Search for excited muons in p
p collisions at
p
s
1:96 TeV
V. M. Abazov,36B. Abbott,76M. Abolins,66B. S. Acharya,29M. Adams,52T. Adams,50M. Agelou,18J.-L. Agram,19 S. H. Ahn,31M. Ahsan,60G. D. Alexeev,36G. Alkhazov,40A. Alton,65G. Alverson,64G. A. Alves,2M. Anastasoaie,35
T. Andeen,54S. Anderson,46B. Andrieu,17M. S. Anzelc,54Y. Arnoud,14M. Arov,53A. Askew,50B. A˚ sman,41 A. C. S. Assis Jesus,3O. Atramentov,58C. Autermann,21C. Avila,8C. Ay,24F. Badaud,13A. Baden,62L. Bagby,53 B. Baldin,51D. V. Bandurin,36P. Banerjee,29S. Banerjee,29E. Barberis,64P. Bargassa,81P. Baringer,59C. Barnes,44 J. Barreto,2J. F. Bartlett,51U. Bassler,17D. Bauer,44A. Bean,59M. Begalli,3M. Begel,72C. Belanger-Champagne,5 A. Bellavance,68J. A. Benitez,66S. B. Beri,27G. Bernardi,17R. Bernhard,42L. Berntzon,15I. Bertram,43M. Besanc¸on,18 R. Beuselinck,44V. A. Bezzubov,39P. C. Bhat,51V. Bhatnagar,27M. Binder,25C. Biscarat,43K. M. Black,63I. Blackler,44 G. Blazey,53F. Blekman,44S. Blessing,50D. Bloch,19K. Bloom,68U. Blumenschein,23A. Boehnlein,51O. Boeriu,56 T. A. Bolton,60F. Borcherding,51G. Borissov,43K. Bos,34T. Bose,78A. Brandt,79R. Brock,66G. Brooijmans,71A. Bross,51
D. Brown,79N. J. Buchanan,50D. Buchholz,54M. Buehler,82V. Buescher,23S. Burdin,51S. Burke,46T. H. Burnett,83 E. Busato,17C. P. Buszello,44J. M. Butler,63S. Calvet,15J. Cammin,72S. Caron,34W. Carvalho,3B. C. K. Casey,78 N. M. Cason,56H. Castilla-Valdez,33S. Chakrabarti,29D. Chakraborty,53K. M. Chan,72A. Chandra,49D. Chapin,78 F. Charles,19E. Cheu,46F. Chevallier,14D. K. Cho,63S. Choi,32B. Choudhary,28L. Christofek,59D. Claes,68B. Cle´ment,19 C. Cle´ment,41Y. Coadou,5J. Coenen,21M. Cooke,81W. E. Cooper,51D. Coppage,59M. Corcoran,81M.-C. Cousinou,15 B. Cox,45S. Cre´pe´-Renaudin,14D. Cutts,78M. C´ wiok,30H. da Motta,2A. Das,63M. Das,61B. Davies,43G. Davies,44 G. A. Davis,54K. De,79P. de Jong,34S. J. de Jong,35E. De La Cruz-Burelo,65C. De Oliveira Martins,3J. D. Degenhardt,65
F. De´liot,18M. Demarteau,51R. Demina,72P. Demine,18D. Denisov,51S. P. Denisov,39S. Desai,73H. T. Diehl,51 M. Diesburg,51M. Doidge,43A. Dominguez,68H. Dong,73L. V. Dudko,38L. Duflot,16S. R. Dugad,29A. Duperrin,15 J. Dyer,66A. Dyshkant,53M. Eads,68D. Edmunds,66T. Edwards,45J. Ellison,49J. Elmsheuser,25V. D. Elvira,51S. Eno,62
P. Ermolov,38J. Estrada,51H. Evans,55A. Evdokimov,37V. N. Evdokimov,39S. N. Fatakia,63L. Feligioni,63 A. V. Ferapontov,60T. Ferbel,72F. Fiedler,25F. Filthaut,35W. Fisher,51H. E. Fisk,51I. Fleck,23M. Ford,45M. Fortner,53
H. Fox,23S. Fu,51S. Fuess,51T. Gadfort,83C. F. Galea,35E. Gallas,51E. Galyaev,56C. Garcia,72A. Garcia-Bellido,83 J. Gardner,59V. Gavrilov,37A. Gay,19P. Gay,13D. Gele´,19R. Gelhaus,49C. E. Gerber,52Y. Gershtein,50D. Gillberg,5 G. Ginther,72N. Gollub,41B. Go´mez,8K. Gounder,51A. Goussiou,56P. D. Grannis,73H. Greenlee,51Z. D. Greenwood,61
E. M. Gregores,4G. Grenier,20Ph. Gris,13J.-F. Grivaz,16S. Gru¨nendahl,51M. W. Gru¨newald,30F. Guo,73J. Guo,73 G. Gutierrez,51P. Gutierrez,76A. Haas,71N. J. Hadley,62P. Haefner,25S. Hagopian,50J. Haley,69I. Hall,76R. E. Hall,48
L. Han,7K. Hanagaki,51K. Harder,60A. Harel,72R. Harrington,64J. M. Hauptman,58R. Hauser,66J. Hays,54 T. Hebbeker,21D. Hedin,53J. G. Hegeman,34J. M. Heinmiller,52A. P. Heinson,49U. Heintz,63C. Hensel,59G. Hesketh,64
M. D. Hildreth,56R. Hirosky,82J. D. Hobbs,73B. Hoeneisen,12M. Hohlfeld,16S. J. Hong,31R. Hooper,78P. Houben,34 Y. Hu,73V. Hynek,9I. Iashvili,70R. Illingworth,51A. S. Ito,51S. Jabeen,63M. Jaffre´,16S. Jain,76K. Jakobs,23C. Jarvis,62 A. Jenkins,44R. Jesik,44K. Johns,46C. Johnson,71M. Johnson,51A. Jonckheere,51P. Jonsson,44A. Juste,51D. Ka¨fer,21
S. Kahn,74E. Kajfasz,15A. M. Kalinin,36J. M. Kalk,61J. R. Kalk,66S. Kappler,21D. Karmanov,38J. Kasper,63 I. Katsanos,71D. Kau,50R. Kaur,27R. Kehoe,80S. Kermiche,15S. Kesisoglou,78A. Khanov,77A. Kharchilava,70 Y. M. Kharzheev,36D. Khatidze,71H. Kim,79T. J. Kim,31M. H. Kirby,35B. Klima,51J. M. Kohli,27J.-P. Konrath,23
M. Kopal,76V. M. Korablev,39J. Kotcher,74B. Kothari,71A. Koubarovsky,38A. V. Kozelov,39J. Kozminski,66 A. Kryemadhi,82S. Krzywdzinski,51T. Kuhl,24A. Kumar,70S. Kunori,62A. Kupco,11T. Kurcˇa,20,* J. Kvita,9S. Lager,41
S. Lammers,71G. Landsberg,78J. Lazoflores,50A.-C. Le Bihan,19P. Lebrun,20W. M. Lee,53A. Leflat,38F. Lehner,42 C. Leonidopoulos,71V. Lesne,13J. Leveque,46P. Lewis,44J. Li,79Q. Z. Li,51J. G. R. Lima,53D. Lincoln,51J. Linnemann,66 V. V. Lipaev,39R. Lipton,51Z. Liu,5L. Lobo,44A. Lobodenko,40M. Lokajicek,11A. Lounis,19P. Love,43H. J. Lubatti,83 M. Lynker,56A. L. Lyon,51A. K. A. Maciel,2R. J. Madaras,47P. Ma¨ttig,26C. Magass,21A. Magerkurth,65A.-M. Magnan,14 N. Makovec,16P. K. Mal,56H. B. Malbouisson,3S. Malik,68V. L. Malyshev,36H. S. Mao,6Y. Maravin,60M. Martens,51
S. E. K. Mattingly,78R. McCarthy,73R. McCroskey,46D. Meder,24A. Melnitchouk,67A. Mendes,15L. Mendoza,8 M. Merkin,38K. W. Merritt,51A. Meyer,21J. Meyer,22M. Michaut,18H. Miettinen,81T. Millet,20J. Mitrevski,71J. Molina,3
N. K. Mondal,29J. Monk,45R. W. Moore,5T. Moulik,59G. S. Muanza,16M. Mulders,51M. Mulhearn,71L. Mundim,3 Y. D. Mutaf,73E. Nagy,15M. Naimuddin,28M. Narain,63N. A. Naumann,35H. A. Neal,65J. P. Negret,8S. Nelson,50 P. Neustroev,40C. Noeding,23A. Nomerotski,51S. F. Novaes,4T. Nunnemann,25V. O’Dell,51D. C. O’Neil,5G. Obrant,40 V. Oguri,3N. Oliveira,3N. Oshima,51R. Otec,10G. J. Otero y Garzo´n,52M. Owen,45P. Padley,81N. Parashar,57S.-J. Park,72 S. K. Park,31J. Parsons,71R. Partridge,78N. Parua,73A. Patwa,74G. Pawloski,81P. M. Perea,49E. Perez,18K. Peters,45
P. Pe´troff,16M. Petteni,44R. Piegaia,1M.-A. Pleier,22P. L. M. Podesta-Lerma,33V. M. Podstavkov,51Y. Pogorelov,56 M.-E. Pol,2A. Pomposˇ,76B. G. Pope,66A. V. Popov,39W. L. Prado da Silva,3H. B. Prosper,50S. Protopopescu,74J. Qian,65
A. Quadt,22B. Quinn,67K. J. Rani,29K. Ranjan,28P. A. Rapidis,51P. N. Ratoff,43P. Renkel,80S. Reucroft,64 M. Rijssenbeek,73I. Ripp-Baudot,19F. Rizatdinova,77S. Robinson,44R. F. Rodrigues,3C. Royon,18P. Rubinov,51 R. Ruchti,56V. I. Rud,38G. Sajot,14A. Sa´nchez-Herna´ndez,33M. P. Sanders,62A. Santoro,3G. Savage,51L. Sawyer,61
T. Scanlon,44D. Schaile,25R. D. Schamberger,73Y. Scheglov,40H. Schellman,54P. Schieferdecker,25C. Schmitt,26 C. Schwanenberger,45A. Schwartzman,69R. Schwienhorst,66S. Sengupta,50H. Severini,76E. Shabalina,52M. Shamim,60 V. Shary,18A. A. Shchukin,39W. D. Shephard,56R. K. Shivpuri,28D. Shpakov,64V. Siccardi,19R. A. Sidwell,60V. Simak,10 V. Sirotenko,51P. Skubic,76P. Slattery,72R. P. Smith,51G. R. Snow,68J. Snow,75S. Snyder,74S. So¨ldner-Rembold,45
X. Song,53L. Sonnenschein,17A. Sopczak,43M. Sosebee,79K. Soustruznik,9M. Souza,2B. Spurlock,79J. Stark,14 J. Steele,61K. Stevenson,55V. Stolin,37A. Stone,52D. A. Stoyanova,39J. Strandberg,41M. A. Strang,70M. Strauss,76 R. Stro¨hmer,25D. Strom,54M. Strovink,47L. Stutte,51S. Sumowidagdo,50A. Sznajder,3M. Talby,15P. Tamburello,46 W. Taylor,5P. Telford,45J. Temple,46B. Tiller,25M. Titov,23V. V. Tokmenin,36M. Tomoto,51T. Toole,62I. Torchiani,23 S. Towers,43T. Trefzger,24S. Trincaz-Duvoid,17D. Tsybychev,73B. Tuchming,18C. Tully,69A. S. Turcot,45P. M. Tuts,71
R. Unalan,66L. Uvarov,40S. Uvarov,40S. Uzunyan,53B. Vachon,5P. J. van den Berg,34R. Van Kooten,55 W. M. van Leeuwen,34N. Varelas,52E. W. Varnes,46A. Vartapetian,79I. A. Vasilyev,39M. Vaupel,26P. Verdier,20
L. S. Vertogradov,36M. Verzocchi,51F. Villeneuve-Seguier,44P. Vint,44J.-R. Vlimant,17E. Von Toerne,60 M. Voutilainen,68,†M. Vreeswijk,34H. D. Wahl,50L. Wang,62J. Warchol,56G. Watts,83M. Wayne,56M. Weber,51 H. Weerts,66N. Wermes,22M. Wetstein,62A. White,79D. Wicke,26G. W. Wilson,59S. J. Wimpenny,49M. Wobisch,51 J. Womersley,51D. R. Wood,64T. R. Wyatt,45Y. Xie,78N. Xuan,56S. Yacoob,54R. Yamada,51M. Yan,62T. Yasuda,51
Y. A. Yatsunenko,36K. Yip,74H. D. Yoo,78S. W. Youn,54C. Yu,14J. Yu,79A. Yurkewicz,73A. Zatserklyaniy,53 C. Zeitnitz,26D. Zhang,51T. Zhao,83Z. Zhao,65B. Zhou,65J. Zhu,73M. Zielinski,72D. Zieminska,55A. Zieminski,55
V. Zutshi,53and E. G. Zverev38 (D0 Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil
3Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil 4Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil
5University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada, York University,
Toronto, Ontario, Canada, and McGill University, Montreal, Quebec, Canada
6Institute of High Energy Physics, Beijing, People’s Republic of China 7University of Science and Technology of China, Hefei, People’s Republic of China
8Universidad de los Andes, Bogota´, Colombia
9Center for Particle Physics, Charles University, Prague, Czech Republic 10Czech Technical University, Prague, Czech Republic
11Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 12Universidad San Francisco de Quito, Quito, Ecuador
13Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Universite´ Blaise Pascal, Clermont-Ferrand, France 14Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France
15CPPM, IN2P3-CNRS, Universite´ de la Me´diterrane´e, Marseille, France 16IN2P3-CNRS, Laboratoire de l’Acce´le´rateur Line´aire, Orsay, France
17LPNHE, IN2P3-CNRS, Universite´s Paris VI and VII, Paris, France 18DAPNIA/Service de Physique des Particules, CEA, Saclay, France
19IReS, IN2P3-CNRS, Universite´ Louis Pasteur, Strasbourg, France, and Universite´ de Haute Alsace, Mulhouse, France 20Institut de Physique Nucle´aire de Lyon, IN2P3-CNRS, Universite´ Claude Bernard, Villeurbanne, France
21III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany 22Physikalisches Institut, Universita¨t Bonn, Bonn, Germany 23Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany
24Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 25Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany 26Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
27Panjab University, Chandigarh, India 28Delhi University, Delhi, India
30University College Dublin, Dublin, Ireland 31Korea Detector Laboratory, Korea University, Seoul, Korea
32SungKyunKwan University, Suwon, Korea 33CINVESTAV, Mexico City, Mexico
34FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 35Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
36Joint Institute for Nuclear Research, Dubna, Russia 37Institute for Theoretical and Experimental Physics, Moscow, Russia
38Moscow State University, Moscow, Russia 39Institute for High Energy Physics, Protvino, Russia 40Petersburg Nuclear Physics Institute, St. Petersburg, Russia 41Lund University, Lund, Sweden, Royal Institute of Technology
and Stockholm University, Stockholm, Sweden, and Uppsala University, Uppsala, Sweden
42Physik Institut der Universita¨t Zu¨rich, Zu¨rich, Switzerland 43Lancaster University, Lancaster, United Kingdom
44Imperial College, London, United Kingdom 45University of Manchester, Manchester, United Kingdom
46University of Arizona, Tucson, Arizona 85721, USA 47Lawrence Berkeley National Laboratory
and University of California, Berkeley, California 94720, USA
48California State University, Fresno, California 93740, USA 49University of California, Riverside, California 92521, USA 50Florida State University, Tallahassee, Florida 32306, USA 51Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
52University of Illinois at Chicago, Chicago, Illinois 60607, USA 53Northern Illinois University, DeKalb, Illinois 60115, USA
54Northwestern University, Evanston, Illinois 60208, USA 55Indiana University, Bloomington, Indiana 47405, USA 56University of Notre Dame, Notre Dame, Indiana 46556, USA
57Purdue University Calumet, Hammond, Indiana 46323, USA 58Iowa State University, Ames, Iowa 50011, USA 59University of Kansas, Lawrence, Kansas 66045, USA 60Kansas State University, Manhattan, Kansas 66506, USA 61Louisiana Tech University, Ruston, Louisiana 71272, USA 62University of Maryland, College Park, Maryland 20742, USA
63Boston University, Boston, Massachusetts 02215, USA 64Northeastern University, Boston, Massachusetts 02115, USA
65University of Michigan, Ann Arbor, Michigan 48109, USA 66Michigan State University, East Lansing, Michigan 48824, USA
67University of Mississippi, University, Mississippi 38677, USA 68University of Nebraska, Lincoln, Nebraska 68588, USA 69Princeton University, Princeton, New Jersey 08544, USA 70State University of New York, Buffalo, New York 14260, USA
71Columbia University, New York, New York 10027, USA 72University of Rochester, Rochester, New York 14627, USA 73State University of New York, Stony Brook, New York 11794, USA
74Brookhaven National Laboratory, Upton, New York 11973, USA 75Langston University, Langston, Oklahoma 73050, USA 76University of Oklahoma, Norman, Oklahoma 73019, USA 77Oklahoma State University, Stillwater, Oklahoma 74078, USA
78Brown University, Providence, Rhode Island 02912, USA 79University of Texas, Arlington, Texas 76019, USA 80Southern Methodist University, Dallas, Texas 75275, USA
81Rice University, Houston, Texas 77005, USA 82University of Virginia, Charlottesville, Virginia 22901, USA
83University of Washington, Seattle, Washington 98195, USA
(Received 19 April 2006; published 2 June 2006)
*On leave from IEP SAS Kosice, Slovakia.
†Visitor from Helsinki Institute of Physics, Helsinki, Finland.
We present the results of a search for the production of an excited state of the muon, , in proton antiproton collisions atps 1:96 TeV. The data have been collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 380 pb1. We search for in the process p p ! , with the subsequently decaying to a muon plus photon. No excess above the standard model expectation is observed in data. Interpreting our data in the context of a model that describes production by four-fermion contact interactions and decay via electroweak processes, we set a 95% confidence level production cross section upper limit ranging from 0.057 to 0.112 pb, depending on the mass of the excited muon. Choosing the scale for contact interactions to be 1 TeV, excited muon masses below 618 GeV are excluded.
DOI:10.1103/PhysRevD.73.111102 PACS numbers: 12.60.Rc, 12.60.i, 13.85.Rm, 14.60.Hi
An open question in particle physics is the observed mass hierarchy of the quark and lepton SU(2) doublets in the standard model (SM). A commonly proposed explana-tion for the three generaexplana-tions is a compositeness model [1] of the known leptons and quarks. According to this ap-proach, a quark or lepton is a bound state of three fermions, or of a fermion and a boson [2]. Because of the underlying substructure, compositeness models imply a large spec-trum of excited states. The coupling of excited fermions to ordinary quarks and leptons, resulting from novel strong interactions, can be described by contact interactions (CI) with the effective four-fermion Lagrangian [3]
LCI g2 22j
j ;
where is the compositeness scale and j is the fermion current
j LfLfL 0LfLfL L00fLfL H:c: L ! R:
The SM and excited fermions are denoted by f and f, respectively; g2 is chosen to be 4, the factors for the left-handed currents are conventionally set to one, and the right-handed currents are set to zero.
Gauge mediated transitions between ordinary and ex-cited fermions can be described by the effective Lagrangian [3] LEW 1 2 fR gsfs a 2 G a gf 2W g0 f0Y 2B fL H:c:; where Ga
, W, and Bare the field strength tensors of the gluon, the SU(2), and U(1) gauge fields, respectively; fs, f, and f0are parameters of order one.
The present analysis considers single production of an excited muon in association with a muon via four-fermion CI, with the subsequent electroweak decay of the into a muon and a photon (Fig. 1). This decay mode leads to the fully reconstructable and almost background-free final state . With the data considered herein, collected with the D0 detector at the Fermilab Tevatron Collider in p p collisions at ps 1:96 TeV, the largest expected SM background is from the Drell-Yan (DY) process p p ! Z= ! , with the final state pho-ton radiated by either a parpho-ton in the initial state p or p, or from one of the final state muons. This background can be strongly suppressed by the application of suitable selection criteria. Other backgrounds are small.
Excited muons have been searched for unsuccessfully previously [4], e.g. at the LEP eecollider; however, the reach has been limited by the center-of-mass energy avail-able to m< 190 GeV. Searches for quark-lepton com-positeness via deviations from the Drell-Yan cross section have excluded values of of up to 6 TeV depending on the chirality [5]. The present analysis is complementary to those results in the sense that an exclusive channel and different couplings ( factors) are probed. The CDF col-laboration has recently presented results [6] for the pro-duction of excited electrons which will be discussed later. For the simulation of the signal, a customized version of
the PYTHIA event generator [7] is used,1 following the
model of [3]. The branching fraction for the decay ! normalized to all gauge particle decay modes is 30% for masses above 300 GeV, and for smaller masses it increases up to 73% at m 100 GeV. Decays via con-tact interactions, not implemented in PYTHIA, contribute
q ¯q m */
EW
CI
1.0 0.5 0 0.2 0.4 0.6 0.8 1.0FIG. 1 (color online). Four-fermion contact interaction q q ! and electroweak decay ! . On the right, the relative
contribution of decays via CI and via electroweak interactions (EW) as a function of m= is shown.
1
PYTHIAv6.225 does not model excited muon production but only excited electrons. We added the excited muon process under the assumption that excited muons differ from excited electrons only in mass.
between a few percent of all decays for mand 92% for m [3,8] (see Fig. 1). This has been taken into account for the signal expectation. The leading order cross section calculated withPYTHIAhas been corrected to next-to-next-to-leading order (NNLO) [9,10]; the corresponding correction factor varies between 1.430 (1.468) for m 100 GeV (200 GeV) and 1.312 for m 1 TeV. The total width is greater than 1 GeV for 100 GeV m 1000 GeV, thus lifetime effects can be neglected. For the values of mand studied here, the total width is always less than 10% of m [3].
The dominant SM background process at all stages of the selection is DY production of pairs. This back-ground, as well as diboson (WW, WZ, ZZ) production, has been simulated with thePYTHIAMonte Carlo program. The DY expectation has been corrected using the NNLO cal-culation from [9]. For diboson production, the next-to-leading order cross sections from [11] are used. Monte Carlo events, both for SM and signal, have been passed through a detector simulation based on theGEANT
[12] package and reconstructed using the same reconstruc-tion program as the data. The CTEQ5L parton distribureconstruc-tion functions (PDF) [13] are used for the generation of all Monte Carlo samples.
The analysis is based on the data collected with the D0 detector [14] between August 2002 and September 2004, corresponding to an integrated luminosity ofL 380 25 pb1. The D0 detector includes a central tracking sys-tem, comprised of a silicon microstrip tracker (SMT) and a central fiber tracker (CFT), both located within a 2 T superconducting solenoidal magnet. The SMT has 800 000 individual strips, with typical pitch of 50–80 m, and a design optimized for tracking and ver-texing capability at pseudorapidities2 of jj < 2:5. The CFT has eight coaxial barrels, each supporting two dou-blets of scintillating fibers of 0.835 mm diameter, one doublet being parallel to the collision axis, and the other alternating by 3 relative to the axis. Three liquid argon and uranium calorimeters provide coverage out to jj 4:2: a central section covering jj up to 1:1 and two end calorimeters. A muon system resides beyond the calorime-try, and consists of a layer of tracking detectors and scin-tillation trigger counters before 1.8 T iron toroids, followed by two similar layers after the toroids. Tracking at jj < 1 relies on 10 cm wide drift tubes, while 1 cm mini-drift tubes are used at 1 < jj < 2. Luminosity is measured using scintillator arrays located in front of the end calo-rimeter cryostats, covering 2:7 < jj < 4:4.
Trigger and data acquisition systems are designed to accommodate the high luminosities of the Tevatron
Run II. Based on information from tracking, calorimetry, and muon systems, the output of the first two levels of the trigger is used to limit the rate for accepted events to <1 kHz, relying on hardware and firmware. The third and final level of the trigger uses software algorithms and a computing farm to reduce the output rate to 50 Hz, which is written to tape.
Efficiencies for muon and photon identification and track reconstruction are determined from the simulation. To verify the simulation and to estimate systematic uncer-tainties, the efficiencies also have been calculated from data samples, using Z ! candidate events and in-clusive dimuon events for muons and tracks and Z ! eeevents to determine the efficiency of reconstructing electrons. We assume that the different response for elec-trons and photons in the calorimeter is properly modeled by the simulation. The transverse (with respect to the beam axis) momentum resolution of the central tracker and the energy resolution of the calorimeter have been tuned in the simulation to reproduce the resolutions observed in the data using Z ! ‘‘‘ e; events.
The process p p ! with ! leads to a final state with two highly energetic isolated muons and a photon. We require two muons to be identified in the muon system and each matched to a track in the central tracking system with transverse momentum pT> 15 GeV. The events have been collected with level 1 trigger con-ditions requiring two muons detected by the muon scintil-lation counters, with at least one muon with tightened criteria identified by the level 2 trigger, and requiring a segment reconstructed in the muon system above certain pT thresholds and/or a track in the central tracking system above certain pT thresholds at level 3. The trigger effi-ciency has been determined from independent data samples for each trigger object (muon) and trigger level separately. The overall trigger efficiency which is applied to the simulation is found to be 88 6% for the signal after application of all selection criteria.
Timing information from the muon scintillation counters is used in order to reject cosmic ray background. Since the signal is expected to produce isolated muons, at least one of the muons is required to be isolated: the amount of energy deposited in the calorimeter along the muon direc-tion in a hollow cone with inner radius R 0:1 (R
2 2 p
) and outer radius R 0:4 is required to be less than 2.5 GeV, and the sum of the transverse momenta of tracks within a cone of R 0:5 has to be below 2.5 GeV, excluding the muon track. The cumulative efficiency of the muon and track reconstruction and muon identification is found to be 88 4% per muon, and the isolation condition is 95 4% efficient. The selected di-muon sample contains 24 853 events, whereas 23 200 2700 events are expected from DY processes, and 34 4 events are expected from diboson production. The invari-ant dimuon mass distribution is shown in Fig. 2(a). 2The pseudorapidity is defined as lntan=2. We
use the polar angle relative to the proton beam direction, and is the azimuthal angle, all measured with respect to the geomet-ric center of the detector.
Next, a photon is identified in the event as an isolated cluster of calorimeter energy with a characteristic shower shape and at least 90% of the energy deposited in the electromagnetic section of the calorimeter. The isolation condition is Etot0:4 Eem0:2=Eem0:2 < 0:15, where Etot0:4 and Eem0:2 denote the energy deposited in the calorimeter and only its electromagnetic section in cones of size R 0:4 and 0.2, respectively. The trans-verse energy ET must be larger than 16 GeV, no track is allowed to be matched to the photon candidate with a 2 probability of greater than 0.1%, and the sum of the trans-verse momenta of tracks within a hollow cone defined by 0:05 < R < 0:4 around the photon direction has to be below 2 GeV to further ensure isolation. The photon can-didate is required to be separated from the muon cancan-didates in the event by at least R 0:4 and has to be recon-structed within the central part of the calorimeter (jj < 1:1).
After this selection, we expect 65 8 events from DY processes, and less than one event from diboson produc-tion. To estimate the possible additional background from jets misidentified as photons and not included in the simu-lation, the misidentification rate has been determined from an inclusive jet data sample; this rate applied to the dimuon plus jet sample results in 39 5 such events in the selection. As a function of ET, the photon fake rate is about 0.5% per jet at low ET and is negligible above 80 GeV. The background from jets misidentified as photons is treated as a systematic uncertainty, resulting in a total SM expectation of 65 8390 events. We find 90 events in the data, in good agreement with the expectation. The invariant mass of the leading muon and the photon is shown in Fig. 2(b) for the data, SM expectation, and signal expectation for m 400 GeV and 1 TeV. The p
T distribution of the leading muon and the ETdistribution of the photon are shown in Fig. 3.
Additional selection criteria are applied to reduce the remaining SM background. The photon ET is required to
be larger than 27 GeV. The efficiency to identify a photon is constant at about 90% above this value. The final dis-criminant to suppress remaining SM backgrounds is the invariant mass of the leading muon and the photon. For masses m above 300 GeV, the leading muon is pre-dominantly the muon from the decay. In order to max-imize the sensitivity of the analysis, the signal expectation is calculated for 1 TeV, the background including DY processes and diboson production is considered, and a cut value is chosen for each value of m. The result is shown in Table I along with the SM expectation for the number of data events and the signal efficiency, which varies between 8% and 15%.
The dominant systematic uncertainties are as follows: The uncertainty on the SM cross sections is dominated by the DY process and the uncertainty from the choice of PDF and renormalization and factorization scales (4%). Muon reconstruction and identification have an uncertainty of 4% per muon, and a 3% error is assigned to the photon
iden-0 0.2 0.4 0.6 0.8 1 1.2 1.4 µ T p 0 50 100 150 200 250 300 Events / 5 GeV [GeV] 0 2 4 6 8 10 12 14 16 18 DO 380 pb-1 Data µ µ → * γ Z misid. γ Signal µ T p [GeV] 0 50 100 150 200 250 300 Events / 5 GeV 0 2 4 6 8 10 12 14 16 18 γ T E [GeV] 0 20 40 60 80 100 120 140 160 180 Events / 5 GeV 0 5 10 15 20 25 30 35 40 -1 380 pb O D Data misid. γ Signal γ T E [GeV] 0 20 40 60 80 100 120 140 160 180 Events / 5 GeV 0 5 10 15 20 25 30 35 40 (b) (a) / Z/γ*→µµ
FIG. 3. For the sample, (a) the distribution of the leading muon pT, and (b) the photon ET. Shown are the data as points with statistical uncertainties, the dominant SM background (DY, shaded histogram, also shown is the uncertainty due to jets misidentified as photons), and the expected signal for m
400 GeV and 1 TeV.
TABLE I. For different values of m, the final selection requirement on the invariant mass of the leading muon and the photon, the remaining data events, the SM expectation, and the signal efficiency. The quoted uncertainties include statistical and systematic uncertainties added in quadrature.
m mcut Data SM Signal eff.
[GeV] [GeV] expectation [%]
100 200 0 0:170 0:126 7:5 1:0 200 200 0 0:170 0:126 12:5 1:5 300 280 0 0:041 0:023 12:1 1:5 400 330 0 0:016 0:011 14:7 1:8 500 440 0 0:003 0:001 11:9 1:5 600 440 0 0:003 0:001 14:4 1:8 700 440 0 0:003 0:001 13:6 1:7 800 440 0 0:003 0:001 14:5 1:8 900 440 0 0:003 0:001 14:7 1:8 1000 440 0 0:003 0:001 14:4 1:8 [GeV] µ µ m 40 60 80 100 120 140 160 180 Events / 2 GeV -2 10 -1 10 1 10 2 10 3 10 -1 380 pb O D [GeV] µ µ m 40 60 80 100 120 140 160 180 Events / 2 GeV -2 10 -1 10 1 10 2 10 3 10 0 50 100 150 200 250 300 350 400 450 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 [GeV] µγ m 100 200 300 400 500 600 Events / 12 GeV 5 10 15 20 25 30 35 -1 380 pb O D Data µ µ → * γ Z/ incl. Z W incl. Z Z incl. W W misid. γ Signal [GeV] m 100 200 300 400 500 600 Events / 12 GeV 5 10 15 20 25 30 35 (b) (a)
FIG. 2. (a) Invariant dimuon mass distribution in the dimuon data sample compared to the SM expectation, (b) invariant mass of the leading muon and the photon in the sample, for data (points with statistical uncertainties), SM backgrounds (DY and diboson production, shaded histograms, as well as the uncer-tainty due to jets misidentified as photons), and the expected signal for m 400 GeV and 1 TeV.
tification. The uncertainty due to the trigger efficiency is 7%. The integrated luminosity is known to a precision of 6.5% [15]. The uncertainty due to jets misidentified as photons is dominant after all selection criteria for m up to 400 GeV: for m 100 GeV (400 GeV), 0.097 (0.008) such ‘‘fake’’ photons are expected, while for m 500 GeV and above this background is negligible ( < 105events). The uncertainty on the signal cross section is estimated to be 10%, consisting of PDF uncertainties and unknown higher order corrections.
Since no events are found in the data, in agreement with the SM expectation, we set 95% confidence level limits on the production cross section times the branching frac-tion into . A Bayesian technique [16] is used, taking into account all uncertainties and treating them as sym-metric for simplicity. The resulting limit as a function of m is shown in Fig. 4 together with predictions of the contact interaction model for different choices of the scale . For 1 TeV ( m), masses below 618 GeV (688 GeV) are excluded. In Fig. 5 the excluded region in terms of and mis shown.
The CDF collaboration has recently searched [6] for the production of excited electrons, and obtained comparable cross section limits, but the CDF mass limit of me> 879 GeV at 95% C.L. for me cannot be directly compared to ours for two reasons. The cross section calcu-lated with the version ofPYTHIAused by CDF is a factor of
2 higher than in subsequent versions corrected by the
PYTHIA authors. Furthermore, CDF assumes that decays
via contact interactions can be neglected, while in our analysis such decays are taken into account in the calcu-lation of the branching fraction ! , following [5,10]. If we adjusted our result for these two differences, we would obtain a limit of m> 890 GeV at 95% C.L. for m.
In summary, we have searched for the production of excited muons in the process p p ! with ! , using 380 pb1of data collected with the D0 detector. We find no events in the data, compatible with the SM expec-tation, and set limits on the production cross section times branching fraction as a function of the mass of the excited muon. For a scale parameter 1 TeV, masses below 618 GeV are excluded, representing the most stringent limit to date.
We thank A. Daleo and M. Kra¨mer for useful discus-sions and A. Daleo for providing us with the NNLO corrections to the production cross section. We thank the staffs at Fermilab and collaborating institutions and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom, and RFBR (Russia); CAPES, CNPq, FAPERJ, FAPESP, and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Argentina); FOM (The Netherlands); PPARC (United Kingdom); MSMT (Czech Republic); CRC Program, CFI, NSERC, and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); Research Corporation; Alexander von Humboldt Foundation; and the Marie Curie Program.
[GeV] * µ m 200 250 300 350 400 450 500 550 600 [TeV] Λ 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -1
380 pb
O
D
at 95% CL
excluded
γ
µ
→
*µ
µ
*µ
→
p
p
[GeV] * µ m 200 250 300 350 400 450 500 550 600 [TeV] Λ 1.0 1.5 2.0 2.5 3.0 3.5 4.0FIG. 5 (color online). The region in the plane of and m
excluded by the present analysis.
[GeV] * µ m 0 200 400 600 800 1000 )γ µ → * µ [pb] * B( σ -5 10 -4 10 -3 10 -2 10 -1 10 1 10 -1
380 pb
O
D
LEP Exclusion Region 190 GeV
=1 TeV Λ =2 TeV Λ =4 TeV Λ Limit (95% CL)
> 618 GeV
* µm
[GeV] * µ m 0 200 400 600 800 1000 )γ µ → * µ [pb] * B( σ -5 10 -4 10 -3 10 -2 10 -1 10 1 10FIG. 4 (color online). The measured cross section branching fraction limit, compared to the contact interaction model pre-diction for different choices of . For the case 1 TeV, the theoretical uncertainty of the model prediction is indicated.
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