Article
Reference
Search for R -Parity Violating Decays of Sneutrinos to eμ , μτ , and eτ Pairs in pp Collisions at √s=1.96 TeV
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present a search for supersymmetric neutrino ν˜ production using the Tevatron pp collision data collected with the CDF II detector and corresponding to an integrated luminosity of 1 fb−1. We focus on the scenarios predicted by the R-parity violating (RPV) supersymmetric models in which sneutrinos decay to two charged leptons of different flavor. With the data consistent with the standard model expectations, we set upper limits on σ(pp→ν˜)×BR(ν˜→eμ,μτ,eτ) and use these results to constrain the RPV couplings as a function of the sneutrino mass.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for R -Parity Violating Decays of Sneutrinos to eμ , μτ , and eτ Pairs in pp Collisions at √s=1.96 TeV. Physical Review
Letters , 2010, vol. 105, no. 19, p. 191801
DOI : 10.1103/PhysRevLett.105.191801
Available at:
http://archive-ouverte.unige.ch/unige:39122
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for R -Parity Violating Decays of Sneutrinos to e , , and e Pairs in p p Collisions at ffiffiffi
p s
¼ 1:96 TeV
T. Aaltonen,24J. Adelman,14B. A´ lvarez Gonza´lez,12,xS. Amerio,44b,44aD. Amidei,35A. Anastassov,39A. Annovi,20 J. Antos,15G. Apollinari,18J. Appel,18A. Apresyan,49T. Arisawa,58A. Artikov,16J. Asaadi,54W. Ashmanskas,18 A. Attal,4A. Aurisano,54F. Azfar,43W. Badgett,18A. Barbaro-Galtieri,29V. E. Barnes,49B. A. Barnett,26P. Barria,47c,47a P. Bartos,15G. Bauer,33P.-H. Beauchemin,34F. Bedeschi,47aD. Beecher,31S. Behari,26G. Bellettini,47b,47aJ. Bellinger,60
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(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3University of Athens, 157 71 Athens, Greece
4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
5Baylor University, Waco, Texas 76798, USA
6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy
6bUniversity of Bologna, I-40127 Bologna, Italy
7Brandeis University, Waltham, Massachusetts 02254, USA
8University of California, Davis, Davis, California 95616, USA
9University of California, Los Angeles, Los Angeles, California 90024, USA
10University of California, San Diego, La Jolla, California 92093, USA
11University of California, Santa Barbara, Santa Barbara, California 93106, USA
12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
17Duke University, Durham, North Carolina 27708, USA
18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
19University of Florida, Gainesville, Florida 32611, USA
20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
21University of Geneva, CH-1211 Geneva 4, Switzerland
22Glasgow University, Glasgow G12 8QQ, United Kingdom
23Harvard University, Cambridge, Massachusetts 02138, USA
24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
191801-2
25University of Illinois, Urbana, Illinois 61801, USA
26The Johns Hopkins University, Baltimore, Maryland 21218, USA
27Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany
28Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea; Seoul National University, Seoul 151-742, Korea; Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon 305-806, Korea;
Chonnam National University, Gwangju 500-757, Korea;
Chonbuk National University, Jeonju 561-756, 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, Montre´al, Que´bec, Canada H3A 2T8; Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6; University of Toronto,
Toronto, Ontario, Canada M5S 1A7; and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3
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
44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44bUniversity of Padova, I-35131 Padova, Italy
45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy
47bUniversity of Pisa, I-56127 Pisa, Italy
47cUniversity of Siena, I-56127 Pisa, Italy
47dScuola 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
52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy
52bSapienza Universita` di Roma, I-00185 Roma, Italy
53Rutgers University, Piscataway, New Jersey 08855, USA
54Texas A&M University, College Station, Texas 77843, USA
55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste,
55bUniversity of Trieste/Udine, I-33100 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
62Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea; Seoul National University, Seoul 151-742, Korea; Sungkyunkwan University, Suwon 440-746, Korea; Korea Institute of Science and Technology Information,
College London Daejeon 305-806, Korea; Chonnam National University, Gwangju 500-757, Korea; Chonbuk National University, Jeonju 561-756, Korea
(Received 10 May 2010; published 5 November 2010)
We present a search for supersymmetric neutrino~production using the Tevatronpp collision data collected with the CDF II detector and corresponding to an integrated luminosity of1 fb1. We focus on the scenarios predicted by theR-parity violating (RPV) supersymmetric models in which sneutrinos decay to two charged leptons of different flavor. With the data consistent with the standard model expectations,
we set upper limits onðpp!Þ ~ BRð~!e; ; eÞand use these results to constrain the RPV couplings as a function of the sneutrino mass.
DOI:10.1103/PhysRevLett.105.191801 PACS numbers: 14.80.Ly, 12.60.Jv, 13.85.Qk, 14.60.St
Supersymmetric (SUSY) extensions of the standard model (SM) are among the leading candidates for a theory which can solve the hierarchy problem and provide a framework for unifying particle interactions [1]. Gauge- invariant and renormalizable interactions introduced in the SUSY models can violate the conservation of baryon (B) and lepton (L) number and lead to a proton lifetime shorter than the current experimental limits [2]. This problem is usually solved by postulating conservation of an additional quantum number,R-parityRp¼ ð1Þ3ðBLÞþ2s, wheresis the particle spin [3]. However, models with R-parity- violating (RPV) interactions conserving spin and eitherB or L can also avoid direct contradiction with the proton lifetime upper limits [4]. Such models have the advantage that they naturally introduce lepton-flavor violation and can generate nonzero neutrino masses and angles [5] con- sistent with neutrino-oscillation data [6]. They can also explain the recently reported anomalous phase of thebtos transition [7]. From an experimental standpoint, RPV in- teractions allow for single production of supersymmetric particles (sparticles) in high-energy particle collisions and for sparticles to decay directly into SM particles only; this makes the lightest sparticle unstable and critically affects the experimental strategy of the SUSY searches. Because of their clean final-state signatures, processes of single slepton production followed by decay to a pair of SM charged leptons become promising search channels for R-parity violating SUSY particles [8].
In this Letter we report a search for a heavy sneutrino, produced in quark-antiquarkddannihilation and decaying via lepton-flavor-violating interactions into e==e final states. The search is performed using data correspond- ing to an integrated luminosity of 1 fb1 collected inpp collisions at ffiffiffi
ps
¼1:96 TeVby the CDF II detector at the Tevatron. The results are analyzed in the framework of the minimal supersymmetric extensions of the SM [1], where the RPV part of the superpotential relevant to the sneutrino production and decay can be written as
WRPV¼0ijkLiQjdkþ12ijkLiLjek: (1) L andQin Eq. (1) are the SUð2Þ doublet superfields of leptons and quarks; eandd are the SUð2Þ singlet super- fields of leptons and quarks; 0 and are the Yukawa couplings at the production and decay vertex respectively;
the indicesi,j, andkdenote the fermion generations. We assume single-coupling dominance [9], heavy squarks, and the tau sneutrino ~ to be the lightest supersymmetric particle. The couplings0311 ¼0:10andi3k ¼0:05, com- patible with the current indirect limits [9], are chosen as a benchmark point. Heavy sneutrinos have been extensively
searched for at LEP [9]. Recently, searches for heavy sneutrinos decaying into the e final state have been performed by the CDF [10] and D0 collaborations [11].
The results in this Letter supersede [10]. This analysis also represents the first search for lepton-flavor-violating de- cays of heavy sneutrinos into final states involving a third generation lepton, the, at the Tevatron.
CDF II is a general-purpose particle detector [12]. This measurement uses information from the central tracker [13], calorimeters [14,15], and muon detectors [16] for charged lepton reconstruction and identification. Recons- truction of photons and0mesons makes extensive use of the central shower maximum detector (CES) [14]. The luminosity is measured by a hodoscopic system of Cherenkov counters [17]. The event geometry and kine- matics are described using the azimuthal angle around the beam line and the pseudorapidity ¼ ln tan2, where is the polar angle with respect to the beam line.
The transverse energy and momentum of the reconstructed particles are defined as:ET ¼Esin ,pT ¼psin , where Eis the energy andpis the momentum.
The data used in this measurement are collected using inclusive high-pT electron and muon triggers which select high-pT electron and muon candidates with jj&1:0. After event reconstruction, electron and muon candidates with pT 20 GeV=care identified using the procedures described in [18]. In addition we use independent mea- surements of the electron energy in the CES to improve the overall electron selection efficiency and identification of electron candidates radiating significant energy due to the bremsstrahlung. Theleptons are identified via their had- ronic decays as narrow calorimeter clusters associated with one or three charged tracks [19]. As the neutrino from the decay escapes detection, the ‘‘visible’’ four-momentum of a candidate, pvis , is reconstructed summing the four- momenta of charged particle tracks and neutral particle CES showers with a pion mass hypotheses. The resolution inpvis is further improved by combining measurements of the track momenta and energies of the CES showers with the energy measurements in the calorimeter. A recon- structed candidate is required to have the visible trans- verse energy, EvisT , greater than 25 GeV and its most energetic track must havepT>10 GeV=c. The invariant mass of its decay products,Mvis¼ ffiffiffiffiffiffiffiffiffiffi
pvis2
p , is required to be consistent with the lepton decay: Mvis <ð1:8þ 0:0455ðEvisT =GeV20ÞÞGeV=c2, where the second term in the formula accounts for a degradation of the resolution inMvis at high energy.
Events selected for the analysis are required to have two identified central (jj<1) lepton candidates of different 191801-4
flavor and opposite electric charge. The leptons have to be isolated: the extra energy measured within a cone of radius R0:4surrounding the leptons must be less than 10%
of the lepton energy. Events with leptons consistent with a photon conversion or a cosmic ray hypothesis are removed from the analysis sample [18].
Signal and background studies are performed using Monte Carlo (MC) samples generated byPYTHIA6.2 [20]
with the Tune A of CTEQ5L parton distribution func- tions [21]. The detector response is simulated with a
GEANT3-based package [22]. The trigger, reconstruction, and identification efficiencies are measured usingZevents as calibration samples [18].
The predicted yield of signal events is calculated using the next-to-leading order (NLO) pp !~ production cross section [23]. The total ~ width is defined by the dd and lilk decay modes as ~ ¼ ð302311þ 22i3kÞM~=16, whereM~ is the~mass.
There are several sources of background events; they are classified based on whether the lepton candidates origi- nated from a ‘‘real’’ lepton (produced from aWorZdecay) or were a result of a hadron being misidentified as a lepton, lepton-flavor misassignment, or a secondary lepton inside a jet. We collectively refer to the lepton candidates of the second category as ‘‘fakes’’ and classify each contributing background process into type I, II, and III according to the typical number of real and fake leptons reconstructed.
Type I contains events with two real leptons and includes Z=!, diboson (WW,WZ,ZZ) andttevents. Type I is therefore called physics background. Type II includes events with one reconstructed fake lepton. They come from either (i) the W=Z=þjetðsÞ events where one of the reconstructed leptons is in fact a jet misidentified as a lepton or (ii) Z= !ee= events with one of the leptons misidentified as a lepton of a different flavor. The backgrounds in type I and II are estimated using MC simulations, and their expected event yields are normalized to the NLO cross sections [24–26]. Events with two fake leptons (type III) are dominated by multijet events with two jets misidentified as leptons andþjets events; in the latter case, a converted photon is not identified as such and gets reconstructed as an electron and a jet is misidentified as aor a. The contribution of the processes in type III is estimated using a data sample with two leptons of the
same-charge and assuming no charge correlation between the two misidentified leptons. Both type II and III are called fake background.
The systematic uncertainties in this search arise from a number of sources. The uncertainty on the luminosity mea- surement is 6% [27]. Uncertainties on lepton identification efficiency are 3% for ’s, 1% for electrons, and 1% for muons [18]. The jet-to- misidentification probability TABLE I. The observed and predicted event yields in the
control region. Uncertainties on the predicted yields include both statistical and systematic contributions.
Control region 50 GeV=c2< Mll<110 GeV=c2
Channel e e
Physics backgrounds 100:26:5 262:421:0 309:624:7 Fake backgrounds 9:43:3 222:631:7 577:837:0 Total SM 109:67:7 485:040:9 887:451:0
Observed 105 477 894
2) (GeV/c µ Invariant Mass of e
100 200 300 400 500 600 700 )2Events/(10 GeV/c
10-2
10-1
1 10 102
103
2) (GeV/c µ Invariant Mass of e
100 200 300 400 500 600 700 )2Events/(10 GeV/c
10-2
10-1
1 10 102
103
) signal (500 GeV/c2
ντ
∼
=0.05 λ132
’=0.10, λ311
Data
Physics Backgrounds Fake Backgrounds
2) (GeV/c τ µ Visible Mass of
100 200 300 400 500 600 700 )2Events/(10 GeV/c
10-2
10-1
1 10 102
2) (GeV/c τ µ Visible Mass of
100 200 300 400 500 600 700 )2Events/(10 GeV/c
10-2
10-1
1 10 102
) signal (500 GeV/c2
ντ
∼
=0.05 λ233
’=0.10, λ311
Data
Physics Backgrounds Fake Backgrounds
2) (GeV/c τ Visible Mass of e
100 200 300 400 500 600 700 )2Events/(10 GeV/c
10-2
10-1
1 10 102
2) (GeV/c τ Visible Mass of e
100 200 300 400 500 600 700 )2Events/(10 GeV/c
10-2
10-1
1 10 102
) signal (500 GeV/c2
ντ
∼
=0.05 λ133
’=0.10, λ311
Data
Physics Backgrounds Fake Backgrounds
FIG. 1 (color online). Expected SM and observed distributions inMllfore,andechannels. Also shown is an expected~
signal for M~¼500 GeV=c2and RPV couplings0311¼0:10 andi3k¼0:05.
measured in jet triggered data is the same as the probability in MC simulations within 15%. Uncertainties in the parton distribution functions (PDF) result in the systematic error on the predicted signal cross section, which varies from 4%
to 20% and increases with the~mass. Variations of the signal acceptance due to PDF uncertainties are less than 1%.
We search for a signal from~decays into lepton pairs in the distributions for dilepton invariant mass, Mll (the visible energy is used in case of the ). The low mass region,50 GeV=c2< Mll<110 GeV=c2, is used to vali- date the event selection and the background normalization.
The observed and expected event yields in this region are in good agreement, as summarized in TableI. The search is performed as a ‘‘blind’’ counting experiment in the region
Mll>110 GeV=c2. Figure 1compares data distributions in Mll to the SM expectations for each of the three chan- nels. With no statistically significant excesses observed, we use the data to set upper limits on ðpp !~Þ BRð~!e==eÞ.
In each of the channels (e,,e), the expected and observed upper limits are calculated using a Bayesian technique [28] at 95% credibility level (C.L.) as a function of M~. For a given M~, the limits are calculated by integrating the differential cross section d=dMll over the region Mll> Mminll , where Mminll optimizes the search sensitivity for a selectedM~. The search results forM~ ¼ 500 GeV=c2 are summarized in TableII. Figure2shows the expected and observed 95% C.L. upper limits on ðpp !~Þ BRð~!e==eÞ as a function of M~. We also set 95% C.L. upper limits on 02311 BRð~!e==eÞas shown in TableIII.
In conclusion, we have searched for production of a massive sneutrino decaying to e, , ore final states via R-parity violating interactions. We find the data con- sistent with the SM predictions and calculate the 95% C.L.
upper limits on theðpp !~Þ BRð~!e==eÞ in the mass range up to 800 GeV=c2. Using these cross section limits, we constrain02311BRð~!e==eÞ as a function of M~. This analysis sets the first Tevatron limits for lepton-flavor violating decays of heavy sneutri- nos into final states involving a third generation lepton. For the RPV couplings 0311¼0:10 and i3k¼0:05 the ob- served 95% C.L. lower limits on~mass are558 GeV=c2 in the e channel, 441 GeV=c2 in the channel, and 442 GeV=c2 in theechannel. In the paper we explicitly refer to~. We note that the analysis is flavour-independent and limits on theðpp!~e;Þ BRð~e;!e==eÞ are the same as presented in Table IIIand Fig.2.
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 Bundesministerium fu¨r Bildung und Forschung, , and e channels. The expected yields of ~ events are
calculated for M~¼500 GeV=c2 and RPV couplings 0311¼ 0:10andi3k¼0:05.
Channel e e
MllminðGeV=c2Þ 440 300 310 Physics backgrounds 0:030:01 0:10:02 0:20:03 Fake backgrounds 0:010:01 0:30:1 0:60:1 Total SM background 0:040:01 0:40:1 0:90:1 Expected signal 5:90:1 2:00:1 2:70:1
Observed 0 0 2
10-2
10-1
1
10 µ
→ e ν∼τ
→ p N.L.O. p
=0.05 λ132
’=0.10, λ311
Observed Expected
σ 1
10-2
10-1
1
10 →ν∼τ→µτ
p N.L.O. p
=0.05 λ233
’=0.10, λ311
Observed Expected
σ 1
100 200 300 400 500 600 700 800 10-2
10-1
1
10 N.L.O. pp→ν∼τ=0.05→ eτ λ133
’=0.10, λ311
Observed Expected
σ 1
BR (pb)×)τν∼→p (pσ
2) Mass (GeV/c ν∼τ
FIG. 2 (color online). The expected and observed 95% C.L.
upper limits onðpp!~Þ BRð~!e==eÞas a func- tion ofM~.
e==eÞ. 311
M~ðGeV=c2Þ e e
300 6105 4104 5104
400 2104 1103 9104
500 7104 2103 3103
600 2103 7103 5103
700 8103 2102 2102
191801-6
Germany; the World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, UK;
the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; and the Academy of Finland.
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