Article
Reference
Search for Excited and Exotic Muons in the μγ Decay Channel in pp Collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We search for excited and exotic muon states μ∗ using an integrated luminosity of 371 pb−1 of pp collision data at s√=1.96 TeV. We search for associated production of μμ∗ followed by the decay μ∗→μγ. We compare the data to model predictions as a function of the mass of the excited muon Mμ∗, the compositeness energy scale Λ, and the gauge coupling factor f. No signal above the standard model expectation is observed. We exclude 107
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for Excited and Exotic Muons in the μγ Decay Channel in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2006, vol.
97, no. 19, p. 191802
DOI : 10.1103/PhysRevLett.97.191802
Available at:
http://archive-ouverte.unige.ch/unige:38362
Disclaimer: layout of this document may differ from the published version.
1 / 1
Search for Excited and Exotic Muons in the Decay Channel in p p Collisions at p s
1:96 TeV
A. Abulencia,23D. Acosta,17J. Adelman,13T. Affolder,10T. Akimoto,55M. G. Albrow,16D. Ambrose,16S. Amerio,43 D. Amidei,34A. Anastassov,52K. Anikeev,16A. Annovi,18J. Antos,1M. Aoki,55G. Apollinari,16J.-F. Arguin,33 T. Arisawa,57A. Artikov,14W. Ashmanskas,16A. Attal,8F. Azfar,42P. Azzi-Bacchetta,43P. Azzurri,46N. Bacchetta,43 H. Bachacou,28W. Badgett,16A. Barbaro-Galtieri,28V. E. Barnes,48B. A. Barnett,24S. Baroiant,7V. Bartsch,30G. Bauer,32
F. Bedeschi,46S. Behari,24S. Belforte,54G. Bellettini,46J. Bellinger,59A. Belloni,32E. Ben Haim,44D. Benjamin,15 A. Beretvas,16J. Beringer,28T. Berry,29A. Bhatti,50M. Binkley,16D. Bisello,43R. E. Blair,2C. Blocker,6B. Blumenfeld,24 A. Bocci,15A. Bodek,49V. Boisvert,49G. Bolla,48A. Bolshov,32D. Bortoletto,48J. Boudreau,47A. Boveia,10B. Brau,10
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T. Rodrigo,11E. Rogers,23S. Rolli,56R. Roser,16M. Rossi,54R. Rossin,17C. Rott,48A. Ruiz,11J. Russ,12V. Rusu,13 H. Saarikko,22S. Sabik,33A. Safonov,53W. K. Sakumoto,49G. Salamanna,51O. Salto´,3D. Saltzberg,8C. Sanchez,3
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T. Vine,30I. Vollrath,33I. Volobouev,28G. Volpi,46F. Wu¨rthwein,9P. Wagner,53R. G. Wagner,2R. L. Wagner,16 W. Wagner,25R. Wallny,8T. Walter,25Z. Wan,52S. M. Wang,1A. Warburton,33S. Waschke,20D. Waters,30 W. C. Wester III,16B. Whitehouse,56D. Whiteson,45A. B. Wicklund,2E. Wicklund,16G. Williams,33H. H. Williams,45
P. Wilson,16B. L. Winer,39P. Wittich,16S. Wolbers,16C. Wolfe,13T. Wright,34X. Wu,19S. M. Wynne,29A. Yagil,16 K. Yamamoto,41J. Yamaoka,52T. Yamashita,40C. Yang,60U. K. Yang,13Y. C. Yang,27W. M. Yao,28G. P. Yeh,16J. Yoh,16
K. Yorita,13T. Yoshida,41G. B. Yu,49I. Yu,27S. S. Yu,16J. C. Yun,16L. Zanello,51A. Zanetti,54I. Zaw,21F. Zetti,46 X. Zhang,23J. Zhou,52and S. Zucchelli5
(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
14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
15Duke University, Durham, North Carolina 27708, USA
16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
17University of Florida, Gainesville, Florida 32611, USA
18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
19University of Geneva, CH-1211 Geneva 4, Switzerland
20Glasgow University, Glasgow G12 8QQ, United Kingdom
21Harvard University, Cambridge, Massachusetts 02138, USA
22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
191802-2
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
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, Korea;
Seoul National University, Seoul 151-742, Korea;
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
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8; and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109, USA
35Michigan State University, East Lansing, Michigan 48824, USA
36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
37University of New Mexico, Albuquerque, New Mexico 87131, USA
38Northwestern University, Evanston, Illinois 60208, USA
39The Ohio State University, Columbus, Ohio 43210, USA
40Okayama University, Okayama 700-8530, Japan
41Osaka City University, Osaka 588, Japan
42University of Oxford, Oxford OX1 3RH, United Kingdom
43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
48Purdue University, West Lafayette, Indiana 47907, USA
49University of Rochester, Rochester, New York 14627, USA
50The Rockefeller University, New York, New York 10021, USA
51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
52Rutgers University, Piscataway, New Jersey 08855, USA
53Texas A&M University, College Station, Texas 77843, USA
54Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
55University of Tsukuba, Tsukuba, Ibaraki 305, Japan
56Tufts University, Medford, Massachusetts 02155, USA
57Waseda University, Tokyo 169, Japan
58Wayne State University, Detroit, Michigan 48201, USA
59University of Wisconsin, Madison, Wisconsin 53706, USA
60Yale University, New Haven, Connecticut 06520, USA (Received 19 June 2006; published 9 November 2006)
We search for excited and exotic muon states using an integrated luminosity of371 pb1 ofpp collision data atps
1:96 TeV. We search for associated production of followed by the decay !. We compare the data to model predictions as a function of the mass of the excited muonM, the compositeness energy scale, and the gauge coupling factorf. No signal above the standard model expectation is observed. We exclude107< M<853 GeV=c2 forMin the contact interaction model, and100< M<410 GeV=c2forf=102 GeV1in the gauge-mediated model, both at the 95% confidence level.
DOI:10.1103/PhysRevLett.97.191802 PACS numbers: 12.60.Rc, 13.85.Qk, 14.60.Hi
In the standard model (SM) the quarks and leptons are treated as fundamental particles. Their generational struc- ture and observed mass hierarchy motivate a model of composite quarks and leptons consisting of fewer elemen- tary particles than contained in the SM [1]. In this model, quarks and leptons are the lowest-energy bound states of these hypothetical particles, and additional excited states exist near the compositeness energy scale.
Exotic fermions are also predicted in the context of grand unified or string theories, in which the known forces are unified into a larger symmetry group [2]. In such models, additional fermions are predicted with properties similar to those of excited fermions.
At app collider, excited or exotic muon states could be observed through the reaction qq !. Excited muon production can be described using a contact interaction
(CI) Lagrangian density [1]:
L4
2qLqLMLLH:c:;
where ML represents the left-handed field, and right- handed currents have been neglected for simplicity. For exotic muon production, the relevant gauge-mediated (GM) Lagrangian density is [3]:
L 1
2MRNR
fg~
2W~f0g0Y 2B
L
L
H:c:;
where NR is the excited neutrino field, W and B are the SU2LandU1Y field strengths,gandg0are the respec- tive electroweak couplings, andf andf0 are phenomeno- logical constants which are set equal to each other by convention to maximize the photonic decay branching ratio. For ff0 (f f0), the relative branching
ratios are BR! 0:3 (0), BR!W
0:6 (0:6), and BR !Z 0:1 (0:4) for M>
200 GeV=c2. The BR! increases to 70% at M 100 GeV=c2 for ff0. We use these branching ratios for both the GM and CI production models [4], and we also quote the CI model result with the branching ratio corrected for CI decays.
In this Letter we describe the first hadron-collider search for associated production in the context of the GM model, and extend existing mass limits in both the GM and CI models. Prior searches for production have been performed by the LEP experiments, which have excluded with M<200 GeV=c2 for f=>102 GeV1 in the GM model [5]. The D0 experiment has excluded withM<688 GeV=c2 for M in a particular CI model [6]. The most recent measurement of (g2) [7]
sets an indirect lower limit on M of O400 GeV=c2, assuming chiral symmetry of the excited state [8].
We use 371 pb1 of data collected with the CDF II detector [9] at the Fermilab Tevatron. The detector’s mag- netic spectrometer consists of silicon microstrip and drift- chamber [10] tracking detectors. Surrounding this are cen- tral [11] (jj<1[12]) and forward [13] (jj>1:1) elec- tromagnetic (EM) and hadronic calorimeters. Embedded in the central EM calorimeter are wire and strip chambers [9]
(used to measure the transverse shower profiles of photons) and a central preshower detector [9] (used for detecting photon conversions). Outside the calorimeters are CMUP (jj<0:6) and CMX (0:6<jj<1) muon detectors [14].
The momentum resolution of beam-constrained drift- chamber tracks ispT=p2T 0:05%=GeV=c. The elec- tromagnetic energy resolution for photons fromdecays is2:5%.
We analyze events passing the trigger requirement of one drift-chamber track with pT>18 GeV=c [12]
matched to a reconstructed track segment in the muon
chambers. In the offline analysis, we require two muon candidates identified by drift-chamber tracks with pT>
20 GeV=cand jj<1, which pass requirements on im- pact parameter and number of hits, have minimum- ionizing particle properties, and at least one of which has a matching muon chamber segment [15]. Both tracks must pass isolation requirements based on calorimetric and tracking energy flow in their vicinity. Finally, we reject cosmic rays based on tracking and track-timing informa- tion [16].
We select dimuon events that have a photon candidate with ET>25 GeV and jj<2:8. Photons are identified by their longitudinal and transverse calorimeter shower profiles, and by the lack of tracks and calorimeter energy in their vicinity [4]. To suppress the initial-state radiation (ISR) Z! background, we reject events with di- muon invariant massmin the range81–101 GeV=c2.
AZ!sample is used to measure the efficiencies of the muon identification criteria and trigger. The efficiency of the calorimeter and tracking identification requirements is measured to be92:6 0:3stat%. We measure the com- bined trigger and muon chamber matching efficiency to be 79:3 1:0stat%for the CMUP and 95:0 0:6stat% for the CMX.
The photon identification efficiency is extracted from a
GEANT-based detector simulation [17]. Since photons and electrons have similar electromagnetic showers, we vali- date the simulated photon efficiency using a control sample of electrons fromZ!eeevents [4].
The geometric acceptance is calculated with theGEANT
detector simulation separately for the CI and GM models.
We use thePYTHIA[18] generator for the CI model and the
LANHEP[19] andCOMPHEP[20] programs to generate GM model events. The two models generate similar kinematic and angular distributions for the final-state particles. The total signal acceptance (including identification efficien- cies) for the CI (GM) model increases from 13% (12%) at M 100 GeV=c2to an asymptotic value of 21% (23%) forM>400300 GeV=c2. The relative systematic un- certainty on the acceptance is 3.1%, which is dominated by the uncertainty in the identification efficiency (2.2%) and simulation statistics (2.0%).
We compute the expected background contributions from the following sources: (1) Z= !, (2) Z= !, where the ’s decay to muons, (3) Z=!jet, where the jet is misidentified as a photon, (4)t!b t! b, where a fermion radi- ates a photon, (5)W!e Z!, where the elec- tron is misidentified as a photon, and (6) Z!ee Z!, where one of the electrons is misidentified as a photon. Other backgrounds (3 jets,W 2jets, W 1 jet, and cosmic rays) were found to be negligible.
The Z, tt, WZ, and ZZ backgrounds are estimated using simulated events, with the ZGAMMA [21] generator 191802-4
for theZ!background andPYTHIAfor the others.
Systematic uncertainties on these background predictions arise due to integrated luminosity (6%) [22], parton distri- bution functions (PDFs) (5%), higher-order QCD correc- tions (5%) [23], acceptance (1%), and identification efficiencies (2%).
TheZjet background is estimated by weightingZ jet events from the data by anET-dependent jet!mis- identification rate. The latter is measured using a jet- triggered data sample, correcting for the fraction of true prompt photons in the jet sample [4]. The prompt photon fraction is estimated using !eeconversions identified with the calorimeter preshower detector [24]. The jet! misidentification rate is applied as a function ofET in the central calorimeter and as a function ofET andin the forward calorimeter.
We observe 17 signal candidates with a background prediction of 8:3 0:9, of which 8:1 0:8are expected fromZproduction. The Poisson probability for the back-
ground to fluctuate up to the observation, or higher, is 0.8%. Eleven events have a 3-body mass m in the 81–101 GeV=c2 range, consistent with being final-state radiation (FSR)Z! events, to be compared to the FSR Z! prediction of 5:5 0:5 events. Figure 1 and Table Ishow them andmdistributions for the data and background.
To test our background prediction, we lower the mini- mum photon ET to 15 GeV and observe 43 events with a prediction of38:5 4:0events. The data show good agree- ment with expectation in this higher statistics sample, as shown in Fig.2. Additionally, we find consistency in the ISR Z control region of 81< m<101 GeV=c2 and 25< ET <50 GeV, where we observe 5 events with a prediction of7:21:20:8 events. Finally, we find our candidate sample to be stable under variations of the muon selection criteria. We conclude that our signal sample has an upward statistical fluctuation, dominantly in the number of FSR Z!events.
For theresonance search, we scan themspectrum with a sliding window of width3, where is the mass width predicted by the simulation. Over almost the entire model parameter space, is dominated by detector reso- lution. The tracker momentum scale and resolution, and the calorimeter energy scale and resolution, are tuned on the well-known Z! and Z!ee mass peaks [25], respectively. For M , the reconstructed mass resolution ranges from 9–90 GeV=c2 for masses ranging from200–800 GeV=c2.
TABLE I. Comparison of data and integrated background predictions above a given cut on the invariant mass of allcombinations (left) and on theinvariant mass (right).
combinations Events
m (GeV=c2) Data Background m(GeV=c2) Data Background
>0 34 16:6 1:8 >0 17 8:3 0:9
>50 22 10:4 1:1 >100 6 2:7 0:3
>100 4 2:1 0:3 >150 2 1:5 0:2
>150 2 0:89 0:14 >200 2 0:9 0:1
>200 0 0:37 0:07 >250 1 0:51 0:09
>300 0 0:29 0:06
2) (GeV/c
γ
Mµ 2 Entries / 20 GeV/c 10-4
10-3 10-2 10-1 1 10
(a)
0 100 200 300 400 500
Data
γ τ) + τ
→ µ, µ
→ Z ( Z+jet
t WZ+ZZ+t
2) (GeV/c
γ
Mµ 2 Entries / 20 GeV/c 10-4
10-3 10-2 10-1 1 10
2) (GeV/c
γ
Mµ 2 Entries / 20 GeV/c 10-4
10-3 10-2 10-1 1 10
2) (GeV/c
γ µ
Mµ
50 100 150 200 250 300 350 400
2 Events / 20 GeV/c 10-3
10-2 10-1 1 10
Data
γ τ) + τ
→ µ, µ
→ Z ( Z+jet
t WZ+ZZ+t
(b)
2) (GeV/c
γ µ
Mµ
50 100 150 200 250 300 350 400
2 Events / 20 GeV/c 10-3
10-2 10-1 1 10
2) (GeV/c
γ µ
Mµ
50 100 150 200 250 300 350 400
2 Events / 20 GeV/c 10-3
10-2 10-1 1 10
FIG. 1. The (a) and (b) mass distributions for the data with background expectations. The total number of ob- served (expected) entries is 17 (8:3 0:9). Both combinations per event are included in (a).
(GeV)
γ
ET
Events / 10 GeV
10-2 10-1 1 10
15 25 35 45 55 65 75 85 95
Data
γ τ) + τ
→ µ, µ
→ Z ( Z+jet
t WZ + ZZ + t
(GeV)
γ
ET
Events / 10 GeV
10-2 10-1 1 10
FIG. 2. The ET distributions for the data (43 events) and background (38:5 4:0events) expectations forET>15 GeV.
The arrow indicates the minimumETrequired for this analysis.
We use a Bayesian [26] approach, with a flat prior on the signal cross section and gamma priors on acceptance and backgrounds, to set limits on the production cross section as a function ofM[27]. The cross section limits are converted toMlimits by comparing them to the next- to-next-to-leading-order (NNLO) theoretical cross sections [23], computed using the MRST set of PDFs [28]. We use the CTEQ prescription [29] to calculate the cross section uncertainty due to PDFs, which varies from 5% (M 100 GeV=c2) to 20% (M 1 TeV=c2). Uncertainties on higher-order QCD corrections (7% –13%) depend on M and the production model.
The 95% confidence level (C.L.)upper limits on the experimental cross sectionBR are shown in Fig. 3,
along with the theoretical curves forM =f(M ) for the GM (CI) model. For both models, we lose sensitivity for M<100 GeV=c2 due to large back- grounds and loss of signal acceptance. In our region of sensitivity, masses below 221 GeV=c2 (853 GeV=c2) are excluded for the GM (CI) model. The CI exclusion reduces to M<696 GeV=c2 if we use branching ratios that account for hypothetical CI decays, as assumed by the D0 collaboration [6]. Figure4shows the limits in the parame- ter space of f= (M=) versus M for the GM (CI) model. These are the world’s strongest limits over much of the parameter space and complement our recent results of a search for excited and exotic electrons [4].
We are grateful to Alejandro Daleo for providing NNLO cross section calculations. 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, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Comisio´n Interministerial 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.
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