Article
Reference
Search for scalar leptoquark pairs decaying to ννqq in pp collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We report on a search for the pair production of scalar leptoquarks (LQ), using 191 pb−1 of proton-antiproton collision data recorded by the CDF experiment during Run II of the Tevatron. The leptoquarks are sought via their decay into a neutrino and quark yielding missing transverse energy and several jets of large transverse energy. No evidence for LQ production is observed, and limits are set on σ(pp→LQLQX→ννqqX). Using a next-to-leading order theoretical prediction of the cross section for LQ production, we exclude first-generation LQ in the mass interval 78 to 117 GeV/c2 at the 95% confidence level for BR(LQ→νq)=100%.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for scalar leptoquark pairs decaying to ννqq in pp collisions at s√=1.96 TeV. Physical Review. D , 2005, vol. 71, no. 11, p.
112001
DOI : 10.1103/PhysRevD.71.112001
Available at:
http://archive-ouverte.unige.ch/unige:38298
Disclaimer: layout of this document may differ from the published version.
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Search for scalar leptoquark pairs decaying to qq in pp collisions at p s
1:96 TeV
D. Acosta,16J. Adelman,12T. Affolder,9T. Akimoto,54M. G. Albrow,15D. Ambrose,15S. Amerio,42D. Amidei,33 A. Anastassov,50K. Anikeev,15A. Annovi,44J. Antos,1M. Aoki,54G. Apollinari,15T. Arisawa,56J-F. Arguin,32 A. Artikov,13W. Ashmanskas,15A. Attal,7F. Azfar,41P. Azzi-Bacchetta,42N. Bacchetta,42H. Bachacou,28W. Badgett,15 A. Barbaro-Galtieri,28G. J. Barker,25V. E. Barnes,46B. A. Barnett,24S. Baroiant,6G. Bauer,31F. Bedeschi,44S. Behari,24 S. Belforte,53G. Bellettini,44J. Bellinger,58A. Belloni,31E. Ben-Haim,15D. Benjamin,14A. Beretvas,15A. Bhatti,48 M. Binkley,15D. Bisello,42M. Bishai,15R. E. Blair,2C. Blocker,5K. Bloom,33B. Blumenfeld,24A. Bocci,48A. Bodek,47
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E. Wicklund,15H. H. Williams,43P. Wilson,15B. L. Winer,38P. Wittich,43S. Wolbers,15C. Wolfe,12M. Wolter,55 M. Worcester,7S. Worm,50T. Wright,33X. Wu,18F. Wu¨rthwein,8A. Wyatt,30A. Yagil,15K. Yamamoto,40T. Yamashita,39
J. Yamaoka,50C. Yang,59U. K. Yang,12W. Yao,28G. P. Yeh,15J. Yoh,15K. Yorita,56T. Yoshida,40I. Yu,27S. Yu,43 J. C. Yun,15L. Zanello,49A. Zanetti,53I. Zaw,20F. Zetti,44J. Zhou,50and S. Zucchelli4
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
5Brandeis University, Waltham, Massachusetts 02254, USA
6University of California at Davis, Davis, California 95616, USA
7University of California at Los Angeles, Los Angeles, California 90024, USA
8University of California at San Diego, La Jolla, California 92093, USA
9University of California at Santa Barbara, Santa Barbara, California 93106, USA
10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
14Duke University, Durham, North Carolina 27708
15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
16University of Florida, Gainesville, Florida 32611, USA
17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
18University of Geneva, CH-1211 Geneva 4, Switzerland
19Glasgow University, Glasgow G12 8QQ, United Kingdom
20Harvard University, Cambridge, Massachusetts 02138, USA
21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
22Hiroshima University, Higashi-Hiroshima 724, Japan
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
D. ACOSTAet al. PHYSICAL REVIEW D71,112001 (2005)
112001-2
25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
27Center for High Energy Physics, Kyungpook National University, Taegu 702-701;
Seoul National University, Seoul 151-742;
and SungKyunKwan University, Suwon 440-746, Korea
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
32Institute of Particle Physics, McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
33University of Michigan, Ann Arbor, Michigan 48109, USA
34Michigan State University, East Lansing, Michigan 48824, USA
35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
36University of New Mexico, Albuquerque, New Mexico 87131, USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
44Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy
45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
46Purdue University, West Lafayette, Indiana 47907, USA
47University of Rochester, Rochester, New York 14627, USA
48The Rockefeller University, New York, New York 10021, USA
49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University di Roma ‘‘La Sapienza,’’ I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Texas Tech University, Lubbock, Texas 79409, USA
53Istituto Nazionale di Fisica Nucleare, University of Trieste, Udine, Italy
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155, USA
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201, USA
58University of Wisconsin, Madison, Wisconsin 53706, USA
59Yale University, New Haven, Connecticut 06520, USA
(Received 25 October 2004; published 2 June 2005; corrected 10 June 2005)
We report on a search for the pair production of scalar leptoquarks (LQ), using191 pb1 of proton- antiproton collision data recorded by the CDF experiment during Run II of the Tevatron. The leptoquarks are sought via their decay into a neutrino and quark yielding missing transverse energy and several jets of large transverse energy. No evidence for LQ production is observed, and limits are set onpp! LQLQX!q qX. Using a next-to-leading order theoretical prediction of the cross section for LQ production, we exclude first-generation LQ in the mass interval 78 to117 GeV=c2at the 95% confidence level forBRLQ!q 100%.
DOI: 10.1103/PhysRevD.71.112001 PACS numbers: 13.85.Rm, 12.60.2i, 14.80.2j
The remarkable symmetry between quarks and leptons in the standard model (SM) suggests that some more fundamental theory may exist, which allows interactions between them. Such interactions may be mediated by a new type of particle, a leptoquark [1], which carries both lepton and baryon number. A leptoquark is a color-triplet boson with spin 0 or 1, and has fractional electric charge.
Leptoquarks are predicted in many extensions of the SM (e.g. grand unification, technicolor, and supersymmetry with R-parity violation). The Yukawa coupling of the
leptoquark to a lepton and quark and the inclusive branch- ing ratio to a charged lepton and quark, denoted by, are model dependent. Usually it is assumed that leptoquarks couple to only one generation to accommodate experimen- tal constraints on flavor-changing neutral currents [2], which allows one to classify leptoquarks as first-, second- , or third-generation, with decay products corresponding to the three generations of fermions in SM. Inpp collisions, leptoquarks can be produced in pairs via the strong inter- action through ggfusion or qq annihilation. The produc- . . .
tion rate for scalar leptoquarks is essentially model- independent and is determined by the known QCD cou- plings and leptoquark mass. On the other hand, vector leptoquark interactions with the gluon field include model-dependent couplings. The production cross section for vector leptoquarks [3] is expected to be about an order of magnitude larger than that for scalar leptoquarks.
We report on a search for pair production of scalar leptoquarks, with LQ decaying to q, resulting in a jets and missing transverse energy (E6 T) topology. We use 19111 pb1 [4] of pp collision data at a center-of- mass energy of 1.96 TeV recorded by the collider detector at Fermilab (CDF) during the Tevatron Run II. This analy- sis is sensitive to leptoquarks of all three generations with 0. However, we would not be able to distinguish between the different generations of leptoquarks, if they are present in the signal, since we do not identify the quark content of the jets. The previous lower mass limit of 98 GeV=c2 [5] on first-generation leptoquarks in this final state was set by the DØ Collaboration. The CDF Collaboration has also published [6] lower mass limits of 123 GeV=c2and148 GeV=c2respectively on second- and third-generation LQ in theE6 T plus heavy-flavor jets final state. Limits on leptoquark production from the Tevatron Run I and HERA experiments as of 1999 are summarized in [7]. The limits from HERA experiments depend on the unknown Yukawa couplings, which are assumed to be of the electro-weak coupling strength. The OPAL collabora- tion published [8] mass limits of97 GeV=c2 independent of the Yukawa couplings for the scalar LQ production into q q final state inee-collision.
CDF is a general-purpose detector that is described in detail elsewhere [9]. The components relevant to this analysis are briefly described here. The charged-particle tracking system is closest to the beam pipe, and consists of multilayer silicon detectors and a large open-cell drift chamber covering the pseudorapidity [10] regionjj<1.
The tracking system is enclosed in a superconducting solenoid, which in turn is surrounded by a calorimeter.
The CDF calorimeter system is organized into electromag- netic and hadronic sections segmented in projective tower geometry, and covers the region jj<3:6. The electro- magnetic calorimeters utilize a lead-scintillator sampling technique, whereas the hadron calorimeters use iron- scintillator technology. The central muon-detection sys- tem, used for this analysis, is located outside of the calorimeter and covers the rangejj<1.
This search centers on selecting events with largeE6 Tand a pair of jets that are acollinear in the transverse plane, because of the neutrinos in the final state. TheE6 T [10] is defined as the energy imbalance in the plane transverse to the beam direction. A jet is defined as a localized energy deposition in the calorimeter and is reconstructed using a cone algorithm with fixed radius R
22 p 0:4inspace [11]. We correct [11] jetET measure- ments andE6 T for detector effects.
The data sample for this analysis was collected using an inclusiveE6 Ttrigger, which is distributed across three levels of online event selection. In the first and second levels of the trigger,E6 T is required to be greater than 25 GeV and is calculated by summing over calorimeter trigger towers [12] with transverse energies above 1 GeV. At Level-3 E6 Tis required to be greater than 45 GeV and is recalculated using full calorimeter segmentation with a tower energy threshold of 100 MeV. We use events from the inclusive high-pT lepton (e or ) samples to measure the trigger efficiency directly from data. To reduce systematic effects associated with the online trigger threshold, we select events offline withE6 T>60 GeV, where the trigger is fully efficient.
The event electromagnetic fraction (Fem) and charged fraction (Fch) [13] are used to remove events associated with beam halo and cosmic ray sources. We reject events that contain little energy in the electromagnetic section of the calorimeter or that have mostly neutral-particle jets, by requiring Fem>0:1 and Fch>0:1. There are 148 462 events in our analysis sample after the initial selection.
The dominant backgrounds to the leptoquark search in the jets andE6 T signature are QCD multijet production,W and Zboson production in association with one or more jets, and top quark pair production. TheALPGENgenerator [14] was used for the simulation of theWandZboson plus parton production, withHERWIG[15] used to model parton showers. As the W=Zjets production cross sections calculated by ALPGEN are only in leading order, we use the exclusiveZ!ee 1jet data and simulation samples to determine a cross section scale-factor between data and simulation, and apply this scale-factor to all W=Zjets simulation samples.HERWIGwas also used to estimate the contribution from ttproduction. The top quark contribu- tion was normalized to the luminosity of the data sample using the predicted theoretical cross section.
Data selection requirements were chosen to maximize the statistical significance of the leptoquark signal over background events based on studies of simulated event samples before the signal region data were examined. In addition to E6 T>60 GeV, the signal region is defined by requiring that the two highest ET jets (EjT1>40 GeV, EjT2>25 GeV) be in the central region jj<1. A third jet with ET>15 GeV and jj<2:5 is allowed, and we veto events with any additional jets withET>15 GeVand jj<3:6. To reject events with E6 T resulting from jet energy mismeasurement, we require that the opening angle in the transverse plane between the two highest ET jets satisfy 80<j1; j2<165. The E6 T direction must not be parallel to any of the jets; we require the minimum azimuthal separation between the direction of the jets and E6 T to satisfy 30<minj; E6 T<135. The E6 T also must not be antiparallel to the leading ET jet: 100<
j1; E6 T<165. These criteria reject most of the QCD multijet background events. To reduce the back-
D. ACOSTAet al. PHYSICAL REVIEW D71,112001 (2005)
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ground contribution from W=Zjets and tt production, we reject events with one or more identified leptons with ET>10 GeV (electron candidates) or pT>10 GeV=c (muon candidates). Criteria similar to those in [16] are used to identify the leptons. To further reduce this back- ground we require each jet not to be highly electromag- netic (jet electromagnetic fraction<0:9) and to have 4 or more associated tracks for central jets (jj<1).
Two methods are employed to estimate the QCD multi- jet contribution in the signal region directly from the inclusive E6 T data sample. Among all the offline analysis selection requirements, the azimuthal angular separation requirement between the E6 T direction and a jet is most effective at removing QCD multijet events. Therefore, for the first method, in addition to the signal region we define a region which is rich in QCD multijet events by requiring that a jet is close to the E6 T direction (20<
minj; E6 T<27). Studies of simulated QCD multijet samples show that the shape of theE6 T distribution in this region is similar to theE6 T distribution in the signal region.
We useE6 T andminj; E6 Trequirements to define four kinematic regions:
(A) 50< E6 T<57 GeV,20<minj; E6 T<27. (B) E6 T >60 GeV,20<minj; E6 T<27. (C) 50< E6 T<57 GeV,30<minj; E6 T<135. (D) E6 T >60 GeV,30<minj; E6 T<135. The regions A, B, and C are used to extrapolate the QCD multijet contribution into the signal region D:NDNNB
ANC, whereNA,NB, andNCare the remaining number of events in regions A, B, and C, after the W=Zjets andttcon- tributions have been subtracted. For the second method, the combined selection requirement efficiency is measured as a function ofE6 T in an independent inclusive jet sample at low E6 T. The extrapolated results of this measurement is then applied to the inclusiveE6 T sample after theW=Z jets andttcontributions have been subtracted. We predict 15:08:0and21:512:4multijet events for the first and second methods, respectively. We take the weighted aver- age and uncertainty of the two methods as our estimate of the multijet background.
We check the simulation predictions for W=Zjets with data in a control region, which is defined by requiring, in addition to 2 or 3 jets,E6 T>60 GeV and at least one electron or muon. We observe 144 events in our inclusive E6 T sample, which is in excellent agreement with154:3 27:9events predicted from SM processes.
The total detection efficiency (LQ) for the scalar lepto- quark signal is estimated using thePYTHIAevent generator [17], and the CDF detector simulation program. The
PYTHIA underlying event simulation was tuned to repro- duce CDF data [18]. The samples were generated using the CTEQ5L [19] parton distribution functions (PDF), with the renormalization and factorization scales set tomLQ. Table I lists the total detection efficiency,LQ1, for the first- generation scalar leptoquark signal and the corresponding
total fractional uncertainty !tot for various leptoquark masses. The acceptances for second- and third-generation leptoquark signals are estimated to be 4% and 10% lower, respectively, than that for the first generation in the gen- erated mass region due to semileptonic decays of heavy- flavor quarks. Also listed are the NLO cross sections [20]
calculated for two choices of thescale. The systematic uncertainty on the signal acceptance includes the uncer- tainties due to modeling gluon radiation from the initial- state or final-state partons (10%), and the choice of the PDF (4%). The limited size of the leptoquark simulation samples gives a 3% statistical uncertainty. The signal acceptance uncertainty due to the jet energy scale varies from 4% to 26%, and the uncertainty on the luminosity is 6%. The uncertainty on the trigger efficiency is 1%. The theoretical uncertainties on the renormalization and facto- rization scales are not included here, since we conserva- tively choose the NLO cross section setting2mLQto extract the limits on leptoquark mass. This choice of scale is found to reduce the cross section prediction by 15%
relative tomLQ[20].
TABLE I. Summary of the first-generation scalar leptoquark detection efficiency (LQ), the relative uncertainty on detection efficiency (!tot), and the next-to-leading order cross section (NLO) for two choices of the renormalization scale as functions of leptoquark mass.
NLO(pb)
mLQ1GeV=c2 LQ1 !tot (%) mLQ1 2mLQ1
75 0.0073 29 69.4 58.8
80 0.0113 26 49.2 41.5
90 0.0187 23 26.0 22.1
100 0.0300 20 14.6 12.5
110 0.0431 16 8.4 7.4
115 0.0482 15 6.7 5.8
125 0.0590 15 4.2 3.6
150 0.0828 13 1.4 1.3
175 0.1010 12 0.57 0.51
TABLE II. The number of expected events from various SM sources in the leptoquark signal region. The first uncertainty is from the limited simulation statistics and the second is from the various systematics.
Source Events expected
W!e jets 6:11:41:5
W! jets 21:72:32:8
W!" jets 28:43:84:1
Z! jets 1:10:20:2
Z!"" jets 0:90:20:2
Z! jets 39:12:83:6
tt 4:30:40:3
QCD 16:96:7
Total events 118:514:5
. . .
In the signal region, we expect118:514:5events from SM processes and observe 124 events. The predicted back- grounds from SM processes are summarized in Table II. In Fig. 1 the predictedE6 T distribution is compared with the distribution observed in data. No evidence for leptoquark production is observed. We calculate the upper limit at the 95% confidence level (C.L.) on the pair production cross section times the square of the branching ratio of the leptoquark to a quark and a neutrino using first-generation LQ acceptance and a Bayesian approach [21] with a flat prior for the signal cross section and Gaussian priors for acceptance and background uncertainties. The upper limit on the cross section times12is shown in Fig. 2 and is compared with the theoretical cross sections. The theoreti- cal cross sections for scalar leptoquark production have been calculated at NLO using CTEQ5M [19] PDFs.
In conclusion, we performed a search for leptoquarks in the jets and E6 T topology using 191 pb1 of CDF Run II data. No evidence for leptoquarks is observed. We set an upper limit on the production cross section at the 95% C.L.
Assuming a leptoquark decays into a neutrino and quark with 100% branching ratio, we exclude the mass interval from 78 to117 GeV=c2 for first-generation LQ indepen- dent of the Yukawa coupling. This extends the previous
limit for the first-generation LQ of 98 GeV=c2 [5]. The limits for the second- and third-generation LQ are weaker than existing limits obtained from an exclusive search [6]
identifying jet flavor.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesmini- sterium fuer Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, U.K.; the Russian Foundation for Basic Research; the Comision Interministerial de Ciencia y Tecnologia, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT- 2002-00292; and the Academy of Finland.
2) Leptoquark mass (GeV/c
80 100 120 140 160
)bp(2 )β-1(×)1QL 1QL →pp(σ
1 10 102
-1) CDF 95% C.L. Upper limit (191 pb
β=0) Theoretical cross section (
LQ1
µ=m CTEQ5M,
LQ1
µ=2m CTEQ5M,
FIG. 2 (color online). The upper limit on the cross section times squared branching ratio for scalar leptoquark production in the jets andE6 Ttopology. Also shown is the NLO cross section for 0 for 2 choices of the factorization/renormalization scale: mLQ1,2mLQ1.
(GeV) Missing ET
60 120 180 240 300
VeG 01 /stnevE
0 10 20 30 40 50
60 CDF Run II Data (191 pb-1)
QCD prediction
prediction t
W/Z + jets and t
2)
= 115 GeV / c
LQ1
Leptoquark (m
(GeV) Missing
60 120 180 240 300
VeG 01 /stnevE
0 10 20 30 40 50 60
FIG. 1 (color online). The E6 T distribution in the leptoquark signal region for data (solid points) compared to SM background (shaded histograms). Also shown is the expected distribution arising from leptoquark production and decay at a mass of 115 GeV=c2(hatched histogram).
D. ACOSTAet al. PHYSICAL REVIEW D71,112001 (2005)
112001-6
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