Article
Reference
Search for Anomalous Production of Multilepton Events in pp Collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We report a search for the anomalous production of events with multiple charged leptons in pp collisions at s√=1.96 TeV using a data sample corresponding to an integrated luminosity of 346 pb−1 collected by the CDF II detector at the Fermilab Tevatron. The search is divided into three-lepton and four-or-more-lepton data samples. We observe six events in the three-lepton sample and zero events in the ≥4-lepton sample. Both numbers of events are consistent with standard model background expectations. Within the framework of an R-parity-violating supergravity model, the results are interpreted as mass limits on the lightest neutralino (χ˜01) and chargino (χ˜±1) particles. For one particular choice of model parameters, the limits are M(χ˜01)>110 GeV/c2 and M(χ˜±1)>203 GeV/c2 at 95% confidence level; the variation of these mass limits with model parameters is presented.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for Anomalous Production of Multilepton Events in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2007, vol. 98, no. 13, p. 131804
DOI : 10.1103/PhysRevLett.98.131804
Available at:
http://archive-ouverte.unige.ch/unige:38376
Disclaimer: layout of this document may differ from the published version.
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Search for Anomalous Production of Multilepton Events in p p Collisions at p s
1:96 TeV
A Abulencia,24J. Adelman,13T. Affolder,10T. Akimoto,56M. G. Albrow,17D. Ambrose,17S. Amerio,44D. Amidei,35 A. Anastassov,53K. Anikeev,17A. Annovi,19J. Antos,14M. Aoki,56G. Apollinari,17J.-F. Arguin,34T. Arisawa,58 A. Artikov,15W. Ashmanskas,17A. Attal,8F. Azfar,43P. Azzi-Bacchetta,44P. Azzurri,47N. Bacchetta,44W. Badgett,17 A. Barbaro-Galtieri,29V. E. Barnes,49B. A. Barnett,25S. Baroiant,7V. Bartsch,31G. Bauer,33F. Bedeschi,47S. Behari,25
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(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
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
18University of Florida, Gainesville, Florida 32611, USA
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138, USA
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
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24University of Illinois, Urbana, Illinois 61801, USA
25The Johns Hopkins University, Baltimore, Maryland 21218, USA
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
28Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
and SungKyunKwan University, Suwon 440-746, Korea
29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
30University of Liverpool, Liverpool L69 7ZE, United Kingdom
31University College London, London WC1E 6BT, United Kingdom
32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
34Institute of Particle Physics: McGill University, Montr´eal, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
38University of New Mexico, Albuquerque, New Mexico 87131, USA
39Northwestern University, Evanston, Illinois 60208, USA
40The Ohio State University, Columbus, Ohio 43210, USA
41Okayama University, Okayama 700-8530, Japan
42Osaka City University, Osaka 588, Japan
43University of Oxford, Oxford OX1 3RH, United Kingdom
44Istituto Nazionale di Fisica Nucleare, University of Padova, Sezione di Padova-Trento, I-35131 Padova, Italy
45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
47Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
49Purdue University, West Lafayette, Indiana 47907, USA
50University of Rochester, Rochester, New York 14627, USA
51The Rockefeller University, New York, New York 10021, USA
52Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
53Rutgers University, Piscataway, New Jersey 08855, USA
54Texas A&M University, College Station, Texas 77843, USA
55Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
56University of Tsukuba, Tsukuba, Ibaraki 305, Japan
57Tufts University, Medford, Massachusetts 02155, USA
58Waseda University, Tokyo 169, Japan
59Wayne State University, Detroit, Michigan 48201, USA
60University of Wisconsin, Madison, Wisconsin 53706, USA
61Yale University, New Haven, Connecticut 06520, USA (Received 16 December 2006; published 30 March 2007)
We report a search for the anomalous production of events with multiple charged leptons in pp collisions atps
1:96 TeVusing a data sample corresponding to an integrated luminosity of346 pb1 collected by the CDF II detector at the Fermilab Tevatron. The search is divided into three-lepton and four-or-more-lepton data samples. We observe six events in the three-lepton sample and zero events in the 4-lepton sample. Both numbers of events are consistent with standard model background expectations.
Within the framework of anR-parity-violating supergravity model, the results are interpreted as mass limits on the lightest neutralino (~01) and chargino (~1) particles. For one particular choice of model parameters, the limits areM~01>110 GeV=c2andM~1>203 GeV=c2at 95% confidence level; the variation of these mass limits with model parameters is presented.
DOI:10.1103/PhysRevLett.98.131804 PACS numbers: 13.85.Qk, 12.60.Jv, 13.85.Rm, 14.80.Ly
Numerous attempts to resolve theoretical problems with the standard model (SM) of elementary particle physics require the existence of new particles with masses at the electroweak scale,100 GeV=c2 [1–4]. At the Tevatron and LHC hadron colliders, the production cross sections
for these particles are predicted to be orders of magnitude smaller than for SM processes. Analysis methods for re- ducing background levels while preserving new physics signals include searching for leptons (‘) [5] with large momentum transverse to the beam axis (pT), like-sign
leptons, or signatures with transverse energy imbalance (6ET). Requiring several leptons effectively reduces the SM backgrounds while maintaining sensitivity to low 6ET regions. In this Letter, we present a search for an excess of events containing three or more leptons above the SM prediction. We use a data sample with an integrated lumi- nosity of 346 pb1 of pp collisions at ps
1:96 TeV collected by the CDF II detector from March 2002 to August 2004.
Several new physics models predict final states with four or more leptons that can be produced in pp collisions, including R-parity-violating supersymmetry (6Rp SUSY) [6], nonminimal supersymmetric models (nMSSM) [7], and production of doubly charged Higgs boson pairs [8].
In 6Rp SUSY, the lightest supersymmetric particle (LSP) may decay directly into two leptons and a neutrino. With a pair of LSPs in each event, there could be at least four leptons plus6ET. Another possibility is the nMSSM, which includes an expanded Higgs sector and their supersymmet- ric partners. ‘‘Cascade’’ decays of these particles may result in multiple leptons, jets, and less 6ET than in other SUSY models. Alternatively, doubly charged Higgs bosons (H) may be produced in pairs in left-right symmetric models [9]. Under certain conditions, the H primarily decays into two leptons, producing a final state of four leptons without 6ET or jets. In order to be sensitive to multiple new physics models, no6ET or jet cuts are applied.
The results of this search are interpreted using an6RpSUSY model within a minimal supergravity (mSUGRA) frame- work only. A brief introduction to mSUGRA and6RpSUSY phenomenology is included below.
In the minimal supersymmetric model [4], there are two Higgs doublets, as well as supersymmetric partners for every SM particle that differ by 1=2 unit of spin. The superpartners of the electroweak gauge bosons and Higgs bosons mix to produce four neutral mass states (~014) and four charged states (~1;2). The mSUGRA framework [3]
has five free parameters: the universal gaugino mass (M1=2), the universal scalar mass (M0), the ratio of the Higgs vacuum expectation values (tan), the trilinear coupling (A0), and the sign of the Higgsino mass parameter ().
R-parity (Rp) is a quantum number defined such that all SM particles have value 1 and all superpartners have value1. Many searches assume thatRp is conserved so that the LSP is stable, although there are viable SUSY models withRp violation. One scenario includes a single 6Rpterm in the renormalizable superpotential, ijkLiLjEk, whereijkare lepton number violating coupling strengths;
i,j, andkare family indices ranging from 1– 3;Li is the left-handed lepton doublet superfield; andEi is the right- handed charged lepton singlet superfield. In this scenario, superpartners are produced in pairs at hadron colliders.
Muon decay measurements [6] constrain 12k&0:14 so that heavier superpartners will cascade decay into the LSP
with nearly 100% branching ratio, producing at least two LSPs per event. The LSP, which is the~01 in mSUGRA for the values ofM1=2 pertinent to this study, will then decay into two leptons plus a neutrino, as shown in Fig. 1, resulting in a final state of four or more leptons. The detector acceptance limits our sensitivity to 12k*3 103, since a smaller value will result in the LSP decaying far from the interaction region [10]. At least two of the four leptons will be electrons (muons) for LSP decays governed by the121 (122) coupling. Final states with taus are not used since they are less efficiently identified. Recently, lower mass limits have been set by the D0 Collaboration on the ~01 ranging from115–119 GeV=c2 and on the ~1 from229–234 GeV=c2in this type of6RpSUSY model, by searching for three leptons plus 6ET [11]. Previously, the limits from LEP were 53 GeV=c2 on the ~01 and 103 GeV=c2 on the~1 [12].
The components of the CDF II detector relevant to this analysis are briefly described here; a more complete de- scription can be found elsewhere [13]. ThepT and[14]
of charged particles are measured by a silicon strip detector [15] and a 96-layer drift chamber (COT) [16] inside a 1.4 T solenoidal magnetic field. The COT provides coverage with high efficiency for jj<1. For 1<jj<2, the silicon detector is mainly used. Electromagnetic (EM) and hadronic calorimeters surround the tracking system.
They are segmented in a projective tower geometry and measure energies of charged and neutral particles in the central (jj<1) and end-plug (1:1<jj<3:6) regions.
Each calorimeter has an EM shower profile detector posi- tioned at the shower maximum. Four layers of planar drift chambers located outside the central hadron calorimeters and another set behind a 60 cm thick steel absorber detect muons withjj<0:6. Additional drift chambers and scin- tillation counters detect muons in the region 0:6<jj<
1:0. Gas Cherenkov counters [17] measure the average number of inelastic pp collisions per bunch crossing and thereby determine the luminosity.
FIG. 1. Lowest order Feynman diagrams for 6Rp SUSY decay of the LSP via virtual sleptons governed by (a) the121coupling and (b) the122coupling.
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Electron identification criteria include a reconstructed track associated with a cluster of energy in the EM calo- rimeter for jj<2:0. ‘‘Tight’’ central electrons and end- plug electrons are required to have high track quality, calorimeter cluster ET that is consistent with the track pT, a high fraction (usually 95%) of the total calorimeter energy deposition in the EM section, and an EM lateral shower profile consistent with test beam measurements. A
‘‘loose’’ central electron sample is also collected by re- moving the shower profile requirements. Muons are iden- tified forjj<1by a charged track consistent with being minimum ionizing in the calorimeters matched to a recon- structed track segment (‘‘stub’’) in one of the muon drift chambers. Stubless muons are also accepted forjj<1if all other criteria are satisfied except for the stub require- ment and the track does not point to any of the muon drift chambers. All tracks associated with electrons or muons must be consistent with being produced at theppcollision point. Leptons are required to be separated from each other
byR
2 2
p >0:4and to havejzj<60 cm at the point of closest approach to the primary vertex.
Leptons are required to be isolated such thatEisoT divided by theETof the electron orpT of the muon is less than 0.1, whereEisoT is the transverse energy within a cone ofR <
0:4that is unassociated with the lepton.
The data were collected with lepton triggers requiring at least one central electron (muon) candidate with ET >
18 GeVpT>18 GeV=c. We select events with two or more leptons that have ET (pT) >20, 8, and 5 GeV (GeV=c) for the first, second, and additional leptons, respectively. At least one identified lepton must pass the trigger requirements. From this sample, trilepton and 4-lepton events that pass additional cuts (described be- low) are used as signal candidates, while dilepton events are used to validate the background prediction.
Several SM processes result in final states with three or more isolated leptons. The dominant contributions include dileptons from the Drell-Yan (DY) Z= process with additional leptons from photon conversions or misidenti- fied jets, and secondary contributions from diboson (WZ; ZZ) production. Except for those due to misidentified jets, the backgrounds are estimated using Monte Carlo (MC) simulation. The simulated detector acceptances are not a 100% accurate representation of the data and there- fore are corrected by data=MC, where data (MC) is the efficiency for lepton reconstruction, lepton identification, and photon conversion removal requirements measured in data (MC). This correction varies from 0.6 –1.0 per event, depending on the number and types of identified leptons.
Lepton reconstruction and identification efficiencies are measured from DY and J= dilepton events in data and MC simulation. Trigger efficiencies are determined from W !e events for electrons and fromZ!events for muons. The probability for a jet to be misidentified as a lepton is determined as a function ofET for electrons and
pT for muons from jet data samples, by counting the numbers of jets that remain after applying the lepton identification requirements. A similar procedure is de- scribed in more detail in [18].
Samples of 6Rp SUSY simulated events are generated using PYTHIA [19] within the mSUGRA framework with input of the supersymmetric couplings and particle masses fromISAJET [20]. For each point in the SUSY parameter space, the next-to-leading-order cross section is calculated with PROSPINO2 [21]. In all cases ~02~1 and~1~1 pro- duction dominate and are the only processes generated.
The ~01 mass, which is roughly proportional to M1=2, has the largest impact on the signal acceptance. The~01mass is also affected by the sgn, but has little dependence on M0,tan, and A0. Therefore, three of the five mSUGRA parameters are held constant:M0 250 GeV=c2,tan 5, andA00; whilesgnandM1=2are allowed to vary.
Since the~01 mass decreases with larger values ofM0and tan, the chosen values are expected to produce conserva- tive results.
To reduce SM background contamination, an event is removed if any of the following conditions are met for opposite-sign, same-flavor lepton pairs: invariant mass between 76–106 GeV=c2 (‘‘Zveto’’), invariant mass be- low15 GeV=c2, or160< <200. Identified cosmic ray events are removed. Electrons are removed if there is a partner track identified as coming from a photon conver- sion. An event is also rejected if the invariant mass of the two highest transverse energy leptons is below20 GeV=c2 to reduce heavy flavor and radiative photon conversion backgrounds. In order to improve signal-to-background in the three-lepton data sample only, we require that one of the two leading leptons be an electron when evaluat- ing the 121 scenario, and a muon for the 122 scenario.
This definition leads to a 22% overlap for the background and 15% overlap for the signal (depending on model parameters) between the trilepton event samples. After all selection criteria, the signal acceptances are 11% and 4% for the trilepton and 4-lepton data samples, respectively.
Uncertainties are determined separately for SM back- ground and 6Rp SUSY expectations, in each data sample.
The sources of systematic uncertainty on the trilepton background prediction include jets misidentified as leptons (12%–13%), lepton identification efficiency (5% –6%), luminosity measurement (6%), choice of parton distribu- tion functions (2%), cross section (5% –6%), photon con- version identification (3%), and initial state radiation (4%).
For events with4-leptons, the uncertainty on the back- ground is dominated by jets misidentified as leptons (41%) and Z= MC statistics (38%). The total systematic uncertainty on the signal acceptance ranges from 12%–
15%, based on the number and types of leptons, where the largest contribution is due to the uncertainty on the low-pT (<20 GeV=c) lepton identification efficiency.
The background prediction is validated through the use of control regions in which one or more of the event selection criteria is inverted. For each control region, the SM prediction is compared to the data, as shown in TableI.
In theeeanddilepton control regions, the background is dominated by DY. In the case ofecontrol regions, the largest background isZ= !; however, there are also significant contributions fromWW,tt, and jets misidenti- fied as leptons. For trilepton control regions, the largest backgrounds originate from DY events where the third lepton is due to either a photon conversion or a misidenti- fied jet. The agreement between predicted and observed events in the control regions indicates that the backgrounds are validated, and we proceed to examine the signal data samples.
In the trilepton data samples, a total of 6 events are observed: 5 events with an expected background of3:1 0:7stat: 0:4syst:events for the121scenario, and one event with an expected background of 1:91:0stat:
0:3syst:events for the122 scenario. The6ET distribution of the observed events are consistent with the background
prediction, as shown in Fig. 2. No events are observed in the 4-lepton data sample, with an expected background of0:0080:003stat: 0:003syst:events. We interpret the results as being consistent with the hypothesis of no signal and therefore set mass limits on the ~01 and ~1 particles. The cross section limits are calculated by com- bining the 3-lepton and 4-lepton signal samples using a multichannel Bayesian method similar to [22], shown for one 6RpSUSY scenario in Fig.3. The limit calculation takes into account correlations between the 3-lepton and 4-lepton samples. The resulting mass limits, at 95%
confidence limit (C.L.), are presented in TableII.
In conclusion, we have performed a search for anoma- lous production of events with three or more leptons using a sample of CDF Run II data corresponding to346 pb1of integrated luminosity. Finding results consistent with the SM, we set limits on the ~01 and ~1 masses within an (GeV) ET
0 20 40 60 80 100
Events / 4 GeV
10-2
10-1
1 10
-1 ) Data (346 pb
SUSY Rp
γ DY + Mis-ID Jets Diboson
Background Uncertainty
FIG. 2 (color online). 6ET for events in the trilepton signal samples. For comparison, an 6Rp SUSY sample is shown with M~01 99 GeV=c2.
2) mass (GeV/c
±
χ1
60 80 100 120 140 160∼ 180 200 220
cross section (pb)
10-1
1
σSUSY
95% CL expected limit 95% CL observed limit
FIG. 3 (color online). Experimental 95% C.L. cross section limits and theoretical6RpSUSY cross sections versus LSP mass,
for 3 103&122<0:14, M0250 GeV=c2, tan5,
A00, and >0.
TABLE I. Predicted and observed events found in the control and signal samples. The leptons are listed in order of decreasing transverse energy. The first lepton must either be a tight central electron or ‘‘stub muon.’’ Systematic and statistical uncertainties are added in quadrature.
Region
Criteria failed
Predicted background
Observed events Two-lepton control samples
ee Zveto 13 9481536 14019
ee 2142230 2125
Zveto 7474809 7499
1264141 1339
e 117:612:9 112
e 186:822:9 203
Three-lepton control samples
ee‘ Zveto 8:81:9 12
‘ Zveto 5:11:2 2
e‘ Zveto 0:550:04 0
‘‘‘ Zveto 14:42:9 14
ee‘ only 2:10:3 2
‘ only 1:20:2 4
e‘ only 0:350:04 0
‘‘‘ only 3:70:3 6
Four-or-more-lepton control samples
‘‘‘‘ Zveto 0:150:02 0
‘‘‘‘ only 0:0060:003 0
Three-lepton signal samples
121 scenario 3:10:8 5
122 scenario 1:91:0 1
Four-or-more-lepton signal sample
121,122scenarios 0:0080:004 0
131804-6
mSUGRA framework. The~01mass limits range from 98 to 110 GeV=c2, while the chargino mass limits range from 185 to203 GeV=c2 at 95% C.L. depending on the choice of model parameters. These results significantly improve upon the LEP limits [12] and are comparable to the D0 limits [11]. While no evidence for new physics was found, the 4-lepton data sample is virtually background free and provides an excellent technique for detecting new physics with more luminosity in the future.
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 Founda- tion; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Re- search Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Comisio´n Inter- ministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT-2002-00292; and the Academy of Finland.
aVisiting scientist from IFIC(CSIC-Universitat de Valencia).
bVisiting scientist from Universidad Iberoamericana.
cVisiting scientist from University of London, Queen Mary and Westfield College.
dVisiting scientist from University de Oviedo.
eVisiting scientist from University Libre de Bruxelles.
fVisiting scientist from University of Athens.
gVisiting scientist from University of Bristol.
hVisiting scientist from Texas Tech University.
iVisiting scientist from University of Edinburgh.
jVisiting scientist from University of Dublin.
kVisiting scientist from University of Heidelberg.
lVisiting scientist from University of Cyprus.
mVisiting scientist from Nagasaki Institute of Applied Science.
nVisiting scientist from Cornell University.
oVisiting scientist from University of Manchester.
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TABLE II. Expected and observed 95% C.L. limits on the ~01 and ~1 masses for 3 103&12k<0:14, with M0250 GeV=c2,tan5, andA00.
6Rp SUSY scenario
M~01 (exp.)
GeV=c2 (obs.)
M~1 (exp.)
GeV=c2 (obs.) 121, >0 105 102 192 185 121, <0 101 98 192 186 122, >0 108 110 198 203 122, <0 103 106 195 202
