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Physics Letters B
www.elsevier.com/locate/physletb
Production of Z
0
bosons in elastic and quasi-elastic ep collisions at HERA
ZEUS Collaboration
H. Abramowicz
as,
52, I. Abt
ai, L. Adamczyk
m, M. Adamus
bb, R. Aggarwal
g,
21, S. Antonelli
d, P. Antonioli
c,
A. Antonov
ag, M. Arneodo
ax, O. Arslan
e, V. Aushev
z,
aa,
45, Y. Aushev
aa,
45,
46, O. Bachynska
o,
A. Bamberger
s, A.N. Barakbaev
y, G. Barbagli
q, G. Bari
c, F. Barreiro
ad, N. Bartosik
o, D. Bartsch
e,
M. Basile
d, O. Behnke
o, J. Behr
o, U. Behrens
o, L. Bellagamba
c, A. Bertolin
am, S. Bhadra
be, M. Bindi
d,
C. Blohm
o, V. Bokhonov
z,
45, T. Bołd
m, K. Bondarenko
aa, E.G. Boos
y, K. Borras
o, D. Boscherini
c, D. Bot
o,
I. Brock
e, E. Brownson
bd, R. Brugnera
an, N. Brümmer
ak, A. Bruni
c, G. Bruni
c, B. Brzozowska
ba,
P.J. Bussey
t, B. Bylsma
ak, A. Caldwell
ai, M. Capua
h, R. Carlin
an, C.D. Catterall
be, S. Chekanov
a,
J. Chwastowski
l,
23, J. Ciborowski
ba,
56, R. Ciesielski
o,
26, L. Cifarelli
d, F. Cindolo
c, A. Contin
d,
A.M. Cooper-Sarkar
al, N. Coppola
o,
27, M. Corradi
c, F. Corriveau
ae, M. Costa
aw, G. D’Agostini
aq,
F. Dal Corso
am, J. del Peso
ad, R.K. Dementiev
ah, S. De Pasquale
d,
19, M. Derrick
a, R.C.E. Devenish
al,
D. Dobur
s,
39, B.A. Dolgoshein
ag,
†, G. Dolinska
aa, A.T. Doyle
t, V. Drugakov
p, L.S. Durkin
ak, S. Dusini
am,
Y. Eisenberg
bc, P.F. Ermolov
ah,
†, A. Eskreys
l,
†, S. Fang
o,
28, S. Fazio
h, J. Ferrando
t, M.I. Ferrero
aw,
J. Figiel
l, B. Foster
al,
48, G. Gach
m, A. Galas
l, E. Gallo
q, A. Garfagnini
an, A. Geiser
o, I. Gialas
u,
42,
A. Gizhko
aa,
47, L.K. Gladilin
ah, D. Gladkov
ag, C. Glasman
ad, O. Gogota
aa, Yu.A. Golubkov
ah,
P. Göttlicher
o,
29, I. Grabowska-Bołd
m, J. Grebenyuk
o, I. Gregor
o, G. Grigorescu
aj, G. Grzelak
ba,
O. Gueta
as, M. Guzik
m, C. Gwenlan
al,
49, T. Haas
o, W. Hain
o, R. Hamatsu
av, J.C. Hart
ar, H. Hartmann
e,
G. Hartner
be, E. Hilger
e, D. Hochman
bc, R. Hori
au, A. Hüttmann
o, Z.A. Ibrahim
j, Y. Iga
ap, R. Ingbir
as,
M. Ishitsuka
at, H.-P. Jakob
e, F. Januschek
o, T.W. Jones
az, M. Jüngst
e, I. Kadenko
aa, B. Kahle
o,
S. Kananov
as, T. Kanno
at, U. Karshon
bc, F. Karstens
s,
40, I.I. Katkov
o,
30, M. Kaur
g, P. Kaur
g,
21,
A. Keramidas
aj, L.A. Khein
ah, J.Y. Kim
i, D. Kisielewska
m, S. Kitamura
av,
54, R. Klanner
v, U. Klein
o,
31,
E. Koffeman
aj, N. Kondrashova
aa,
47, O. Kononenko
aa, P. Kooijman
aj, Ie. Korol
aa, I.A. Korzhavina
ah,
A. Kota ´nski
n,
24, U. Kötz
o, H. Kowalski
o, O. Kuprash
o, M. Kuze
at, A. Lee
ak, B.B. Levchenko
ah, A. Levy
as,
∗
,
V. Libov
o, S. Limentani
an, T.Y. Ling
ak, M. Lisovyi
o, E. Lobodzinska
o, W. Lohmann
p, B. Löhr
o,
E. Lohrmann
v, K.R. Long
w, A. Longhin
am,
50, D. Lontkovskyi
o, O.Yu. Lukina
ah, J. Maeda
at,
53, S. Magill
a,
I. Makarenko
o, J. Malka
o, R. Mankel
o, A. Margotti
c, G. Marini
aq, J.F. Martin
ay, A. Mastroberardino
h,
M.C.K. Mattingly
b, I.-A. Melzer-Pellmann
o, S. Mergelmeyer
e, S. Miglioranzi
o,
32, F. Mohamad Idris
j,
V. Monaco
aw, A. Montanari
o, J.D. Morris
f,
20, K. Mujkic
o,
33, B. Musgrave
a, K. Nagano
x, T. Namsoo
o,
34,
R. Nania
c, A. Nigro
aq, Y. Ning
k, T. Nobe
at, D. Notz
o, R.J. Nowak
ba, A.E. Nuncio-Quiroz
e, B.Y. Oh
ao,
N. Okazaki
au, K. Olkiewicz
l, Yu. Onishchuk
aa, K. Papageorgiu
u, A. Parenti
o, E. Paul
e, J.M. Pawlak
ba,
B. Pawlik
l, P.G. Pelfer
r, A. Pellegrino
aj, W. Perla ´nski
ba,
57, H. Perrey
o, K. Piotrzkowski
ac, P. Pluci ´nski
bb,
58,
N.S. Pokrovskiy
y, A. Polini
c, A.S. Proskuryakov
ah, M. Przybycie ´n
m, A. Raval
o, D.D. Reeder
bd, B. Reisert
ai,
Z. Ren
k, J. Repond
a, Y.D. Ri
av,
55, A. Robertson
al, P. Roloff
o,
32, I. Rubinsky
o, M. Ruspa
ax, R. Sacchi
aw,
U. Samson
e, G. Sartorelli
d, A.A. Savin
bd, D.H. Saxon
t, M. Schioppa
h, S. Schlenstedt
p, P. Schleper
v,
W.B. Schmidke
ai, U. Schneekloth
o, V. Schönberg
e, T. Schörner-Sadenius
o, J. Schwartz
ae, F. Sciulli
k,
L.M. Shcheglova
ah, R. Shehzadi
e, S. Shimizu
au,
32, I. Singh
g,
21, I.O. Skillicorn
t, W. Słomi ´nski
n,
25,
W.H. Smith
bd, V. Sola
v, A. Solano
aw, D. Son
ab, V. Sosnovtsev
ag, A. Spiridonov
o,
35, H. Stadie
v,
L. Stanco
am, N. Stefaniuk
aa, A. Stern
as, T.P. Stewart
ay, A. Stifutkin
ag, P. Stopa
l, S. Suchkov
ag, G. Susinno
h,
L. Suszycki
m, J. Sztuk-Dambietz
v, D. Szuba
v, J. Szuba
o,
36, A.D. Tapper
w, E. Tassi
h,
22, J. Terrón
ad,
0370-2693/$ – see front matter ©2012 Elsevier B.V. All rights reserved.
T. Theedt
o, H. Tiecke
aj, K. Tokushuku
x,
43, J. Tomaszewska
o,
37, V. Trusov
aa, T. Tsurugai
af, M. Turcato
v,
O. Turkot
aa,
47, T. Tymieniecka
bb,
59, M. Vázquez
aj,
32, A. Verbytskyi
o, O. Viazlo
aa, N.N. Vlasov
s,
41,
R. Walczak
al, W.A.T. Wan Abdullah
j, J.J. Whitmore
ao,
51, K. Wichmann
o,
38, L. Wiggers
aj, M. Wing
az,
M. Wlasenko
e, G. Wolf
o, H. Wolfe
bd, K. Wrona
o, A.G. Yagües-Molina
o, S. Yamada
x, Y. Yamazaki
x,
44,
R. Yoshida
a, C. Youngman
o, O. Zabiegalov
aa,
47, A.F. ˙Zarnecki
ba, L. Zawiejski
l, O. Zenaiev
o, W. Zeuner
o,
32,
B.O. Zhautykov
y, N. Zhmak
z,
45, A. Zichichi
d, Z. Zolkapli
j, D.S. Zotkin
ahaArgonne National Laboratory, Argonne, IL 60439-4815, USA1 bAndrews University, Berrien Springs, MI 49104-0380, USA cINFN Bologna, Bologna, Italy2
dUniversity and INFN Bologna, Bologna, Italy2
ePhysikalisches Institut der Universität Bonn, Bonn, Germany3
fH.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom4 gPanjab University, Department of Physics, Chandigarh, India
hCalabria University, Physics Department and INFN, Cosenza, Italy2
iInstitute for Universe and Elementary Particles, Chonnam National University, Kwangju, South Korea jJabatan Fizik, Universiti Malaya, 50603 Kuala Lumpur, Malaysia5
kNevis Laboratories, Columbia University, Irvington on Hudson, NY 10027, USA6
lThe Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland7 mAGH – University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland8 nDepartment of Physics, Jagellonian University, Cracow, Poland
oDeutsches Elektronen-Synchrotron DESY, Hamburg, Germany pDeutsches Elektronen-Synchrotron DESY, Zeuthen, Germany qINFN Florence, Florence, Italy2
rUniversity and INFN Florence, Florence, Italy2
sFakultät für Physik der Universität Freiburg i.Br., Freiburg i.Br., Germany tSchool of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom4 uDepartment of Engineering in Management and Finance, Univ. of the Aegean, Chios, Greece vHamburg University, Institute of Experimental Physics, Hamburg, Germany9
wImperial College London, High Energy Nuclear Physics Group, London, United Kingdom4 xInstitute of Particle and Nuclear Studies, KEK, Tsukuba, Japan10
yInstitute of Physics and Technology of Ministry of Education and Science of Kazakhstan, Almaty, Kazakhstan zInstitute for Nuclear Research, National Academy of Sciences, Kyiv, Ukraine
aaDepartment of Nuclear Physics, National Taras Shevchenko University of Kyiv, Kyiv, Ukraine abKyungpook National University, Center for High Energy Physics, Daegu, South Korea11 acInstitut de Physique Nucléaire, Université Catholique de Louvain, Louvain-la-Neuve, Belgium12 adDepartamento de Física Teórica, Universidad Autónoma de Madrid, Madrid, Spain13 aeDepartment of Physics, McGill University, Montréal, QC, Canada H3A 2T814 afMeiji Gakuin University, Faculty of General Education, Yokohama, Japan10 agMoscow Engineering Physics Institute, Moscow, Russia15
ahLomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics, Moscow, Russia16 aiMax-Planck-Institut für Physik, München, Germany
ajNIKHEF and University of Amsterdam, Amsterdam, Netherlands17 akPhysics Department, Ohio State University, Columbus, OH 43210, USA1 alDepartment of Physics, University of Oxford, Oxford, United Kingdom4 amINFN Padova, Padova, Italy2
anDipartimento di Fisica dell’ Università and INFN, Padova, Italy2
aoDepartment of Physics, Pennsylvania State University, University Park, PA 16802, USA6 apPolytechnic University, Tokyo, Japan10
aqDipartimento di Fisica, Università ‘La Sapienza’ and INFN, Rome, Italy2 arRutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom4
asRaymond and Beverly Sackler Faculty of Exact Sciences, School of Physics, Tel Aviv University, Tel Aviv, Israel18 atDepartment of Physics, Tokyo Institute of Technology, Tokyo, Japan10
auDepartment of Physics, University of Tokyo, Tokyo, Japan10 avTokyo Metropolitan University, Department of Physics, Tokyo, Japan10 awUniversità di Torino and INFN, Torino, Italy2
axUniversità del Piemonte Orientale, Novara, and INFN, Torino, Italy2 ayDepartment of Physics, University of Toronto, Toronto, ON, Canada M5S 1A714
azPhysics and Astronomy Department, University College London, London, United Kingdom4 baFaculty of Physics, University of Warsaw, Warsaw, Poland
bbNational Centre for Nuclear Research, Warsaw, Poland
bcDepartment of Particle Physics and Astrophysics, Weizmann Institute, Rehovot, Israel bdDepartment of Physics, University of Wisconsin, Madison, WI 53706, USA1 beDepartment of Physics, York University, ON, Canada M3J 1P314
a r t i c l e
i n f o
a b s t r a c t
Article history:
Received 18 October 2012
Received in revised form 22 November 2012 Accepted 22 November 2012
Available online 23 November 2012 Editor: W.-D. Schlatter
The production of Z0 bosons in the reaction ep→e Z0p(∗), where p(∗) stands for a proton or a
low-mass nucleon resonance, has been studied in ep collisions at HERA using the ZEUS detector. The analysis is based on a data sample collected between 1996 and 2007, amounting to 496 pb−1 of integrated luminosity. The Z0was measured in the hadronic decay mode. The elasticity of the events was ensured by a cut on
η
max<3.0, whereη
max is the maximum pseudorapidity of energy deposits in the calorimeter defined with respect to the proton beam direction. A signal was observed at the0.13±0.06(stat.)±0.01(syst.)pb, in agreement with the Standard Model prediction of 0.16 pb. This is the first measurement of Z0production in ep collisions.
©2012 Elsevier B.V. All rights reserved.
*
Corresponding author.E-mail address:levy@alzt.tau.ac.il(A. Levy).
1 Supported by the US Department of Energy.
2 Supported by the Italian National Institute for Nuclear Physics (INFN). 3 Supported by the German Federal Ministry for Education and Research (BMBF),
under contract No. 05 H09PDF.
4 Supported by the Science and Technology Facilities Council, UK.
5 Supported by HIR and UMRG grants from Universiti Malaya, and an ERGS grant
from the Malaysian Ministry for Higher Education.
6 Supported by the US National Science Foundation. Any opinion, findings and
conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
7 Supported by the Polish Ministry of Science and Higher Education as a scientific
project No. DPN/N188/DESY/2009.
8 Supported by the Polish Ministry of Science and Higher Education and its grants
for Scientific Research.
9 Supported by the German Federal Ministry for Education and Research (BMBF),
under contract No. 05h09GUF, and the SFB 676 of the Deutsche Forschungsgemein-schaft (DFG).
10 Supported by the Japanese Ministry of Education, Culture, Sports, Science and
Technology (MEXT) and its grants for Scientific Research.
11 Supported by the Korean Ministry of Education and Korea Science and
Engineer-ing Foundation.
12 Supported by FNRS and its associated funds (IISN and FRIA) and by an
Inter-University Attraction Poles Programme subsidised by the Belgian Federal Science Policy Office.
13 Supported by the Spanish Ministry of Education and Science through funds
pro-vided by CICYT.
14 Supported by the Natural Sciences and Engineering Research Council of Canada
(NSERC).
15 Partially supported by the German Federal Ministry for Education and Research
(BMBF).
16 Supported by RF Presidential grant N 3920.2012.2 for the Leading Scientific
Schools and by the Russian Ministry of Education and Science through its grant for Scientific Research on High Energy Physics.
17 Supported by the Netherlands Foundation for Research on Matter (FOM). 18 Supported by the Israel Science Foundation.
19 Now at University of Salerno, Italy.
20 Now at Queen Mary University of London, United Kingdom. 21 Also funded by Max Planck Institute for Physics, Munich, Germany.
22 Also Senior Alexander von Humboldt Research Fellow at Hamburg University,
Institute of Experimental Physics, Hamburg, Germany.
23 Also at Cracow University of Technology, Faculty of Physics, Mathematics and
Applied Computer Science, Poland.
24 Supported by the research grant No. 1 P03B 04529 (2005–2008).
25 Supported by the Polish National Science Centre, project No. DEC-2011/01/
BST2/03643.
26 Now at Rockefeller University, New York, NY 10065, USA. 27 Now at DESY group FS-CFEL-1.
28 Now at Institute of High Energy Physics, Beijing, China. 29 Now at DESY group FEB, Hamburg, Germany. 30 Also at Moscow State University, Russia. 31 Now at University of Liverpool, United Kingdom. 32 Now at CERN, Geneva, Switzerland.
33 Also affiliated with University College London, UK. 34 Now at Goldman Sachs, London, UK.
35 Also at Institute of Theoretical and Experimental Physics, Moscow, Russia. 36 Also at FPACS, AGH-UST, Cracow, Poland.
37 Partially supported by Warsaw University, Poland. 38 Supported by the Alexander von Humboldt Foundation. 39 Now at Istituto Nazionale di Fisica Nucleare (INFN), Pisa, Italy. 40 Now at Haase Energie Technik AG, Neumünster, Germany. 41 Now at Department of Physics, University of Bonn, Germany. 42 Also affiliated with DESY, Germany.
43 Also at University of Tokyo, Japan.
1. Introduction
The production of electroweak bosons in ep collisions is a good benchmark process for testing the Standard Model (SM). Even though the expected numbers of events for W±and Z0production are low, the measurement of the cross sections of these processes is important as some extensions of the SM predict anomalous couplings and thus changes in these cross sections. A measure-ment of the cross section for W± production at HERA has been performed by H1 and ZEUS [1] in events containing an isolated lepton and missing transverse momentum, giving a cross section
σ
(
ep→
W±X)
=
1.06±
0.17(stat.⊕
syst.)pb, in good agreement with the SM prediction. The cross section for Z0 production is pre-dicted to be 0.4 pb.This Letter reports on a measurement of the production of Z0 bosons in e±p collisions using an integrated luminosity of about 0.5 fb−1. The hadronic decay mode was chosen61 because of its large branching ratio. The excellent resolution of the ZEUS hadronic calorimeter makes this measurement possible. The anal-ysis was restricted to elastic and quasi-elastic Z0 production in order to suppress QCD multi-jet background. The selected process is ep
→
e Z0p(∗), where p(∗)stands for a proton (elastic process) or a low-mass nucleon resonance (quasi-elastic process).Fig. 1 shows a leading-order (LO) diagram of Z0 production with subsequent hadronic decay. In such events, there are at least two hadronic jets with high transverse energies, and no hadronic energy deposits around the forward62 direction, in contrast to what would be expected in inelastic collisions.
2. Experimental set-up
HERA was the world’s only high-energy ep collider, with an electron63 beam of 27.6 GeV and a proton beam of 920 GeV
44 Now at Kobe University, Japan. 45 Supported by DESY, Germany.
46 Member of National Technical University of Ukraine, Kyiv Polytechnic Institute,
Kyiv, Ukraine.
47 Member of National University of Kyiv–Mohyla Academy, Kyiv, Ukraine. 48 Alexander von Humboldt Professor; also at DESY and University of Oxford. 49 STFC Advanced Fellow.
50 Now at LNF, Frascati, Italy.
51 This material was based on work supported by the National Science Foundation,
while working at the Foundation.
52 Also at Max Planck Institute for Physics, Munich, Germany, External Scientific
Member.
53 Now at Tokyo Metropolitan University, Japan. 54 Now at Nihon Institute of Medical Science, Japan. 55 Now at Osaka University, Osaka, Japan. 56 Also at Łód ´z University, Poland. 57 Member of Łód ´z University, Poland.
58 Now at Department of Physics, Stockholm University, Stockholm, Sweden. 59 Also at Cardinal Stefan Wyszy ´nski University, Warsaw, Poland.
† Deceased.
61 The Z0 decay into charged lepton pairs was studied in a previous ZEUS
publi-cation[2].
62 The ZEUS coordinate system is a right-handed Cartesian system, with the Z axis
pointing in the proton beam direction, referred to as the forward direction, and the
X axis pointing towards the centre of HERA. The coordinate origin is at the nominal
interaction point. The pseudorapidity is defined asη= −ln(tanθ2), where the polar
angle,θ, is measured with respect to the proton beam direction.
Fig. 1. Example of a leading-order diagram of Z0boson production and subsequent
hadronic decay (into quark q and antiquarkq) in ep¯ →e Z0p.
(820 GeV until 1997). For this analysis, e±p collision data col-lected with the ZEUS detector between 1996 and 2007, amounting to 496 pb−1of integrated luminosity, have been used. They consist of 289 pb−1 of e+p data and 207 pb−1of e−p data.
After 2003, HERA was operated with a polarised lepton beam. When combining the data taken with negative and positive polar-isations, the average polarisation is less than 1% and its effect was neglected in this analysis.
A detailed description of the ZEUS detector can be found else-where[3]. A brief outline of the components that are most relevant for this analysis is given below.
Charged particles were tracked in the central tracking detector (CTD) [4], which operated in a magnetic field of 1.43 T provided by a thin superconducting solenoid. The CTD consisted of 72 cylin-drical drift chamber layers, organised in nine superlayers covering the polar-angle region 15◦
< θ <
164◦. For the data taken after 2001, the CTD was complemented by a silicon microvertex detec-tor (MVD) [5], consisting of three active layers in the barrel and four disks in the forward region.The high-resolution uranium–scintillator calorimeter (CAL) [6] consisted of three parts: the forward (FCAL), the barrel (BCAL) and the rear (RCAL) calorimeters. Each part was subdivided trans-versely into towers and longitudinally into one electromagnetic section (EMC) and either one (in RCAL) or two (in BCAL and FCAL) hadronic sections (HAC). The smallest subdivision of the calorime-ter was called a cell. The CAL energy resolutions, as measured under test-beam conditions, were
σ
(
E)/
E=
0.18/√
E for electrons andσ
(
E)/
E=
0.35/√
E for hadrons, with E in GeV.The luminosity was measured using the Bethe–Heitler reaction ep
→
eγ
p by a luminosity detector which consisted of indepen-dent lead-scintillator calorimeter [7] and magnetic spectrometer [8]systems. The fractional systematic uncertainty on the measured luminosity was 2%.3. Monte Carlo simulations
Monte Carlo (MC) simulations were made to simulate the Z0 production process. They were used to correct for instrumental effects and selection acceptance and to provide a template for the shape of the invariant-mass distribution of the Z0 signal. The EPVEC program[9]was used to generate the signal events at the parton level. The following Z0production processes are considered in EPVEC:
•
elastic scattering, ep→
e Z0p, where the proton stays intact;•
quasi-elastic scattering, ep→
e Z0p∗, where the proton is transformed into a nucleon resonance p∗;•
deep inelastic scattering (DIS),γ
∗p→
Z0X , in the region Q2>
4 GeV2, where Q2 is the virtuality of the photon ex-changed between the electron and proton;•
resolved photoproduction,γ
p→ (
qq¯
→
Z0)
X , where one of the quarks is a constituent of the resolved photon and the other quark is a constituent of the proton.In EPVEC the first two processes are calculated using form factors and structure functions fitted directly to experimental data. Note that, even if the virtuality of the exchanged photon is small, the scattered electron could receive a large momentum transfer when the Z0 is radiated from the lepton line. In the last two processes, the proton breaks up. The DIS process is calculated in the quark– parton model using a full set of leading-order Feynman diagrams. Resolved photoproduction is parametrised using a photon structure function and is carefully matched to the DIS region. The cross sec-tion of Z0 production is calculated to be 0.16 pb for elastic and quasi-elastic processes and 0.24 pb for DIS and resolved photo-production. The difference between e+p and e−p cross sections is negligible for this analysis (<1% for the DIS process). A correction, based on the MC cross section, was made to account for the part of data taken at the centre-of-mass energy
√
s=
300 GeV, so that the result is quoted at√
s=
318 GeV.After the parton-level generation by EPVEC, PYTHIA 5.6 [10] was used to simulate final-state parton showers with the fragmen-tation into hadrons using the Lund string model [11] as imple-mented in JETSET 7.3 [10]. The generated MC events were passed through the ZEUS detector and trigger simulation programs based on GEANT 3.13[12]. They were reconstructed and analysed by the same programs as the data.
A reliable prediction of background events with the signal topology, which are predominantly due to the diffractive photo-production of jets of high transverse momentum, is currently not available. Therefore, the background shape of the invariant-mass distribution was estimated with a data-driven method. The nor-malisation was determined by a fit to the data.
4. Event reconstruction and selection
The events used in this analysis were selected online by the ZEUS three-level trigger system [13], using a combination of sev-eral trigger chains which were mainly based on requirements of large transverse energy deposited in the calorimeter. In the offline selection, further criteria were imposed in order to separate the signal from the background.
The events are characterised by the presence of at least two jets of high transverse energy and, for a fraction of events, by the presence of a reconstructed scattered electron. In order to select events with a Z0 decaying hadronically, jets were reconstructed in the hadronic final state using the kT cluster algorithm [14] in the longitudinally invariant inclusive mode[15]. The algorithm was applied to the energy clusters in the CAL after excluding those associated with the scattered-electron candidate [16–18]. Energy corrections[19–21] were applied to the jets in order to compen-sate for energy losses in the inactive material in front of the CAL.
In this analysis, only jets with ET
>
4 GeV and|
η
| <
2.0 were used. Here ET is the jet transverse energy andη
its pseudorapid-ity. The hadronic Z0 decay sample was selected by the following requirements on the reconstructed jets:•
at least two jets in the event had to satisfy ET>
25 GeV;• |φ
j| >
2 rad, whereφ
jis the azimuthal difference between the first and second highest-ET jet, as the two leading jets from the Z0 boson decays are expected to be nearly back-to-back in the X–Y plane.Electrons were reconstructed using an algorithm that combined information from clusters of energy deposits in the CAL and from tracks[16]. To be defined as well-reconstructed electrons, the can-didates were required to satisfy the following selection:
•
Ee>
5 GeV and Ein<
3 GeV, where Ee is the scattered elec-tron energy and Ein is the total energy in all CAL cells not associated with the cluster of the electron but lying within a cone inη
andφ
of radius R=
η
2+ φ
2=
0.8, centred on the cluster;•
If the electron was in the acceptance region of the track-ing system, a matched track was required with momentum ptrack>
3 GeV. After extrapolating the track to the CAL surface, its distance of closest approach (DCA) to the electron cluster had to be within 10 cm.The following cuts were applied to suppress low- Q2 neutral-current and direct-photoproduction backgrounds:
•
ERCAL<
2 GeV, where ERCAL is the total energy deposit in RCAL;•
50<
E−
pZ<
64 GeV, where E−
pZ=
iEi(1
−
cosθi); E
iis the energy of the i-th CAL cell,θ
i is its polar angle and the sum runs over all cells64;• θ
e<
80◦ for well-reconstructed electrons, whereθ
e is the po-lar angle of the scattered electron, motivated by the fact that, due to the large mass of the produced system, the electron is backscattered to the forward calorimeter or forward beam pipe;•
the event was rejected if more than one electron candidate was found;•
jets were regarded as a misidentified electron or photon and were discarded from the list of jets if the direction of the jet candidate was matched within R<
1.0 with that of an elec-tron candidate identified by looser criteria65 than those de-scribed above. This cut causes a loss of acceptance of about 3%. To remove cosmic and beam–gas backgrounds, events fulfilling any of the conditions listed below were rejected:• |
Zvtx| >
50 cm, where Zvtx is the Z position of the primary vertex reconstructed from CTD+
MVD tracks;•
175◦< (θ
jet1+ θ
jet2) <
185◦ andφ
j>
175◦ simultaneously, whereθ
jet1 andθ
jet2are the polar angles of the first and sec-ond highest-ET jet, respectively, andφ
j is the azimuthal difference between them;• |
tu−
td| >
6.0 ns, where|
tu−
td|
is the timing difference be-tween the upper and the lower halves of the BCAL;• /
pT>
25 GeV, where/
pT is the missing transverse momentum calculated from the energy clusters in the CAL;•
Ntrkvtx<
0.25(Ntrkall−
20), where Nvtxtrk is the number of tracks as-sociated with the primary vertex and Nalltrk is the total number of tracks[22].
The number of events passing the above selection was 5257. Fi-nally, to select the elastic and quasi-elastic processes preferentially, a cut on
η
max was introduced,64 For fully contained events, or events in which the particles escape only in the
forward beam pipe, the E−pZvalue peaks around twice the electron beam energy,
55 GeV.
65 Candidates were selected by less stringent requirements and clusters with no
tracks were also accepted to find photons and electrons.
•
η
max<
3.0.The quantity
η
maxwas defined as the pseudorapidity of the energy deposit in the calorimeter closest to the proton beam direction with energy greater than 400 MeV as determined by calorimeter cells. This cut also rejected signal events which have energy de-posits from the scattered electron in the calorimeter around the forward beam pipe, causing an acceptance loss of about 30%.After all selection cuts, 54 events remained. The total selection efficiency was estimated by the MC simulation to be 22% for elastic and quasi-elastic processes and less than 1% for DIS and resolved photoproduction events. The number of expected signal events in the final sample, as predicted by EPVEC, is 18.3. The contribution from elastic and quasi-elastic processes amounts to 17.9 events. 5. Background-shape study
Fig. 2(a) shows the distribution of the invariant mass, Mjets, af-ter all the selection criaf-teria except for the requirement
η
max<
3.0. The variable Mjets was calculated using all jets passing the se-lection criteria described in Section 4. Figs. 2(b)–(d) show Mjets for variousη
max slices in the inelastic region (η
max>
3.0) for the same selection. No significant dependence onη
max of the Mjets distribution beyond that expected from statistical fluctua-tions was observed in the inelastic region. In addition, the shape of the Mjets distribution outside the Z0 mass window in the regionη
max<
3.0 was found to be consistent with that in the inelastic region (Fig. 3). Therefore, the Mjetsdistribution in the inelastic re-gion was adopted as a background template in a fit to the data in the elastic region as described in the following section.6. Cross-section extraction
A fit to the sum of the signal and a background template for the Mjets distribution was used for the cross-section extraction. The template Nref,i is defined according to:
Nref,i
=
aNsgMC,i(
)
+
bNbgdata,i,
(1)where i is the bin number of the Mjets distribution. The parame-ter
accounts for a possible energy shift, i.e. Mjets
= (
1+
)
MMCjets, where MMCjets is the invariant-mass distribution of the signal Z0 MC. The quantity NMC
sg,i is a signal template estimated from the Z0 MC distribution after all cuts, normalised to data luminosity. The quantity Ndata
bg,i is a background template determined from the data outside the selected region. The parameters a and b are the nor-malisation factors for the signal and background, respectively. The likelihood of the fit,
L
, is defined as follows:L
=
L
1(
Nobs,
Nref)
×
L
2(
,
σ
),
(2)with
L
1=
i
exp
(
−
Nref,i)(
Nref,i)
Nobs,iNobs,i
!
andL
2=
exp−
2 2
σ
2.
Here
L
1(
Nobs,
Nref)
is the product of Poisson probabilities to ob-serve Nobs,ievents for the bin i when Nref,iis expected. The termL
2(
,
σ
)
represents the Gaussian probability density for a shiftof the jet energy scale from the nominal scale, which has a known systematic uncertainty of
σ
=
3%. From the likelihood, a chi-squared function is defined as˜
χ
2= −
2 lnL
1(
Nobs,
Nref)
L
1(
Nobs,
Nobs)
−
2 lnL
2=
2 fi+
σ
2
,
(3)Fig. 2. The Mjetsdistribution of the data (a) after all selection criteria, except for theηmaxcut, (b)–(d) in severalηmaxslices.
Fig. 3. The Mjetsdistribution and the fit result. The data are shown as points, and
the fitting result of signal+background (background component) is shown as solid (dashed) line. The signal contribution is also indicated by the shaded area and amounts to a total number of Nobsevents. The error bars represent the approximate
Poissonian 68% CL intervals, calculated as±√n+0.25+0.5 for a given entry n.
with fi
=
Nref,i
−
Nobs,i+
Nobs,iln(
Nobs,i/
Nref,i) (
if Nobs,i>
0)
Nref,i
(
if Nobs,i=
0).
The best combination of (a,b
,
) is found by minimising
χ
˜
2. The value of a after this optimisation gives the ratio between the ob-served and expected cross section, i.e.σ
obs=
aσ
SM. The maximum and minimum values of a in the intervalχ
˜
2<
1 define the range of statistical uncertainty.7. Systematic uncertainties
Several sources of systematic uncertainties were considered and their impact on the measurement estimated.
•
An uncertainty of 3% was assigned to the energy scale of the jets and the effect on the acceptance correction was estimated using the signal MC. The uncertainty on the Z0 cross-section measurement was estimated to be+
2.1% and−
1.7%.•
The uncertainty associated with the elastic and quasi-elastic selection was considered. In a control sample of diffractive DIS candidate events, theη
maxdistribution of the MC agreed with the data to within a shift ofη
max of 0.2 units [23]. Thus, theη
max threshold was changed in the signal MC by±
0.2, and variations of the acceptance were calculated accordingly. The uncertainty on the cross-section measurement was+
6.4% and−
5.4%.•
The background shape uncertainty was estimated by using dif-ferent slices ofη
max in the fit. The background shape was obtained using only the regions of 4.0<
η
max<
4.2 or 4.2<
a small number of signal events is expected in this
η
max region.66 The resulting uncertainty in the cross-section mea-surement was±
1.5%.•
The uncertainty associated with the luminosity measurement was estimated to be 2%, as described in Section2.•
The Z0 mass distribution from the MC used as a signal tem-plate has a Gaussian core width of 6 GeV. A possible system-atic uncertainty coming from the width of the MC signal peak was studied. The mass fit was repeated after smearing the Z0 mass distribution in the MC by a Gaussian function with different widths. The measured cross section did not change significantly after smearing the distribution up to the point where the fitχ
˜
2changed by 1. No systematic uncertainty from this source was assigned.The total systematic uncertainty was calculated by summing the individual uncertainties in quadrature and amounts to
+
7.2% and−
6.2%.8. Results and conclusions
Fig. 3 shows the invariant-mass distribution of the selected events. It also shows the fit result for the signal plus background and the background separately. As described in the previous sec-tions, the background is mainly from diffractive multi-jet produc-tion, and the template of its invariant-mass distribution is deter-mined from the data. The fit yielded a result for the parameter a from Eq. (1) of a
=
0.82+−00..3835. That translates into a number of observed Z0 events of 15.0+7.0−6.4
(stat.), which corresponds to a
sig-nal with a 2.3σ
statistical significance. The fit yielded a value for, the potential energy shift with respect to the signal MC, of 0.028+−00..021020, which is compatible with zero. The correlation be-tween the parameters a and
is rather weak; when fixing the value of
to zero the minimum
χ
˜
2 is observed at a=
0.65. The quality was evaluated according to Eq.(3); the value ofχ
˜
2/ndf
=
17.6/22, where ndf is the number of degrees of freedom, indi-cates a good fit. The cross section for the elastic and quasi-elastic production of Z0bosons, ep→
e Z0p(∗), at√
s=
318 GeV, was cal-culated to beσ
ep
→
e Z0p(∗)=
0.
13±
0.
06(
stat.)
±
0.
01(
syst.)
pb.
(4)This result is consistent with the SM cross section calculated with EPVEC of 0.16 pb. This represents the first observation of Z0 pro-duction in ep collisions.
66 The ratio of the expected number of signal MC events to the observed data in
this region was estimated to be 2.6% for 80<Mjets<100 GeV, while in the other slices it was less than 0.4%.
Acknowledgements
We appreciate the contributions to the construction and main-tenance of the ZEUS detector of many people who are not listed as authors. The HERA machine group and the DESY computing staff are especially acknowledged for their success in providing excel-lent operation of the collider and the data-analysis environment. We thank the DESY directorate for their strong support and en-couragement.
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