Search for the Supersymmetric Partner of the Top Quark in Dilepton Events from <em>pp</em> Collisions at s√=1.8   TeV

Article

Reference

Search for the Supersymmetric Partner of the Top Quark in Dilepton Events from pp Collisions at s√=1.8   TeV

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

We have searched for pair production of the supersymmetric partner of the top quark (stop) in 107   pb−1 of pp collisions at s√=1.8   TeV collected by the Collider Detector at Fermilab (CDF).

Each stop is assumed to decay into a lepton, bottom quark, and supersymmetric neutrino.

Such a scenario would give rise to events with two leptons, two hadronic jets, and a substantial imbalance of transverse energy. No evidence of such a stop signal has been found. We exclude stop masses in the region (80≤mt˜≤135   GeV/c2) in the mass plane of stop versus sneutrino.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for the Supersymmetric Partner of the Top Quark in Dilepton Events from pp Collisions at s√=1.8   TeV. Physical Review Letters , 2003, vol. 90, no. 25, p. 251801

DOI : 10.1103/PhysRevLett.90.251801

Available at:

http://archive-ouverte.unige.ch/unige:38054

Disclaimer: layout of this document may differ from the published version.

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Search for the Supersymmetric Partner of the Top Quark in Dilepton Events from pp Collisions at

p s

1:8 TeV

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S. Zucchelli³ (CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

4Brandeis University, Waltham, Massachusetts 02254, USA

5University of California at Davis, Davis, California 95616, USA

6University of California at Los Angeles, Los Angeles, California 90024, USA

7University of California at Santa Barbara, Santa Barbara, California 93106, USA

8Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

9Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

10Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

11Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

12Duke University, Durham, North Carolina 27708, USA

13Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

14University of Florida, Gainesville, Florida 32611, USA

15Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

16University of Geneva, CH-1211 Geneva 4, Switzerland

17Glasgow University, Glasgow G12 8QQ, United Kingdom

18Harvard University, Cambridge, Massachusetts 02138, USA

19Hiroshima University, Higashi-Hiroshima 724, Japan

20University of Illinois, Urbana, Illinois 61801, USA

21The Johns Hopkins University, Baltimore, Maryland 21218, USA

22Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

23High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

24Center for High Energy Physics: Kyungpook National University, Taegu 702-701; Seoul National University, Seoul 151-742;

and SungKyunKwan University, Suwon 440-746; Korea

25Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

26University College London, London WC1E 6BT, United Kingdom

27Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

28University of Michigan, Ann Arbor, Michigan 48109, USA

29Michigan State University, East Lansing, Michigan 48824, USA

30Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

31University of New Mexico, Albuquerque, New Mexico, 87131, USA

251801-2 251801-2

32Northwestern University, Evanston, Illinois 60208, USA

33The Ohio State University, Columbus, Ohio 43210, USA

34Osaka City University, Asaka 588, Japan

35University of Oxford, Oxford OX1 3RH, United Kingdom

36Universita di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy

37University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

38Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy

39University of Pittsburgh, Pennsylvania 15260, USA

40Purdue University, West Lafayette, Indiana 47907, USA

41University of Rochester, Rochester, New York 14627, USA

42Rockefeller University, New York, New York 10021, USA

43Instituto Nazionale de Fisica Nucleare, Sezione di Roma, University de Roma I, ‘‘La Sapienza,’’ I-00185 Roma, Italy

44Rutgers University, Piscataway, New Jersey 08855, USA

45Texas A&M University, College Station, Texas 77843, USA

46Texas Tech University, Lubbock, Texas 79409, USA

47Institute of Particle Physics, University of Toronto, Toronto, Canada M5S 1A7

48Istituto Nazionale de Fisica Nucleare, University of Trieste/Udine, Italy

49University of Tsukuba, Tsukuba, Ibaraki 305, Japan

50Tufts University, Medford, Massachusetts 02155, USA

51Waseda University, Tokyo 169, Japan

52University of Wisconsin, Madison, Wisconsin 53706, USA

53Yale University, New Haven, Connecticut 06520, USA (Received 7 February 2003; published 24 June 2003)

We have searched for pair production of the supersymmetric partner of the top quark (stop) in 107 pb¹ofppcollisions atps

1:8 TeVcollected by the Collider Detector at Fermilab (CDF). Each stop is assumed to decay into a lepton, bottom quark, and supersymmetric neutrino. Such a scenario would give rise to events with two leptons, two hadronic jets, and a substantial imbalance of transverse energy. No evidence of such a stop signal has been found. We exclude stop masses in the region (80 m~tt135 GeV=c²) in the mass plane of stop versus sneutrino.

DOI: 10.1103/PhysRevLett.90.251801 PACS numbers: 14.80.Ly, 12.60.Jv, 13.85.Rm

Some of the most promising extensions of the standard model (SM) are based on supersymmetry, e.g., the mini- mal supersymmetric standard model (MSSM) [1]. It pre- dicts that each SM particle has a superpartner (sparticle) with the same quantum numbers, except for spin which differs by one-half unit. Experimental results indicate that supersymmetric (SUSY) particles are generally not as light as their SM partners. SUSY, therefore, is broken at or above the electroweak scale, and we treat the sparticle masses as free parameters. Because of the large top quark mass, there may be a large mixing between the super- partners of the left and right helicity states of the top quark [2]. This can lead to substantial mass splitting of the squark (stop) mass eigenstates (~tt₁;~tt₂) with the lighter one (denoted~ttfrom now on) potentially being the lightest squark.

Stop-antistop pairs (~tt~tt) are strongly produced in the ppcollisions at the Fermilab Tevatron if kinematically accessible. The production cross section has been calcu- lated using QCD in the next-to-leading order (NLO) approximation [3]. For a given stop mass (m~tt), the cross section depends weakly on the other parameters of the MSSM. In the mass region of interest to our search (m~tt 80–140 GeV=c²), the cross section drops from 44 to 1 pb.

We assume SUSY R-parity [4] conservation, from which the stability of the lightest supersymmetric par-

ticle (LSP) follows. All SUSY particles, including the stop, eventually decay into this LSP. Stop decays into the top quark are kinematically not accessible in our region of interest due to the high top mass (m~tt< m_t). For the stop decay into a bottom quark and an on-shell chargino (~₁), only a very small window of opportunity remains at the Fermilab Tevatron due to the ~₁ mass limit from LEP2 [5]. Another possible two-body stop decay would be the flavor-changing, ~tt!c~⁰₁, decay [6]. It would proceed via higher order loop diagrams and is highly suppressed. The three-body decay into a charged super- symmetric lepton,~tt!~llb, is closed for most of the stop region currently within the reach of the Collider Detector at Fermilab (CDF) because of the slepton mass limit of LEP2 [5]. The existing mass limit of the supersymmetric neutrino, m_~45 GeV=c² [7], leaves the decay into sneutrino, ~tt!l~b, open. We assume equal e, , and branching ratios.

Stop pair production with the~tt!l~bdecay yields two leptons with opposite electric charge, two hadronic jets from the bottom quarks, and considerable transverse energy imbalance (6E_T) in the detector [8] due to the escaping sneutrinos. CDF has reported earlier on an analysis based on B identification [9]. In this Letter, we use dilepton events. Only a few SM processes yield di- leptons and can thus mimic our stop signature. The most

significant ones arettproduction,bbandcc with semi- leptonic decays, Drell-Yan production with hadronic jets from higher order processes, diboson production (WW, WZ, and ZZ), lepton pairs from the decay of vector mesons, such as J= and , and events without two genuine prompt leptons, where a hadron is misidentified as a lepton, or decays in flight to a lepton.

The search presented here is based on107 pb¹ ofpp collisions at

ps

1:8 TeV collected by the CDF during the 1992 to 1995 running period of the Tevatron. A de- tailed description of the CDF detector can be found in Ref. [10]. Online triggers selected approximately 6:4 10⁶ single lepton events and an additional 3:3 10⁶ di- lepton events. All of those events have been recon- structed, and 13 295 events were selected as a dilepton sample, by requiring at least one tight electron (E_T 10 GeV,jj 1:0) or muon (p_T 10 GeV=c,jj 0:6) candidate, and a second loose electron (E_T 6 GeV, jj 1:0) or muon (p_T 6 GeV=c, jj 1:0) candi- date. No explicit tau lepton identification was done, but taus can enter the search sample if they decay leptoni- cally. Electrons are identified by energy deposition in the electromagnetic calorimeter with a track of correspond- ing energy in the central drift chamber (CTC) pointing to it. Muons are identified by track segments in both the CTC and the muon drift chambers that are located behind 4.5 to 10 interaction lengths of absorber. Standard lepton identification cuts are used and described elsewhere [11].

Each lepton is required to be isolated; i.e., we require the total p_T of all other tracks within a cone R

^{2 2}

p 0:4 around the lepton’s track not to exceed4 GeV=c. The jets were reconstructed with a cone algorithm with cone radiusR0:7[12]. We require at least one jet in the central region of the calorimeter (jj 1:0) withE_T 15 GeV, that is separated byR 0:7 from both leptons in the event. For increased effi- ciency, we require only one of the two jets to be identified.

SequentialBdecays,J= ,, andZevents were removed requiring the invariant dilepton massesm_ll⁰ 6 GeV=c² or m_ll12 GeV=c² and excluding 76< m_ll<

106 GeV=c² (where prime indicates any mixture of e and flavors and no prime indicates same-flavor dilep- tons). At the preselection level, we start with 6E_T 15 GeV. Experimental backgrounds, like electrons from conversions and muons from cosmic rays are removed with additional cuts [13]. 176 events fulfill the above preselection requirements.

To estimate the number of SM and stop events in the sample, events of the various physics processes are gen- erated byISAJET[14] and simulated for the CDF detector.

We have used CTEQ-3 parton distribution functions (PDF) [15]. The stop production cross section was calcu- lated withPROSPINO[16] and theISAJETcross section was adjusted accordingly. We have generated events over a large range of stop (80–140 GeV=c²) and sneutrino (45–90 GeV=c²) masses.

The Drell-Yan and tt production cross sections were normalized to CDF measurements [17]. The Monte Carlo (MC)bbandcccross sections were verified by inclusive electron-muon samples. TheB⁰B⁰ oscillation effect was added based on the CDF measured inclusive mixing fraction [18]. The diboson production cross sections of the MC data were scaled to those of NLO calculations [19].

For lowp_T leptons, the contribution due to misidenti- fication can be significant and is calculated in two steps [13]. First, we measure in various data samples the so-called ‘‘fake lepton probabilities’’ (momentum- dependent, separately for electrons and muons, and dependent on detector region). These fake lepton proba- bilities include hadrons being misidentified as electrons or muons, and also include leptons from in-flight decays of pions and kaons. We measure misidentification proba- bilities between0:4%and7%for botheand[13].

Second, in a single lepton sample we use these ‘‘fake lepton probabilities’’ successively on each track in the event to simulate dilepton events. We use the ‘‘fake lepton probabilities’’ to simulate both the number of misidenti- fied-lepton events as well as their kinematic properties.

The major background to the preselection sample comes from heavy flavor production, with about a quarter of the events having leptons of the same charge. Another significant background comes from Drell-Yan processes.

In those events, the 6E_T comes from decays or jet and lepton energy mismeasurements due to uninstrumented detector regions. We expect a total background of155 55events, while a stop and sneutrino mass combination of 100 and 75 GeV=c² would contribute 249 events.

Table I shows the expected contributions for like-sign (LS) and opposite-sign (OS) charge leptons. To verify our background calculation further, we compare kine- matic distributions of the data and the expected back- ground. Figure 1 shows a few such distributions. Top and diboson production yield generally more energetic leptons thanbb,cc, or misidentified leptons. Thep_T distributions of the leptons show that both high and low p_T lepton sources agree well with the data. The 6E_T distribution

TABLE I. Data, expected backgrounds for the preselec- tion sample, and expected stop signal for m~ttm_~ 10075GeV=c². The stop event acceptance is 2:5% at this stage.

Source OS LS

Drell-Yan 52:213:7 0:40:4

bb,cc 43:532:1 16:417:6

tt 9:52:9 0:60:2

WW,WZ,ZZ 3:80:9 0:40:1

Misidentified leptons 16:34:4 12:43:4 Total background 125:246:7 30:118:4

Data 128 48

Expected~tt~tt 22:68:9 1:00:4

251801-4 251801-4

agrees both at low 6E_T, where detector effects dominate, and at high 6E_T, where neutrinos from W and Z bosons determine the spectrum. The parton shower MC program describes well the observed jet multiplicity. From the signal (S) to background (B) ratio, it is clear that the preselection sample does not have sufficient sensitivity to answer the question of stop pair production. In contrast to an earlier search [20], we select a kinematic region in which we expect higherS=B.

In less than5%of stop events, the two leptons are LS due to the semileptonic decay of one of the b quarks.

However, 20% of the SM background yields LS lepton events. We thus focus our search on events with OS leptons. For R_p-conserving supersymmetry, we expect large missing energy from the rather heavy sneutrinos.

In Fig. 1, we see most of the background events clustering at low missing 6E_T. A6E_T cut of 30 GeV removes77%of the SM background but keeps about 57% of the stop events. Energy mismeasurement of leptons, or the pres- ence of neutrinos from Drell-Yan decays, would cause the leptons (and the dilepton system as well) to be aligned with the 6E_T direction. This is not typical for the signal, where we expect true 6E_T and the individual leptons and

the dilepton system^6E_l^T

1 ,_l^6ET

2 , and_l^6ET

1l₂ to be larger than30.

In Drell-Yan plus jets events or whenbb orcc events originate from gluon splitting (initial or final state) events, the two leptons balance the jets in the transverse plane. We veto events where the angle between either lepton and the most energetic central jet, _l^6ET

1 and

_l^6ET

2 , is larger than90.

Events from top pair production pass the above cuts with efficiencies similar to stop pair events and are now the dominant source of SM background. In top events, the leptons come fromWdecay and are very energetic. In the case of stop, we have three-body decays containing a very heavy sneutrino. The amount of available energy in the decay depends on the stop-sneutrino mass difference, m~tt~. For small mass difference, the leptons and jets are quite soft and a large fraction of the event energy escapes detection through the sneutrinos, unlike a tt event. For best stop sensitivity at smallm_~tt~, we require the scalar sum of lepton p_T,p^l_T¹p^l_T²75 GeV=c, and the p_T of the dilepton system, p^l_T^1l2 30 GeV=c.

Although a large amount of energy escapes undetected, the sneutrinos tend to be back to back, thus reducing the Tight lepton p_T (GeV/c)

Events/5 GeV/c

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70 80

Second lepton p_T (GeV/c)

Events/5 GeV/c

Data

Background Background Uncertainty

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70 80

Missing E_T (GeV)

Events/5 GeV

Stop Signal x 10

0 10 20 30 40 50 60 70 80

0 10 20 30 40 50 60 70 80

Jet Multiplicity

Events

0 20 40 60 80 100 120

0 1 2 3 4 5

FIG. 1 (color online). Data and expected background after preselection. Tight and second lepton transverse energies, missing transverse energy [for comparison we also show the missing E_T distribution for a 10 times stop signal of m~ttm_~ 10075GeV=c²], and jet multiplicity shown for events with opposite charge leptons. The last high bins contain overflows.

measured6E_T. We also require the sum of the most ener- getic central jet E_T and the missing E_T, E^jet_T 6E_T 160 GeV.

For large stop-sneutrino mass difference, the leptons are more energetic and we can increase our lepton p_T requirement to10 GeV=cwithout much loss in stop effi- ciency. However, leptons and jets are still significantly softer than inttevents. We place the same jet, missingE_T, and lepton requirements as at smallm~tt~,E^jet_T 6E_T 160 GeV, and p^l_T¹p^l_T²75 GeV=c but loosen the re- quirement on the dileptonp_T top^l_T^1l2 55 GeV=c.

Table II shows the expected number of stop events for the two search regions. We start our search at stop masses of80 GeV=c² to overlap with previous LEP lim- its. Near the kinematic limit of the stop decay, m~tt m_~m_b, lepton and jet energies become very soft, limit- ing our stop detection capabilities. At high stop mass, our sensitivity is limited by the steeply falling produc- tion cross section. In the region of interest to this search, the final stop event acceptance varies between 0:3%

and2:3%.

The biggest source of uncertainty on the number of expected stop events arises from the choice of the renor- malization and factorization scale,Q², which character- izes the amount of energy transferred during the collision. The6E_T is reduced (due to the sneutrinos being more back to back) whenQ is increased, and the jet E_T gets softer whenQis decreased. By varyingQby a factor of 2 up and down, we determine the uncertainty due to the choice of Q² to be 32%. Other significant sources of uncertainty are the choice of PDF (11%); the absolute energy scale of the detector (11%); the amount of gluon radiation (7%); the trigger, lepton, and isolation efficiency (5%), and the luminosity measurement (4%). The statis- tical uncertainties of the MC samples are about 8%.

Combining the statistical and systematic uncertainties, we obtained a total uncertainty of 38% for the signal expectation. Similarly, we evaluated the uncertainty of the background calculation to be30%.

After establishing the selection cuts by using a ‘‘blind’’

analysis technique, we apply the cuts to the preselection data. We observe zero events for both the small and the largem~tt~sets of cuts, consistent with our background

expectation of1:520:47and2:070:46events. We use the frequentist method with zero observed events, no background subtraction, and a total uncertainty of 38%

on the predicted signal to calculate a95%confidence level (C.L.) upper limit of 4.01 stop events. Consequently, we exclude all stop-sneutrino mass combinations that would yield more than 4.01 events. Figure 2 shows our result compared to LEP2 [5] and DØ [20].

In conclusion, we have searched for stop pair produc- tion in 107 pb¹ of data from pp collisions at

ps 1:8 TeV collected by CDF. The observed dilepton, jet, and missing E_T events are consistent with expectations from SM sources. We exclude stop masses up to m~tt 135 GeV=c² (at m_~ of 72–79 GeV=c²) and sneutrino masses up to 88:4 GeV=c² (at m~tt of 126 GeV=c²) at 95%C.L.

We thank the Fermilab staff and the technical staffs of the participating institutions for their contributions. This

M(t˜) (GeV/c²) M(ν˜) (GeV/c2 )

CDF ∫L dt = 107 pb^-1

∑Br(t˜ → l ν˜ b) = 100%

e,µ,τ

θ˜ =0t ^o

θ˜ =56t ^o

M(t

˜) < M(

ν˜) + M(b)

Aleph L3 Opal

D0/

CDF Excluded at 95% C.L.

40 LEP 50 60 70 80 90 100 110

60 80 100 120 140

FIG. 2 (color online). Stop and sneutrino mass plane showing the CDF 95% C.L. excluded region as hatched area. For the three-body stop decay, ~tt!l~b, a 33:3% branching ratio to each of the three lepton flavors is used.

TABLE II. Data, expected background, and expected stop signals after final cuts. Stop A scenario represents a smallm~tt~ withm_tm 10075GeV=c². Stop B scenario repre- sents a largem~tt~withm~ttm_~ 12060GeV=c²

Selection Data Background Stop A Stop B

Preselection 176 155:350:2 23:68:9 34:513:0

OS and6E_T 26 28:78:6 12:94:9 25:19:5

^6E_l;ll^T and^jet_l 4 8:12:4 6:72:5 14:85:6

Smallm~tt~ 0 1:50:5 5:72:1

Largem~tt~ 0 2:10:5 8:23:1

95%C.L. cross section limit 9.0 pb 2.2 pb

251801-6 251801-6

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 Re- search Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation;

the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korea Science and Engineering Foundation; the Korea Research Foundation; and the Comision Interministerial de Ciencia y Tecnologia, Spain.

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