arXiv:hep-ex/0104025v1 13 Apr 2001
EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
1 February 2001
Search for a fermiophobic Higgs at
Higgs bosons predicted by the fermiophobic scenario within Two Higgs Doublets
Models were searched for in the data collected by the DELPHI detector at
centre-of-mass energies between 189 GeV and 202 GeV, corresponding to a
total integrated luminosity of 380 pb−1. No signal was found and confidence
limits were derived in the framework of possible extensions of the Standard
Model Higgs sector.
(Accepted by Phys.Lett.B)
P.Abreu22, W.Adam51, T.Adye37, P.Adzic12, I.Ajinenko43, Z.Albrecht19, T.Alderweireld2, G.D.Alekseev18, R.Alemany9,
T.Allmendinger19, P.P.Allport23, S.Almehed25, U.Amaldi29, N.Amapane46, S.Amato48, E.Anashkin36, E.G.Anassontzis3,
P.Andersson45, A.Andreazza28, S.Andringa22,N.Anjos22,
J-E.Augustin24, A.Augustinus9, P.Baillon9, A.Ballestrero46, P.Bambade9,21, F.Barao22, G.Barbiellini47, R.Barbier26,
D.Y.Bardin18, G.Barker19, A.Baroncelli39, M.Battaglia17, M.Baubillier24, K-H.Becks53, M.Begalli6, A.Behrmann53,
Yu.Belokopytov9, N.C.Benekos32, A.C.Benvenuti5, C.Berat16, M.Berggren24, L.Berntzon45, D.Bertrand2, M.Besancon40,
G.Borisov21, C.Bosio42, O.Botner49, E.Boudinov31, B.Bouquet21, T.J.V.Bowcock23, I.Boyko18, I.Bozovic12, M.Bozzo15,
M.Bracko44, P.Branchini39, R.A.Brenner49, P.Bruckman9, J-M.Brunet8, L.Bugge33, P.Buschmann53, M.Caccia28,
F.R.Cavallo5,M.Chapkin43, Ph.Charpentier9, P.Checchia36,
P.Chochula7, V.Chorowicz26, J.Chudoba30, K.Cieslik20, P.Collins9, E.Cortina50, G.Cosme21, F.Cossutti9, M.Costa50,
H.B.Crawley1,D.Crennell37,J.Croix10, G.Crosetti15, J.Cuevas Maestro34,
M.Davenport9, W.Da Silva24,G.Della Ricca47, P.Delpierre27,
C.De Clercq2, B.De Lotto47, A.De Min9, L.De Paula48, H.Dijkstra9, L.Di Ciaccio38, K.Doroba52, M.Dracos10, J.Drees53,
D.Fassouliotis12, M.Feindt19, J.Fernandez41, A.Ferrer50, E.Ferrer-Ribas21, F.Ferro15, A.Firestone1, U.Flagmeyer53,
H.Foeth9, E.Fokitis32, F.Fontanelli15, B.Franek37, A.G.Frodesen4, R.Fruhwirth51, F.Fulda-Quenzer21, J.Fuster50,
Ph.Gavillet9, E.N.Gazis32, D.Gele10, T.Geralis12, N.Ghodbane26, I.Gil50, F.Glege53, R.Gokieli9,52, B.Golob9,44,
G.Gomez-Ceballos41, P.Goncalves22, I.Gonzalez Caballero41, G.Gopal37, L.Gorn1, Yu.Gouz43, V.Gracco15, J.Grahl1,
P.Herquet2, H.Herr9, O.Hertz19, E.Higon50, S-O.Holmgren45, P.J.Holt35, S.Hoorelbeke2, M.Houlden23, J.Hrubec51,
G.J.Hughes23, K.Hultqvist9,45, J.N.Jackson23, R.Jacobsson9, P.Jalocha20, Ch.Jarlskog25, G.Jarlskog25, P.Jarry40,
B.Jean-Marie21, D.Jeans35, E.K.Johansson45, P.Jonsson26, C.Joram9, P.Juillot10, L.Jungermann19, F.Kapusta24,
P.Kokkinias12, V.Kostioukhine43, C.Kourkoumelis3,O.Kouznetsov18,
A.Leisos12, R.Leitner30, J.Lemonne2, G.Lenzen53, V.Lepeltier21, T.Lesiak20, M.Lethuillier26, J.Libby35, W.Liebig53,
D.Liko9, A.Lipniacka45, I.Lippi36, J.G.Loken35, J.H.Lopes48, J.M.Lopez41, R.Lopez-Fernandez16, D.Loukas12, P.Lutz40,
L.Lyons35, J.MacNaughton51, J.R.Mahon6, A.Maio22, A.Malek53, S.Maltezos32, V.Malychev18, F.Mandl51, J.Marco41,
R.Marco41, B.Marechal48, M.Margoni36, J-C.Marin9, C.Mariotti9, A.Markou12, C.Martinez-Rivero9, S.Marti i Garcia9,
J.Masik13, N.Mastroyiannopoulos12, F.Matorras41, C.Matteuzzi29, G.Matthiae38, F.Mazzucato36,14, M.Mazzucato36,
M.Mc Cubbin23, R.Mc Kay1, R.Mc Nulty23, G.Mc Pherson23, E.Merle16, C.Meroni28, W.T.Meyer1, A.Miagkov43,
J.Montenegro31, D.Moraes48, P.Morettini15, G.Morton35, U.Mueller53, K.Muenich53, M.Mulders31, L.M.Mundim6,
V.Obraztsov43, A.G.Olshevski18, A.Onofre22, R.Orava17, K.Osterberg9, A.Ouraou40, A.Oyanguren50, M.Paganoni29,
S.Paiano5, R.Pain24, R.Paiva22, J.Palacios35, H.Palka20, Th.D.Papadopoulou32, L.Pape9, C.Parkes9, F.Parodi15,
A.Petrolini15, H.T.Phillips37, F.Pierre40, M.Pimenta22, E.Piotto28, T.Podobnik44, V.Poireau40, M.E.Pol6, G.Polok20,
P.Poropat47, V.Pozdniakov18, P.Privitera38, N.Pukhaeva18, A.Pullia29, D.Radojicic35, S.Ragazzi29, H.Rahmani32,
A.L.Read33, P.Rebecchi9, N.G.Redaelli29, M.Regler51, J.Rehn19, D.Reid31, R.Reinhardt53, P.B.Renton35, L.K.Resvanis3,
F.Richard21, J.Ridky13, G.Rinaudo46, I.Ripp-Baudot10, A.Romero46, P.Ronchese36, E.I.Rosenberg1, P.Rosinsky7,
T.Rovelli5, V.Ruhlmann-Kleider40, A.Ruiz41, H.Saarikko17, Y.Sacquin40, A.Sadovsky18, G.Sajot16, L.Salmi17, J.Salt50,
D.Sampsonidis12, M.Sannino15, A.Savoy-Navarro24, C.Schwanda51, Ph.Schwemling24, B.Schwering53, U.Schwickerath19,
F.Scuri47, Y.Sedykh18, A.M.Segar35, R.Sekulin37, G.Sette15, R.C.Shellard6, M.Siebel53, L.Simard40, F.Simonetto36,
A.N.Sisakian18, G.Smadja26, N.Smirnov43, O.Smirnova25, G.R.Smith37, O.Solovianov43, A.Sopczak19, R.Sosnowski52,
T.Spassov9, E.Spiriti39, S.Squarcia15, C.Stanescu39, M.Stanitzki19, K.Stevenson35, A.Stocchi21, J.Strauss51, R.Strub10,
B.Stugu4, M.Szczekowski52, M.Szeptycka52, T.Tabarelli29, A.Taffard23, F.Tegenfeldt49, F.Terranova29, J.Timmermans31,
N.Tinti5, L.G.Tkatchev18, M.Tobin23, S.Todorova9, B.Tome22, A.Tonazzo9, L.Tortora39, P.Tortosa50, D.Treille9,
G.Tristram8, M.Trochimczuk52, C.Troncon28, M-L.Turluer40, I.A.Tyapkin18, P.Tyapkin25, S.Tzamarias12, O.Ullaland9,
V.Uvarov43, G.Valenti9,5,E.Vallazza47,P.Van Dam31,
N.van Remortel2,I.Van Vulpen31,G.Vegni28,L.Ventura36,
W.De Boer19,N.Demaria46,A.De Angelis47,
T.Moa45, M.Moch19,K.Moenig9,11, M.R.Monge15,
W.Van den Boeck2,
A.Zintchenko18, Ph.Zoller10, G.Zumerle36, M.Zupan12
1Department of Physics and Astronomy, Iowa State University, Ames IA 50011-3160, USA
2Physics Department, Univ. Instelling Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium
and IIHE, ULB-VUB, Pleinlaan 2, B-1050 Brussels, Belgium
and Facult´ e des Sciences, Univ. de l’Etat Mons, Av. Maistriau 19, B-7000 Mons, Belgium
3Physics Laboratory, University of Athens, Solonos Str. 104, GR-10680 Athens, Greece
4Department of Physics, University of Bergen, All´ egaten 55, NO-5007 Bergen, Norway
5Dipartimento di Fisica, Universit` a di Bologna and INFN, Via Irnerio 46, IT-40126 Bologna, Italy
6Centro Brasileiro de Pesquisas F´ ısicas, rua Xavier Sigaud 150, BR-22290 Rio de Janeiro, Brazil
and Depto. de F´ ısica, Pont. Univ. Cat´ olica, C.P. 38071 BR-22453 Rio de Janeiro, Brazil
and Inst. de F´ ısica, Univ. Estadual do Rio de Janeiro, rua S˜ ao Francisco Xavier 524, Rio de Janeiro, Brazil
7Comenius University, Faculty of Mathematics and Physics, Mlynska Dolina, SK-84215 Bratislava, Slovakia
8Coll` ege de France, Lab. de Physique Corpusculaire, IN2P3-CNRS, FR-75231 Paris Cedex 05, France
9CERN, CH-1211 Geneva 23, Switzerland
10Institut de Recherches Subatomiques, IN2P3 - CNRS/ULP - BP20, FR-67037 Strasbourg Cedex, France
11Now at DESY-Zeuthen, Platanenallee 6, D-15735 Zeuthen, Germany
12Institute of Nuclear Physics, N.C.S.R. Demokritos, P.O. Box 60228, GR-15310 Athens, Greece
13FZU, Inst. of Phys. of the C.A.S. High Energy Physics Division, Na Slovance 2, CZ-180 40, Praha 8, Czech Republic
14Currently at DPNC, University of Geneva, Quai Ernest-Ansermet 24, CH-1211, Geneva, Switzerland
15Dipartimento di Fisica, Universit` a di Genova and INFN, Via Dodecaneso 33, IT-16146 Genova, Italy
16Institut des Sciences Nucl´ eaires, IN2P3-CNRS, Universit´ e de Grenoble 1, FR-38026 Grenoble Cedex, France
17Helsinki Institute of Physics, HIP, P.O. Box 9, FI-00014 Helsinki, Finland
18Joint Institute for Nuclear Research, Dubna, Head Post Office, P.O. Box 79, RU-101 000 Moscow, Russian Federation
19Institut f¨ ur Experimentelle Kernphysik, Universit¨ at Karlsruhe, Postfach 6980, DE-76128 Karlsruhe, Germany
20Institute of Nuclear Physics and University of Mining and Metalurgy, Ul. Kawiory 26a, PL-30055 Krakow, Poland
21Universit´ e de Paris-Sud, Lab. de l’Acc´ el´ erateur Lin´ eaire, IN2P3-CNRS, Bˆ at. 200, FR-91405 Orsay Cedex, France
22LIP, IST, FCUL - Av. Elias Garcia, 14-1o, PT-1000 Lisboa Codex, Portugal
23Department of Physics, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK
24LPNHE, IN2P3-CNRS, Univ. Paris VI et VII, Tour 33 (RdC), 4 place Jussieu, FR-75252 Paris Cedex 05, France
25Department of Physics, University of Lund, S¨ olvegatan 14, SE-223 63 Lund, Sweden
26Universit´ e Claude Bernard de Lyon, IPNL, IN2P3-CNRS, FR-69622 Villeurbanne Cedex, France
27Univ. d’Aix - Marseille II - CPP, IN2P3-CNRS, FR-13288 Marseille Cedex 09, France
28Dipartimento di Fisica, Universit` a di Milano and INFN-MILANO, Via Celoria 16, IT-20133 Milan, Italy
29Dipartimento di Fisica, Univ. di Milano-Bicocca and INFN-MILANO, Piazza delle Scienze 3, IT-20126 Milan, Italy
30IPNP of MFF, Charles Univ., Areal MFF, V Holesovickach 2, CZ-180 00, Praha 8, Czech Republic
31NIKHEF, Postbus 41882, NL-1009 DB Amsterdam, The Netherlands
32National Technical University, Physics Department, Zografou Campus, GR-15773 Athens, Greece
33Physics Department, University of Oslo, Blindern, NO-1000 Oslo 3, Norway
34Dpto. Fisica, Univ. Oviedo, Avda. Calvo Sotelo s/n, ES-33007 Oviedo, Spain
35Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
36Dipartimento di Fisica, Universit` a di Padova and INFN, Via Marzolo 8, IT-35131 Padua, Italy
37Rutherford Appleton Laboratory, Chilton, Didcot OX11 OQX, UK
38Dipartimento di Fisica, Universit` a di Roma II and INFN, Tor Vergata, IT-00173 Rome, Italy
39Dipartimento di Fisica, Universit` a di Roma III and INFN, Via della Vasca Navale 84, IT-00146 Rome, Italy
40DAPNIA/Service de Physique des Particules, CEA-Saclay, FR-91191 Gif-sur-Yvette Cedex, France
41Instituto de Fisica de Cantabria (CSIC-UC), Avda. los Castros s/n, ES-39006 Santander, Spain
42Dipartimento di Fisica, Universit` a degli Studi di Roma La Sapienza, Piazzale Aldo Moro 2, IT-00185 Rome, Italy
43Inst. for High Energy Physics, Serpukov P.O. Box 35, Protvino, (Moscow Region), Russian Federation
44J. Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia and Laboratory for Astroparticle Physics,
Nova Gorica Polytechnic, Kostanjeviska 16a, SI-5000 Nova Gorica, Slovenia,
and Department of Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia
45Fysikum, Stockholm University, Box 6730, SE-113 85 Stockholm, Sweden
46Dipartimento di Fisica Sperimentale, Universit` a di Torino and INFN, Via P. Giuria 1, IT-10125 Turin, Italy
47Dipartimento di Fisica, Universit` a di Trieste and INFN, Via A. Valerio 2, IT-34127 Trieste, Italy
and Istituto di Fisica, Universit` a di Udine, IT-33100 Udine, Italy
48Univ. Federal do Rio de Janeiro, C.P. 68528 Cidade Univ., Ilha do Fund˜ ao BR-21945-970 Rio de Janeiro, Brazil
49Department of Radiation Sciences, University of Uppsala, P.O. Box 535, SE-751 21 Uppsala, Sweden
50IFIC, Valencia-CSIC, and D.F.A.M.N., U. de Valencia, Avda. Dr. Moliner 50, ES-46100 Burjassot (Valencia), Spain
51Institut f¨ ur Hochenergiephysik,¨Osterr. Akad. d. Wissensch., Nikolsdorfergasse 18, AT-1050 Vienna, Austria
52Inst. Nuclear Studies and University of Warsaw, Ul. Hoza 69, PL-00681 Warsaw, Poland
53Fachbereich Physik, University of Wuppertal, Postfach 100 127, DE-42097 Wuppertal, Germany
The spontaneous symmetry breaking mechanism is a fundamental component of the
Standard Model (SM) but no direct experimental evidence for the Higgs particles has
been presented so far. Many of the proposed extensions of the Standard Model change
the properties of the Higgs particles, either by the effect of new interactions at higher
energy scales or directly by assuming a non-minimal Higgs sector. The introduction of a
second Higgs doublet is a natural assumption and it can lead to a scenario where a light
Higgs particle with suppressed couplings to fermions arises .
In the Two Higgs Doublets Models (2HDM), the lightest scalar Higgs boson (h0) can
be produced at LEP either in association with a CP-odd Higgs particle or in association
with a Z0boson. The decay branching ratios for the lightest scalar Higgs change with
respect to Standard Model ones and its decay to a pair of photons becomes dominant in
large regions of the parameter space, while in the Standard Model this branching ratio
is ≤ 10−3. Events with isolated photons in the final state constitute rather distinctive
signatures of this fermiophobic scenario.
We present analyses of final states with isolated photons using the data collected by
DELPHI at centre-of-mass energies ranging between 189 GeV and 202 GeV, correspond-
ing to a total integrated luminosity of about 380 pb−1. In this paper we include also the
results from an analysis of 6-jet events relevant to the 2HDM scenario. The h0Z0produc-
tion with h0→ γγ has been investigated previously and interpreted in other frameworks:
an analysis of previous DELPHI data can be found in reference  and results from other
LEP experiments can be found in . Results obtained at LEP 1 will be discussed in
22HDM: the fermiophobic scenario
The Two Higgs Doublets Models (2HDM) without explicit CP violation  are charac-
terised by five physical Higgs bosons: two neutral CP-even bosons (h0, H0), two charged
bosons (H±), and one neutral CP-odd boson (A0). The important parameters for de-
scribing the 2HDM are the angles α and β, where α is the mixing angle in the neutral
CP-even Higgs sector and tanβ is the ratio of the vacuum expectation values of the two
Higgs doublets. A seventh parameter is fixed in the symmetry breaking, and is related to
the masses of the vector bosons Z0and W±which are nowadays extremely well measured
In the framework of 2HDM there are four different ways in which the Higgs doublets
can couple to fermions . The most common choice is the structure assumed in the
Minimal Supersymmetric extension to the Standard Model (MSSM)  : one of the
Higgs doublets couples both to up type quarks and to leptons, and the other doublet
couples to down type quarks.
In this paper a model is explored where only one of the Higgs doublets is allowed to
couple to fermions (model type I) . The coupling of the lightest CP-even boson, h0,
to a fermion pair is then proportional to cosα. If α =π
becomes a fermiophobic Higgs.
In general 2HDM, the main mechanisms for the production of neutral Higgs bosons at
LEP are e+e−→ h0Z0and e+e−→ h0A0. These processes have complementary cross-
sections, proportional to sin2δ and to cos2δ respectively, where δ = α − β. The high δ
region can be studied by analysing the Higgs-strahlung process, while the small δ region
is dominated by the associated h0A0production. The combination of both processes
2this coupling vanishes and h0
leads to an interpretation of the results as a function of mh0 and mA0. The region in
the plane (mh0, mA0) that is relevant for the present analyses corresponds to a band
mlow< mA0 + mh0 < mhigh. The upper constraint represents the sensitivity accessible
with the present LEP 2 integrated luminosity and centre-of-mass energies and the lower
constraint corresponds to the region excluded by previous analyses, namely at LEP 1.
The Higgs-Higgs interactions, namely the h0H+H−vertex, depend on the specific
2HDM potential. In fact, there are two different potentials, defined by seven parameters,
which assure no CP violation. They are referred to as potential A and potential B
. These potentials are equivalent so far as the Higgs couplings to gauge bosons and
fermions are concerned. However, differences in the Higgs-Higgs interactions lead to
different phenomenologies and can alter the decay width of h0→ γγ, for which the H+
loop has a fundamental contribution. On the other hand, the relevant tree-level decays
of A0are completely independent of the chosen potential. The two potentials also give
rise to different forbidden regions in the parameter space accessible at LEP. Namely, a
small value of δ implies a light h0for potential A and a small difference between mh0 and
mA0 for potential B (which is also the one assumed in the MSSM).
In this paper the results are interpreted for both potentials. For Potential A, the
branching ratio of the lightest scalar Higgs (h0) to two photons, BR(h0→ γγ), depends
only on mh0 and mildly on the value of δ, provided that mH± is above the experimental
limit of 78.6 GeV/c2 and the heavier neutral scalar Higgs boson (H0) has a mass of
the order of 1 TeV/c2. For potential B, the same branching ratio depends also on mA0
and mH±, and there can be large cancellations between the several loop contributions for
some values of these parameters. For higher values of mH± (above 400 GeV/c2) or higher
values of δ (sin2δ > 0.02), there are again regions free of such cancellations.
The dominant decay modes for mh0 < mZ0 in the fermiophobic limit (Model I and
The decays of h0to other boson pairs can be important when mh0 > mZ0, namely the
one loop decay h0→ Z0γ can have a BR as large as 20% for very small δ values, while
the decay to WW∗is important for large δ values.
The tree level decay modes of the A0boson are: A0→ ff, A0→ Z0h0, and A0→
W±H±(when kinematicaly allowed). The main decay of A0is into a fermion-antifermion
pair, namely a bb pair if mA0 > 10 GeV. However, above the Z0h0threshold, the decay
A0→ Z0h0dominates for all δ < 1.3 rad. Finally it should be noted that in the region of
very low δ values (δ < 10−3rad) and mA0 < mZ0 +mh0, the A0total width is very small
and A0can leave the detector before decaying . While for potential B, final states with
invisible A0are important only for a small band of mA0 ∼ mh0, for potential A they can
give rise to totally invisible final states.
The several topologies contributing to the analyses are summarised in table 1. For
mh0 > 2mA0, the final states will not involve photons but rather 6 b-jets or only invis-
ible particles (stable A0). In this region the analysis of  was used together with the
interpretation of LEP 1 data.
2) are h0→ A0A0(tree level) if mh0 > 2mA0 and h0→ γγ (one-loop) otherwise.
3 Data samples, event selection and analysis
The analysed data from the LEP runs of 1998 and 1999 were taken at centre-of-mass
energies of 189 GeV, 192 GeV, 196 GeV, 200 GeV and 202 GeV, with integrated lumi-
nosities of about 153, 26, 77, 85 and 42 pb−1, respectively. A detailed description of
the DELPHI detector and its performance can be found in references [9,10]. The most
relevant subdetectors for the present analyses were the electromagnetic calorimeters: the
Relevant mass region
mA0 < mZ0 + mh0
mh0 + mA0 > 10 GeV/c2
mA0 > mZ0 + mh0
e+e−→ h0A0→ h0h0Z0
mh0 < 110 GeV/c2
Table 1: Topologies of the final states considered in the framework of the explored fermio-
phobic scenario in 2HDM.
High density Projection Chamber (HPC) in the barrel region, the Forward ElectroMag-
netic Calorimeter (FEMC) in the endcaps and the Small angle TIle Calorimeter (STIC)
for the very-forward region; the Hadronic CALorimeter (HCAL, covering polar angles
down to 11 degrees), and the tracking devices, namely: the Vertex Detector (VD), the
Inner Detector (ID), the Time Projection Chamber (TPC) and the Outer Detector (OD)
in the barrel and the Forward Chambers A and B (FCA, FCB) in the forward region. The
Vertex Detector is crucial for the determination of secondary vertices and the tagging of
b-quark jets and also for the identification of photons which convert inside the tracking
system but after the VD.
The effects of experimental resolution on background and signal events were stud-
ied by generating Monte Carlo events and passing them through the full DELPHI
simulation and reconstruction chain .
to simulate the background processes:e+e−→ Z0(Nγ) → q¯ q(Nγ), e+e−→ W+W−,
e+e−→ W±e∓ν, e+e−→ Z0Z0/γ∗, and e+e−→ Z0e+e−. The e+e−→ Z0(Nγ) → ν¯ ν(Nγ),
e+e−→ Z0(Nγ) → µ¯ µ(Nγ) and e+e−→ Z0(Nγ) → τ¯ τ(Nγ) processes were generated with
the KoralZ generator . Bhabha events were generated with the BHWIDE generator
, e+e−→ γγ(γ) events according to , and Compton events according to . The
two-photon (“γγ”) physics events were generated with the TWOGAM  generator.
The two main backgrounds in the analysis are e+e−→ q¯ q(Nγ) and e+e−→ ν¯ ν(Nγ).
The matrix-element in KoralZ generator (used for ν¯ ν(Nγ)) has the complete order α
complemented with a third order leading-log expansion. The Pyhia generator (used for
q¯ q(Nγ)) was verified to be compatible with KoralZ for events with up to two visible
photons. The absence of a complete description of the multiple photon radiation in MC
generators may be a problem for very high luminosity analysis.
The analysis of events with isolated photons was done in several steps. First a general
selection was applied and isolated leptons, isolated photons and jets were reconstructed.
Events with isolated leptons were removed from the analysis.
Charged particles were considered only if they had momentum greater than 0.1 GeV/c
and impact parameters below 4 cm in the transverse plane and below 4 cm /sinθ in the
beam direction (θ is the polar angle, defined in relation to the beam axis). Energy
deposits in the calorimeters unassociated to charged particle tracks were required to be
above 0.3 GeV.
Isolated particles were defined by constructing double cones centered around the axis
of the neutral cluster (charged particle track) with half opening angles of 5◦and 15◦
(5◦and 25◦), and requiring that the average energy density in the outer cone was below
10 MeV/degree ( 15 MeV/degree), to assure isolation. In the case of neutral deposits,
The PYTHIA  generator was used
no charged particle with more than 250 MeV was allowed inside the inner cone. The
energy of the isolated particle was then re-evaluated as the sum of the energies (charged
particle track momenta) inside the inner cone. For well identified photons or leptons, the
above requirements were weakened: the external angle was allowed to be smaller and one
energetic particle was allowed in the outer cone.
Photons were further required to have no HPC layer with more than 90% of the photon
electromagnetic energy. Alternatively, energy deposits above 3 GeV in the hadronic
calorimeter were considered as photon candidates if at least 90% of the deposited energy
was in the first layer of the HCAL.
Photons converting within the tracking system were recovered only in the non-hadronic
3.1Photonic final states
Photons converting inside the tracking system, but after the Vertex Detector, are
characterized by charged particle tracks and will be referred to as converted photons.
Photons reaching the electromagnetic calorimeters before converting, yielding no recon-
structed charged particles tracks, will be referred to as unconverted photons. According
to this classification, two different algorithms were applied in the photon reconstruction
Energy deposits were considered unconverted photons if the following requirements
• The energy was above 3 GeV.
• The polar angle of the energy deposit was inside one of the intervals [20◦,35◦],
[42◦,88◦], [92◦,138◦] or [145◦,160◦] in order to reduce calorimeter edge effects.
• No charged particle tracks were associated to the energy deposit.
• There was no VD track element pointing to the energy deposit direction within 3◦
(10◦) in azimuth in the barrel (forward) region of DELPHI (a VD track element was
defined as at least two hits in different VD layers aligned within an azimuthal angle
interval of 0.5◦, assuming the charged particle track originated from the beam spot).
• If the polar angle of the energy deposit was below 30◦(above 150◦), it had to be out
of the 6 TPC φ intermodular divisions by 2.5◦.
Photons converting after the VD in the polar angle range between 25◦and 155◦were
recovered. They were reconstructed with the help of the DURHAM jet clustering algo-
rithm . All particles in the event, with exception of isolated neutral particles were
clustered in jets, using as the resolution variable ycut= 0.003. Low multiplicity jets with
less than 6 charged particles were treated as converted photon candidates if they were
associated to energy deposits fulfilling the same requirements imposed on unconverted
A common preselection was defined for all the photonic final states (level 1). It was
required that the visible energy in the polar angle region between 20◦and 160◦was greater
than 0.1√s. The number of charged particle tracks was required to be less than 6, all
without VD track elements. At least two photons had to have energy greater than 5
GeV and polar angles between 25◦and 155◦. No particles (with the exception of isolated
photons) with energy above 3 GeV were allowed in the event; no more than one photon
converting in the tracking system was allowed.
Specific criteria were then applied to the photonic preselected sample according to the
final state topology under study.
3.1.1Events with two photons and missing energy
The level 2 selection of the γγ + Emisssample consisted of requiring events with two
and only two photons. The acoplanarity1between the two photons in these events is
compared to the Standard Model prediction in figure 1a).
Final selection criteria (level 3), aiming at the enhancement of a possible signal con-
tribution were then imposed and consisted of the following conditions:
• Whenever the missing momentum was greater than 0.1√s the polar angle of the
direction of the missing momentum was required to be greater than 10◦and less
than 170◦and no signal in the set of lead/scintillator counters placed between the
barrel and forward electromagnetic calorimeters was allowed.
• The acoplanarity between the two photons was required to be greater then 10◦.
• The sum of the energies of the two photons had to be lower than 0.7√s.
In the case of the search for the Higgs-strahlung production, h0Z0, with h0→ γγ and
Z0→ ν¯ ν, it was further required that the mass recoiling against the two photons was
above 20 GeV/c2. The invariant masses of the photon pairs are displayed for these events
in figure 1b). The background comes mainly from double radiative returns to the Z0with
Z → ν¯ ν.
The efficiencies are about 60% for both h0Z0and h0A0, for all centre-of-mass energies
and mass ranges considered. For mh0 = 90 GeV/c2and δ = π/4, the number of expected
signal events from h0Z0production is 1.7.
3.1.2Events with four photons and missing energy
Different criteria were imposed on the level 1 photonic sample in order to get a wide
sample of candidates for the associated production of h0A0, in which the CP-odd bo-
son decays to h0Z0, the Z0going to two neutrinos. The specific criteria for selecting
γγγγ + Emissevents (level 2), consisted of demanding that the events had at least three
photons, all but one between 25◦and 155◦in polar angle. Moreover, whenever the miss-
ing momentum was greater than 0.1√s the polar angle of the direction of the missing
momentum was required to be between 10◦and 170◦.
A final set of requirements was imposed in order to enhance a possible signal (selection
• The acoplanarity between the two most energetic photons had to be greater then
• If the missing energy was below 70 GeV, the missing transverse momentum had to
be greater that 50 GeV/c.
• The energy of the most energetic photon had to be less than√s/2 − 20 GeV.
The average efficiency of this selection is around 50%. For mh0 = 10 GeV/c2and
mA0 = 120 GeV/c2and a δ = π/4, the signal expectation is of 3.6 events, for a total
background expectation of 2.9±0.5 events, coming both from Z0γγ and γγ producton.
3.2 Final states with jets and photons
Selection criteria were implemented to identify events with two jets and at least two
isolated photons (level 1). Isolated photons were reconstructed as explained in the be-
1acoplanarity is defined as the complement of the angle between the projections of the two photons in the plane
perpendicular to the beam
gining of section 3. Their energy was further required to be above 5 GeV to avoid large
contamination from photons coming from the hadronization.
Events were selected in the hadronic topologies if at least six charged particles were
present, the visible energy in the polar angle region between 20◦and 160◦was greater
than 0.2√s and there was at least one charged particle or one electromagnetic cluster with
an energy greater than 5 GeV. All selected charged particles and neutrals not associated
to isolated photons were forced to be clustered into two jets using the DURHAM jet
For qqγγ, (qqγγγγ) final states two (at least three) photons with polar angle above
40◦and below 140◦were required. In order to improve momentum and energy resolu-
tion for the q¯ qγγ final states, a kinematic fit  imposing total energy and momentum
conservation (with the two jets and two photons) was performed on the selected events.
Only events with a χ2per degree of freedom lower than 5 were accepted. This defined
the selection level 2. The jet-jet mass resolution at this level was 3 GeV/c2.
Selection level 2 was used for the search for h0Z0. A selection level 3 was defined for
the search for h0A0with A0→ b¯b, in which flavour tagging was performed based on the
identification of the final state quark. Events with a high probability of containing a b
quark (using the variable defined in ) were thus accepted, allowing for a reduction of
50% in the background while keeping 90% of the signal.
The γγ invariant masses reconstructed for events with two jets and two photons are
displayed in figure 2, both for h0Z0(a) and h0A0(b) searches.
The average efficiencies for masses near the upper kinematic limit are 36% and 33%
for two photon events from h0Z0and h0A0production, respectively, and 30% for the
final state with at least three photons. These numbers correspond to expectations of
2.4 events from h0Z0in the q¯ qγγ selection and 2.1 from h0A0in the b¯bγγ selection, for
mA0 = mh0=90 GeV/c2and δ = π/4. For the final state with at least three photons and
for masses of mA0=120 GeV/c2and mh0=10 GeV and δ = π/4, the signal expectation is
of 5.0 events to be compared with 3.0±0.6 background events coming mainly from q¯ qγγγ.
The number of candidates at different selection levels for the relevant topologies are
given in table 2. The numbers in parentheses correspond to the Standard Model expec-
tations which, in the case of final states with only photons, were corrected for trigger
efficiencies (of the order of 98% in the barrel region of the detector and above 99% in
the forward region considered in the analysis). Overall, there is a reasonable agreement
between data and MC expectations.
Small excesses appear in topologies with low statistics. For instance, in the q¯ qγγγγ
final state at 189 GeV there is a slight excess not confirmed at higher energies. The
reconstructed mZ0 (missing mass or invariant mass of the two jets) for events selected in
the last level of the two topologies with four photons are shown in figure 3. It should be
remarked that the good description of the Z0(Nγ) background has been confirmed only
for final states with at most two visible photons.
Signal selection efficiencies were calculated for each final state topology according
to the specific process to be studied. Several (mh0,mA0) points covering the relevant
parameter space were considered. For all these masses, the width of the Higgs bosons is
smaller than the mass resolution.
These results were then combined and interpreted within the 2HDM fermiophobic
framework giving limits on the cross-sections of the studied processes. The Modified
7 (5.6±0.9) (h0Z0)
8 (6.4±0.9) (h0A0)
1 (1.0±0.1) (h0Z0)
1 (1.1±0.1) (h0A0)
1 (1.9±0.3) (h0Z0)
1 (2.3±0.3) (h0A0)
3 (3.1±0.4) (h0Z0)
3 (3.2±0.4) (h0A0)
2 (1.2±0.2) (h0Z0)
2 (1.3±0.2) (h0A0)
714 (707 ± 6) 561 (555±5)
77 (89±1)91 (119±1)
264 (259 ± 3)343 (347±3)
263 (264±3)334 (356±3)
126 (128±1)171 (170±2)
Table 2: Number of events passing the sets of cuts corresponding to the selection levels
described in the text for each topology and centre-of-mass energy. The MC predicted
numbers of events and their statistical errors are displayed between parentheses. The
second selection level of b¯bγγ is the last level for the selection of q¯ qγγ.
Frequentist Likelihood Ratio method described in  was used. The method is based
on the measured and expected mass distributions. A test statistic is constructed as the
ratio of the probability density functions of the signal plus background to background
4.1Constraints from LEP 1
The production of Higgs bosons at LEP 1 energies would have the effect of increasing
the Z0width. Since the Z0parameters are very well measured, tight bounds can be
derived on the Higgs mass. However these results should be used with some care .
The Higgs production would change the result for the hadronic cross-section, which plays
an important role in the fitting of the electroweak parameters. In  a fit with more
independent variables is performed, assuming that only the e+e−→ Z0→ e+e−and
e+e−→ Z0→ µ+µ−have no contribution from new physics and that the new physics
corrections to the other processes are not strongly flavour dependent. The limit thus
obtained is almost model independent and completely independent of the efficiency with
which the new modes could be selected for the e+e−→ Z0→ hadrons or e+e−→ Z0→
τ+τ−samples. A 95% Confidence Level (CL) upper limit of 6.3 MeV/c2is obtained for
the change in the total Z0width, which yields a limit of 149.4 pb for the production
cross-section of unknown particles.
This cross-section limit can be used to constrain both h0Z0∗production if sin2δ = 1
and h0A0production if sin2δ = 0. The first case corresponds to the exclusion of mh0 <
9 GeV/c2at 95% CL and the second to the exclusion of a band of values corresponding
approximately to mh0+mA0 < 70 GeV/c2at 95% CL. The intersection of the two regions
is excluded for all δ values.
On the other hand, for processes where all the decay products are invisible, the mea-
surement of the Z0invisible width can be used, leading to tighter limits on their cross-
section. LEP 1 data  leads to a cross-section upper limit of 67 pb at 95% CL. This
limit allows us to exclude the totally invisible final state arising from h0A0→ A0A0A0
and A0stable (using potential A in the 2HDM fermiophobic scenario), excluding a band
of values corresponding approximately to mh0 + mA0 < 80 GeV/c2at 95% CL.
4.2Constraints from 6-fermion final states
In general 2HDM, the decay h0→ A0A0is the dominant one when kinematically
allowed. This gives rise to final states with 6-jets: 6 b-jets for h0A production and 4 b-jets
+Z0for the Higgs-strahlung process.
To cover this kinematic region (mh0 > 2mA0), the results from  were used. In this
paper, there is no dedicated analysis of 6-jet events, but it is shown that the analysis of
4-jet events (relevant for the SM and MSSM Higgs searches) has enough sensitivity to
exclude this region almost up to the kinematic limit.
A 95% CL exclusion in the plane (mh0,mA0), valid for all possible combinations of α
and β (all values of δ), is obtained by combining the numbers of expected events in h0Z0
and h0A0channels and minimizing the CL with respect to sinδ and cosδ.
5Limits on fermiophobic Higgs boson production
In figure 4, the 95% CL limits on the production of a resonance X in the process
Z0X → Z0γγ as a function of the γγ invariant mass are presented in terms of the
product of BR(X → γγ) and ξ = σ(Z0X)/σ(Z0H)SM. Here the analyses of q¯ qγγ and
ν¯ νγγ were used and the limit is valid for resonances with width smaller than the analyses
mass resolution. For a value of ξ.BR = 1, a limit of 107 GeV/c2is obtained for mh0.
Also shown is BR(h0→ γγ) as a function of mh0, obtained for potential A of the 2HDM.
This branching fraction is comparable to the one obtained in the SM by setting to zero
the value of the couplings of the Higgs boson to fermion pairs.
In the 2HDM scenario, ξ corresponds to sin2δ and the BR (which is a function of
mh0,mA0,δ) must be taken into account to determine the excluded (mh0,sin2δ) region.
The result for potential A is shown in figure 5. The lower limit thus obtained for mh0
is 96 GeV/c2at 95% CL , for sin2δ=1. For small values of sin2δ, the Higgs-strahlung
cross-section vanishes but an exclusion region can be obtained from the h0A0associated
production. Such 95% CL exclusion regions are shown for two different A0masses. For
mA0 < 60 GeV/c2, mh0 < 9 GeV/c2is excluded by the Z0width measurements.
Due to the complementarity of the h0Z0and h0A0cross-sections, regions of the plane
(mh0,mA0) can be excluded at 95 % CL for all δ values. Figures 6(a) and 6(b) show the
excluded regions in this plane in the framework of potentials A and B, respectively. In
both plots, Region I corresponds to the zone where h0→ γγ and A0→ b¯b. In the case of
potential A, results of γγA0(stable) must also be taken into account, however, the γγ final
state gives stronger limits and the unexcluded region is still defined by the b¯bγγ search.
In Region II, corresponding to h0→ γγ and A0→ h0Z0, both γγγγ and q¯ qγγγγ are
considered, together with the Higgs-strahlung process. In Region III, corresponding to
h0→ A0A0and A0→ b¯b giving rise to 6-jet final states, the 95% CL limits on (mh0,mA0)
from  are used. In the case of potential A, the A0boson can be stable and the limits
from the Z0invisible width provide the most conservative exclusion region. As discussed
previously, the measurement of the Z0width at LEP 1 allows the exclusion of a band of
values for low masses of both h0and A0which is common for both potentials, this is also
indicated in figures 6(a) and 6(b).
DELPHI data corresponding to a total integrated luminosity of 380 pb−1, at centre-
of-mass energies between 189 GeV and 202 GeV, have been analysed and a search for
a neutral Higgs boson with predominantly non-fermionic couplings was performed. The
final states γγ, γγγγ, b¯bγγ, q¯ qγγ and q¯ qγγγγ were considered. A large region of the
parameter space in a 2HDM fermiophobic scenario was excluded.
We would like to thank L.Br¨ ucher and R.Santos for very interesting and long discus-
sions in exploring the 2HDM fermiophobic scenario.
We are greatly indebted to our technical collaborators, to the members of the CERN-
SL Division for the excellent performance of the LEP collider, and to the funding agencies
for their support in building and operating the DELPHI detector.
We acknowledge in particular the support of
Austrian Federal Ministry of Education, Science and Culture, GZ 616.364/2-III/2a/98,
FNRS–FWO, Flanders Institute to encourage scientific and technological research in the
industry (IWT), Belgium,
FINEP, CNPq, CAPES, FUJB and FAPERJ, Brazil,
Czech Ministry of Industry and Trade, GA CR 202/96/0450 and GA AVCR A1010521,
Danish Natural Research Council,
Commission of the European Communities (DG XII),
Direction des Sciences de la Mati` ere, CEA, France,
Bundesministerium f¨ ur Bildung, Wissenschaft, Forschung und Technologie, Germany,
General Secretariat for Research and Technology, Greece,
National Science Foundation (NWO) and Foundation for Research on Matter (FOM),
Norwegian Research Council,
State Committee for Scientific Research, Poland, 2P03B06015, 2P03B11116 and
JNICT–Junta Nacional de Investiga¸ c˜ ao Cient´ ıfica e Tecnol´ ogica, Portugal,
Vedecka grantova agentura MS SR, Slovakia, Nr. 95/5195/134,
Ministry of Science and Technology of the Republic of Slovenia,
CICYT, Spain, AEN96–1661 and AEN96-1681,
The Swedish Natural Science Research Council,
Particle Physics and Astronomy Research Council, UK,
Department of Energy, USA, DE–FG02–94ER40817.
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Figure 1: Acoplanarity between the two photons at selection level 2 (a) and invariant
γγ mass at selection level 3 (b) of the two photon analysis. The dots represent the
data. The shaded area represents the expected standard model background, which comes
mainly from the process e+e−→ Z0→ ν¯ ν with two visible ISR photons and from the
QED process e+e−→ γγ(γ). The darker line corresponds to signal distributions for a
Higgs mass of 90 GeV/c2, with arbitrary normalization. The invariant mass distribution
corresponds to level 3 of the h0Z0selection, with 14 data events and 13±1 expected from