1. Art and Diseño

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F erm ilab-P ub-06/081-E
arXiv:hep-ex/0604040v1
19 Apr 2006
S earch for E x c ite d M u o n s in p p C ollisions a t
y /s
= 1.96 TeV
V.M . A bazov,36 B. A b b o tt,76 M. A bolins,66 B.S. A charya,29 M. A dam s,52 T. A dam s,50 M. A gelou,18 J.-L. A g ram ,19
S.H. A hn,31 M. A h san ,60 G.D. Alexeev,36 G. A lkhazov,40 A. A lton,65 G. A lverson,64 G.A. Alves,2 M. A nastasoaie,35
T. A ndeen,54 S. A nderson,46 B. A n d rieu ,17 M.S. Anzelc,54 Y. A rn o u d ,14 M. A rov,53 A. Askew,50 B. Â sm an,41
A.C.S. Assis Jesu s,3 O. A tram en to v ,58 C. A u te rm a n n ,21 C. A vila,8 C. Ay,24 F. B a d a u d ,13 A. B ad en ,62 L. Bagby,53
B. B ald in ,51 D.V. B a n d u rin ,36 P. B anerjee,29 S. B anerjee,29 E. B arb eris,64 P. B argassa,81 P. B aringer,59 C. B arnes,44
J. B a rre to ,2 J.F . B a rtle tt,51 U. B assler,17 D. B auer,44 A. B ean,59 M. Begalli,3 M. Begel,72 C. B elanger-C ham pagne,5
A. Bellavance,68 J.A . B en itez,66 S.B. B eri,27 G. B e rn a rd i,17 R. B ern h ard ,42 L. B ern tzo n ,15 I. B e rtra m ,43
M. B esan co n ,18 R. B euselinck,44 V.A. B ezzubov,39 P.C. B h a t,51 V. B h a tn a g a r,27 M. B inder,25 C. B isc ara t,43
K.M . B lack,63 I. B lackler,44 G. Blazey,53 F . B lekm an,44 S. B lessing,50 D. B loch,19 K. B loom ,68 U. B lum enschein,23
A. B oehnlein,51 O. B oeriu,56 T .A . B o lto n ,60 F . B orcherding,51 G. B orissov,43 K. Bos,34 T. B ose,78 A. B ra n d t,79
R. B rock,66 G. B rooijm ans,71 A. B ross,51 D. B row n,79 N .J. B u chanan,50 D. Buchholz,54 M. B uehler,82
V. B uescher,23 S. B u rd in ,51 S. B urke,46 T.H . B u rn e tt,83 E. B u sa to ,17 C.P. Buszello,44 J.M . B u tle r,63 S. C alv e t,15
J. C am m in,72 S. C aro n ,34 W . C arvalho,3 B .C .K . Casey,78 N.M. C ason,56 H. C astilla-V aldez,33 S. C h a k ra b a rti,29
D. C h ak rab o rty ,53 K .M . C h a n ,72 A. C h a n d ra ,49 D. C h ap in ,78 F. C h arles,19 E. C heu,46 F . C hevallier,14 D.K. C ho,63
S. C hoi,32 B. C h o u d h ary ,28 L. C hristofek,59 D. C laes,68 B. C lem en t,19 C. C lem ent,41 Y. C oadou,5 J. C oenen,21
M. C ooke,81 W .E . C ooper,51 D. C oppage,59 M. C orcoran,81 M .-C. C ousinou,15 B. Cox,45 S. C rep e-R en au d in ,14
D. C u tts ,78 M. Cw iok,30 H. d a M o tta ,2 A. D as,63 M. D as,61 B. D avies,43 G. D avies,44 G .A. D avis,54 K. D e,79
P. de Jo n g ,34 S.J. de Jo n g ,35 E. De L a C ruz-B urelo,65 C. De O liveira M artin s,3 J.D . D eg en h ard t,65 F. D eliot,18
M. D em arteau ,51 R. D em ina,72 P. D em ine,18 D. D enisov,51 S.P. D enisov,39 S. D esai,73 H .T . D iehl,51 M. D iesburg,51
M. D oidge,43 A. D om inguez,68 H. D ong,73 L.V. D udko,38 L. D uflot,16 S.R. D ugad,29 A. D u p e rrin ,15 J. D yer,66
A. D yshkant,53 M. E a d s,68 D. E d m u n d s,66 T. E dw ards,45 J. E llison,49 J. E lm sheuser,25 V.D. E lv ira,51 S. E n o ,62
P. E rm olov,38 J. E s tra d a ,51 H. E vans,55 A. E vdokim ov,37 V.N. E vdokim ov,39 S.N. F a ta k ia ,63 L. Feligioni,63
A.V. F erap o n to v ,60 T. F erbel,72 F. F ied ler,25 F. F ilth a u t,35 W . F isher,51 H .E. F isk,51 I. Fleck,23 M. F ord,45
M. F o rtn e r,53 H. Fox,23 S. F u,51 S. Fuess,51 T. G ad fo rt,83 C .F. G alea,35 E. G allas,51 E. G alyaev,56 C. G arcia,72
A. G arcia-B ellido,83 J. G ard n er,59 V. G avrilov,37 A. G ay,19 P. G ay,13 D. G ele,19 R. G elhaus,49 C .E. G erb er,52
Y. G ershtein,50 D. G illberg,5 G. G in th e r,72 N. G ollub,41 B. G om ez,8 K. G ounder,51 A. G oussiou,56 P.D. G ran n is,73
H.
G reenlee,51 Z.D. G reenw ood,61 E.M . G regores,4 G. G renier,20 P h. G ris,13 J.-F . G rivaz,16 S. G rän e n d a h l,51
M .W . G rünew ald,30 F. G uo,73 J. G uo,73 G. G u tierrez,51 P. G u tierrez,76 A. H aas,71 N .J. H adley,62 P. H aefner,25
S. H agopian,50 J. H aley,69 I. H all,76 R .E. H all,48 L. H an ,7 K. H anagaki,51 K. H a rd e r,60 A. H arel,72
R. H a rrin g to n ,64 J.M . H a u p tm a n ,58 R. H auser,66 J. H ays,54 T. H ebbeker,21 D. H edin,53 J.G . H egem an,34
J.M . H einm iller,52 A.P. H einson,49 U. H eintz,63 C. H ensel,59 G. H esketh,64 M.D. H ild reth ,56 R. H irosky,82
J.D . H obbs,73 B. H oeneisen,12 M. H ohlfeld,16 S.J. H ong,31 R. H ooper,78 P. H ouben,34 Y. H u ,73 V. H ynek,9
I. Iashvili,70 R. Illingw orth,51 A.S. Ito ,51 S. Ja b e e n ,63 M. Jaffre,16 S. Ja in ,76 K. Jak o b s,23 C. Ja rv is,62 A. Jen k in s,44
R. Jesik,44 K. Jo h n s,46 C. Jo h n so n ,71 M. Johnson,51 A. Jonckheere,51 P. Jonsson,44 A. Ju ste ,51 D. K üfer,21
S. K a h n ,74 E. K a jfasz,15 A.M . K alinin,36 J.M . K alk ,61 J.R . K alk ,66 S. K ap p le r,21 D. K arm anov,38 J. K a sp er,63
I. K a tsa n o s,71 D. K a u ,50 R. K a u r,27 R. K ehoe,80 S. K erm iche,15 S. K esisoglou,78 A. K hanov,77 A. K harchilava,70
Y.M . K h arzheev,36 D. K h atid ze,71 H. K im ,79 T .J. K im ,31 M.H. K irby,35 B. K lim a,51 J.M . K ohli,27 J.-P. K o n ra th ,23
M. K o p al,76 V.M . K orablev,39 J. K o tch er,74 B. K o th a ri,71 A. K oubarovsky,38 A.V. K ozelov,39 J. K ozm inski,66
A. K ry em ad h i,82 S. K rzyw dzinski,51 T. K u h l,24 A. K u m a r,70 S. K u n o ri,62 A. K u p c o ,11 T. K u rca ,20’* J. K v ita ,9
S. L ager,41 S. L am m ers,71 G. L an d sb erg ,78 J. Lazoflores,50 A.-C. Le B ih a n ,19 P. L ebrun,20 W .M . Lee,53 A. Leflat,38
F.
L ehner,42 C. L eonidopoulos,71 V. L esne,13 J. Leveque,46 P. Lewis,44 J. L i,79 Q.Z. Li,51 J.G .R . L im a,53
D. L incoln,51 J. L in n em an n ,66 V.V. L ipaev,39 R. L ip to n ,51 Z. L iu,5 L. Lobo,44 A. L obodenko,40 M. L okajicek,11
A. L ounis,19 P. Love,43 H .J. L u b a tti,83 M. L ynker,56 A.L. Lyon,51 A.K.A. M aciel,2 R .J. M ad aras,47 P. M üttig ,26
C. M agass,21 A. M ag erk u rth ,65 A.-M . M ag n an ,14 N. M akovec,16 P.K. M al,56 H.B. M albouisson,3 S. M alik,68
V.L. M alyshev,36 H.S. M ao,6 Y. M arav in ,60 M. M arten s,51 S.E.K . M attingly,78 R. M cC arthy,73 R. M cCroskey,46
D. M eder,24 A. M elnitchouk,67 A. M endes,15 L. M endoza,8 M. M erkin,38 K .W . M e rritt,51 A. M eyer,21 J. M eyer,22
M. M ich au t,18 H. M iettin en ,81 T. M illet,20 J. M itrevski,71 J. M olina,3 N.K. M ondal,29 J. M onk,45 R.W . M oore,5
T. M oulik,59 G.S. M u an za,16 M. M ulders,51 M. M ulhearn,71 L. M undim ,3 Y.D. M u taf,73 E. N agy,15
M. N aim uddin,28 M. N arain ,63 N.A. N aum ann,35 H.A. N eal,65 J.P . N egret,8 S. N elson,50 P. N eustroev,40
C. N oeding,23 A. N om erotski,51 S.F. Novaes,4 T. N u nnem ann,25 V. O ’Dell,51 D.C. O ’Neil,5 G. O b ra n t,40
2
V. O guri,3 N. O liveira,3 N. O shim a,51 R. O te c ,10 G .J. O tero y G arzon,52 M. O wen,45 P. Padley,81 N. P a ra sh a r,57
S.-J. P a rk ,72 S.K. P a rk ,31 J. P arso n s,71 R. P a rtrid g e ,78 N. P a ru a ,73 A. P a tw a ,74 G. Paw loski,81 P.M . P erea,49
E.
P erez,18 K. P eters,45 P. P etro ff,16 M. P e tte n i,44 R. P ieg a ia,1 M.-A. P leier,22 P.L.M . P o d esta-L erm a,33
V.M . P o dstavkov,51 Y. Pogorelov,56 M .-E. P ol,2 A. P om pos,76 B .G . P o p e,66 A.V. P opov,39 W .L. P ra d o d a Silva,3
H.B. P ro sp e r,50 S. P ro to p o p escu ,74 J. Q ian ,65 A. Q u a d t,22 B. Q u in n ,67 K .J. R an i,29 K. R a n jan ,28 P.A. R apidis,51
P.N. R atoff,43 P. R enkel,80 S. R eucroft,64 M. R ijssenbeek,73 I. R ip p -B au d o t,19 F . R izatdinova,77 S. R obinson,44
R .F. R odrigues,3 C. R oyon,18 P. R ubinov,51 R. R uchti,56 V.I. R ud,38 G. S a jo t,14 A. Sânchez-H ernandez,33
M .P. S anders,62 A. S antoro,3 G. Savage,51 L. Saw yer,61 T. Scanlon,44 D. Schaile,25 R.D . Scham berger,73
Y. Scheglov,40 H. Schellm an,54 P. Schieferdecker,25 C. S ch m itt,26 C. Schw anenberger,45 A. Schw artzm an,69
R. Schw ienhorst,66 S. S en g u p ta,50 H. Severini,76 E. S habalina,52 M. Sham im ,60 V. S hary,18 A.A. Shchukin,39
W .D . S h ep h ard ,56 R .K . S hivpuri,28 D. Shpakov,64 V. S iccardi,19 R.A. Sidwell,60 V. S im ak,10 V. Sirotenko,51
P. Skubic,76 P. S lattery ,72 R.P. S m ith,51 G .R . Snow,68 J. Snow,75 S. Snyder,74 S. Soldner-R em bold,45 X. Song,53
L. S onnenschein,17 A. Sopczak,43 M. Sosebee,79 K. S oustruznik,9 M. Souza,2 B. Spurlock,79 J. S ta rk ,14 J. Steele,61
K. Stevenson,55 V. S tolin,37 A. S tone,52 D.A. Stoyanova,39 J. S tra n d b e rg ,41 M.A. S tran g ,70 M. S trau ss,76
R. S tro h m er,25 D. S tro m ,54 M. Strovink,47 L. S tu tte ,51 S. Sum ow idagdo,50 A. S znajder,3 M. T alby,15
P. T am burello,46 W . T aylor,5 P. T elford,45 J. Tem ple,46 B. T ille r,25 M. T ito v ,23 V.V. Tokm enin,36 M. T om oto,51
T. Toole,62 I. T orchiani,23 S. Towers,43 T. Trefzger,24 S. T rincaz-D uvoid,17 D. T sybychev,73 B. T uchm ing,18
C. Tully,69 A.S. T u rco t,45 P.M . T u ts,71 R. U n alan ,66 L. U varov,40 S. U varov,40 S. U zunyan,53 B. V achon,5
P .J. van den B erg,34 R. V an K o o ten ,55 W .M . van Leeuwen,34 N. V arelas,52 E.W . V arnes,46 A. V a rta p e tia n ,79
I.A. Vasilyev,39 M. V aupel,26 P. V erdier,20 L.S. V ertogradov,36 M. Verzocchi,51 F. V illeneuve-Seguier,44 P. V in t,44
J.-R . V lim a n t,17 E. Von T oerne,60 M. V outilainen,68’^ M. Vreeswijk,34 H.D. W ahl,50 L. W ang,62 J. W archol,56
G. W a tts,83 M. W ayne,56 M. W eber,51 H. W eerts,66 N. W erm es,22 M. W etstein,62 A. W h ite,79 D. W icke,26
G .W . W ilson,59 S.J. W im penny,49 M. W obisch,51 J. W omersley,51 D.R. W ood,64 T .R . W y a tt,45 Y. X ie,78
N. X u an ,56 S. Y acoob,54 R. Y am ada,51 M. Y an,62 T. Y asuda,51 Y.A. Y atsunenko,36 K. Y ip,74 H.D. Yoo,78
S.W . Y oun,54 C. Y u,14 J. Y u,79 A. Y urkewicz,73 A. Zatserklyaniy,53 C. Z eitnitz,26 D. Z hang,51 T. Z hao,83
Z. Z hao,65 B. Z hou,65 J. Z hu,73 M. Zielinski,72 D. Ziem inska,55 A. Ziem inski,55 V. Z utshi,53 and E .G . Zverev38
(D 0 C ollaboration)
1 Universidad de Buenos Aires, Buenos Aires, Argentina
2LAFEX, Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brazil
3 Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
4Instituto de Fisica Teorica, Universidade Estadual Paulista, Sao Paulo, Brazil
5 University 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
7 University of Science and Technology of China, Hefei, People ’s Republic of China
8 Universidad de los Andes, Bogota, Colombia,
9 Center fo r Particle Physics, Charles University, Prague, Czech Republic
10 Czech Technical University, Prague, Czech Republic
11 Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
12 Universidad San Francisco de Quito, Quito, Ecuador
13Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Université Blaise Pascal, Clermont-Ferrand, France
14Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France
15 CPPM, IN2P3-CNRS, Université de la Méditerranée, Marseille, France
16IN2P3-CNRS, Laboratoire de l ’Accélérateur Linéaire, Orsay, France
17LPNHE, IN2P3-CNRS, Universités Paris VI and VII, Paris, France
18DAPNIA/Service de Physique des Particules, CEA, Saclay, France
19IReS, IN2P3-CNRS, Université Louis Pasteur, Strasbourg, France, and Université de Ha,ute Alsace, Mulhouse, France
20Institut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, Villeurbanne, France
21III. Physikalisches Institut A, R W TH Aachen, Aachen, Germany
22Physikalisches Institut, Universitat Bonn, Bonn, Germany
23Physikalisches Institut, Universität Freiburg, Freiburg, Germany
24Institut fü r Physik, Universitat Mainz, Mainz, Germany
25Ludwig-Maximilians-Universitat München, München, Germany
26Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
27 Pa,nja,b University, Chandigarh, India,
28Delhi University, Delhi, India,
29 Tata Institute of Fundamental Research, Mumbai, India,
30 University College Dublin, Dublin, Ireland
31Korea Detector Laboratory, Korea University, Seoul, Korea,
3
32
SungKyunKwan University, Suwon, Korea
33 CINVESTAV, Mexico City, Mexico
34FOM-Institute NIKH EF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands
35 Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
36 Joint Institute fo r Nuclear Research, Dubna, Russia,
37Institute fo r Theoretical and Experimental Physics, Moscow, Russia
38 Moscow State University, Moscow, Russia,
39Institute fo r High Energy Physics, Protvino, Russia
40 Petersburg Nuclear Physics Institute, St. Petersburg, Russia
41Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden, and
Uppsala University, Uppsala, Sweden
42 Physik Institut der Universität Zurich, Zurich, Switzerland,
43Lancaster University, Lancaster, United Kingdom
44Imperial College, London, United Kingdom
45 University of Manchester, Manchester, United Kingdom
46 University of Arizona, Tucson, Arizona 85721, USA
47Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
48 California State University, Fresno, California 93740, USA
49 University of California, Riverside, California 92521, USA
50Florida State University, Tallahassee, Florida 32306, USA
51Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
52 University 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
56 University of Notre Dame, Notre Dame, Indiana 46556, USA
57Purdue University Calumet, Hammond, Indiana 46323, USA
58Iowa State University, Ames, Iowa 50011, USA
59 University of Kansas, Lawrence, Kansas 66045, USA
60Kansas State University, Manhattan, Kansas 66506, USA
61Louisiana Tech University, Ruston, Louisiana 71272, USA
62 University of Maryland, College Park, Maryland 20742, USA
63Boston University, Boston, Massachusetts 02215, USA
64Northeastern University, Boston, Massachusetts 02115, USA
65 University of Michigan, A nn Arbor, Michigan 48109, USA
66Michigan State University, East Lansing, Michigan 48824, USA
67 University of Mississippi, University, Mississippi 38677, USA
68 University of Nebraska, Lincoln, Nebraska 68588, USA
69 Princeton University, Princeton, New Jersey 08544, USA
70State University of New York, Buffalo, New York 14260, USA
71
Columbia University, New York, New York 10027, USA
72 University of Rochester, Rochester, New York 14627, USA
73State University of New York, Stony Brook, New York 11794, USA
74 Brookhaven National Laboratory, Upton, New York 11973, USA
75Langston University, Langston, Oklahoma 73050, USA
76 University of Oklahoma, Norman, Oklahoma 73019, USA
77 Oklahoma State University, Stillwater, Oklahoma 74078, USA
78 Brown University, Providence, Rhode Island 02912, USA
79 University of Texas, Arlington, Texas 76019, USA
80Southern Methodist University, Dallas, Texas 75275, USA
81 Rice University, Houston, Texas 77005, USA
82 University of Virginia, Charlottesville, Virginia 22901, USA
83 University of Washington, Seattle, Washington 98195, USA
(Dated: April 19, 2006)
We present the results of a search for the production of an excited state of the muon, ß *, in proton
antiproton collisions at *Js = 1.96 TeV. The data have been collected with the DO experiment at the
Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 380 pb- 1 .
We search for ß* in the process pp ^ ß*ß, with the ß* 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 th a t describes ß* production by four-fermion contact interactions and
ß* decay via electroweak processes, we exclude production cross sections higher than 0.057 pb 0.112 pb at the 95% confidence level, depending on the mass of the excited muon. Choosing the
4
scale for contact interactions to be A = 1 TeV, excited muon masses below 618 GeV are excluded.
PACS numbers: 12.60.Rc, 14.60.Hi, 12.60.-i, 13.85.Rm
A n open question in p article physics is th e observed
m ass h ierarchy of th e q u a rk an d lepton SU(2) doublets
in th e sta n d a rd m odel (SM). A com m only proposed ex­
p lan atio n for th e th ree generations is a com positeness
m odel [1] of th e know n leptons an d quarks. A ccording
to th is approach, a q u ark or lep to n is a b o u n d sta te of
th ree ferm ions, or of a ferm ion an d a boson [2]. Due to
th e underlying su b stru c tu re , com positeness m odels im ­
ply a large sp ectru m of excited states. T h e coupling of
excited ferm ions to o rd in ary q u ark s an d leptons, resu lt­
ing from novel stro n g interactions, can be described by
co n tact in teractio n s (CI) w ith th e effective four-ferm ion
L agrangian [3]
£cl =
reach has been lim ited by th e center-of-m ass energy avail­
able to m M* < 190 GeV. Searches for quark-lepton com­
positeness via deviations from th e Drell-Yan cross section
have excluded values of A of up to « 6 TeV depending on
th e chirality [5]. T he present analysis is com plem entary
to those results in th e sense th a t an exclusive channel
and different couplings (n factors) are probed. T he CD F
collaboration has recently presented results [6 ] for the
pro d u ctio n of excited electrons which will be discussed
later.
2 X 5 ^ ^ ’
w here j M is th e ferm ion cu rren t
= nt f L
L + n l f l Ymf L + n l /L Ym/l
+ h.c. + (L ——R).
T h e SM an d excited ferm ions are den o ted by f and f *,
respectively; g 2 is chosen to be 4n, th e n factors for the
left-handed cu rren ts are conventionally set to one, and
th e rig h t-h an d ed cu rren ts are set to zero. T h e com pos­
iteness scale is A.
G auge m ed iated tra n sitio n s betw een o rd in a ry and ex­
cited ferm ions can be described by th e effective Lagran g ian [3]
C EW
\a
f t + h.c.
where G M
a V, WMV, an d B mv are th e field stre n g th tensors of
th e gluon, th e SU(2) an d U(1) gauge fields, respectively;
f s, f an d f ' are p a ra m e te rs of o rd er one.
T h e p resent analysis considers single p ro d u ctio n of an
excited m uon u * in association w ith a m uon via fourferm ion CI, w ith th e subsequent electrow eak decay of
th e u* in to a m uon an d a p h o to n (Fig. 1). T his de­
cay m ode leads to th e fully reco n stru ctab le and alm ost
background-free final s ta te UUY. W ith th e d a ta consid­
ered herein, collected w ith th e D0 d etecto r a t th e Ferm ilab T evatron C ollider in pp collisions a t a/s = 1.96 TeV,
th e largest expected SM background is from th e DrellYan (DY) process pp — Z / y * — u+ U - (y), w ith th e final
sta te p h o to n ra d ia te d by eith er a p a rto n in th e initial
sta te p or p, or from one of th e final s ta te m uons. T his
background can be stro n g ly suppressed by th e applica­
tio n of suitable selection criteria. O th er backgrounds are
small.
E xcited m uons have been searched for unsuccessfully
previously [4], e.g. a t th e L E P e + e - collider; however the
FIG. 1: Four-fermion contact interaction qq ^ ß*ß, and electroweak decay ß* ^ ßY. On the right, the relative contribu­
tion of decays via CI and via electroweak interactions (EW)
as a function of m M*/A is shown.
For th e sim ulation of th e signal a custom ized ver­
sion of th e PYTHIA event generator [7] is used, following
th e m odel of [3]. T he branching fraction for th e decay
U* — UY norm alized to all gauge particle decay m odes is
30% for m asses above 300 GeV, and for sm aller u* m asses
it increases up to 73% a t m M* = 100 GeV. Decays via con­
ta c t interactions, n o t im plem ented in PYTHIA, co n tribute
betw een a few percent of all decays for A ^ m M* and
92% for A = m M* [3, 8 ] (see Fig. 1). T his has been tak en
into account for th e signal expectation. T he leading or­
der cross section calculated w ith PYTHIA has been cor­
rected to n ex t-to-next-to-leading order (NNLO) [9, 10];
th e corresponding correction factor varies betw een 1.430
(1.468) for m M* = 100 GeV (200 GeV) and 1.312 for
m M* = 1 TeV. T he to ta l w idth is greater th a n 1 GeV for
100 G eV < m M* < 1000 GeV, th u s lifetim e effects can be
neglected. For th e values of m M* and A studied here, the
to ta l w idth is always less th a n 10% of m M* [3].
T he dom inant SM background process a t all stages of
th e selection is DY p ro d u ctio n of u+ U - pairs. T his back­
ground, as well as diboson ( W W , W Z , Z Z ) production,
has been sim ulated w ith th e PY TH IA M onte C arlo (MC)
program . T he DY ex p ectatio n has been corrected us­
ing th e NNLO calculation from [9]. For diboson produc­
tion, th e next-to-leading order cross sections from [11 ]
are used. M onte C arlo events, b o th for SM and signal,
have been passed th ro u g h a detecto r sim ulation based
on th e G EA N T [12] package, and reco n stru cted using the
sam e reco n stru ctio n p rogram as th e d a ta . T he C T E Q 5L
p a rto n d istrib u tio n functions (P D F ) [13] are used for the
generation of all MC sam ples.
T he analysis is based on th e d a ta collected w ith th e D0
detecto r [14] betw een A ugust 2002 and Septem ber 2004,
5
corresponding to an in te g ra te d lum inosity of 380 p b - 1 .
T he D0 d etecto r includes a cen tral track in g system , com­
prised of a silicon m icrostrip track er (SM T) an d a central
fiber track er (C F T ), b o th located w ithin a 2 T supercon­
d u ctin g solenoidal m agnet. T he SM T has « 800,000
individual strips, w ith typical p itch of 50 — 80 ^m , and
a design optim ized for track in g an d vertexing capability
a t p seudorapidities [15] of |n| < 2.5. T he C F T has eight
coaxial barrels, each su p p o rtin g tw o d oublets of scintil­
latin g fibers of 0.835 m m diam eter, one doublet being
parallel to th e collision axis, an d th e o th er a lte rn a tin g by
± 3 ° relative to th e axis. T hree liquid argon an d uran iu m
calorim eters provide coverage o u t to |n| « 4.2: a central
section covering |n| u p to « 1 . 1 , an d tw o end calorim e­
ters. A m uon system resides beyond th e calorim etry, and
consists of a layer of track in g d etecto rs an d scintillation
trig g er counters before 1.8 T iron toroids, followed by two
sim ilar layers after th e toroids. Tracking a t |n| < 1 relies
on 10 cm wide d rift tub es, while 1 cm m ini-drift tu b es
are used a t 1 < |n| < 2. L um inosity is m easured using
scin tillato r array s located in front of th e end calorim eter
cry o stats, covering 2.7 < |n| < 4.4.
Trigger an d d a ta acquisition system s are designed to
accom m odate th e high lum inosities of th e T evatron R un
II. B ased on inform ation from tracking, calorim etry, and
m uon system s, th e o u tp u t of th e first tw o levels of the
trig g er is used to lim it th e ra te for accepted events to
< 1 kHz, relying on hard w are an d firm ware. T he th ird
an d final level of th e trigger uses softw are algorithm s and
a com puting farm to reduce th e o u tp u t ra te to « 50 Hz,
which is w ritte n to tap e.
Efficiencies for m uon an d p h o to n identification and
tra c k reco n stru ctio n are determ in ed from th e sim ula­
tion. To verify th e sim ulation an d to estim ate system atic
un certain ties, th e efficiencies have also been calculated
from d a ta sam ples, using Z ^
can d id ate events
and inclusive dim uon events for m uons an d tracks, and
Z ^ e + e - events to determ ine th e efficiency of recon­
stru c tin g electrons. We assum e th a t th e different re­
sponse for electrons an d photo n s in th e calorim eter is
p ro p erly m odelled by th e sim ulation. T he transverse
(w ith respect to th e beam axis) m om entum resolution
of th e cen tral track er an d th e energy resolution of the
calorim eter have been tu n e d in th e sim ulation to repro­
duce th e resolutions observed in th e d a ta using Z ^ I I
(I = e, ^ ) events.
T he process pp ^
w ith
^ ^ 7 leads to a final
sta te w ith tw o highly energetic isolated m uons and a pho­
ton. We require tw o m uons to be identified in th e m uon
system an d each m atch ed to a tra c k in th e central track ­
ing system w ith tran sv erse m om entum p T > 15 GeV.
T he events have been collected w ith Level 1 trigger con­
ditions requiring tw o m uons d etected by th e m uon scin­
tillatio n counters, w ith a t least one m uon w ith tightened
criteria identified by th e Level 2 trigger, an d requiring a
segm ent reco n stru cted in th e m uon system above certain
p T thresholds a n d /o r a tra c k in th e cen tral tracking sys­
tem above c ertain p T thresholds a t Level 3. T he trigger
DO 3 8 0 p b -1
■ Data
h{ ®
Z
mm
W Z incl.
■
I
100
200
300
Z Z incl.
W W incl.
I g misid.
400
500
600
[GeV]
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 ßßY sam­
ple, for data (points with statistical uncertainties), SM back­
grounds (DY and diboson production, shaded histograms, as
well as the uncertainty due to jets misidentified as photons),
and the expected signal for m M* = 400 GeV and A = 1 TeV.
efficiency has been determ ined from independent d a ta
sam ples for each trigger object (muon) and trigger level
separately. T he overall trigger efficiency which is applied
to th e sim ulation is found to be 88 ± 6 % for th e signal
after application of all selection criteria.
T im ing inform ation from th e m uon scintillation coun­
ters is used in order to reject cosmic ray background.
Since th e signal is expected to produce isolated m uons,
a t least one of th e m uons is required to be isolated:
th e am ount of energy deposited in th e calorim eter along
th e m uon direction in a, hollow cone w ith inner radius
A T I = 0.1 (A7Z =
{Ai])2 + (A <f>)2) and o u te r radius
A R = 0.4 is required to be less th a n 2.5 GeV, and
th e sum of th e transverse m om enta of track s w ithin a
cone of A R = 0.5 has to be below 2.5 GeV, excluding
th e m uon track. T he cum ulative efficiency of th e m uon
and trac k reco n stru ctio n and m uon identification is found
to be 88 ± 4% per m uon, and th e isolation condition is
95 ± 4% efficient. T he selected dim uon sam ple contains
24853 events, w hereas 23200 ± 2700 events are expected
from DY processes, and 34 ± 4 events are expected from
diboson production. T he invariant dim uon m ass d istri­
b u tio n is show n in Fig. 2 a).
N ext, a ph o to n is identified in th e event as an iso­
la ted cluster of calorim eter energy w ith a characteristic
shower shape an d a t least 90% of th e energy deposited in
th e electrom agnetic section of th e calorim eter. T he isola­
tio n condition is (E tot (0.4) —E em(0 .2 ))/E em(0.2) < 0.15,
where E to t(0.4) and E em(0.2) denote th e energy de­
posited in th e calorim eter and only its electrom agnetic
section in cones of size A R = 0.4 and 0.2, respectively.
T he transverse energy E T m ust be larger th a n 16 GeV,
no trac k is allowed to be m atched to th e ph o to n candi­
d ate w ith a x 2 p ro b ab lility of g reater th a n 0 . 1%, and the
sum of th e transverse m om enta of tracks w ithin a hollow
cone defined by 0.05 < A R < 0.4 aro u n d th e ph o to n di­
rection has to be below 2 G eV to fu rth er ensure isolation.
T he ph o to n can d id ate is required to be sep a ra ted from
6
DO 3 8 0 p b -1
■ Data
z/g' ® mm
I
I g misid.
Signal
Et g [GeV]
m ln cut
«V*
[GeV]
[GeV]
100
200
200
200
300
280
400
330
500
440
600
440
700
440
800
440
900
440
1000
440
D ata
0
0
0
0
0
0
0
0
0
0
SM
expectation
0.170 ±0.126
0.170 ±0.126
0.041 ±0.023
0.016 ±0.011
0.003 ±0.001
0.003 ±0.001
0.003 ±0.001
0.003 ±0.001
0.003 ±0.001
0.003 ±0.001
Signal eff.
[%]
7.5 ± 1.0
12.5 ± 1.5
12.1 ± 1.5
14.7 ± 1.8
11.9 ± 1.5
14.4 ± 1 .8
13.6 ± 1 .7
14.5 ± 1 .8
14.7 ± 1.8
14.4 ± 1 .8
FIG. 3: For the ßßY sample, a) the distribution of the leading
muon pT, and b) the photon E T. Shown are the d ata 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 u* = 400 GeV and A = 1 TeV.
TABLE I: For different values of m M*, the final selection re­
quirement 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 statis­
tical and systematic uncertainties added in quadrature.
th e m uon can d id ates in th e event by a t least A R = 0.4,
and has to be reco n stru cted w ithin th e cen tral p a rt of
th e calorim eter (|n| < 1.1).
A fter th is selection, we expect 65 ± 8 events from DY
processes, an d less th a n one event from diboson produc­
tion. To estim ate th e possible ad d itio n al background
from je ts m isidentified as photo n s an d n o t included in
th e sim ulation, th e m isidentification ra te has been d eter­
m ined from an inclusive je t d a ta sam ple; th is ra te applied
to th e dim uon plus je t sam ple resu lts in 39 ± 5 such events
in th e UUY selection. As a function of E T , th e p h o to n fake
ra te is a b o u t 0.5% p er je t a t low E T , an d is negligible
above « 80 GeV. T he b ackground from je ts m isidentified
as photo n s is tre a te d as a system atic uncertainty, resu lt­
ing in a to ta l SM ex p ectatio n of 65 ± 8 - 0 9 events. We
find 90 events in th e d a ta , in good agreem ent w ith the
expectatio n . T he invariant m ass of th e leading m uon and
th e p h o to n is show n in Fig. 2 b) for th e d a ta , SM expec­
ta tio n , an d signal ex p ectatio n for m M* = 400 G eV and
A = 1 TeV. T he p T d istrib u tio n of th e leading m uon and
th e E t d istrib u tio n of th e p h o to n are show n in Fig. 3.
A dditional selection criteria are applied to reduce the
rem aining SM background. T he p h o to n E T is required to
be larger th a n 27 GeV. T he efficiency to identify a pho­
to n is co n stan t a t a b o u t 90% above th is value. T he final
discrim in an t to suppress rem aining SM backgrounds is
th e invariant m ass of th e leading m uon an d th e photon.
For m asses m M* above « 300 GeV, th e leading m uon is
pred o m in an tly th e m uon from th e u * decay. In order to
m axim ize th e sensitivity of th e analysis, th e signal ex­
p ec ta tio n is calcu lated for A = 1 TeV, th e background
including DY processes an d diboson p ro d u ction is con­
sidered, an d a c u t value is chosen for each value of m M*.
T he resu lt is shown in Table I along w ith th e SM ex­
p ec ta tio n for th e num ber of d a ta events an d th e signal
efficiency, w hich varies betw een 8% an d 15%.
T he d o m in an t sy stem atic u n certain ties are as follows.
T he u n c e rta in ty on th e SM cross sections is dom inated
by th e DY process an d th e u n c e rta in ty from th e choice of
P D F and renorm alization and factorization scales (4%).
M uon reco n stru ctio n and identification have an uncer­
ta in ty of 4% per m uon, and a 3% erro r is assigned to the
ph o to n identification. T he u n certa in ty due to th e trig ­
ger efficiency is 7%. T he in te g ra ted lum inosity is know n
to a precision of 6.5% [16]. T he u n c e rtain ty due to je ts
m isidentified as photons is dom inant after all selection
criteria for m M* u p to 400 GeV: for m M* = 100 GeV
(400 GeV), 0.097 (0.008) such “fake” photons are ex­
pected, while for m M* = 500 GeV and above th is back­
ground is negligible (< 10-5 events). T he u n ce rtain ty
on th e signal cross section is e stim ated to be 10%, con­
sisting of P D F u n certainties and unknow n higher order
corrections.
Since no events are found in th e d a ta , in agreem ent
w ith th e SM expectation, we set 95% confidence level lim ­
its on th e u * p ro d u ctio n cross section tim es th e branching
fraction into UY. A B ayesian technique [17] is used, ta k ­
ing into account all u n certainties and tre a tin g th em as
sym m etric for simplicity. T he resulting lim it as a func­
tio n of m M* is show n in Fig. 4 to g eth er w ith predictions
of th e co n tact in teractio n m odel for different choices of
th e scale A. For A = 1 TeV (A = m M* ), m asses below
618 GeV (688 GeV) are excluded. In Fig. 5 th e excluded
region in term s of A and m M* is shown.
T he C D F collaboration has recently searched [6] for
th e pro d u ctio n of excited electrons, and ob tain ed com­
p arable cross section lim its, b u t th e C D F m ass lim it of
m e* > 879 GeV a t 95% C.L. for A = m e* can n o t be
d irectly com pared to ours for two reasons. T he cross
section calculated w ith th e version of PY TH IA used by
C D F is a factor of two higher th a n in subsequent ver­
sions corrected by th e PY TH IA authors. F urtherm ore,
C D F assum es th a t decays via co n tact in teractio n s can
be neglected, while in our analysis such decays are tak en
into account in th e calculation of th e branching frac­
tio n u* ^ UY, following [3, 8]. If we ad ju sted our re­
sult for these two differences, we would o b tain a lim it of
m M* > 890 GeV a t 95% C.L. for A = m M*.
7
In sum m ary, we have searched for th e pro d u ctio n of
excited m uons in th e process p p ^ u* U w ith u* ^ UY,
using 380 p b -1 of d a ta collected w ith th e D0 detector.
We find no events in th e d a ta , com patible w ith th e SM
expectation, and set lim its on th e p ro d u ctio n cross sec­
tio n tim es branching fraction as a function of th e m ass
of th e excited m uon. For a scale p ara m e ter A = 1 TeV,
m asses below 618 G eV are excluded, representing the
m ost strin g e n t lim it to date.
FIG. 4: The measured cross section x branching fraction
limit, compared to the contact interaction model prediction
for different choices of A. For the case A = 1 TeV, the theo­
retical uncertainty of the model prediction is indicated.
:
D O 3 8 0 p b -1
p p ® m"m
m ® mg
'
e x c lu d e d
at 95% CL
......................I . . . . I . . . ......................................
200 250 300 350 400 450 500 550 600
mm[GeV]
FIG. 5: The region in the plane of A and m M* excluded by
the present analysis.
We th a n k A. Daleo and M. K ram er for useful discus­
sions, and A. Daleo for providing us w ith th e NNLO cor­
rections to th e u* pro d u ctio n cross section. We th a n k
th e staffs a t Ferm ilab and collaborating in stitu tio n s, and
acknowledge su p p o rt from th e D O E and NSF (USA);
C E A and C N R S /IN 2 P 3 (France); FASI, R osatom and
R F B R (Russia); C A PE S, C N Pq, F A P E R J, F A P E S P and
FU N D U N E SP (Brazil); DAE and D ST (India); Colciencias (Colom bia); C O N A C yT (Mexico); K R F and
K O S E F (K orea); C O N IC E T and U B A C yT (A rgentina);
FO M (T he N etherlands); P PA R C (U nited K ingdom );
M SM T (Czech Republic); CRC P rogram , C FI, NSERC
and W estG rid P ro je c t (C anada); B M B F and D FG (G er­
m any); SFI (Ireland); T he Swedish R esearch Council
(Sweden); Research C orporation; A lexander von H um ­
b o ld t F oundation; and th e M arie C urie P rogram .
[*] On leave from IEP SAS Kosice, Slovakia.
[f] Visitor from Helsinki Institute of Physics, Helsinki, Fin­
land.
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The pseudorapidity n is defined as n = —ln(tan(6/2)).
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8
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