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arXiv:hep-ex/0002038v2 17 Jul 2000
Search for Resonances Decaying to e+-jet in e+p
Interactions at HERA
ZEUS Collaboration
Abstract
The e+-jet invariant mass spectrum produced in the reaction e+p→e+Xhas been
studied at a center-of-mass energy of 300 GeV. The data were collected using the
ZEUS detector operating at the HERA collider, and correspond to an integrated
luminosity of 47.7 pb−1. The observed mass spectrum is in good agreement with
Standard Model expectations up to an e+-jet mass of 210 GeV. Above this mass,
some excess is seen. The angular distribution of these events is typical of high-Q2
neutral current events and does not give convincing evidence for the presence of a
narrow scalar or vector state. Limits are presented on the product of cross section
and branching ratio for such a state and are interpreted as limits on leptoquark
or R-parity-violating squark production. Specific leptoquark types are ruled out
at 95% confidence level for coupling strength λ= 0.3 for masses between 150 and
280 GeV.
DESY 00-023
February 2000
The ZEUS Collaboration
J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, B. Musgrave, A. Pellegrino,
J. Repond, R. Stanek, R. Yoshida
Argonne National Laboratory, Argonne, IL, USA p
M.C.K. Mattingly
Andrews University, Berrien Springs, MI, USA
G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini1,
A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini2, L. Cifarelli3, F. Cindolo, A. Contin,
N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi,
A. Margotti, T. Massam, R. Nania, F. Palmonari, A. Pesci, A. Polini, G. Sartorelli,
Y. Zamora Garcia4, A. Zichichi
University and INFN Bologna, Bologna, Italy f
C. Amelung, A. Bornheim, I. Brock, K. Cob¨oken, J. Crittenden, R. Deffner, H. Hartmann,
K. Heinloth, E. Hilger, P. Irrgang, H.-P. Jakob, A. Kappes, U.F. Katz, R. Kerger, E. Paul,
H. Schnurbusch,
A. Stifutkin, J. Tandler, K.Ch. Voss, A. Weber, H. Wieber
Physikalisches Institut der Universit¨at Bonn, Bonn, Germany c
D.S. Bailey, O. Barret, N.H. Brook5, B. Foster6, G.P. Heath, H.F. Heath, J.D. McFall,
D. Piccioni, E. Rodrigues, J. Scott, R.J. Tapper
H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o
M. Capua, A. Mastroberardino, M. Schioppa, G. Susinno
Calabria University, Physics Dept.and INFN, Cosenza, Italy f
H.Y. Jeoung, J.Y. Kim, J.H. Lee, I.T. Lim, K.J. Ma, M.Y. Pac7
Chonnam National University, Kwangju, Korea h
A. Caldwell, W. Liu, X. Liu, B. Mellado, S. Paganis, R. Sacchi, S. Sampson, F. Sciulli
Columbia University, Nevis Labs., Irvington on Hudson, N.Y., USA q
J. Chwastowski, A. Eskreys, J. Figiel, K. Klimek, K. Olkiewicz, K. Piotrzkowski8, M.B. Przy-
bycie´n, P. Stopa, L. Zawiejski
Inst. of Nuclear Physics, Cracow, Poland j
L. Adamczyk, B. Bednarek, K. Jele´n, D. Kisielewska, A.M. Kowal, T. Kowalski, M. Przy-
bycie´n,
E. Rulikowska-Zar¸ebska, L. Suszycki, D. Szuba
Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow,
Poland j
A. Kota´nski
Jagellonian Univ., Dept. of Physics, Cracow, Poland k
I
L.A.T. Bauerdick, U. Behrens, J.K. Bienlein, C. Burgard9, K. Desler, G. Drews, A. Fox-Murphy,
U. Fricke, F. Goebel, P. G¨ottlicher, R. Graciani, T. Haas, W. Hain, G.F. Hartner,
D. Hasell10, K. Hebbel, K.F. Johnson11, M. Kasemann12 , W. Koch, U. K¨otz, H. Kowal-
ski, L. Lindemann13, B. L¨ohr, M. Mart´ınez, M. Milite, T. Monteiro8, M. Moritz, D. Notz,
F. Pelucchi, M.C. Petrucci, M. Rohde, P.R.B. Saull, A.A. Savin, U. Schneekloth, F. Selonke,
M. Sievers, S. Stonjek, E. Tassi, G. Wolf, U. Wollmer,
C. Youngman, W. Zeuner
Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
C. Coldewey, H.J. Grabosch, A. Lopez-Duran Viani, A. Meyer, S. Schlenstedt, P.B. Straub
DESY Zeuthen, Zeuthen, Germany
G. Barbagli, E. Gallo, P. Pelfer
University and INFN, Florence, Italy f
G. Maccarrone, L. Votano
INFN, Laboratori Nazionali di Frascati, Frascati, Italy f
A. Bamberger, A. Benen, S. Eisenhardt14, P. Markun, H. Raach, S. W¨olfle
Fakult¨at f¨ur Physik der Universit¨at Freiburg i.Br., Freiburg i.Br., Germany c
P.J. Bussey, A.T. Doyle, S.W. Lee, N. Macdonald, G.J. McCance, D.H. Saxon, L.E. Sin-
clair,
I.O. Skillicorn, R. Waugh
Dept. of Physics and Astronomy, University of Glasgow, Glasgow, U.K. o
I. Bohnet, N. Gendner, U. Holm, A. Meyer-Larsen, H. Salehi, K. Wick
Hamburg University, I. Institute of Exp. Physics, Hamburg, Germany c
D. Dannheim, A. Garfagnini, I. Gialas15, L.K. Gladilin16 , D. K¸cira17, R. Klanner, E. Lohrmann,
G. Poelz, F. Zetsche
Hamburg University, II. Institute of Exp. Physics, Hamburg, Germany c
R. Goncalo, K.R. Long, D.B. Miller, A.D. Tapper, R. Walker
Imperial College London, High Energy Nuclear Physics Group, London, U.K. o
U. Mallik
University of Iowa, Physics and Astronomy Dept., Iowa City, USA p
P. Cloth, D. Filges
Forschungszentrum J¨ulich, Institut f¨ur Kernphysik, J¨ulich, Germany
T. Ishii, M. Kuze, K. Nagano, K. Tokushuku18 , S. Yamada, Y. Yamazaki
Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan g
S.H. Ahn, S.B. Lee, S.K. Park
Korea University, Seoul, Korea h
H. Lim, I.H. Park, D. Son
Kyungpook National University, Taegu, Korea h
II
F. Barreiro, G. Garc´ıa, C. Glasman19 , O. Gonzalez, L. Labarga, J. del Peso, I. Redondo20,
J. Terr´on
Univer. Aut´onoma Madrid, Depto de F´ısica Te´orica, Madrid, Spain n
M. Barbi, F. Corriveau, D.S. Hanna, A. Ochs, S. Padhi, M. Riveline, D.G. Stairs, M. Wing
McGill University, Dept. of Physics, Montr´eal, Qu´ebec, Canada a,b
T. Tsurugai
Meiji Gakuin University, Faculty of General Education, Yokohama, Japan
V. Bashkirov21, B.A. Dolgoshein
Moscow Engineering Physics Institute, Moscow, Russia l
R.K. Dementiev, P.F. Ermolov, Yu.A. Golubkov, I.I. Katkov, L.A. Khein, N.A. Korotkova,
I.A. Korzhavina, V.A. Kuzmin, O.Yu. Lukina, A.S. Proskuryakov, L.M. Shcheglova,
A.N. Solomin,
N.N. Vlasov, S.A. Zotkin
Moscow State University, Institute of Nuclear Physics, Moscow, Russia m
C. Bokel, M. Botje, N. Br¨ummer, J. Engelen, S. Grijpink, E. Koffeman, P. Kooijman,
S. Schagen, A. van Sighem, H. Tiecke, N. Tuning, J.J. Velthuis, J. Vossebeld, L. Wiggers,
E. de Wolf
NIKHEF and University of Amsterdam, Amsterdam, Netherlands i
D. Acosta22, B. Bylsma, L.S. Durkin, J. Gilmore, C.M. Ginsburg, C.L. Kim, T.Y. Ling
Ohio State University, Physics Department, Columbus, Ohio, USA p
S. Boogert, A.M. Cooper-Sarkar, R.C.E. Devenish, J. Große-Knetter23, T. Matsushita,
O. Ruske,
M.R. Sutton, R. Walczak
Department of Physics, University of Oxford, Oxford U.K. o
A. Bertolin, R. Brugnera, R. Carlin, F. Dal Corso, U. Dosselli, S. Dusini, S. Limentani,
M. Morandin, M. Posocco, L. Stanco, R. Stroili, C. Voci
Dipartimento di Fisica dell’ Universit`a and INFN, Padova, Italy f
L. Iannotti24, B.Y. Oh, J.R. Okrasi´nski, W.S. Toothacker, J.J. Whitmore
Pennsylvania State University, Dept. of Physics, University Park, PA, USA q
Y. Iga
Polytechnic University, Sagamihara, Japan g
G. D’Agostini, G. Marini, A. Nigro
Dipartimento di Fisica, Univ. ’La Sapienza’ and INFN, Rome, Italy f
C. Cormack, J.C. Hart, N.A. McCubbin, T.P. Shah
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K. o
D. Epperson, C. Heusch, H.F.-W. Sadrozinski, A. Seiden, R. Wichmann, D.C. Williams
University of California, Santa Cruz, CA, USA p
III
N. Pavel
Fachbereich Physik der Universit¨at-Gesamthochschule Siegen, Germany c
H. Abramowicz25, S. Dagan26, S. Kananov26, A. Kreisel, A. Levy26
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics, Tel-Aviv
University,
Tel-Aviv, Israel e
T. Abe, T. Fusayasu, K. Umemori, T. Yamashita
Department of Physics, University of Tokyo, Tokyo, Japan g
R. Hamatsu, T. Hirose, M. Inuzuka, S. Kitamura27, T. Nishimura
Tokyo Metropolitan University, Dept. of Physics, Tokyo, Japan g
M. Arneodo28, N. Cartiglia, R. Cirio, M. Costa, M.I. Ferrero, S. Maselli, V. Monaco,
C. Peroni, M. Ruspa, A. Solano, A. Staiano
Universit`a di Torino, Dipartimento di Fisica Sperimentale and INFN, Torino, Italy f
M. Dardo
II Faculty of Sciences, Torino University and INFN - Alessandria, Italy f
D.C. Bailey, C.-P. Fagerstroem, R. Galea, T. Koop, G.M. Levman, J.F. Martin, R.S. Orr,
S. Polenz, A. Sabetfakhri, D. Simmons
University of Toronto, Dept. of Physics, Toronto, Ont., Canada a
J.M. Butterworth, C.D. Catterall, M.E. Hayes, E.A. Heaphy, T.W. Jones, J.B. Lane,
B.J. West
University College London, Physics and Astronomy Dept., London, U.K. o
J. Ciborowski, R. Ciesielski, G. Grzelak, R.J. Nowak, J.M. Pawlak, R. Pawlak, B. Smalska,
T. Tymieniecka, A.K. Wr´oblewski, J.A. Zakrzewski, A.F. ˙
Zarnecki
Warsaw University, Institute of Experimental Physics, Warsaw, Poland j
M. Adamus, T. Gadaj
Institute for Nuclear Studies, Warsaw, Poland j
O. Deppe, Y. Eisenberg26 , D. Hochman, U. Karshon26
Weizmann Institute, Department of Particle Physics, Rehovot, Israel d
W.F. Badgett, D. Chapin, R. Cross, C. Foudas, S. Mattingly, D.D. Reeder, W.H. Smith,
A. Vaiciulis29, T. Wildschek, M. Wodarczyk
University of Wisconsin, Dept. of Physics, Madison, WI, USA p
A. Deshpande, S. Dhawan, V.W. Hughes
Yale University, Department of Physics, New Haven, CT, USA p
S. Bhadra, C. Catterall, J.E. Cole, W.R. Frisken, R. Hall-Wilton, M. Khakzad, S. Menary,
W.B. Schmidke
York University, Dept. of Physics, Toronto, Ont., Canada a
IV
1now visiting scientist at DESY
2also at IROE Florence, Italy
3now at Univ. of Salerno and INFN Napoli, Italy
4supported by Worldlab, Lausanne, Switzerland
5PPARC Advanced fellow
6also at University of Hamburg, Alexander von Humboldt Research Award
7now at Dongshin University, Naju, Korea
8now at CERN
9now at Barclays Capital PLC, London
10 now at Massachusetts Institute of Technology, Cambridge, MA, USA
11 visitor from Florida State University
12 now at Fermilab, Batavia, IL, USA
13 now at SAP A.G., Walldorf, Germany
14 now at University of Edinburgh, Edinburgh, U.K.
15 visitor of Univ. of Crete, Greece, partially supported by DAAD, Bonn - Kz. A/98/16764
16 on leave from MSU, supported by the GIF, contract I-0444-176.07/95
17 supported by DAAD, Bonn - Kz. A/98/12712
18 also at University of Tokyo
19 supported by an EC fellowship number ERBFMBICT 972523
20 supported by the Comunidad Autonoma de Madrid
21 now at Loma Linda University, Loma Linda, CA, USA
22 now at University of Florida, Gainesville, FL, USA
23 supported by the Feodor Lynen Program of the Alexander von Humboldt foundation
24 partly supported by Tel Aviv University
25 an Alexander von Humboldt Fellow at University of Hamburg
26 supported by a MINERVA Fellowship
27 present address: Tokyo Metropolitan University of Health Sciences, Tokyo 116-8551,
Japan
28 now also at Universit`a del Piemonte Orientale, I-28100 Novara, Italy
29 now at University of Rochester, Rochester, NY, USA
V
asupported by the Natural Sciences and Engineering Research Council of
Canada (NSERC)
bsupported by the FCAR of Qu´ebec, Canada
csupported by the German Federal Ministry for Education and Science,
Research and Technology (BMBF), under contract numbers 057BN19P,
057FR19P, 057HH19P, 057HH29P, 057SI75I
dsupported by the MINERVA Gesellschaft f¨ur Forschung GmbH, the German
Israeli Foundation, and by the Israel Ministry of Science
esupported by the German-Israeli Foundation, the Israel Science Foundation,
the U.S.-Israel Binational Science Foundation, and by the Israel Ministry of
Science
fsupported by the Italian National Institute for Nuclear Physics (INFN)
gsupported by the Japanese Ministry of Education, Science and Culture (the
Monbusho) and its grants for Scientific Research
hsupported by the Korean Ministry of Education and Korea Science and Engi-
neering Foundation
isupported by the Netherlands Foundation for Research on Matter (FOM)
jsupported by the Polish State Committee for Scientific Research, grant No.
112/E-356/SPUB/DESY/P03/DZ 3/99, 620/E-77/SPUB/DESY/P-03/ DZ
1/99, 2P03B03216, 2P03B04616, 2P03B03517, and by the German Federal
Ministry of Education and Science, Research and Technology (BMBF)
ksupported by the Polish State Committee for Scientific Research (grant No.
2P03B08614 and 2P03B06116)
lpartially supported by the German Federal Ministry for Education and Science,
Research and Technology (BMBF)
msupported by the Fund for Fundamental Research of Russian Ministry for
Science and Education and by the German Federal Ministry for Education
and Science, Research and Technology (BMBF)
nsupported by the Spanish Ministry of Education and Science through funds
provided by CICYT
osupported by the Particle Physics and Astronomy Research Council
psupported by the US Department of Energy
qsupported by the US National Science Foundation
VI
1 Introduction
The e+-jet mass spectrum in e+pscattering has been investigated with the ZEUS detec-
tor at HERA. An excess of events relative to Standard Model expectations has previously
been reported by the H1 [1, 2] and ZEUS [3, 4] collaborations in neutral current deep
inelastic scattering at high xand high Q2. These events contain high-mass e+-jet final
states. Several models have been discussed [5] as possible sources of these events, includ-
ing leptoquark production [6] and R-parity-violating squark production [7]. This paper
presents an analysis of ZEUS data specifically aimed at searching for high mass states
decaying to e+-jet.
Candidate events with high transverse energy, an identified final-state positron, and at
least one jet are selected. The measured energies (E′
e, Ej) and angles of the final-state
positron and the jet with highest transverse momentum are used to calculate an invariant
mass
M2
ej = 2E′
eEj·(1 −cos ξ),(1)
where ξis the angle between the positron and jet. The angle between the outgoing and
incoming positron in the e+-jet rest frame, θ∗, is also reconstructed using the measured
energies and angles. No assumptions about the production process are made in the
reconstruction of either Mej or θ∗.
The search was performed using 47.7 pb−1of data collected in the 1994-1997 running
periods. In the following, expectations from the Standard Model, leptoquark produc-
tion and R-parity-violating squark production are summarized. After a discussion of the
experimental conditions, the analysis is described and the Mej and cos θ∗distributions
presented. Since these distributions do not show a clear signal for a narrow resonance,
limits on the cross section times branching ratio are extracted for the production of such
a state. Limits are also presented in the mass versus coupling plane which can be applied
to leptoquark and squark production.
2 Model Expectations
High-mass e+-jet pairs, produced in the Standard Model (SM) via neutral current (NC)
scattering, form the principal background to the search for heavy states. This process is
reviewed first. Leptoquark (LQ) production and squark production in R-parity-violating
(6RP) supersymmetry are used as examples of physics beyond the SM that could generate
the e+-jet final state. The diagrams for NC and LQ processes are shown in Fig. 1. The
squark production diagrams are similar to the LQ diagrams, but different decay modes
are possible, as discussed below.
2.1 Standard Model Expectations
The kinematic variables used to describe the deep inelastic scattering (DIS) reaction
e+p→e+X
1
e+e+
γ
q q
p
a)
e+e+
Z0
q q
p
b)
e+
q
p
LQ e+
q
c)
e+
q
p
LQ
q
e+
d)
Figure 1: Diagrams for NC scattering via a) photon exchange and b) Z0exchange. The
leptoquark diagrams for the same initial and final states are c) s-channel LQ production
for fermion number F= 0 LQ and d) u-channel LQ exchange for an F= 2 LQ.
are
Q2=−q2=−(k−k′)2,(2)
y=q·P
k·Pand (3)
x=Q2
2q·P,(4)
where kand k′are the four-momenta of the incoming and outgoing positron, respectively,
and Pis the four-momentum of the incoming proton. The center-of-mass energy is given
by s= (k+P)2≈(300 GeV)2. The NC interaction occurs between the positron and a
parton (quark) inside the proton (see Fig. 1). The production of the large e+-jet masses of
interest requires high xpartons, where the valence quarks dominate the proton structure.
In leading-order electroweak theory, the cross section for the NC DIS reaction can be
expressed as [8]
d2σ(e+p)
dxdy =2πα2
sx2y2hY+F2−Y−xF3+y2FLi(5)
with Y±= 1 ±(1 −y)2and αthe fine structure constant. The contribution from the
longitudinal structure function, FL, is expected to be negligible in the kinematic range
considered here.
The xdependence of the NC cross section is very steep. In addition to the explicit 1/x2
factor, the structure functions F2and xF3are dominated at large xby valence-quark
2
densities that fall quickly for x > 0.3. The ydependence of the cross section is dominated
by the 1/y2term. The structure functions vary slowly with yat fixed x. The uncertainty
in the NC cross section predicted by Eq. 5 is dominated by the uncertainty in the structure
functions (parton densities), and is small, about 5% at the high-xand moderate-yranges
of this analysis [4]. The quantity of interest in this paper is the e+-jet cross section, which
is sensitive to QCD corrections. The uncertainty arising from these corrections has been
estimated to be small for this analysis.
For DIS or LQ events produced via the diagrams shown in Fig. 1 (i.e. assuming no QED
or QCD radiation), the mass of the eq system is related to xvia
M2=sx (6)
and θ∗is related to yvia
cos θ∗= 1 −2y . (7)
The steeply-falling xand ydependences of DIS events will therefore produce distributions
falling sharply with mass and peaking towards cos θ∗= 1.
2.2 Leptoquark Production and Exchange
Leptoquark production is an example of new physics that could generate high-mass e+-jet
pairs. The set of leptoquarks with SU(3) ×SU (2) ×U(1)-invariant couplings has been
specified [6]. Only LQs with fermion number F=L+ 3B= 0 are considered here, where
Land Bdenote the lepton and baryon number, respectively. These leptoquarks are listed
in Table 1 together with some of their properties. The F= 0 LQs have higher cross
sections in e+pscattering than e−pscattering since in the e+pcase a valence quark can
fuse with the positron.
In principle, additional LQ types can be defined [9] which depend on the generations of
the quarks and leptons to which they couple. Only LQs which preserve lepton flavor and
which couple to first-generation quarks are considered in this analysis.
As shown in Fig. 1, leptoquark production can generate an s-channel resonance provided
mLQ <√s. Contributions to the e+pcross section would also result from u-channel
exchange and interference of LQ diagrams with photon and Z0exchange. The cross
section in the presence of a leptoquark can be written as
d2σ(e+p)
dxdy =d2σNC
dxdy +d2σInt
u/NC
dxdy +d2σInt
s/NC
dxdy +d2σLQ
u
dxdy +d2σLQ
s
dxdy .(8)
The first term on the right-hand side of Eq. 8 represents the SM contribution discussed
previously. The second (third) term arises from the interference between the SM and u-
3
LQ species qProduction Decay Branching ratio
SL
1/2-5/3 eL¯u e¯u1
SR
1/2-5/3 eR¯u e¯u1
-2/3 eR¯
d e ¯
d1
˜
SL
1/2-2/3 eL¯
d e ¯
d1
VL
0-2/3 eL¯
d e ¯
d1/2
νe¯u1/2
VR
0-2/3 eR¯
d e ¯
d1
˜
VR
0-5/3 eR¯u e¯u1
VL
1-5/3 eL¯u e¯u1
-2/3 eL¯
d e ¯
d1/2
νe¯u1/2
Table 1: The F= 0 leptoquarks that can be produced at HERA. The LQ species are divided
according to their spin (Sfor scalar and Vfor vector), their chirality (Lor R) and their
weak isospin (0,1/2,1). The leptoquarks ˜
Sand ˜
Vdiffer by two units of hypercharge from S
and V, respectively. In addition, the electric charge, q, of the leptoquarks, the production
channel, as well as their allowed decay channels assuming lepton-flavor conservation,
are displayed. The quantum numbers and decay channels correspond to an electron-type
LQ. For positrons, the corresponding anti-leptoquarks have the sign of the electric charge
reversed, the helicity of the incoming lepton reversed and antiquarks are replaced by the
corresponding quark. The nomenclature follows the Aachen convention [10].
channel (s-channel) LQ diagram, and the fourth (fifth) term represents the u-channel (s-
channel) LQ diagram alone. The additional contributions to the SM cross section depend
on two parameters: mLQ , and λRor λL, the coupling to e+
L,R and quark. Leptoquarks of
well-defined helicity (λR·λL= 0) are assumed for simplicity in the limit-setting procedure,
and one species of LQ is assumed to dominate the cross section. The cos θ∗dependence
varies strongly for the different terms: it is flat for scalar-LQ production in the s-channel,
and for vector-LQ exchange in the u-channel, while it varies as (1 + cos θ∗)2for vector-LQ
production in the s-channel or scalar-LQ exchange in the u-channel. The interference
terms produce a cos θ∗dependence which is steeper due to the sharply-peaking cos θ∗
distribution in NC DIS.
In general, the s-channel term dominates the additional contributions to the SM cross
section if mLQ <√s, the coupling λis small, and the LQ is produced from a quark rather
than an antiquark. However, there are conditions for which the other terms can become
significant, or even dominant [11], leading to important consequences for the expected
mass spectra and decay angular distributions. The u-channel and interference terms
cannot produce a resonance peak in the mass spectrum and the angular distributions
from such terms can behave more like those of NC deep inelastic scattering. Limits are
presented in this paper for narrow-width LQ and under conditions for which the s-channel
term dominates.
4
The width of a LQ depends on its spin and decay modes, and is proportional to mLQ
times the square of the coupling. In the narrow-width approximation, the LQ production
cross section is given by integrating the s-channel term [6]:
σN W A = (J+ 1) π
4sλ2q(x0, µ) (9)
where Jrepresents the spin of the LQ, q(x0, µ) is the quark density evaluated at x0=
m2
LQ/s and with the scale µ=m2
LQ. In the limit-setting procedure (Sect. 8), this cross
section was corrected for expected QED and QCD (for scalar LQ only) radiative effects.
The QCD corrections [12] enhance the cross section by 20 - 30% for the F= 0 LQ
considered here. The effect of QED radiation on the LQ production cross section was
calculated and was found to decrease the cross section by 5-25% as mLQ increases from
100 →290 GeV.
2.3 R-Parity-Violating Squark Production
In the supersymmetry (SUSY) superpotential, R-parity-violating terms of the form
λ′
ijk Li
LQj
LDk
R[7] are of particular interest for lepton-hadron collisions. Here, LL,QL,
and DRdenote left-handed lepton and quark doublets and the right-handed down-type
quark-singlet chiral superfields, respectively. The indices i,j, and klabel their respective
generations.
For i= 1, which is the case for ep collisions, these operators can lead to ˜u- and ˜
d-type
squark production. There are 9 possible production couplings probed in e+pcollisions,
corresponding to the reactions [13]
e++ ¯uj→˜
¯
dk,(10)
e++dk→˜uj.(11)
For production and decay via the λ′
1jk coupling, squarks behave like scalar leptoquarks
and the final state is indistinguishable, event by event, from Standard Model neutral and
charged current events. However, as for the scalar leptoquarks, the angular distributions
of the final-state lepton and quark will be different and this fact can be exploited in
performing searches. Limits derived for scalar LQ production can then be directly related
to limits on squark production and decay via λ′
ijk . In addition to the Yukawa couplings,
gauge couplings also exist whereby ˜qcan decay by radiating a neutralino or chargino
which can subsequently decay. The final-state signature depends on the properties of the
neutralino or chargino. The search for such decay topologies from a squark is outside the
scope of this analysis.
3 Experimental Conditions
In 1994-97, HERA operated with protons of energy Ep= 820 GeV and positrons of
energy Ee= 27.5 GeV. The ZEUS detector is described in detail in [14]. The main com-
5
ponents used in the present analysis were the central tracking detector (CTD) positioned
in a 1.43 T solenoidal magnetic field and the uranium-scintillator sampling calorimeter
(CAL). The CTD was used to establish an interaction vertex with a typical resolution
along (transverse to) the beam direction of 0.4 (0.1) cm. It was also used in the positron-
finding algorithm that associated a charged track with an energy deposit in the calorime-
ter. The CAL was used to measure the positron and hadronic energies. The CAL consists
of a forward part (FCAL), a barrel part (BCAL) and a rear part (RCAL), with depths of
7,5 and 4 interaction lengths, respectively. The FCAL and BCAL are segmented longitu-
dinally into an electromagnetic section (EMC), and two hadronic sections (HAC1,2). The
RCAL has one EMC and one HAC section. The cell structure is formed by scintillator
tiles; cell sizes range from 5 ×20 cm2(FEMC) to 24.4×35.2 cm2at the front face of a
BCAL HAC2 cell. The light generated in the scintillator is collected on both sides of the
module by wavelength-shifter bars, allowing a coordinate measurement based on knowl-
edge of the attenuation length in the scintillator. The light is converted into an electronic
signal by photomultiplier tubes. The cells are arranged into towers consisting of 4 EMC
cells, a HAC1 cell and a HAC2 cell (in FCAL and BCAL). The transverse dimensions of
the towers in FCAL are 20 ×20 cm2. One tower is absent at the center of the FCAL and
RCAL to allow space for passage of the beams. The outer boundary of the inner ring
of FCAL towers, used to define a fiducial cut for the jet reconstruction, defines a box of
60 ×60 cm2.
Under test beam conditions, the CAL has energy resolutions of σ/E = 0.18/√Efor
positrons hitting the center of a calorimeter cell and σ/E = 0.35/√Efor single hadrons,
where energies are in GeV. In the ZEUS detector, the energy measurement is affected
by the energy loss in the material between the interaction point and the calorimeter.
For the events selected in this analysis, the positrons predominantly strike the BCAL,
while the jets hit the FCAL. The in-situ positron-energy resolution in the BCAL has
been determined to average σ/E = 0.32/√E⊕0.03 while the jet-energy resolution in
the FCAL averages σ/E = 0.55/√E⊕0.02. The jet-energy resolution was determined
by comparing reconstructed jet energies in the calorimeter with the total energy of the
particles in the hadronic final-state using Monte Carlo simulation, and therefore includes
small contributions from the jet-finding algorithm.
In the reconstruction of the positron and jet energies, corrections were applied for inactive
materials located in front of the calorimeter and for non-uniformities in the calorimeter
response [4]. For the high energies important in this analysis, the overall energy scale is
known to 1% for positrons in BCAL and 2% for hadrons in FCAL and BCAL. The elec-
tromagnetic energy scale was determined by a comparison with momentum measurements
in the central tracking detector (using lower-energy electrons and positrons). Its linearity
was checked with energies reconstructed from the double angle (DA) method [15]. The
hadronic-energy scales in the FCAL and BCAL were determined by using transverse-
momentum balance in NC DIS events.
The angular reconstruction was performed using a combination of tracking and calorimeter
information. From Monte Carlo studies, the polar-angle resolutions were found to be
2.5 mrad for positrons and approximately (220/√E−4) mrad for jets with energies
6
above 100 GeV.
The luminosity was measured from the rate of the bremsstrahlung process e+p→e+pγ [16],
and has an uncertainty of 1.6%.
The ZEUS coordinate system is right-handed and centered on the nominal interaction
point, with the Zaxis pointing in the direction of the proton beam (forward) and the X
axis pointing horizontally toward the center of HERA. The polar angle θis defined with
respect to the Zaxis.
4 Event Selection
The events of interest with large e+-jet mass contain a final-state positron at a large angle
and of much higher energy than that of the incident positron beam, as well as one or more
energetic jets. The only important SM source of such events is NC scattering with large
Q2. Other potential backgrounds, such as high transverse-energy (ET) photoproduction,
were determined to be negligible.
The following requirements selected events of the desired topology:
•A reconstructed event vertex was required in the range |Z|<50 cm.
•The total transverse energy, ET, was required to be at least 60 GeV.
•An identified [4] positron was required with energy E′
e>25 GeV, located either
in the FCAL or BCAL. The positron was required to be well-contained in the
BCAL or FCAL and not to point to the BCAL/FCAL interface, at approximately
31◦< θ < 36◦. Positrons within 1.5 cm of the boundary between adjacent BCAL
modules, as determined by tracking information, were also discarded to remove
showers developing in the wavelength-shifter bars.
•A hadronic jet with transverse momentum Pj
T>15 GeV, located in a region of good
containment, was required. The jets were reconstructed using the longitudinally-
invariant kT-clustering algorithm [17] in the inclusive mode [18]. Only jets with
a reconstructed centroid outside the inner ring of FCAL towers were considered.
In events where multiple jets were reconstructed, the jet with highest transverse
momentum was used. After all cuts, 12% of the events had more than one jet, both
in the data and Monte Carlo simulation (see below).
The ETcut, the jet-containment cut and the positron-containment cut define the available
kinematic region for further analysis, as shown in Fig. 2. The jet containment cut, in
particular, limits the values of cos θ∗that can be measured at the highest e+-jet masses.
Because most such events have cos θ∗near 1, the acceptance for NC DIS events (with
ET>60 GeV) falls below 10% for masses beyond 220 GeV. In the region allowed by the
cuts shown in Fig. 2, the acceptance is typically 80%.
7
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200 250 300
Mass(GeV)
cos(θ*)
Et cut
BCAL/FCAL interface
jet containment
Figure 2: The acceptance region (unshaded) in the cos θ∗versus Mej plane allowed by the
ET, jet-containment and positron-fiducial-volume cuts, assuming eq →eq scattering at
the nominal interaction point. No detector simulation is included.
8
A total of 7103 events remained after applying all cuts, compared to 6949±445 events pre-
dicted by the NC Monte Carlo simulation based on the measured luminosity of 47.7 pb−1
(the sources of uncertainty on the expected number of events are described in Sect. 7.1).
The ETdistributions for data and NC simulation are compared in Fig. 3a. The positron
transverse-momentum (Pe
T) spectrum, jet transverse-momentum (Pj
T) spectrum and the
ratio Pe
T/P h
T, where Ph
Tis the transverse momentum of the hadronic system, are shown in
Figs. 3(b-d), respectively. The missing transverse momentum, 6PT, and the longitudinal
momentum variable, E−PZ, are compared in Figs. 3(e,f). The global properties of the
events are well reproduced by the simulation.
5 Event Simulation
The SM deep inelastic scattering events were simulated using the HERACLES 4.5.2 [19]
program with the DJANGO 6 version 2.4 [20] interface to the hadronization programs.
In HERACLES, corrections for initial- and final-state electroweak radiation, vertex and
propagator corrections, and two-boson exchange are included. The NC DIS hadronic final
state was simulated using the MEPS model of LEPTO 6.5 [21], which includes order αS
matrix elements and models of higher-order QCD radiation. As a systematic check, the
NC final state was simulated using the color-dipole model of ARIADNE 4.08 [22]. The
CTEQ4 parton-distribution set [23] was used to evaluate the expected number of events
from NC DIS scattering.
The leptoquark events were generated using PYTHIA 6.1 [24]. This program takes into
account the finite width of the LQ, but only includes the s-channel diagrams. Initial- and
final-state QCD radiation from the quark and the effect of LQ hadronization before decay
are taken into account, as are initial- and final-state QED radiation from the positron.
The generated events were input into a GEANT-based [25] program which simulated the
response of the ZEUS detector. The trigger and offline processing requirements applied
to the data were applied to the simulated events. The luminosity of the NC Monte Carlo
samples ranges from 46 pb−1at Q2= 400 GeV2to 7.3·106pb−1at Q2= 50000 GeV2.
6 Mass and θ∗Reconstruction
The mass of each e+-jet pair was reconstructed from the measured energies and angles of
the positron and jet as described by Eq. 1. This formula makes no correction for the finite
jet mass. Possible mass shifts and the resolutions for resonant lepton-hadron states were
estimated from PYTHIA. Narrow scalar LQ events in the mass range 150 −290 GeV were
simulated. The mean mass for reconstructed events was found to be within 6% of the
generated value, while the peak position as determined by a Gaussian fit was typically
lower than the generated value by only 1%. The average mass resolution, determined
from a Gaussian fit to the peak of the reconstructed mass spectrum, ranged from 5.5% to
9
1
10
10 2
10 3
100 150 200 250
1
10
10 2
10 3
0 50 100
1
10
10 2
10 3
0 50 100
1
10
10 2
10 3
0 0.5 1 1.5 2
1
10
10 2
10 3
0 10 20 30
1
10
10 2
10 3
0 20 40 60 80
ZEUS 1994-97
ET(GeV)
Events/bin
a)
PT
e(GeV)
Events/bin
b)
PT
j(GeV)
Events/bin
c)
PT
e/PT
h
Events/bin
d)
P/T(GeV)
Events/bin
e)
E-Pz(GeV)
Events/bin
f)
Figure 3: Comparison of data (points) with Standard Model expectations (histograms) for
selected distributions: a) total transverse energy, ET; b) positron transverse momentum,
Pe
T; c) jet transverse momentum, Pj
T; d) the ratio of the positron to hadron transverse
momenta, Pe
T/P h
T, (e) the missing transverse momentum, 6PT, and (f) the longitudinal-
momentum variable, E−PZ, for the event.
10
3% for masses from 150 to 290 GeV. The RMS of the distribution was typically twice as
large.
The positron scattering angle in the e+-jet rest frame, θ∗, was reconstructed as the angle
between the incoming and outgoing positron directions in this frame. These directions
were determined by performing a Lorentz transformation using the measured positron and
jet energies and angles in the laboratory frame. The resolution in cos θ∗near |cos θ∗|= 1,
as determined from a Gaussian fit, was 0.01 degrading to 0.03 as |cos θ∗|decreases. The
shift in cos θ∗was less than 0.01 for both the NC MC and the leptoquark MC.
In order to determine limits on leptoquark and squark production, the mass of the
electron-hadron system was reconstructed by the constrained-mass method. This method
reconstructs the e+-hadron mass as
MCJ =q2Ee(E+PZ) (12)
where (E+PZ) is the sum of the energy and PZcontributions from the positron and
all jets satisfying Pj
T>15 GeV and pseudorapidity ηj<3 (with the highest PTjet
required to be outside the FCAL inner ring). The ηjcut removes contributions from
the proton remnant. The constraints 6PT= 0 and E−PZ= 2Ee, which are satisfied by
fully contained events, have been assumed in arriving at this equation. When using this
mass-reconstruction method, events with measured E−PZ<40 GeV were removed to
avoid large initial-state QED radiation.
The MCJ method gave, on average, improved resolution over the Mej method for narrow
LQ MC events. The improved resolution occurred at smaller cos θ∗(for cos θ∗≈0 the
mass resolution determined from a Gaussian fit to the reconstructed mass distribution
for mLQ = 200 GeV was about 1.5% in the MC J method and 3% for the Mej method);
at the larger cos θ∗values where NC DIS events are concentrated, the resolutions of the
two methods were similar (about 3% for mLQ = 200 GeV). The MC J method relies on
constraints which do not necessarily apply to a resonant state whose properties cannot be
anticipated in detail. We therefore choose to use the Mej method as our primary search
method. The MC J method is used in the limit-setting procedure.
7Mej and cos θ∗Distributions
The reconstructed values of Mej are plotted versus cos θ∗for the selected events in Fig. 4.
Most of the events are concentrated at large cos θ∗and small mass, as expected from
Standard Model NC scattering. The five events indicated as open circles are from data
taken in 1994-96, with total luminosity 20 pb−1. They were the subject of a previous
publication [3]. In this earlier analysis, the kinematic variables were reconstructed with
the DA method. The five events also stand out with the Mej reconstruction technique.
The average value of Mej for these events is 224 GeV, or 7 GeV less than the corresponding
mass calculated previously via M=√s·xDA, where xDA is the estimator of Bjorken-x
calculated with the DA method. This mass shift is compatible with expectations based
11
on resolution and initial state radiation effects. With the present luminosity of 47.7 pb−1,
7 events are observed in the region of Mej >200 GeV and cos θ∗<0.5, where 5.0 events
are expected.
The Mej spectrum for events with Mej >100 GeV is shown in Fig. 5a on a logarithmic
scale. The high-mass part of the spectrum is shown on a linear scale in the inset. The
predicted number of events (Npred) from NC processes is shown as the histogram. The
ratio of the measured mass spectrum to the expectation is shown in Fig. 5b. The shaded
band indicates the systematic uncertainty on the expectations.
10 -1
1
10
10 2
10 3
100 120 140 160 180 200 220 240 260
0
10
20
30
40
50
60
180 190 200 210 220 230 240 250 260 270 280
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
100 120 140 160 180 200 220 240 260
ZEUS 1994-97
Mej(GeV)
Events
a)
Mej(GeV)
Ndata/Npred
b)
Figure 5: a) Comparison of observed events (points) and SM expectations (histogram) for
the reconstructed e+-jet invariant mass. The inset shows the region with Mej >180 GeV
on a linear scale. b) The ratio of the number of observed events to the Standard Model
expectations. The shaded band shows the systematic uncertainty in the predicted number
of events. The error bars on the data points are calculated from the square root of the
number of events in the bin.
13
7.1 Systematic Uncertainties
The uncertainty on Npred varies with mass from 7% at 100 GeV up to 30% at 250 GeV.
The most important uncertainties are on the energy scale and the jet position. The NC
DIS cross section given in Eq. 5 (neglecting FL) can be rewritten in terms of the e+q
invariant mass, M, and the polar angle of the outgoing struck quark in the laboratory
frame, γ:
d2σ(e+p)
dMdγ =32πα2E2
esin γ
M5(1 −cos γ)2[Y+F2−Y−xF3].(13)
The mass dependence is very steep. In addition to the explicit M−5dependence, there is
also a strong suppression of high masses implicit in the structure functions. An incorrect
energy scale will produce a shift in the mass spectrum and potentially a significant error
in the number of expected events at a given mass. The dependence on the quark angle is
also steep, approximately γ−3at small γ. The number of events passing the jet fiducial
cut is therefore strongly dependent on the accuracy of the jet position reconstruction.
The jet fiducial-volume cut requires the highest-PTjet to point outside the inner ring
of FCAL towers. Many distributions from data and MC were compared to search for
possible systematic biases.
The dominant sources of uncertainty are itemized below in order of decreasing importance:
1. Knowledge of the calorimeter energy scales:
The scale uncertainties discussed in Sect. 3 are 1% for BCAL positrons and 2% for
hadrons, leading to an uncertainty of 5(18)% in Npred at Mej = 100(210) GeV.
2. Uncertainties in the simulation of the hadronic energy flow, including simulation of
the proton remnant, the energy flow between the struck quark and proton remnant,
and possible detector effects in the innermost calorimeter towers:
Many distributions of data and MC were compared and no important systematic
differences were found. Figure 6 shows the fraction of the jet energy in the inner
ring of FCAL towers associated with the highest PTjet as a function of ηj. This is
shown for all events in Fig. 6a, as well as for those with Mej >210 GeV in Fig. 6b.
For the highest ηjvalues considered, this ratio is about 20%. The energy located
in the innermost towers of the FCAL and not associated with the highest PTjet
is shown in Fig. 6c,d, and compared to the MC simulation. No large differences
are seen between data and MC (the lowest ηbin in Fig 6d contains only five data
events). The innermost towers of the FCAL have a larger uncertainty in the energy
scale than the rest of the FCAL owing to their slightly different construction and
proximity to the beam. The energy in these cells has been varied by ±10%. As
a test of the simulation of the forward energy flow, the ARIADNE MC has been
used instead of the LEPTO MC. These tests yielded variations in Npred of 13% at
Mej = 210 GeV.
3. Uncertainty in the parton density functions:
14
0
0.05
0.1
0.15
0.2
0.25
0 1 2 0
0.05
0.1
0.15
0.2
0.25
0 1 2
0
10
20
30
40
50
0 1 2 0
10
20
30
40
50
0 1 2
ZEUS 1994-97
a)
ηj
EIRj/Ej
b)
ηj
EIRj/Ej
c)
ηj
EIR not in jet(GeV)
d)
ηj
EIR not in jet(GeV)
Figure 6: The ratio of the jet energy in the innermost towers of the FCAL, EIRj, to
the total jet energy, Ej, as a function of ηof the jet for a) the full sample, and b) for
those events with Mej >210 GeV. The energy deposited in the innermost FCAL towers,
excluding that associated with the highest PTjet, is shown in c) for the full sample, and
in d) for those events with Mej >210 GeV. The data are shown as points, while the NC
Monte Carlo predictions are shown as a histogram.
15
The parton density functions were estimated as in [4], and led to an uncertainty of
5% in Npred at Mej = 210 GeV.
4. Uncertainties in the acceptance:
The alignment of the FCAL was determined to better than 5 mm, and various
jet position reconstruction algorithms were compared. These studies yielded an
uncertainty of 2% in Npred.
5. Uncertainties in the energy resolution functions.
These were studied by comparing tracking information with calorimeter information
for individual events, as well as by comparing different reconstruction methods. The
MC energies were smeared by additional amounts to represent these uncertainties,
leading to a variation of less than 5% in Npred.
Other uncertainties include positron finding efficiency, luminosity determination, vertex
simulation, multijet production rates, and hadronization simulation. These were found to
be small in comparison to the items listed above. The overall systematic uncertainty was
obtained by summing the contributions from all these sources in quadrature.
7.2 Discussion
The data in Fig. 5 are in good agreement with the SM expectations up to Mej ≈210 GeV.
Some excess is seen at higher masses. For Mej >210 GeV, 49 events were observed in
the data, while 24.7±5.6 events are expected. A careful study of individual events in
this mass region uncovered no signs of reconstruction errors. Rather, the events always
contain clear examples of a high-energy positron (typically 70 GeV) near 90◦and a high-
energy jet (typically 400 GeV) in the forward direction (2 events have a second jet, in
accord with NC DIS Monte Carlo expectations). The distributions shown in Fig. 3 for
all the data are restricted to the events with Mej >210 GeV in Fig. 7. Whereas the
shapes of the distributions are similar, the data lie systematically above the MC, which
is normalized to the integrated luminosity.
The events with large Mej have characteristics similar on average to NC DIS events. In
particular, the cos θ∗projection of the events with Mej >210 GeV is shown in Fig. 8
and compared to the MC expectations for neutral current DIS (solid histogram). The
expectations for narrow s-channel scalar and vector LQ production are also shown for
comparison. For F= 0 LQs with λ < 1, the u-channel and interference terms would
not significantly affect these expectations. The shape of the data and NC MC cos θ∗
distributions are qualitatively similar, peaking at high values of cos θ∗.
In summary, there is some excess of events with Mej >210 GeV above the Standard
Model predictions. The probability of observing such an excess depends strongly on
possible systematic biases. The most important of these are biases in the energy scales.
As a test, many MC experiments were generated where the jet energy scale was shifted by
16
1
10
100 150 200 250
1
10
0 50 100
1
10
0 50 100
1
10
0 0.5 1 1.5 2
1
10
0 10 20 30
1
10
0 20 40 60 80
ZEUS 1994-97
ET(GeV)
Events/bin
a)
PT
e(GeV)
Events/bin
b)
PT
j(GeV)
Events/bin
c)
PT
e/PT
h
Events/bin
d)
P/T(GeV)
Events/bin
e)
E-Pz(GeV)
Events/bin
f)
Figure 7: Comparison of data (points) with Standard Model expectations (histograms) for
selected distributions and requiring Mej >210 GeV: a) total event transverse energy, ET;
b) positron transverse momentum, Pe
T; c) jet transverse momentum, Pj
T; d) the ratio of
the positron to hadron transverse momenta, Pe
T/P h
T, (e) the net (or missing) transverse
momentum, 6PT, and (f ) the longitudinal momentum variable, E−PZ, for the event.
17
+2% and the electron energy scale by +1%. A window of width 3σ(Mej), where σ(Mej )
is the mass resolution at mass Mej , was moved over the accessible mass range. For
each simulated experiment, the number of observed events within the mass window was
compared with the nominal expectations as a function of Mej , seeking the excess which
gave the largest statistical significance. The same procedure was applied to the data. As
a result, it was found that 5% of the simulated experiments would observe, somewhere
in the mass spectrum, an excess of statistical significance at least as large as the one
found in the data. The excess is therefore not statistically compelling. Furthermore, the
events have the characteristics of neutral current scattering. Limits are therefore set on
the production of narrow scalar or vector states, as discussed below.
1
10
10 2
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
ZEUS 1994-97
cos(θ*)
dN/dcos(θ*)
Mej > 210GeV
data
neutral current
scalar leptoquark
vector leptoquark
Figure 8: Comparison of data (points) and NC Monte Carlo expectations (solid histogram)
for the cos θ∗distribution for events with Mej >210 GeV. The predictions for narrow
scalar and vector resonant leptoquarks are shown with arbitrary normalization for com-
parison.
8 Limits on Narrow Scalar and Vector States
Limits are set on the production cross section times branching ratio into positron+jet,
σB, for a narrow scalar or vector state. For definiteness, limits on coupling strength
versus mass for F= 0 leptoquarks are presented, as well as limits on λ√Bversus mass
for scalar states coupling to uor dquarks, such as 6RPsquarks. The limits are extracted
for λ≤1, allowing the use of the narrow-width approximation assumed in Eq. 9.
The MCJ mass reconstruction method was used to set limits as described in Sect. 6. The
18
10 -1
1
10
10 2
10 3
100 120 140 160 180 200 220 240 260
10 -1
1
10
10 2
10 3
100 120 140 160 180 200 220 240 260
ZEUS 1994-97
MCJ(GeV)
Events
a)
MCJ(GeV)
Events
b)
Figure 9: The reconstructed mass spectrum using the MCJ method for data (points) and
SM expectations (histogram): a) shows the spectrum for events satisfying all cuts, while
b) shows the mass spectrum after the cut (cos θ∗<cos θ∗
cut) for the scalar LQ search has
been applied.
positron fiducial cuts were removed since this method is less sensitive to the positron-
energy measurement, while the cut E−PZ>40 GeV was applied to reduce radiative
effects. The mass spectrum reconstructed with this technique is shown in Fig. 9a. In
total, 8026 events passed all selection cuts while 7863 events are predicted by the MC.
The leptoquark MC described in Sect. 5 was used to determine the event selection effi-
ciency and the acceptance of the fiducial cuts, as well as to estimate the mass resolution.
This MC and the NC background simulation were used to calculate an optimal bin width,
∆MCJ , for each MCJ , and optimal cos θ∗range, cos θ∗<cos θ∗
cut, to obtain on average
the best limits on LQ couplings. The bin widths were typically 20 GeV. The values of
cos θ∗
cut for setting limits range from 0.5 to 0.9 for vector leptoquarks with masses between
150 −290 GeV, and from 0.1 to 0.9 for scalar leptoquarks in the same mass range. The
mass spectrum after applying the optimal cos θ∗cut for the scalar search is shown in
Fig. 9b. No significant deviations from expectations are seen after applying this cut.
The 95% confidence level (CL) limits on σB were obtained directly from the observed
number of data events with cos θ∗<cos θ∗
cut in the particular mass window [26]. The
procedure described in [26] was extended to include the systematic uncertainties in the
numbers of predicted events. This was found to have negligible effect on the limits. The
limits for a narrow scalar or vector state are shown in Fig. 10. These limits lie between 1
and 0.1 pb as the mass increases from 150 to 290 GeV.
19
0
0.2
0.4
0.6
0.8
1
160 180 200 220 240 260 280
ZEUS 1994-97
scalar
vector
EXCLUDED
m(GeV)
σB(pb)
ZEUS 1994-97
0
0.2
0.4
0.6
0.8
1
160 180 200 220 240 260 280
Figure 10: Limits on the production cross section times branching ratio for decay into
e+-jet(s) for a scalar or vector state, as a function of the mass of the state. The shaded
regions are excluded.
20
The 95% CL exclusion limits for different species of LQ are given in the coupling versus
mass plane in Fig. 11. The limits exclude leptoquarks with coupling strength λ=√4πα ≈
0.3 for masses up to 280 GeV for specific types of F=0 leptoquarks. The H1 collaboration
has recently published similar limits [2]. In Fig. 11, the ZEUS results are compared to
recent limits from OPAL. At LEP [27–29], sensitivity to a high-mass LQ arises from effects
of virtual LQ exchange on the hadronic cross section. The HERA and LEP limits are
complementary to Tevatron limits [30, 31], which are independent of the coupling λL,R.
The limits by D0 (CDF) extend up to 225 (213) GeV for a scalar LQ with 100% branching
ratio to eq. The D0 limits are shown as vertical lines in Fig. 11. The Tevatron limits for
vector LQs are model dependent [32], but are expected to be considerably higher than for
scalar LQs.
The ZEUS limits presented in Fig. 11 can also be applied to any narrow state which couples
to a positron and a uor dquark and with unknown branching ratio to e+-jet(s). These
states correspond to the leptoquark types as labelled in the figure. For these states, the
limits are on the quantity λ√B. Examples of scalar states for which these limits apply
are 6RP-squarks (e.g.,the limit on the ˜
SL
1/2(e+d) LQ can be read as a limit on the λ′
1j1
R-parity-violating coupling).
9 Conclusion
Data from 47.7 pb−1of e+pcollisions at a center-of-mass energy of 300 GeV have been
used to search for a resonance decaying into e+-jet. The invariant mass of the e+-jet pair
was calculated directly from the measured energies and angles of the positron and jet.
This approach makes no assumptions about the production mechanism of such a state.
The observed mass spectrum is in good agreement with Standard Model expectations up
to e+-jet masses of about 210 GeV. Above this mass, some excess is seen. The angular
distribution of these events is typical of high-Q2neutral current events and does not give
convincing evidence for the presence of a narrow scalar or vector state. By applying
restrictions on the decay angle to optimize sensitivity to a narrow state in the presence
of NC background, limits have been derived on the cross section times decay branching
fraction for a scalar or vector state decaying into positron and jet(s). These limits can be
interpreted, for example, as limits on leptoquark or R-parity-violating squark production.
Limits on the production of leptoquarks and squarks are presented in the coupling strength
versus mass plane. At a coupling strength λ= 0.3, new states are ruled out at 95%
confidence level for masses between 150 and 280 GeV.
Acknowledgements
We thank the DESY Directorate for their strong support and encouragement, and the
HERA machine group for their diligent efforts. We are grateful for the support of the
21
10 -2
10 -1
1
150 200 250
10 -2
10 -1
1
150 200 250
ZEUS 1994-97
D0(B=1.0)
D0(B=0.5)
ZEUS
LEP
SL
1/2
(e+u)
SR
1/2
S
~L
1/2
(e+d)
a)
mLQ(GeV)
λ
λ√B(LQ→eq)
ZEUS
LEP
VL
0
VR
0(e+d)
V
~R
0(e+u)
VL
1
b)
mLQ(GeV)
λ
λ√B(LQ→eq)
Figure 11: Coupling limits as a function of leptoquark mass for F= 0 leptoquarks. The
results from this analysis are compared to representative limits from LEP [28] and the
Tevatron [31]. The areas above the ZEUS and LEP curves are excluded, while the area to
the left of the Tevatron line is excluded for scalar leptoquarks with the indicated branching
ratio to e+jet. The limits on scalars are shown in a) while the limits on vectors are shown
in b).
22
DESY computing and network services. The design, construction and installation of the
ZEUS detector have been made possible by the ingenuity and effort of many people from
DESY and home institutes who are not listed as authors. It is also a pleasure to thank
W. Buchm¨uller, R. R¨uckl and M. Spira for useful discussions.
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