. Hepp15, E.E. Kluge15, A. Putzer15, H. Stenzel15, K. Tittel15,
M. Wunsch15,45, R. Beuselinck16, W. Cameron16, G. Davies16, P.J. Dornan16, M. Girone16,36, N. Marinelli16,
J. Nowell16, S.A. Rutherford16, J.K. Sedgbeer16, J.C. Thompson16b, R. White16, V.M. Ghete17, P. Girtler17,
E. Kneringer17, D. Kuhn17, G. Rudolph17, E. Bouhova-Thacker18, C.K. Bowdery18, D.P. Clarke18, G. Ellis18,
A.J. Finch18, F. Foster18, G. Hughes18, R.W.L. Jones18, M.R. Pearson18, N.A. Robertson18, M. Smizanska18,
O. van der Aa19, C. Delaere19c, G. Leibenguth19d, V. Lemaitre19e, U. Blumenschein20, F. H¨ olldorfer20, K. Jakobs20,
F. Kayser20, A.-S. M¨ uller20, B. Renk20, H.-G. Sander20, S. Schmeling20, H. Wachsmuth20, C. Zeitnitz20, T. Ziegler20,
A. Bonissent21, P. Coyle21, C. Curtil21, A. Ealet21, D. Fouchez21, P. Payre21, A. Tilquin21, F. Ragusa22, A. David23,
H. Dietl23,53, G. Ganis23,51, K. H¨ uttmann23, G. L¨ utjens23, W. M¨ anner23,53, H.-G. Moser23, R. Settles23,
M. Villegas23, G. Wolf23, J. Boucrot24, O. Callot24, M. Davier24, L. Duflot24, J.-F. Grivaz24, P. Heusse24,
A. Jacholkowska24,41, L. Serin24, J.-J. Veillet24, P. Azzurri25, G. Bagliesi25, T. Boccali25, L. Fo` a25, A. Giammanco25,
A. Giassi25, F. Ligabue25, A. Messineo25, F. Palla25, G. Sanguinetti25, A. Sciab` a25, G. Sguazzoni25, P. Spagnolo25,
R. Tenchini25, A. Venturi25, P.G. Verdini25, O. Awunor26, G.A. Blair26, G. Cowan26, A. Garcia-Bellido26,
M.G. Green26, T. Medcalf26,†, A. Misiejuk26, J.A. Strong26, P. Teixeira-Dias26, R.W. Clifft27, T.R. Edgecock27,
P.R. Norton27, I.R. Tomalin27, J.J. Ward27, B. Bloch-Devaux28, D. Boumediene28, P. Colas28, B. Fabbro28,
E. Lan¸ con28, M.-C. Lemaire28, E. Locci28, P. Perez28, J. Rander28, B. Tuchming28, B. Vallage28, A.M. Litke29,
G. Taylor29, C.N. Booth30, S. Cartwright30, F. Combley30,†, P.N. Hodgson30, M. Lehto30, L.F. Thompson30,
A. B¨ ohrer31, S. Brandt31, C. Grupen31, J. Hess31, A. Ngac31, G. Prange31, C. Borean32, G. Giannini32, H. He33,
J. Putz33, J. Rothberg33, S.R. Armstrong34, K. Berkelman34, K. Cranmer34, D.P.S. Ferguson34, Y. Gao34,43,
S. Gonz´ alez34, O.J. Hayes34, H. Hu34, S. Jin34, J. Kile34, P.A. McNamara III34, J. Nielsen34, Y.B. Pan34,
J.H. von Wimmersperg-Toeller34, W. Wiedenmann34, J. Wu34, S.L. Wu34, X. Wu34, G. Zobernig34,
G. Dissertori35
The DELPHI Collaboration
J. Abdallah82, P. Abreu79, W. Adam111, P. Adzic68, T. Albrecht74, T. Alderweireld55,56,57, R. Alemany-Fernandez65,
T. Allmendinger74, P.P. Allport80, U. Amaldi86, N. Amapane104, S. Amato108, E. Anashkin93, A. Andreazza85,
S. Andringa79, N. Anjos79, P. Antilogus82, W.-D. Apel74, Y. Arnoud71, S. Ask83, B. Asman103, J.E. Augustin82,
A. Augustinus65, P. Baillon65, A. Ballestrero105, P. Bambade77, R. Barbier84, D. Bardin73, G.J. Barker74,
A. Baroncelli96, M. Battaglia65, M. Baubillier82, K.-H. Becks113, M. Begalli61,62,63, A. Behrmann113,
E. Ben-Haim77, N. Benekos89, A. Benvenuti60, C. Berat71, M. Berggren82, L. Berntzon103, D. Bertrand55,56,57,
M. Besancon97, N. Besson97, D. Bloch66, M. Blom88, M. Bluj112, M. Bonesini86, M. Boonekamp97,
P.S.L. Booth80,†, G. Borisov78, O. Botner109, B. Bouquet77, T.J.V. Bowcock80, I. Boyko73, M. Bracko100,101,102,

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548The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
R. Brenner109, E. Brodet92, P. Bruckman75, J.M. Brunet64, B. Buschbeck111, P. Buschmann113, M. Calvi86,
T. Camporesi65, V. Canale95, F. Carena65, N. Castro79, F. Cavallo60, M. Chapkin99, P. Charpentier65,
P. Checchia93, R. Chierici65, P. Chliapnikov99, J. Chudoba65, S.U. Chung65, K. Cieslik75, P. Collins65, R. Contri70,
G. Cosme77, F. Cossutti106,107, M.J. Costa110, D. Crennell94, J. Cuevas91, J. D’Hondt55,56,57, J. Dalmau103,
T. da Silva108, W. Da Silva82, G. Della Ricca106,107, A. De Angelis106,107, W. De Boer74, C. De Clercq55,56,57,
B. De Lotto106,107, N. De Maria104, A. De Min93, L. de Paula108, L. Di Ciaccio95, A. Di Simone96, K. Doroba112,
J. Drees113,65, G. Eigen59, T. Ekelof109, M. Ellert109, M. Elsing65, M.C. Espirito Santo79, G. Fanourakis68,
D. Fassouliotis68,58, M. Feindt74, J. Fernandez98, A. Ferrer110, F. Ferro70, U. Flagmeyer113, H. Foeth65,
E. Fokitis89, F. Fulda-Quenzer77, J. Fuster110, M. Gandelman108, C. Garcia110, P. Gavillet65, E. Gazis89,
R. Gokieli65,112, B. Golob100,101,102, G. Gomez-Ceballos98, P. Goncalves79, E. Graziani96, G. Grosdidier77,
K. Grzelak112, J. Guy94, C. Haag74, A. Hallgren109, K. Hamacher113, K. Hamilton92, S. Haug90, F. Hauler74,
V. Hedberg83, M. Hennecke74, H. Herr65,†, J. Hoffman112, S.-O. Holmgren103, P.J. Holt65, M.A. Houlden80,
K. Hultqvist103, J.N. Jackson80, G. Jarlskog83, P. Jarry97, D. Jeans92, E.K. Johansson103, P.D. Johansson103,
P. Jonsson84, C. Joram65, L. Jungermann74, F. Kapusta82, S. Katsanevas84, E. Katsoufis89, G. Kernel100,101,102,
B.P. Kersevan65,100,101,102, U. Kerzel74, B.T. King80, N.J. Kjaer65, P. Kluit88, P. Kokkinias68, C. Kourkoumelis58,
O. Kouznetsov73, Z. Krumstein73, M. Kucharczyk75, J. Lamsa54, G. Leder111, F. Ledroit71, L. Leinonen103,
R. Leitner87, J. Lemonne55,56,57, V. Lepeltier77, T. Lesiak75, W. Liebig113, D. Liko111, A. Lipniacka103,
J.H. Lopes108, J.M. Lopez91, D. Loukas68, P. Lutz97, L. Lyons92, J. MacNaughton111, A. Malek113, S. Maltezos89,
F. Mandl111, J. Marco98, R. Marco98, B. Marechal108, M. Margoni93, J.-C. Marin65, C. Mariotti65, A. Markou68,
C. Martinez-Rivero98, J. Masik69, N. Mastroyiannopoulos68, F. Matorras98, C. Matteuzzi86, F. Mazzucato93,
M. Mazzucato93, R. Mc Nulty80, C. Meroni85, E. Migliore104, W. Mitaroff111, U. Mjoernmark83, T. Moa103,
M. Moch74, K. Moenig65,67, R. Monge70, J. Montenegro88, D. Moraes108, S. Moreno79, P. Morettini70, U. Mueller113,
K. Muenich113, M. Mulders88, L. Mundim61,62,63, W. Murray94, B. Muryn76, G. Myatt92, T. Myklebust90,
M. Nassiakou68, F. Navarria60, K. Nawrocki112, R. Nicolaidou97, M. Nikolenko73,66, A. Oblakowska-Mucha76,
V. Obraztsov99, A. Olshevski73, A. Onofre79, R. Orava72, K. Osterberg72, A. Ouraou97, A. Oyanguren110,
M. Paganoni86, S. Paiano60, J.P. Palacios80, H. Palka75, T.D. Papadopoulou89, L. Pape65, C. Parkes81, F. Parodi70,
U. Parzefall65, A. Passeri96, O. Passon113, L. Peralta79, V. Perepelitsa110, A. Perrotta60, A. Petrolini70,
J. Piedra98, L. Pieri96, F. Pierre97, M. Pimenta79, E. Piotto65, T. Podobnik100,101,102, V. Poireau65,
M.E. Pol61,62,63, G. Polok75, V. Pozdniakov73, N. Pukhaeva55,56,57,73, A. Pullia86, J. Rames69, A. Read90,
P. Rebecchi65, J. Rehn74, D. Reid88, R. Reinhardt113, P. Renton92, F. Richard77, J. Ridky69, M. Rivero98,
D. Rodriguez98, A. Romero104, P. Ronchese93, P. Roudeau77, T. Rovelli60, V. Ruhlmann-Kleider97,
D. Ryabtchikov99, A. Sadovsky73, L. Salmi72, J. Salt110, C. Sander74, A. Savoy-Navarro82, U. Schwickerath65,
A. Segar92,†, R. Sekulin94, M. Siebel113, A. Sisakian73, G. Smadja84, O. Smirnova83, A. Sokolov99, A. Sopczak78,
R. Sosnowski112, T. Spassov65, M. Stanitzki74, A. Stocchi77, J. Strauss111, B. Stugu59, M. Szczekowski112,
M. Szeptycka112, T. Szumlak76, T. Tabarelli86, A.C. Taffard80, F. Tegenfeldt109, J. Timmermans88, L. Tkatchev73,
M. Tobin80, S. Todorovova69, B. Tome79, A. Tonazzo86, P. Tortosa110, P. Travnicek69, D. Treille65, G. Tristram64,
M. Trochimczuk112, C. Troncon85, M.-L. Turluer97, I.A. Tyapkin73, P. Tyapkin73, S. Tzamarias68, V. Uvarov99,
G. Valenti60, P. Van Dam88, J. Van Eldik65, N. van Remortel72, I. Van Vulpen65, G. Vegni85, F. Veloso79,
W. Venus94, P. Verdier84, V. Verzi95, D. Vilanova97, L. Vitale106,107, V. Vrba69, H. Wahlen113, A.J. Washbrook80,
C. Weiser74, D. Wicke65, J. Wickens55,56,57, G. Wilkinson92, M. Winter66, M. Witek75, O. Yushchenko99,
A. Zalewska75, P. Zalewski112, D. Zavrtanik100,101,102, V. Zhuravlov73, N.I. Zimin73, A. Zintchenko73, M. Zupan68
The L3 Collaboration
P. Achard133, O. Adriani130, M. Aguilar-Benitez138, J. Alcaraz138, G. Alemanni136, J. Allaby131, A. Aloisio142,
M.G. Alviggi142, H. Anderhub162, V.P. Andreev119,147, F. Anselmo121, A. Arefiev141, T. Azemoon116, T. Aziz122,
P. Bagnaia152, A. Bajo138, G. Baksay139, L. Baksay139, S.V. Baldew115, S. Banerjee122, Sw. Banerjee117,
A. Barczyk162,160, R. Barill` ere131, P. Bartalini136, M. Basile121, N. Batalova159, R. Battiston146, A. Bay136,
F. Becattini130, U. Becker126, F. Behner162, L. Bellucci130, R. Berbeco116, J. Berdugo138, P. Berges126,
B. Bertucci146, B.L. Betev162, M. Biasini146, M. Biglietti142, A. Biland162, J.J. Blaising117, S.C. Blyth148,
G.J. Bobbink115, A. B¨ ohm114, L. Boldizsar125, B. Borgia152, S. Bottai130, D. Bourilkov162, M. Bourquin133,
S. Braccini133, J.G. Branson154, F. Brochu117, J.D. Burger126, W.J. Burger146, X.D. Cai126, M. Capell126,
G. Cara Romeo121, G. Carlino142, A. Cartacci130, J. Casaus138, F. Cavallari152, N. Cavallo149, C. Cecchi146,
M. Cerrada138, M. Chamizo133, Y.H. Chang157, M. Chemarin137, A. Chen157, G. Chen120, G.M. Chen120,
H.F. Chen135, H.S. Chen120, G. Chiefari142, L. Cifarelli153, F. Cindolo121, I. Clare126, R. Clare151, G. Coignet117,
N. Colino138, S. Costantini152, B. de la Cruz138, S. Cucciarelli146, R. de Asmundis142, P. D´ eglon133,
J. Debreczeni125, A. Degr´ e117, K. Dehmelt139, K. Deiters160, D. della Volpe142, E. Delmeire133, P. Denes150,
F. DeNotaristefani152, A. De Salvo162, M. Diemoz152, M. Dierckxsens115, C. Dionisi152, M. Dittmar162, A. Doria142,
M.T. Dova123,f, D. Duchesneau117, M. Duda114, B. Echenard133, A. Eline131, A. El Hage114, H. El Mamouni137,
A. Engler148, F.J. Eppling126, P. Extermann133, M.A. Falagan138, S. Falciano152, A. Favara145, J. Fay137,
O. Fedin147, M. Felcini162, T. Ferguson148, H. Fesefeldt114, E. Fiandrini146, J.H. Field133, F. Filthaut144,

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The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP549
P.H. Fisher126, W. Fisher150, G. Forconi126, K. Freudenreich162, C. Furetta140, Yu. Galaktionov141,126,
S.N. Ganguli122, P. Garcia-Abia138, M. Gataullin145, S. Gentile152, S. Giagu152, Z.F. Gong135, G. Grenier137,
O. Grimm162, M.W. Gruenewald129, M. Guida153, V.K. Gupta150, A. Gurtu122, L.J. Gutay159, D. Haas118,
D. Hatzifotiadou121, T. Hebbeker114, A. Herv´ e131, J. Hirschfelder148, H. Hofer162, M. Hohlmann139, G. Holzner162,
S.R. Hou157, J. Hu144, B.N. Jin120, P. Jindal127, L.W. Jones116, P. de Jong115, I. Josa-Mutuberr´ ıa138, M. Kaur127,
M.N. Kienzle-Focacci133, J.K. Kim156, J. Kirkby131, W. Kittel144, A. Klimentov126,141, A.C. K¨ onig144, M. Kopal159,
V. Koutsenko126,141, M. Kr¨ aber162, R.W. Kraemer148, A. Kr¨ uger161, A. Kunin126, P. Ladron de Guevara138,
I. Laktineh137, G. Landi130, M. Lebeau131, A. Lebedev126, P. Lebrun137, P. Lecomte162, P. Lecoq131,
P. Le Coultre162, J.M. Le Goff131, R. Leiste161, M. Levtchenko140, P. Levtchenko147, C. Li135, S. Likhoded161,
C.H. Lin157, W.T. Lin157, F.L. Linde115, L. Lista142, Z.A. Liu120, W. Lohmann161, E. Longo152, Y.S. Lu120,
C. Luci152, L. Luminari152, W. Lustermann162, W.G. Ma135, L. Malgeri131, A. Malinin141, C. Ma na138, J. Mans150,
J.P. Martin137, F. Marzano152, K. Mazumdar122, R.R. McNeil119, S. Mele131,142, L. Merola142, M. Meschini130,
W.J. Metzger144, A. Mihul124, H. Milcent131, G. Mirabelli152, J. Mnich114, G.B. Mohanty122, G.S. Muanza137,
A.J.M. Muijs115, B. Musicar154, M. Musy152, S. Nagy128, S. Natale133, M. Napolitano142, F. Nessi-Tedaldi162,
H. Newman145, A. Nisati152, T. Novak144, H. Nowak161, R. Ofierzynski162, G. Organtini152, I. Pal159,
C. Palomares138, P. Paolucci142, R. Paramatti152, G. Passaleva130, S. Patricelli142, T. Paul123, M. Pauluzzi146,
C. Paus126, F. Pauss162, M. Pedace152, S. Pensotti140, D. Perret-Gallix117, D. Piccolo142, F. Pierella121, M. Pieri154,
M. Pioppi146, P.A. Pirou´ e150, E. Pistolesi140, V. Plyaskin141, M. Pohl133, V. Pojidaev130, J. Pothier131,
D. Prokofiev147, G. Rahal-Callot162, M.A. Rahaman122, P. Raics128, N. Raja122, R. Ramelli162, P.G. Rancoita140,
R. Ranieri130, A. Raspereza161, P. Razis143, S. Rembeczki139, D. Ren162, M. Rescigno152, S. Reucroft123,
S. Riemann161, K. Riles116, B.P. Roe116, L. Romero138, A. Rosca161, C. Rosemann114, C. Rosenbleck114,
S. Rosier-Lees117, S. Roth114, J.A. Rubio131, G. Ruggiero130, H. Rykaczewski162, A. Sakharov162, S. Saremi119,
S. Sarkar152, J. Salicio131, E. Sanchez138, C. Sch¨ afer131, V. Schegelsky147, H. Schopper134, D.J. Schotanus144,
C. Sciacca142, L. Servoli146, S. Shevchenko145, N. Shivarov155, V. Shoutko126, E. Shumilov141, A. Shvorob145,
D. Son156, C. Souga137, P. Spillantini130, M. Steuer126, D.P. Stickland150, B. Stoyanov155, A. Straessner133,
K. Sudhakar122, G. Sultanov155, L.Z. Sun135, S. Sushkov114, H. Suter162, J.D. Swain123, Z. Szillasi139,g,
X.W. Tang120, P. Tarjan128, L. Tauscher118, L. Taylor123, B. Tellili137, D. Teyssier137, C. Timmermans144,
S.C.C. Ting126, S.M. Ting126, S.C. Tonwar122, J. T´ oth125, C. Tully150, K.L. Tung120, J. Ulbricht162, E. Valente152,
R.T. Van de Walle144, R. Vasquez159, G. Vesztergombi125, I. Vetlitsky141, G. Viertel162, M. Vivargent117,
S. Vlachos118, I. Vodopianov139, H. Vogel148, H. Vogt161, I. Vorobiev148,141, A.A. Vorobyov147, M. Wadhwa118,
Q. Wang144, X.L. Wang135, Z.M. Wang135, M. Weber131, S. Wynhoff150, L. Xia145, Z.Z. Xu135, J. Yamamoto116,
B.Z. Yang135, C.G. Yang120, H.J. Yang116, M. Yang120, S.C. Yeh158, An. Zalite147, Yu. Zalite147, Z.P. Zhang135,
J. Zhao135, G.Y. Zhu120, R.Y. Zhu145, H.L. Zhuang120, A. Zichichi121,131,132, B. Zimmermann162, M. Z¨ oller114
The OPAL Collaboration
G. Abbiendi164, C. Ainsley167, P.F.˚ Akesson165,219, G. Alexander183, J. Allison177, P. Amaral170, G. Anagnostou163,
K.J. Anderson170, S. Asai184,185, D. Axen189, G. Azuelos179,196, I. Bailey188, E. Barberio169,210, T. Barillari194,
R.J. Barlow177, R.J. Batley167, P. Bechtle187,197, T. Behnke187, K.W. Bell181, P.J. Bell163, G. Bella183,
A. Bellerive168, G. Benelli166, S. Bethke194, O. Biebel193, O. Boeriu171, P. Bock172, M. Boutemeur193, S. Braibant169,
L. Brigliadori164, R.M. Brown181, K. Buesser187, H.J. Burckhart169, S. Campana166, R.K. Carnegie168,
A.A. Carter174, J.R. Carter167, C.Y. Chang178, D.G. Charlton163, C. Ciocca164, A. Csilling191, M. Cuffiani164,
S. Dado182, S. de Jong173,214, A. De Roeck169, E.A. De Wolf169,213h, K. Desch187, B. Dienes192, M. Donkers168,
J. Dubbert193, E. Duchovni186, G. Duckeck193, I.P. Duerdoth177, E. Etzion183, F. Fabbri164, L. Feld171, P. Ferrari169,
F. Fiedler193, I. Fleck171, M. Ford167, A. Frey169, P. Gagnon173, J.W. Gary166, S.M. Gascon-Shotkin178,
G. Gaycken187, C. Geich-Gimbel165, G. Giacomelli164, P. Giacomelli164, M. Giunta166, J. Goldberg182, E. Gross186,
J. Grunhaus183, M. Gruw´ e169, P.O. G¨ unther165, A. Gupta170, C. Hajdu191, M. Hamann187, G.G. Hanson166,
A. Harel182, M. Hauschild169, C.M. Hawkes163, R. Hawkings169, R.J. Hemingway168, G. Herten171, R.D. Heuer187,
J.C. Hill167, K. Hoffman170, D. Horv´ ath191,198, P. Igo-Kemenes172, K. Ishii184,185, H. Jeremie179, U. Jost172,
P. Jovanovic163, T.R. Junk168,203, N. Kanaya188, J. Kanzaki184,185,215, D. Karlen188, K. Kawagoe184,185,
T. Kawamoto184,185, R.K. Keeler188, R.G. Kellogg178, B.W. Kennedy181, S. Kluth194, T. Kobayashi184,185,
M. Kobel165, S. Komamiya184,185, T. Kr¨ amer187, P. Krieger168,206, J. von Krogh172, K. Kruger169, T. Kuhl187,
M. Kupper186, G.D. Lafferty177, H. Landsman182, D. Lanske175, J.G. Layter166, D. Lellouch186, J. Letts209,
L. Levinson186, J. Lillich171, S.L. Lloyd174, F.K. Loebinger177, J. Lu189,217, A. Ludwig165, J. Ludwig171,
W. Mader165, S. Marcellini164, A.J. Martin174, G. Masetti164, T. Mashimo184,185, P. M¨ attig207, J. McKenna189,
R.A. McPherson188, F. Meijers169, W. Menges187, F.S. Merritt170, H. Mes168,196, N. Meyer187, A. Michelini164,
S. Mihara184,185, G. Mikenberg186, D.J. Miller176, S. Moed182, W. Mohr171, T. Mori184,185, A. Mutter171,
K. Nagai174, I. Nakamura184,185,216, H. Nanjo184,185, H.A. Neal195, R. Nisius194, S.W. O’Neale163,†, A. Oh169,
M.J. Oreglia170, S. Orito184,185,†, C. Pahl194, G. P´ asztor166,201, J.R. Pater177, J.E. Pilcher170, J. Pinfold190,
D.E. Plane169, B. Poli164, O. Pooth175, M. Przybycie´ n169,208, A. Quadt165, K. Rabbertz169,212, C. Rembser169,
P. Renkel186, J.M. Roney188, Y. Rozen182, K. Runge171, K. Sachs168, T. Saeki184,185, E.K.G. Sarkisyan169,204,

Page 4

550 The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
A.D. Schaile193, O. Schaile193, P. Scharff-Hansen169, J. Schieck194, T. Sch¨ orner-Sadenius169,220, M. Schr¨ oder169,
M. Schumacher165, W.G. Scott181, R. Seuster175,200, T.G. Shears169,202, B.C. Shen166, P. Sherwood176, A. Skuja178,
A.M. Smith169, R. Sobie188, S. S¨ oldner-Rembold176, F. Spano170, A. Stahl165,218, D. Strom180, R. Str¨ ohmer193,
S. Tarem182, M. Tasevsky169,213,g, R. Teuscher170, M.A. Thomson167, E. Torrence180, D. Toya184,185, P. Tran166,
I. Trigger169, Z. Tr´ ocs´ anyi192,199, E. Tsur183, M.F. Turner-Watson163, I. Ueda184,185, B. Ujv´ ari192,199,
C.F. Vollmer193, P. Vannerem171, R. V´ ertesi192,199, M. Verzocchi178, H. Voss169,211, J. Vossebeld169,202,
C.P. Ward167, D.R. Ward167, P.M. Watkins163, A.T. Watson163, N.K. Watson163, P.S. Wells169, T. Wengler169,
N. Wermes165, G.W. Wilson177,205, J.A. Wilson163, G. Wolf186, T.R. Wyatt177, S. Yamashita184,185, D. Zer-Zion166,
L. Zivkovic186,
S. Heinemeyer221,222, A. Pilaftsis223, G. Weiglein224
1Physikalisches Institut das RWTH-Aachen, 52056 Aachen, Germany
2Laboratoire de Physique des Particules (LAPP), IN2P3-CNRS, 74019 Annecy-le-Vieux Cedex, France
3Institut de F´isica d’Altes Energies, Universitat Aut` onoma de Barcelona, 08193 Bellaterra (Barcelona), Spaini
4Dipartimento di Fisica, INFN Sezione di Bari, 70126 Bari, Italy
5Institute of High Energy Physics, Academia Sinica, Beijing, P.R. Chinaj
6European Laboratory for Particle Physics (CERN), 1211 Geneva 23, Switzerland
7Laboratoire de Physique Corpusculaire, Universit´ e Blaise Pascal, IN2P3-CNRS, Clermont-Ferrand, 63177 Aubi` ere, France
8Niels Bohr Institute, 2100 Copenhagen, Denmarkk
9Nuclear Research Center Demokritos (NRCD), 15310 Attiki, Greece
10Laoratoire Leprince-Ringuet, Ecole Polytechnique, IN2P3-CNRS, 91128 Palaiseau Cedex, France
11Dipartimento di Fisica, Universit` a di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy
12Laboratori Nazionali dell’INFN (LNF-INFN), 00044 Frascati, Italy
13Department of Physics and Astronomy, University of Glasgow, Glasgow G128QQ, UKl
14Utah Valley State College, Orem, UT 84058, U.S.A.
15Kirchhoff-Institut f¨ ur Physik, Universit¨ at Heidelberg, 69120 Heidelberg, Germanym
16Department of Physics, Imperial College, London SW7 2BZ, UKk
17Institut f¨ ur Experimentalphysik, Universit¨ at Innsbruck, 6020 Innsbruck, Austrian
18Department of Physics, University of Lancaster, Lancaster LA1 4YB, UKk
19Institut de Physique Nucl´ eaire, D´ epartement de Physique, Universit´ e Catholique de Louvain, 1348 Louvain-la-Neuve,
Belgium
20Institut f¨ ur Physik, Universit¨ at Mainz, 55099 Mainz, Germanyo
21Centre de Physique des Particules de Marseille, Univ M´ editerran´ ee, IN2P3-CNRS, 13288 Marseille, France
22Dipartimento di Fisica, Universit` a di Milano e INFN Sezione di Milano, 20133 Milano, Italy
23Max-Planck-Institut f¨ ur Physik, Werner-Heisenberg-Institut, 80805 M¨ unchen, Germanyn
24Laboratoire de l’Acc´ el´ erateur Lin´ eaire, Universit´ e de Paris-Sud, IN2P3-CNRS, 91898 Orsay Cedex, France
25Dipartimento di Fisica dell’Universit` a, INFN Sezione di Pisa, e Scuola Normale Superiore, 56010 Pisa, Italy
26Department of Physics, Royal Holloway & Bedford New College, University of London, Egham, Surrey TW20 OEX, UKk
27Particle Physics Dept., Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UKk
28CEA, DAPNIA/Service de Physique des Particules, CE-Saclay, 91191 Gif-sur-Yvette Cedex, Francep
29Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USAq
30Department of Physics, University of Sheffield, Sheffield S3 7RH, UKk
31Fachbereich Physik, Universit¨ at Siegen, 57068 Siegen, Germanyn
32Dipartimento di Fisica, Universit` a di Trieste e INFN Sezione di Trieste, 34127 Trieste, Italy
33Experimental Elementary Particle Physics, University of Washington, Seattle, WA 98195, U.S.A.
34Department of Physics, University of Wisconsin, Madison, WI 53706, USAl
35Institute for Particle Physics, ETH H¨ onggerberg, 8093 Z¨ urich, Switzerland
36Also at CERN, 1211 Geneva 23, Switzerland
37Now at Fermilab, PO Box 500, MS 352, Batavia, IL 60510, USA
38Also at Dipartimento di Fisica di Catania and INFN Sezione di Catania, 95129 Catania, Italy
39Now at University of Florida, Department of Physics, Gainesville, Florida 32611-8440, USA
40Also IFSI sezione di Torino, INAF, Italy
41Also at Groupe d’Astroparticules de Montpellier, Universit´ e de Montpellier II, 34095 Montpellier, France
42Now at Departement de Physique Corpusculaire, Universit´ e de Gen` eve, 1211 Gen` eve 4, Switzerland
43Also at Department of Physics, Tsinghua University, Beijing, P.R. China
44Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain
45Now at SAP AG, 69185 Walldorf, Germany
The ALEPH Collaboration

Page 5

The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP 551
46Now at Groupe d’ Astroparticules de Montpellier, Universit´ e de Montpellier II, 34095 Montpellier, France
47Now at BNP Paribas, 60325 Frankfurt am Mainz, Germany
48Now at Institut Inter-universitaire des hautes Energies (IIHE), CP 230, Universit´ e Libre de Bruxelles, 1050 Bruxelles,
Belgique
49Now at Dipartimento di Fisica e Tecnologie Relative, Universit` a di Palermo, Palermo, Italy
50Now at SLAC, Stanford, CA 94309, USA
51Now at CERN, 1211 Geneva 23, Switzerland
52Now at Liverpool University, Liverpool L69 7ZE, UK
53Now at Henryk Niewodnicznski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland
The DELPHI Collaboration
54Department of Physics and Astronomy, Iowa State University, Ames IA 50011-3160, USA
55Physics Department, Universiteit Antwerpen, Universiteitsplein 1, 2610 Antwerpen, Belgium
56IIHE, ULB-VUB, Pleinlaan 2, 1050 Brussels, Belgium
57Facult´ e des Sciences, Univ. de l’Etat Mons, Av. Maistriau 19, 7000 Mons, Belgium
58Physics Laboratory, University of Athens, Solonos Str. 104, 10680 Athens, Greece
59Department of Physics, University of Bergen, All´ egaten 55, 5007 Bergen, Norway
60Dipartimento di Fisica, Universit` a di Bologna and INFN, Via Irnerio 46, 40126 Bologna, Italy
61Centro Brasileiro de Pesquisas F´ ısicas, rua Xavier Sigaud 150, 22290 Rio de Janeiro, Brazil
62Depto. de F´ ısica, Pont. Univ. Cat´ olica, C.P. 38071, 22453 Rio de Janeiro, Brazil
63Inst. de F´ ısica, Univ. Estadual do Rio de Janeiro, rua S˜ ao Francisco Xavier 524, Rio de Janeiro, Brazil
64Coll` ege de France, Lab. de Physique Corpusculaire, IN2P3-CNRS, 75231 Paris Cedex 05, France
65CERN, 1211 Geneva 23, Switzerland
66Institut de Recherches Subatomiques, IN2P3 - CNRS/ULP - BP20, 67037 Strasbourg Cedex, France
67Now at DESY-Zeuthen, Platanenallee 6, 15735 Zeuthen, Germany
68Institute of Nuclear Physics, N.C.S.R. Demokritos, P.O. Box 60228, 15310 Athens, Greece
69FZU, Inst. of Phys. of the C.A.S. High Energy Physics Division, Na Slovance 2, 180 40 Praha 8, Czech Republic
70Dipartimento di Fisica, Universit` a di Genova and INFN, Via Dodecaneso 33, 16146 Genova, Italy
71Institut des Sciences Nucl´ eaires, IN2P3-CNRS, Universit´ e de Grenoble 1, 38026 Grenoble Cedex, France
72Helsinki Institute of Physics and Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland
73Joint Institute for Nuclear Research, Dubna, Head Post Office, P.O. Box 79, 101 000 Moscow, Russian Federation
74Institut f¨ ur Experimentelle Kernphysik, Universit¨ at Karlsruhe, Postfach 6980, 76128 Karlsruhe, Germany
75Institute of Nuclear Physics PAN,Ul. Radzikowskiego 152, 31142 Krakow, Poland
76Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, 30055 Krakow, Poland
77Universit´ e de Paris-Sud, Lab. de l’Acc´ el´ erateur Lin´ eaire, IN2P3-CNRS, Bˆ at. 200, 91405 Orsay Cedex, France
78School of Physics and Chemistry, University of Lancaster, Lancaster LA1 4YB, UK
79LIP, IST, FCUL - Av. Elias Garcia, 14-1o, 1000 Lisboa Codex, Portugal
80Department of Physics, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK
81Dept. of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK
82LPNHE, IN2P3-CNRS, Univ. Paris VI et VII, Tour 33 (RdC), 4 place Jussieu, 75252 Paris Cedex 05, France
83Department of Physics, University of Lund, S¨ olvegatan 14, 223 63 Lund, Sweden
84Universit´ e Claude Bernard de Lyon, IPNL, IN2P3-CNRS, 69622 Villeurbanne Cedex, France
85Dipartimento di Fisica, Universit` a di Milano and INFN-MILANO, Via Celoria 16, 20133 Milan, Italy
86Dipartimento di Fisica, Univ. di Milano-Bicocca and INFN-MILANO, Piazza della Scienza 2, 20126 Milan, Italy
87IPNP of MFF, Charles Univ., Areal MFF, V Holesovickach 2, 180 00, Praha 8, Czech Republic
88NIKHEF, Postbus 41882, 1009 DB Amsterdam, The Netherlands
89National Technical University, Physics Department, Zografou Campus, 15773 Athens, Greece
90Physics Department, University of Oslo, Blindern, 0316 Oslo, Norway
91Dpto. Fisica, Univ. Oviedo, Avda. Calvo Sotelo s/n, 33007 Oviedo, Spain
92Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
93Dipartimento di Fisica, Universit` a di Padova and INFN, Via Marzolo 8, 35131 Padua, Italy
94Rutherford Appleton Laboratory, Chilton, Didcot OX11 OQX, UK
95Dipartimento di Fisica, Universit` a di Roma II and INFN, Tor Vergata, 00173 Rome, Italy
96Dipartimento di Fisica, Universit` a di Roma III and INFN, Via della Vasca Navale 84, 00146 Rome, Italy
97DAPNIA/Service de Physique des Particules, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
98Instituto de Fisica de Cantabria (CSIC-UC), Avda. los Castros s/n, 39006 Santander, Spain
99Inst. for High Energy Physics, Serpukov P.O. Box 35, Protvino, (Moscow Region), Russian Federation
100J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
101Laboratory for Astroparticle Physics, Nova Gorica Polytechnic, Kostanjeviska 16a, 5000 Nova Gorica, Slovenia
102Department of Physics, University of Ljubljana, 1000 Ljubljana, Slovenia
103Fysikum, Stockholm University, Box 6730, 113 85 Stockholm, Sweden
104Dipartimento di Fisica Sperimentale, Universit` a di Torino and INFN, Via P. Giuria 1, 10125 Turin, Italy

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552 The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
105INFN,Sezione di Torino and Dipartimento di Fisica Teorica, Universit` a di Torino, Via Giuria 1, 10125 Turin, Italy
106Dipartimento di Fisica, Universit` a di Trieste and INFN, Via A. Valerio 2, 34127 Trieste, Italy
107Istituto di Fisica, Universit` a di Udine, 33100 Udine, Italy
108Univ. Federal do Rio de Janeiro, C.P. 68528 Cidade Univ., Ilha do Fund˜ ao 21945-970 Rio de Janeiro, Brazil
109Department of Radiation Sciences, University of Uppsala, P.O. Box 535, 751 21 Uppsala, Sweden
110IFIC, Valencia-CSIC, and D.F.A.M.N., U. de Valencia, Avda. Dr. Moliner 50, 46100 Burjassot (Valencia), Spain
111Institut f¨ ur Hochenergiephysik,¨Osterr. Akad. d. Wissensch., Nikolsdorfergasse 18, 1050 Vienna, Austria
112Inst. Nuclear Studies and University of Warsaw, Ul. Hoza 69, 00681 Warsaw, Poland
113Fachbereich Physik, University of Wuppertal, Postfach 100 127, 42097 Wuppertal, Germany
The L3 Collaboration
114III. Physikalisches Institut, RWTH, 52056 Aachen, Germanyr
115National Institute for High Energy Physics, NIKHEF, and University of Amsterdam, 1009 DB Amsterdam, The
Netherlands
116University of Michigan, Ann Arbor, MI 48109, USA
117Laboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, 74941 Annecy-le-Vieux
CEDEX, France
118Institute of Physics, University of Basel, 4056 Basel, Switzerland
119Louisiana State University, Baton Rouge, LA 70803, USA
120Institute of High Energy Physics, IHEP, 100039 Beijing, Chinas
121University of Bologna and INFN-Sezione di Bologna, 40126 Bologna, Italy
122Tata Institute of Fundamental Research, Mumbai (Bombay) 400 005, India
123Northeastern University, Boston, MA 02115, USA
124Institute of Atomic Physics and University of Bucharest, 76900 Bucharest, Romania
125Central Research Institute for Physics of the Hungarian Academy of Sciences, 1525 Budapest 114, Hungaryt
126Massachusetts Institute of Technology, Cambridge, MA 02139, USA
127Panjab University, Chandigarh 160 014, India
128KLTE-ATOMKI, 4010 Debrecen, Hungaryf
129UCD School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
130INFN Sezione di Firenze and University of Florence, 50125 Florence, Italy
131European Laboratory for Particle Physics, CERN, 1211 Geneva 23, Switzerland
132World Laboratory, FBLJA Project, 1211 Geneva 23, Switzerland
133University of Geneva, 1211 Geneva 4, Switzerland
134University of Hamburg, 22761 Hamburg, Germany
135Chinese University of Science and Technology, USTC, Hefei, Anhui 230 029, Chinar
136University of Lausanne, 1015 Lausanne, Switzerland
137Institut de Physique Nucl´ eaire de Lyon, IN2P3-CNRS, Universit´ e Claude Bernard, 69622 Villeurbanne, France
138Centro de Investigaciones Energ´ eticas, Medioambientales y Tecnol´ ogicas, CIEMAT, 28040 Madrid, Spainu
139Florida Institute of Technology, Melbourne, FL 32901, USA
140INFN-Sezione di Milano, 20133 Milan, Italy
141Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia
142INFN-Sezione di Napoli and University of Naples, 80125 Naples, Italy
143Department of Physics, University of Cyprus, Nicosia, Cyprus
144Radboud University and NIKHEF, 6525 ED Nijmegen, The Netherlands
145California Institute of Technology, Pasadena, CA 91125, USA
146INFN-Sezione di Perugia and Universit` a Degli Studi di Perugia, 06100 Perugia, Italy
147Nuclear Physics Institute, St. Petersburg, Russia
148Carnegie Mellon University, Pittsburgh, PA 15213, USA
149INFN-Sezione di Napoli and University of Potenza, 85100 Potenza, Italy
150Princeton University, Princeton, NJ 08544, USA
151University of Californa, Riverside, CA 92521, USA
152INFN-Sezione di Roma and University of Rome, “La Sapienza”, 00185 Rome, Italy
153University and INFN, Salerno, 84100 Salerno, Italy
154University of California, San Diego, CA 92093, USA
155Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, 1113 Sofia, Bulgaria
156The Center for High Energy Physics, Kyungpook National University, 702-701 Taegu, Republic of Korea
157National Central University, Chung-Li, Taiwan, China
158Department of Physics, National Tsing Hua University, Taiwan, China
159Purdue University, West Lafayette, IN 47907, USA
160Paul Scherrer Institut, PSI, 5232 Villigen, Switzerland
161DESY, 15738 Zeuthen, Germany
162Eidgen¨ ossische Technische Hochschule, ETH Z¨ urich, 8093 Z¨ urich, Switzerland

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The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP553
The OPAL Collaboration
163School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
164Dipartimento di Fisica dell’ Universit` a di Bologna and INFN, 40126 Bologna, Italy
165Physikalisches Institut, Universit¨ at Bonn, 53115 Bonn, Germany
166Department of Physics, University of California, Riverside CA 92521, USA
167Cavendish Laboratory, Cambridge CB3 0HE, UK
168Ottawa-Carleton Institute for Physics, Department of Physics, Carleton University, Ottawa, Ontario K1S 5B6,
Canada
169CERN, European Organisation for Nuclear Research, 1211 Geneva 23, Switzerland
170Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago IL 60637, USA
171Fakult¨ at f¨ ur Physik, Albert-Ludwigs-Universit¨ at Freiburg, 79104 Freiburg, Germany
172Physikalisches Institut, Universit¨ at Heidelberg, 69120 Heidelberg, Germany
173Indiana University, Department of Physics, Bloomington IN 47405, USA
174Queen Mary and Westfield College, University of London, London E1 4NS, UK
175Technische Hochschule Aachen, III Physikalisches Institut, Sommerfeldstrasse 26–28, 52056 Aachen, Germany
176University College London, London WC1E 6BT, UK
177Department of Physics, Schuster Laboratory, The University, Manchester M13 9PL, UK
178Department of Physics, University of Maryland, College Park, MD 20742, USA
179Laboratoire de Physique Nucl´ eaire, Universit´ e de Montr´ eal, Montr´ eal, Qu´ ebec H3C 3J7, Canada
180University of Oregon, Department of Physics, Eugene OR 97403, USA
181CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
182Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel
183Department of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
184International Centre for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo 113-0033,
Japan
185Kobe University, Kobe 657-8501, Japan
186Particle Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel
187Universit¨ at Hamburg/DESY, Institut f¨ ur Experimentalphysik, Notkestrasse 85, 22607 Hamburg, Germany
188University of Victoria, Department of Physics, P O Box 3055, Victoria BC V8W 3P6, Canada
189University of British Columbia, Department of Physics, Vancouver BC V6T 1Z1, Canada
190University of Alberta, Department of Physics, Edmonton AB T6G 2J1, Canada
191Research Institute for Particle and Nuclear Physics, 1525 Budapest, P O Box 49, Hungary
192Institute of Nuclear Research, 4001 Debrecen, P O Box 51, Hungary
193Ludwig-Maximilians-Universit¨ at M¨ unchen, Sektion Physik, Am Coulombwall 1, 85748 Garching, Germany
194Max-Planck-Institute f¨ ur Physik, F¨ ohringer Ring 6, 80805 M¨ unchen, Germany
195Yale University, Department of Physics, New Haven, CT 06520, USA
196Now at TRIUMF, Vancouver V6T 2A3, Canada
197Now at SLAC, Stanford, CA 94309, USA
198And Institute of Nuclear Research, Debrecen, Hungary
199And Department of Experimental Physics, University of Debrecen, Hungary
200And MPI M¨ unchen, M¨ unchen, Germany
201And Research Institute for Particle and Nuclear Physics, Budapest, Hungary
202Now at University of Liverpool, Dept. of Physics, Liverpool L69 3BX, UK
203Now at Dept. Physics, University of Illinois at Urbana-Champaign, USA
204And Manchester University, Manchester, UK
205Now at University of Kansas, Dept. of Physics and Astronomy, Lawrence, KS 66045, USA
206Now at University of Toronto, Dept. of Physics, Toronto, Canada
207Current address Bergische Universit¨ at, Wuppertal, Germany
208Now at University of Mining and Metallurgy, Cracow, Poland
209Now at University of California, San Diego, USA
210Now at The University of Melbourne, Victoria, Australia
211Now at IPHE Universit´ e de Lausanne, 1015 Lausanne, Switzerland
212Now at IEKP Universit¨ at Karlsruhe, Germany
213Now at University of Antwerpen, Physics Department, 2610 Antwerpen, Belgium
214Now at University of Nijmegen, Nijmegen, The Netherlands
215And High Energy Accelerator Research Organisation (KEK), Tsukuba, Ibaraki, Japan
216Now at University of Pennsylvania, Philadelphia, Pennsylvania, USA
217Now at TRIUMF, Vancouver, Canada
218Now at DESY-Zeuthen, Zeuthen, Germany
219Now at CERN, 1211 Geneva 23, Switzerland
220Now at DESY, Hamburg, Germany

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554 The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
221Departamento de Fisica Teorica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain
222CERN TH division, Dept. of Physics, 1211 Geneva 23, Switzerland
223Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
224Institute for Particle Physics Phenomenology, University of Durham, Durham DH1 3LE, UK
†
Deceased
Received: 17 January 2006 / Revised version: 12 April 2006 /
Published online: 18 July 2006 − © Springer-Verlag / Societ` a Italiana di Fisica 2006
Abstract. The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neu-
tral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The
data of the four collaborations are statistically combined and examined for their consistency with the
background hypothesis and with a possible Higgs boson signal. The combined LEP data show no signifi-
cant excess of events which would indicate the production of Higgs bosons. The search results are used
to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are inter-
preted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating
scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Ab-
solute limits are set on the parameter tanβ and, in some scenarios, on the masses of neutral Higgs
bosons.
aThe LEP Working Group for Higgs Boson Searches consists
of members of the four LEP Collaborations and of theorists
among whom S. Heinemeyer, A. Pilaftsis and G. Weiglein are
authors of this paper.
bSupported by the Leverhulme Trust.
cResearch Fellow of the Belgium FNRS.
dSupported by the Federal Office for Scientific, Technical and
Cultural Affairs through the Interuniversity Attraction Pole
P5/27.
eResearch Associate of the Belgium FNRS.
fAlso supported by CONICET and Universidad Nacional de
La Plata, CC 67, 1900 La Plata, Argentina.
gAlso supported by the Hungarian OTKA fund under con-
tract number T026178.
hSupported by Interuniversity Attraction Poles Programme –
Belgian Science Policy.
iSupported by CICYT, Spain.
jSupported by the National Science Foundation of China.
kSupported by the Danish Natural Science Research Council.
lSupported by the UK Particle Physics and Astronomy Re-
search Council.
mSupported by the US Department of Energy, grant DE-
FG0295-ER40896.
nSupported by the Austrian Ministry for Science and Trans-
port.
oSupported by Bundesministerium f¨ ur Bildung und For-
schung, Germany.
pSupported by the Direction des Sciences de la Mati` ere,
C.E.A.
qSupported by the US Department of Energy, grant DE-
FG03-92ER40689.
rSupported by the German Bundesministerium f¨ ur Bildung,
Wissenschaft, Forschung und Technologie.
sSupported by the National Natural Science Foundation of
China.
tSupported by the Hungarian OTKA fund under contract
numbers T019181, F023259 and T037350.
uSupported also by the Comisi´ on Interministerial de Ciencia
y Tecnolog´ ıa.
1 Introduction
One of the outstanding questions in particle physics is
that of electroweak symmetry breaking and the origin of
mass. The leading candidate for an answer is the Higgs
mechanism [1] whereby fundamental scalar Higgs fields
acquire nonzero vacuum expectation values and sponta-
neously break the electroweak symmetry. Gauge bosons
and fermions obtain their masses by interacting with the
resulting vacuum Higgs fields. Associated with this de-
scription is the existence of massive scalar particles, the
Higgs bosons.
The standard model [2] requires one complex Higgs
field doublet and predicts a single neutral Higgs boson of
unknown mass. After extensive searches at LEP, a lower
bound of 114.4GeV/c2has been established for the mass
of the standard model Higgs boson, at the 95% confidence
level (CL) [3].
Supersymmetric (SUSY) [4] extensions of the standard
model are of interest since they provide a consistent frame-
work for the unification of the gauge interactions at a high
energy scale and for the stability of the electroweak scale.
SUSY is also a basic ingredient of models such as Super-
gravity and Superstring which aim at a unified description
of all fundamental forces, including the gravitational force.
It is remarkable that the predictions of SUSY extensions of
the standard model are found to be compatible with exist-
ing high-precision data [5].
In SUSY, each of the standard model fermions has
a bosonic superpartner “sfermion”, and each boson has
a fermionic superpartner “gaugino” or “Higgsino”. These
additional particles, if they exist, have several virtues.
Their presence modifies the renormalisation group evolu-
tion of the gauge couplings, improving the convergence of
the couplings at a unique (GUT) energy. The new par-
ticles lead to a naturally light Higgs boson (close to the
electroweak energy scale) since the divergent loop contri-
butions of the standard model particles are almost com-
pletely compensated by corresponding contributions from

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The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP555
the superpartners (the compensation would be perfect if
SUSY were an exact symmetry). SUSY models also pro-
vide a perfect cold dark matter candidate, the neutralino,
which arises from the mixing of the neutral gaugino and
Higgsino fields.
The Minimal Supersymmetricstandardmodel(MSSM)
(reviewed, e.g., in [6]) is the SUSY extension with minimal
new particle content. It requires two Higgs field doublets
and predicts the existence of three neutral and two charged
Higgs bosons. The lightest of the neutral Higgs bosons is
predicted to have a mass less than about 140GeV/c2in-
cluding radiative corrections [7]. This prediction provided
a strong motivation for the searches at LEP energies.
Most of the experimental investigations carried out in
the past at LEP and elsewhere were interpreted in MSSM
scenarios where CP conservation in the Higgs sector was
assumed. In such scenarios the neutral Higgs bosons are
CP eigenstates. However, CP violation in the Higgs sector
cannot be a priori excluded [8]. Scenarios with CP vio-
lation are theoretically appealing since they provide one
of the ingredients needed to explain the observed cosmic
matter-antimatter asymmetry. The observed size of CP
violation in B and K meson systems is not sufficient to
drive this asymmetry. In the MSSM, however, substan-
tial CP violation can be induced by complex phases in
the soft SUSY-breaking sector, through radiative correc-
tions, especially from third-generation scalar quarks [9]. In
such scenarios the three neutral Higgs mass eigenstates
are mixtures of CP-even and CP-odd fields, with pro-
duction and decay properties different from those in the
CP-conserving scenarios. Hence, the experimental exclu-
sions published so far for the CP-conserving MSSM sce-
narios may be weakened by CP-violating effects. There is
currently one publication on searches interpreted in CP-
violating scenarios [10].
In this paper we describe the results of a statistical
combination based on the searchesof the four LEPcollabo-
rations[10–13],which wascarriedout by the LEPWorking
Group for Higgs BosonSearches. These searchesinclude all
LEP2 data up to the highest energy, 209GeV; in the case
of [10,12] they also include the LEP1 data collected at en-
ergies in the vicinity of 91GeV (the Z boson resonance).
The combined LEP data show no significant signal for
Higgs boson production. The search results are used to set
upper bounds on topological cross-sections for a number of
Higgs-likefinal states. Furthermore,they areinterpreted in
a set of representative MSSM “benchmark” models, with
and without CP-violating effects in the Higgs sector.
2 The MSSM framework
The LEP searches and their statistical combination pre-
sented in this paper are interpreted in a constrained MSSM
model. At tree level, two parameters are sufficient (besides
the known parameters of the standard model fermion and
gauge sectors) to fully describe the Higgs sector. A con-
venient choice is one Higgs boson mass (mAis chosen in
CP-conserving scenarios and mH± in CP-violating scenar-
ios), and the ratio tanβ = v2/v1of the vacuum expecta-
tion values of the two Higgs fields (v2and v1refer to the
fields which couple to the up- and down-type fermions).
Additional parameters, MSUSY, M2, µ, A and m˜ g, enter at
the level of radiative corrections. MSUSYis a soft SUSY-
breaking mass parameter and represents a common mass
for all scalar fermions (sfermions) at the electroweak scale.
Similarly, M2represents a common SU(2) gaugino mass at
the electroweakscale.The “Higgsmass parameter”µ is the
strengthofthe supersymmetricHiggsmixing; A=At=Ab
is a common trilinear Higgs-squark coupling at the elec-
troweak scale and m˜ gis the mass of the gluino (the super-
partner of the gluons). Three of these parameters define
the stop and sbottom mixing parameters Xt= A−µcotβ
and Xb= A−µtanβ. In CP-violating scenarios, the com-
plex phases related to A and m˜ g, arg(A) and arg(m˜ g), are
supplementary parameters. In addition to all these MSSM
parameters,the top quarkmassalsohasa strongimpact on
the predictions through radiative corrections. In this pa-
per, three fixed values are used in the calculations: mt=
169.3, 174.3 and 179.3GeV/c2. For the purposes of illus-
tration, mt= 174.3GeV/c2is used in producing the figures
(unless explicitly specified otherwise), which is a previous
world-average value [14] and which is within the current
experimental range of 172.7±2.9GeV/c2[15]. The influ-
ence of the top quark mass on the exclusion limits is dis-
cussed in Sects. 5 and 6 along with the other results.
The combined LEP data are compared to the predic-
tions of a number of MSSM “benchmark” models [16].
Within each of these models, the two tree-level parame-
ters, tanβ and mA (in the CP-conserving scenarios) or
mH± (in the CP-violating scenarios) are scanned while the
other parameters are set to fixed values. Each scan point
thus represents a specific MSSM model. The ranges of the
scanned parameters and the values of the fixed parameters
are listed in Table 1 for the main scenarios studied. The
first five models represent the main benchmarks for CP-
conserving scenarios while the last model, labelled CPX,
is a benchmark model for CP-violating scenarios. Some
variants of these benchmark scenarios, which are also in-
vestigated, are presented in the text below.
The scan range of tanβ is limited by the following con-
siderations. For values of tanβ below the indicated lower
bounds, the calculations of the observables in the Higgs
sector (masses, cross-sections and decay branching ratios)
become uncertain; for values above the upper bounds, the
decay width of the Higgs bosons may become larger than
the experimental mass resolution (typically a few GeV/c2)
and the modelling of the kinematic distributions of the sig-
nal becomes inaccurate1. The scan range of mAis limited
in most casesto less than 1000GeV/c2; at higher values the
Higgs phenomenology is insensitive to the choice of mA.
1The DELPHI Collaboration included the variation of the
Higgs boson decay width with tanβ in their simulation for
tanβ between 30 and 50. With increasing tanβ, DELPHI ob-
served an increase of the mass resolutions and hence a loss in
the signal detection efficiencies; but this was compensated by
the increase of the cross-sections, such that DELPHI found no
significant drop in the overall sensitivity.

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556The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
Table 1. Parameters of the main benchmark scenarios investigated in this paper. The values of tanβ and the mass
parameters mA(in the CP-conserving scenarios) or mH± (in the CP-violating scenarios) are scanned within the
indicated ranges. For the definitions of A and Xt, the Feynman-diagrammatic on-shell renormalisation scheme is
used in the CP-conserving scenarios and the MS renormalisation scheme in the CP-violating scenarios
Benchmark parameters
(1)(2)(3) (4)(5)(6)
CPX mh-maxno-mixing large-µgluophobicsmall-αeff
Parameters varied in the scan
0.4–40
0.1–1000
–
tanβ
mA(GeV/c2)
mH± (GeV/c2)
0.4–40
0.1–1000
–
0.7–50
0.1–400
–
0.4–40
0.1–1000
–
0.4–40
0.1–1000
–
0.6–40
–
4–1000
Fixed parameters
MSUSY(GeV)
M2(GeV)
µ (GeV)
m˜ g(GeV/c2)
Xt(GeV)
1000
200
−200
800
1000
200
−200
800
400
400
1000
200
350
300
300
500
800
500
2000
500
500
200
2000
1000
2MSUSY
Xt+µcotβ
–
0
−300
−750
−1100A−µcotβ
A (GeV)
arg(A) = arg(m˜ g)
Xt+µcotβ
–
Xt+µcotβ
–
Xt+µcotβ
–
Xt+µcotβ
–
1000
90◦
For a given scan point, the observables in the Higgs sec-
tor are calculated using two theoretical approaches, both
includingone-andtwo-loopcorrections.TheFeynHiggs2.0
code [17] is based on a Feynman-diagrammatic approach
andusestheon-shellrenormalizationscheme.TheSUBHPOLE
calculation and its CP-violating variant CPH [18] are based
onarenormalization-groupimprovedeffectivepotentialcal-
culation [19]andusetheMSscheme2.
In the CP-conserving case, the FeynHiggs calcula-
tion is retained for the presentation of the results since it
yields slightly more conservative results (the theoretically
allowed parameter space is wider) than SUBHPOLE does.
Also, FeynHiggs is preferred on theoretical grounds since
its radiative corrections are more detailed than those of
SUBHPOLE.
In the CP-violatingcase, neither of the two calculations
is preferred on theoretical grounds. While FeynHiggs con-
tains moreadvancedone-loopcorrections,the CPHcode has
a more precise phase dependence at the two-loop level. We
opted therefore for a solution where, in each scan point, the
CPH and FeynHiggscalculations are compared and the cal-
culation yielding the weaker exclusion (more conservative)
is retained. However, we also discuss in Sect. 6 the effect of
using separately either one or the other of the two calcula-
tions. Rather largediscrepanciesbetween the twocodes are
found in calculating the partial width for the Higgs boson
cascade decay Γ(H2→ H1H1) (H1and H2are the lightest
andthesecond-lightestneutralMSSM Higgsbosons).Aim-
ing at conservative exclusion limits, therefore, the CPH for-
mulaforthisdecaywasalsousedwithintheFeynHiggscode.
All codes are implemented in a modified version of
the HZHA program package [21], which takes into account
initial-state radiation and the interference between iden-
tical final states from Higgsstrahlung and boson fusion
processes.
2New developments in this approach are implemented in the
code CPsuperH [20].
2.1 CP-conserving scenarios
Assuming CP conservation, the spectrum of MSSM Higgs
bosons consists of two CP-even neutral scalars, h and H
(h is defined to be the lighter of the two), one CP-odd
neutral scalar, A, and one pair of charged Higgs bosons,
H±. The following ordering of masses is valid at tree level:
mh< (mZ,mA) < mH and mW± < mH±. This ordering
may be substantially modified by radiative corrections [7]
where the largest contribution arises from the incomplete
cancellation between top and scalar top (stop) loops. The
corrections affect mainly the neutral Higgs boson masses
and decay branching ratios.
In e+e−collisions at LEP energies, the main produc-
tion processes of h, H and A are the Higgsstrahlung pro-
cesses e+e−→ hZ and HZ and the pair production pro-
cesses e+e−→ hA and HA (in most of the MSSM param-
eter space only the hZ and hA processes are possible by
kinematics). The fusion processes e+e−→ (WW → h)νe¯ νe
ande+e−→ (ZZ→h)e+e−playamarginalroleat LEPen-
ergiesbut they are alsotaken into account in the derivation
of the results.
The cross-sections for Higgsstrahlung and pair produc-
tion can be expressed in terms of the standard model Higgs
boson production cross-section σSM
sions hold for the processes involving the lightest scalar
boson h:
HZ. The following expres-
σhZ= sin2(β−α)σSM
σhA= cos2(β −α)¯λσSM
HZ
(1)
HZ.(2)
Here α is the mixing angle which diagonalises the CP-even
Higgs mass matrix (at lowest order it can be expressed in
terms of mA, MZand tanβ) and¯λ is a kinematic factor:
¯λ = λ3/2
Ah/
?
λ1/2
Zh
?12M2
Z/s+λZh
??
(3)

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The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP557
with
λij= [1−(mi+mj)2/s][1−(mi−mj)2/s],(4)
where s is the square of the centre-of-mass energy. The
cross-sections for the processes involving the heavy scalar
boson H are obtained by interchanging the MSSM sup-
pression factors sin2(β −α) and cos2(β −α) in (1) and
(2) and replacing the index h by H in (1)–(3). The Hig-
gsstrahlung and pair production cross-sectionsare comple-
mentary, as seen from (1) and (2). At LEP energies, the
process e+e−→ hZ is typically more abundant at small
tanβ and e+e−→ hA at large tanβ, but the latter process
can be suppressed also by the kinematic factor¯λ.
The following decay features are relevant to the neu-
tral MSSM Higgs bosons. The h boson decays mainly to
fermion pairs, with only a small fraction of WW∗and ZZ∗
decays, since its mass is below the threshold of the on-shell
processes h → WW and h → ZZ. However, for particular
choices of the parameters, the fermionic final states may
be strongly suppressed. The A boson also decays predom-
inantly to fermion pairs, independently of its mass, since
its coupling to vector bosons is zero at leading order. For
tanβ > 1, decays of h and A to b¯b and τ+τ−pairs are
preferred while the decays to c¯ c and gluon pairs are sup-
pressed. Decays to c¯ c may become important for tanβ < 1.
The decay h → AA may be dominant if allowed by kine-
matics [22]. Higgs boson decays into SUSY particles, such
as sfermions, charginos or invisible neutralinos, are sup-
pressed due to the high values of the SUSY-breaking scale
MSUSYwhich have been chosen.
In the following we describe the CP-conserving bench-
mark scenarios [16] which are examined in this paper. The
corresponding parameters are listed in Table 1.
2.1.1 The mh-max scenario
In the mh-max scenario the stop mixing parameter is set
to a large value, Xt= 2MSUSY. This model is designed to
maximise the theoretical upper bound on mhfor a given
tanβ and fixed mtand MSUSY(uncertainties due to un-
known higher-order corrections are ignored). This model
thus provides the largest parameter space in the mhdirec-
tion and conservative exclusion limits for tanβ.
We also examine a variant of this scenario where the
sign of µ is changed to positive, since, in the context of
SUSY extensions of the standard model, a positive sign of
µ is favoured by presently available results on (g−2)µ[23,
24]. This variant is labelled mh-max (a) below. Further-
more, we examine the case where, besides changing the
sign of µ to positive, the sign of the mixing parameter Xt
is changed to negative. This choice of parametersgives bet-
ter agreement with measurements of the branching ratios
and of the CP- and isospin-asymmetries for the process
b → sγ [16,25]. This variant is labelled mh-max (b) below.
2.1.2 The no-mixing scenario
In the no-mixing scenario the stop mixing parameter Xt
is set to zero, giving rise to a relatively restricted MSSM
parameter space. Like in the mh-max scenario, we also ex-
amine a variant of the no-mixing scenario where the sign
of µ is changed to positive. At the same time, we raise
MSUSYto 2TeV in order to enlarge the allowed parameter
space [16]. In the case of this variant, which is labelled no-
mixing (a) below, the scan in tanβ is done only from 0.7
upward, due to numerical instabilities, at lower values, in
the diagonalisation of the mass matrix.
2.1.3 Special scenarios
Some scenarios were designed to illustrate choices of the
MSSM parameters for which the detection of Higgs bosons
at LEP, at the Tevatron and at the LHC is expected to be
difficult a priori due to the suppression of some main dis-
covery channels [16].
– The large-µ scenario is constructed in such a way that,
while the h boson is accessible by kinematics at LEP
for all scan points, the decay h → b¯b, on which most
of the searches at LEP and at the Tevatron are based,
is typically strongly suppressed. For many of the scan
pointsthe decayh → τ+τ−isalsosuppressed,suchthat
the dominant decay modes are h → c¯ c, gg and WW∗.
The detection of Higgs bosons thus relies mainly on
flavour- and decay-mode-independent searches. More-
over,forsomeofthe scanpoints, the e+e−→ hZ process
is suppressed altogether by a small value of sin2(β−α).
In such cases, however, the heavy neutral scalar H is
within reach (mH< 111GeV/c2) and the cross-section
fore+e−→ HZ,proportionaltocos2(β−α),islarge;the
searchmaythusproceedviatheheavyHiggsbosonH.
– The gluophobic scenario is constructed in such a way
that the Higgs boson coupling to gluons is suppressed
due to acancellationbetween the top and the stop loops
at the hgg vertex. Since at the LHC the searches will
rely heavily on producing the Higgs boson in gluon-
gluon fusion, and since the mass determination will rely
in part on the decays into gluon pairs, such a scenario
may present experimental difficulties.
– In the small-αeffscenario the couplings governing the
decays h → b¯b and h → τ+τ−are suppressed with
respect to their standard model values by a factor
−sinαeff/cosβ (αeff is the effective mixing angle of
the neutral CP-even Higgs sector including radiative
corrections). This suppression is most prominent for
tanβ > 15 and 170 < mA< 350GeV/c2. (One should
note that in most models which fall in this domain, all
three neutral Higgs bosons are beyond the kinematic
reach of LEP.)
2.2 CP-violating scenarios
In CP-violating MSSM scenarios the three neutral Higgs
mass eigenstates Hi(i = 1, 2, 3) do not have well defined
CP quantum numbers. Each of them can thus be pro-
duced by Higgsstrahlung (e+e−→ HiZ) via the CP-even
field component and in pairs (e+e−→ HiHj(i ?= j)). The
relative rates depend on the choice of the parameters de-
scribing the CP-even/odd mixing.

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558 The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
Table 2. A typical parameter set which is difficult to address
by the present searches. The results of the two calculations,
FeynHiggs and CPH, are given for a centre-of-mass energy of
206 GeV. The main input parameters are listed in the first two
lines; all other input parameters correspond to the CPX bench-
mark scenario and are listed in the last column of Table 1. The
output masses mH1, mH2and the relevant topological cross-
sections are listed in the lower part
Parameters
FeynHiggsCPH
H+(GeV/c2)
tanβ
mH1(GeV/c2)
mH2(GeV/c2)
σ(H1Z → b¯bZ) (pb)
σ(H2Z → b¯bZ) (pb)
σ(H2Z → H1H1Z → b¯bb¯bZ) (pb)
σ(H1H2→ b¯bb¯b) (pb)
129.0
5.0
129.0
5.0
38.1
105.4
0.0051
0.0156
0.0866
0.0066
33.4
102.4
0.0019
0.0197
0.0978
0.0094
Experimentally, the CP-violating scenarios are more
challenging than the CP-conserving scenarios. For a wide
range of model parameters, the coupling of the lightest
Higgs boson H1to the Z boson may be suppressed. Fur-
thermore, the second- and third-lightest H2and H3bosons
may both have masses close to or beyond the kinematic
reach of LEP. Also, in CP-violating scenarios, the decays
to the main “discovery channels”, H1→ b¯b, H2→ b¯b and
H2→ H1H1→ b¯bb¯b3, may have lower branching ratios.
One therefore anticipates less search sensitivity in the CP-
violating scenarios than in the CP-conserving scenarios.
An example illustrating this situation is given in Table 2.
The cross-sections for Higgsstrahlung and pair produc-
tion are given by [9]
σHiZ= g2
σHiHj= g2
HiZZσSM
HiHjZ¯λσSM
HZ
(5)
HZ
(6)
(in the expression for¯λ, (3), the indices h and A have to be
replaced by Hiand Hj). The couplings
gHiZZ= cosβO1i+sinβO2i
gHiHjZ= O3i(cosβO2j−sinβO1j)
−O3j(cosβO2i−sinβO1i)
(7)
(8)
obey the complementarity relation
3
?
i=1
g2
HiZZ= 1(9)
gHkZZ= εijkgHiHjZ
(10)
where εijkis the usual Levi–Civita symbol.
3Regarding the decay properties, the CP-violating scenarios
maintain a certain similarity to the CP-conserving scenarios al-
though the branching ratios are, in general, different. The light-
est mass eigenstate H1predominantly decays to b¯b if allowed
by kinematics, with a small fraction decaying to τ+τ−and c¯ c.
The second-lightest Higgs boson H2may decay to H1H1when
allowed by kinematics; otherwise it decays preferentially to b¯b.
In CP-violating scenarios, the orthogonal matrix Oij
(i,j = 1, 2, 3) relating the weak CP eigenstates to the mass
eigenstates has non-vanishing off-diagonal elements. These
elements, giving rise to CP-even/odd mixing, are propor-
tional to
m4
v2M2
tIm(µA)
SUSY
(11)
with v =?v2
and large Im(µA), which are obtained if the CP-violating
phase arg(A) takes values close to 90◦. Furthermore, the
effects from CP violation strongly depend on the precise
value of the top quark mass [15].
The parameters of the benchmark model CPX have
been chosen [18] to maximise the phenomenological differ-
ences with respect to the CP-conserving scenarios. Con-
straints from measurements of the electron and neutron
electric dipole moments [26] were also taken into account.
The basic set of parameters is listed in the last column
of Table 1. Note that the scan of mH± started at 4GeV/c2
but values less than about 100GeV/c2give unphysical re-
sults and are thus considered as theoretically inaccessible.
The parameters which follow have been varied one-by-
one while all the other parameters were kept at their stan-
dard CPX value.
– Top quark mass: mt= 169.3, 174.3, and 179.3GeV/c2,
embracing the current experimental value, mt= 172.7
±2.9GeV/c2[15].
– The CP-violating phases: arg(A) = arg(m˜ g) = 0◦, 30◦,
60◦, 90◦(CPX value), 135◦and 180◦(the values 0◦and
180◦correspond to CP-conserving limits).
– The Higgs mass parameter: µ = 0.5, 1.0, 2.0 (CPX
value) and 4.0TeV.
– The SUSY-breaking scale: MSUSY= 0.5TeV (CPX
value) and 1.0TeV. The proposal of the CPX sce-
nario [18] predicts a weak dependence on MSUSY if
the relations|A| =|m˜ g| =µ/2=2MSUSYare preserved.
This behaviour is examined by studying a model where
MSUSY is increased from 0.5TeV to 1TeV and the
values of A,m˜ gand µ are scaled to 2000GeV, 2000GeV
and 4000GeV, respectively.
1+v2
2. Substantial deviations from the CP-
conserving scenarios are thus expected for small MSUSY
3 Experimental searches
The searches carried out by the four LEP collaborations
are based on e+e−collision data which span a large range
of centre-of-mass energies, from 91GeV to 209GeV. The
searches include the Higgsstrahlung and pair production
processes, ensuring, by their complementarity, a high sen-
sitivity over the accessible MSSM parameter space. It is
important to note that the kinematic properties of the sig-
nal processes are to a large extent independent of the CP
composition of the Higgs bosons. This implies that the
same topological searches can be applied to study the CP-
conservingand CP-violating scenarios.For Higgsstrahlung
this is natural since only the CP-even components of the

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The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP 559
Higgs fields couple to the Z boson. In pair production in-
volving CP-even and CP-odd field components, the simi-
larity of the kinematic properties (e.g., angular distribu-
tions) arises from the scalar nature of the Higgs bosons.
Small differences may occur fromspin-spin correlationsbe-
tween final-state particles but these were found to have
no noticeable effect on the signal detection efficiencies. We
therefore adopt in the following a common notation for
the CP-conserving and CP-violating processes in which Hi
(i = 1, 2, 3) designate three generic neutral Higgs bosons of
increasing mass, with undefined CP properties; in the CP-
conserving limit (arg(A) = arg(m˜ g) = 0◦), these become
the CP eigenstates h, A, H (the correspondence depends
on the mass hierarchy).
In each of the four LEP experiments, the data analysis
is done in several steps. A preselection is applied to re-
duce some of the largest backgrounds, in particular, from
two-photon processes. The remaining background, mainly
from production of fermion pairs and WW or ZZ (pos-
sibly accompaniedby photon or gluonradiation), is further
reduced by more selective cuts or by applying multivari-
ate techniques such as likelihood analyses and neural net-
works. The identification of b-quarks in the decay of the
Higgs bosons plays an important role in the discrimina-
tion between signal and background, as does the kinematic
reconstruction of the Higgs boson masses. The detailed
implementation of these analyses, as well as the data sam-
ples used by the four collaborations, are described in the
individual publications. A full catalog of the searches pro-
vided by the four LEP collaborations for this combination,
with corresponding references to the detailed descriptions,
is given in Appendix A.
3.1 Search topologies
Searches have been carried out for the two main signal
processes, the Higgsstrahlungprocess e+e−→ H1Z (which
also apply in some cases to e+e−→ H2Z) and the pair pro-
duction process e+e−→ H2H1.
(a) Considering first the Higsstrahlung process e+e−→
H1Z, the principal signal topologies are those used in the
search for the standard model Higgs boson at LEP [3],
namely:
– the four-jet topology, (H1→ b¯b)(Z → q¯ q), in which the
invariant mass of two jets is close to the Z boson mass
mZwhile the other two jets contain b-flavour;
– the missing energy topology, (H1→ b¯b,τ+τ−)(Z →
ν¯ ν), in which the event consists of two b-jets or identi-
fied tau decays and substantial missing momentum and
missing mass, compatible with mZ;
– the leptonic final states, (H1→ b¯b)(Z → e+e−,µ+µ−),
in which the invariant mass of the two leptons is close to
mZ;
– the final states with tau-leptons, (H1→ τ+τ−)(Z →
q¯ q) and (H1→ b¯b,τ+τ−)(Z → τ+τ−), in which either
the τ+τ−or the q¯ q pair has an invariant mass close to
mZ.
Most of these signatures are relevant for Higgs boson
masses above the b¯b threshold and rely on the identifi-
cation of b-quarks in the final state. Searches for lighter
Higgs bosons, listed in Appendix A, use signatures which
are described in the specific publications. In some regions
of the MSSM parameter space, the H1→ b¯b decay may
be suppressed while decays into other quark flavours or
gluon pairs are favoured. The above searches are there-
fore complemented or replaced4by flavour-independent
searches for (H1→ q¯ q)Z in which there is no require-
ment on the quark-flavour of the jets. Finally, the searches
for Higgsstrahlung also include the Higgs cascade decay
e+e−→ H2Z → (H1H1)Z, giving rise to a new class of
event topologies. These processes may play an important
role in those regions of the parameter space where they are
allowed by kinematics.
(b) In the case of the pair production process, e+e−→
H2H1, the principal signal topologies at LEP are:
– the four-b final state (H2→ b¯b)(H1→ b¯b);
– the mixed final states (H2→ τ+τ−)(H1→ b¯b) and
(H2→ b¯b)(H1→ τ+τ−);
– the four-tau final state (H2→ τ+τ−)(H1→ τ+τ−).
The Higgs cascade decay, e+e−→ H2H1→ (H1H1)H1,
gives rise to event topologies ranging from six b-jets to six
tau-leptons. Most of these searches are relevant for Higgs
boson masses above the τ+τ−threshold. Similarly to the
Higgsstrahlung case, the above searches for pair produc-
tion are complemented or replaced, whenever more effi-
cient, by flavour-independent searches.
3.2 Additional experimental constraints
If the combination of the above searches is not sufficiently
sensitive for excluding a given model point, additional con-
straints are applied; these are listed below.
– Constraint from the measured decay width of the Z bo-
son, ΓZ, and its possible deviation, ∆ΓZ, from the stan-
dard model prediction. The model point is regarded as
excluded if the following relation between the relevant
cross-sections is found to be true:
?
i
σHiZ(mZ)+
?
i,j
σHiHj(mZ) >∆ΓZ
ΓZ
·σtot
Z (mZ),
(12)
where∆ΓZ= 2.0MeV [27]stands forthe 95%CL upper
bound on the possible additional decay width of the Z
boson, beyond the standard model prediction, and σtot
is the Z pole cross-section.
– Constraint from a decay mode independent search for
e+e−→ H1Z [28]. The model point is regarded as ex-
cluded if the condition
Z
σHiZ> k(mHi)·σSM
HZ
(13)
is fulfilled, where k(mHi) is a mass-dependent factor
whichscalesthestandardmodelHiggsproductioncross-
sectiontothe value thatis excludedat the 95%CL.
4The replacement is necessary whenever the overlap in terms
of selected events is important, in order to avoid double-
counting.

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560The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP
– ConstraintfromasearchforlightHiggsbosonsproduced
by the Yukawa process5. The model point is regarded
asexcludedifthepredictedYukawaenhancementfactor
ξ(mH1), defined in[29],isexcludedbythis search.Tobe
conservative,theweakerofthetwoenhancementfactors,
forCP-evenandCP-oddcouplings,isused.
These additional constraints are particularly useful at
small mH1and mH2, below the b¯b threshold.
3.3 Statistical combination of search channels
The statistical method by which the topological searches
are combined is described in [3,30].
After selection, the combined data configuration (dis-
tribution of all selected events in several discriminating
variables) is compared in a frequentist approach to a large
number of simulated configurations generated separately
for two hypotheses: the background (b) hypothesis and the
signal-plus-background (s+b) hypothesis. The ratio
Q = Ls+b/Lb,(14)
of the correspondinglikelihoods is used as the test statistic.
The predicted, normalised, distributions of Q (probability
density functions) are integratedto obtain the p-values [31]
1−CLb= 1−Pb(Q ≤ Qobserved) and CLs+b= Ps+b(Q ≤
Qobserved); these measure the compatibility of the observed
data configuration with the two hypotheses. Here Pband
Ps+b are the probabilities for a single experiment to ob-
tain a value of Q smaller than or equal to the observed
value, given the background or the signal-plus-background
hypothesis. More details can be found in [3].
Systematic errors are incorporated in the calculation of
the likelihoods by randomly varying the signal and back-
ground estimates in each channel6according to Gaussian
error distributions and widths corresponding to the sys-
tematic errors. For a given source of uncertainty, correla-
tions are addressed by applying these random variations
simultaneously to all those channels for which the source of
uncertainty is relevant. Errors which are correlated among
the experiments arise mainly from using the same Monte
Carlo generators and cross-section calculations for the sig-
nal and background processes. The uncorrelated errors
arise mainly from the limited statistics of the simulated
background event samples.
In a purely frequentist approach, the exclusion limit is
computed from the confidence CLs+bfor the signal-plus-
background hypothesis: a signal is regarded as excluded at
the 95% CL, for example, if an observation is made such
that CLs+b is lower than 0.05. However, this procedure
5Note that, in the case of DELPHI, the Yukawa channels are
not usedasexternalconstraintsbutarecombined with theother
search channels.
6The word “channel” designates any subset of the data in
which a search has been carried out. These subsets may cor-
respond to specific final-state topologies, to data sets collected
at different centre-of-mass energies or to the subsets of data
collected by different experiments.
may lead to the undesired situation in which a large down-
ward fluctuation of the background would exclude a sig-
nal hypothesis for which the experiment has no sensitivity
since the expected signal rate is too small. This problem is
avoided by using the ratio
CLs= CLs+b/CLb
(15)
instead of CLs+b. We adopt this quantity for setting exclu-
sionlimits and consideragivenmodel to be excluded at the
95% CL if the corresponding value of CLsis less than 0.05.
Since CLbis a positive number less than one, CLsis always
larger than CLs+band the limits obtained in this way are
therefore conservative.
3.4 Comparisons of the data
with the expected background
The distribution of the p-value 1−CLbover the parameter
space covered by the searches provides a convenient way of
studying the agreement between the data and the expected
background and of discussing the statistical significance of
any local excess in the data. While a purely background-
like behaviour7would yield p-values close to 0.5, much
smaller values are expected in the case of a signal-like
excess. For example, a local excess of three or five stan-
dard deviations would give rise to a p-value 1−CLb of
2.7×10−3or 5.7×10−7, respectively.
One has to be careful, however, when interpreting these
numbersasprobabilitiesforlocalexcessesoccurringoverthe
extendeddomainscoveredbythesearches.Forexample,the
probabilityfora fluctuation of three standarddeviations to
occur anywhere in the parameter space is much larger than
the number 2.7×10−3just quoted. A multiplication factor
has to be applied to the probability 1−CLbwhich reflects
the number of independent “bins” of the parameter space;
thisfactorcanbeestimatedfromthetotalsizeoftheparame-
terspaceandtheexperimentalresolutions.Forexample,the
searchesfortheHiggsstrahlungprocesse+e−→H1Z,cover-
ingtherange0< mH1< 120GeV/c2withamassresolution
∆mH1of about 3 GeV/c2, would yield about twenty fairly
independent mass-bins of width 2∆mH1; hence, a multipli-
cation factor of about twenty. Much bigger multiplication
factors are expected in the searches for the pair production
processe+e−→ H2H1withtwoindependentsearchparam-
eters(masses).
These simple considerations do not take into account,
for example, possible correlations from resolution tails ex-
tending over several adjacent bins or correlations between
different searches sharing candidate events. A more elab-
orate evaluation of the multiplication factor has therefore
been performed. A large number of background experi-
ments was simulated, covering the whole parameter space,
using realistic resolution functions and taking correlations
into account. From these random experiments, the prob-
ability to obtain 1−CLbsmaller than a given value, any-
where in the parameter space of a given scenario, has
7Single, background-like, experiments have values of 1−CLb
uniformly distributed between zero and one.

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The LEP Collaborations et al.: Search for neutral MSSM Higgs bosons at LEP561
Fig. 1. Contours of the observed p-values, 1−CLb, indicating the statistical significances of local excesses in the data. Plots a
and b refer to the CP-conserving MSSM benchmark scenario mh-max and plots c and d to the CP-violating scenario CPX. For
each scenario, the parameter space is shown in two projections. Regions which are not part of the parameter space (labelled “The-
oretically Inaccessible”) are shown in light-grey or yellow. In the medium-grey or light-green regions the data show an excess of
less than one standard deviation above the expected background. Similarly, in the dark-grey or dark-green regions the excess is
between one and two standard deviations while in the darkest-grey or blue regions it is between two and three standard devia-
tions. In plots c and d, two small regions with excesses larger than three standard deviations are shown in white. The dashed lines
show the expected exclusion limit at 95% CL. The hatched areas represent regions where the median expected value of CLsin the
background hypothesis is larger than 0.4; apparent excesses in these regions would not be significant
been determined (the mh-max scenario was taken for this
study). A scale factor of at least 60 was obtained in this
manner. According to this estimate, the probability of ob-
serving a background fluctuation of three standard devia-
tions anywhere in the parameter space of a given scenario
(e.g., mh-max) can be 16% or more. Also, to observe two
fluctuations with two standard deviations turns out to be
more likely than to observe only one.
Figure 1 shows the distribution of the p-value 1−CLb,
determined from the present combined searches, for the
CP-conserving benchmark scenario mh-max and the CP-
violating scenario CPX. Over the largest part of the pa-
rameter space, the local excessesare smaller than two stan-
dard deviations. In the mh-max scenario, the lowest value,
1−CLb= 1.3×10−2, lies within the vertical band at mh
around 100GeV/c2and corresponds to 2.5 standard devi-
ations. This excess, and a less significant excess at about
115GeV/c2, come from the Higgsstrahlung search; both
are discussed in [3] in the context of the search for the
standard model Higgs boson. In the CPX scenario, one ob-
serves two small regions at mH1≈ 35–40GeV/c2, mH2≈
105GeV/c2and tanβ ≈ 10, where the significance exceeds
three standard deviations; they arise from the search for
the pair production process.
The exact position and size of these fluctuations may
vary from one scenario to the other. In Tables 3 and 4 we
list the parameters of the most significant excesses for all
CP-conservingandCP-violatingbenchmarkscenarioscon-
sidered in this paper. The largest fluctuation of all has
a significance of 3.5 standard deviations; its probability is
estimated as 3.6% at least, when the scale factor of 60 or
more is applied.
From these studies one can conclude that there is a rea-
sonable agreement between the data and the simulated