Search for the Flavor-Changing Neutral Current Decay D^0tomu^+mu^- in pbar{p} Collisions at
a r X i v :h e p -e x /0308059v 1 25 A u g 2003
Search for the Flavor-Changing Neutral Current Decay
D 0→μ+μ?in p
s =1.96TeV D.Acosta,14T.A?older,7M.H.Ahn,25T.Akimoto,52M.G.Albrow,13 D.Ambrose,40
D.Amidei,30 A.Anastassov,47K.Anikeev,29 A.Annovi,41J.Antos,1M.Aoki,52G.Apollinari,13J-F.Arguin,50T.Arisawa,54A.Artikov,11T.Asakawa,52W.Ashmanskas,2
A.Attal,6F.Azfar,38P.Azzi-Bacchetta,39N.Bacchetta,39H.Bachacou,26W.Badgett,13S.Bailey,18A.Barbaro-Galtieri,26G.Barker,23V.E.Barnes,43
B.A.Barnett,22S.Baroiant,5M.Barone,15G.Bauer,29 F.Bedeschi,41S.Behari,22S.Belforte,51W.H.Bell,17G.Bellettini,41J.Bellinger,55D.Benjamin,12A.Beretvas,13A.Bhatti,45M.Binkley,13
D.Bisello,39M.Bishai,13R.
E.Blair,2C.Blocker,4K.Bloom,30B.Blumenfeld,22A.Bocci,45
A.Bodek,44G.Bolla,43 A.Bolshov,29P.S.L.Booth,27 D.Bortoletto,43J.Boudreau,42S.Bourov,13C.Bromberg,31M.Brozovic,12E.Brubaker,26J.Budagov,11H.S.Budd,44K.Burkett,18G.Busetto,39P.Bussey,17K.L.Byrum,2S.Cabrera,12P.Cala?ura,26M.Campanelli,16M.Campbell,30A.Canepa,43D.Carlsmith,55S.Carron,12R.Carosi,41M.Casarsa,51W.Caskey,5A.Castro,3P.Catastini,41D.Cauz,51A.Cerri,26C.Cerri,41L.Cerrito,21J.Chapman,30 C.Chen,40Y.C.Chen,1M.Chertok,5G.Chiarelli,41G.Chlachidze,11F.Chlebana,13K.Cho,25D.Chokheli,11M.L.Chu,1J.Y.Chung,35W-H.Chung,55Y.S.Chung,44 C.I.Ciobanu,21M.A.Ciocci,41 A.G.Clark,16M.N.Coca,44
A.Connolly,26M.E.Convery,45J.Conway,47M.Cordelli,15G.Cortiana,39J.Cranshaw,49R.Culbertson,13C.Currat,26D.Cyr,55D.Dagenhart,4S.DaRonco,39S.D’Auria,17P.de Barbaro,44S.De Cecco,46S.Dell’Agnello,15M.Dell’Orso,41S.Demers,44L.Demortier,45M.Deninno,3D.De Pedis,46P.F.Derwent,13C.Dionisi,46J.R.Dittmann,13P.Doksus,21
A.Dominguez,26S.Donati,41M.D’Onofrio,16T.Dorigo,39V.Drollinger,33K.Ebina,54N.Eddy,21R.Ely,26R.Erbacher,13M.Erdmann,23D.Errede,21S.Errede,21R.Eusebi,44H-C.Fang,26S.Farrington,17I.Fedorko,41R.G.Feild,56M.Feindt,23J.P.Fernandez,43
C.Ferretti,30R.
D.Field,14I.Fiori,41G.Flanagan,31 B.Flaugher,13L.R.Flores-Castillo,42A.Foland,18S.Forrester,5G.W.Foster,13M.Franklin,18H.Frisch,10Y.Fujii,24I.Furic,29A.Gallas,34M.Gallinaro,45J.Galyardt,9M.Garcia-Sciveres,26A.F.Gar?nkel,43
C.Gay,56H.Gerberich,12 E.Gerchtein,9
D.W.Gerdes,30S.Giagu,46P.Giannetti,41
A.Gibson,26K.Gibson,9 C.Ginsburg,55K.Giolo,43M.Giordani,5G.Giurgiu,9
V.Glagolev,11D.Glenzinski,13M.Gold,33N.Goldschmidt,30D.Goldstein,6J.Goldstein,13 G.Gomez,8G.Gomez-Ceballos,29M.Goncharov,48I.Gorelov,33 A.T.Goshaw,12 Y.Gotra,42K.Goulianos,45A.Gresele,3G.Grim,5C.Grosso-Pilcher,10M.Guenther,43 J.Guimaraes da Costa,18C.Haber,26K.Hahn,40S.R.Hahn,13E.Halkiadakis,44C.Hall,18 R.Handler,55 F.Happacher,15K.Hara,52M.Hare,53R.F.Harr,30R.M.Harris,13 F.Hartmann,23K.Hatakeyama,45J.Hauser,6C.Hays,12E.Heider,53B.Heinemann,27 J.Heinrich,40M.Hennecke,23M.Herndon,22 C.Hill,7 D.Hirschbuehl,23 A.Hocker,44 K.D.Ho?man,10A.Holloway,18S.Hou,1M.A.Houlden,27B.T.Hu?man,38R.E.Hughes,35 J.Huston,31K.Ikado,54J.Incandela,7G.Introzzi,41M.Iori,46Y.Ishizawa,52C.Issever,7 A.Ivanov,44Y.Iwata,20 B.Iyutin,29 E.James,30 D.Jang,47J.Jarrell,33 D.Jeans,46 H.Jensen,13M.Jones,40S.Y.Jun,9T.Junk,21T.Kamon,48J.Kang,30M.Karagoz Unel,34 P.E.Karchin,30S.Kartal,13Y.Kato,37Y.Kemp,23R.Kephart,13U.Kerzel,23D.Khazins,12 V.Khotilovich,48 B.Kilminster,44 B.J.Kim,25 D.H.Kim,25H.S.Kim,21J.E.Kim,25 M.J.Kim,9M.S.Kim,25S.B.Kim,25S.H.Kim,52T.H.Kim,29Y.K.Kim,10B.T.King,27 M.Kirby,12M.Kirk,4L.Kirsch,4S.Klimenko,14B.Knuteson,10H.Kobayashi,52P.Koehn,35 K.Kondo,54J.Konigsberg,14K.Kordas,50 A.Korn,29 A.Korytov,14K.Kotelnikov,32 A.V.Kotwal,12 A.Kovalev,40J.Kraus,21I.Kravchenko,29 A.Kreymer,13J.Kroll,40 M.Kruse,12V.Krutelyov,48S.E.Kuhlmann,2N.Kuznetsova,https://www.sodocs.net/doc/6f2088848.html,asanen,https://www.sodocs.net/doc/6f2088848.html,i,50 https://www.sodocs.net/doc/6f2088848.html,mi,https://www.sodocs.net/doc/6f2088848.html,mmel,https://www.sodocs.net/doc/6f2088848.html,ncaster,https://www.sodocs.net/doc/6f2088848.html,ncaster,https://www.sodocs.net/doc/6f2088848.html,nder,https://www.sodocs.net/doc/6f2088848.html,nnon,21 https://www.sodocs.net/doc/6f2088848.html,th,https://www.sodocs.net/doc/6f2088848.html,tino,https://www.sodocs.net/doc/6f2088848.html,uhakangas,https://www.sodocs.net/doc/6f2088848.html,zzizzera,39Y.Le,22 C.Lecci,23 T.LeCompte,2J.Lee,25J.Lee,44S.W.Lee,48N.Leonardo,29S.Leone,41J.D.Lewis,13 K.Li,56 C.S.Lin,13M.Lindgren,6T.M.Liss,21 D.O.Litvintsev,13T.Liu,13Y.Liu,16 N.S.Lockyer,40 A.Loginov,32J.Loken,38M.Loreti,39P.Loverre,46 D.Lucchesi,39 P.Lukens,13L.Lyons,38J.Lys,26 D.MacQueen,50R.Madrak,18K.Maeshima,13 P.Maksimovic,22L.Malferrari,3G.Manca,38R.Marginean,35A.Martin,56M.Martin,22 V.Martin,34M.Martinez,13T.Maruyama,10H.Matsunaga,52M.Mattson,30P.Mazzanti,3 K.S.McFarland,44 D.McGivern,28P.M.McIntyre,48P.McNamara,47R.McNulty,27 S.Menzemer,23 A.Menzione,41P.Merkel,13C.Mesropian,45A.Messina,46A.Meyer,13 T.Miao,https://www.sodocs.net/doc/6f2088848.html,ler,https://www.sodocs.net/doc/6f2088848.html,ler,https://www.sodocs.net/doc/6f2088848.html,ler,30R.Miquel,26S.Miscetti,15M.Mishina,13 G.Mitselmakher,14A.Miyamoto,24Y.Miyazaki,37N.Moggi,3R.Moore,13M.Morello,41 T.Moulik,43A.Mukherjee,13M.Mulhearn,29T.Muller,23R.Mumford,22 A.Munar,40 P.Murat,13S.Murgia,31J.Nachtman,13S.Nahn,56I.Nakamura,40I.Nakano,36A.Napier,53
R.Napora,22V.Necula,14 F.Niell,30J.Nielsen,26 C.Nelson,13T.Nelson,13 C.Neu,35 M.S.Neubauer,29 C.Newman-Holmes,13A-S.Nicollerat,16T.Nigmanov,42H.Niu,4 L.Nodulman,2K.Oesterberg,19T.Ogawa,54S.Oh,12Y.D.Oh,25T.Ohsugi,20R.Oishi,52 T.Okusawa,37R.Oldeman,40R.Orava,19W.Orejudos,26C.Pagliarone,41F.Palmonari,41 R.Paoletti,41V.Papadimitriou,49D.Partos,4S.Pashapour,50J.Patrick,13G.Pauletta,51 M.Paulini,9T.Pauly,38C.Paus,29D.Pellett,5A.Penzo,51T.J.Phillips,12G.Piacentino,41 J.Piedra,8K.T.Pitts,21 A.Pompoˇs,43L.Pondrom,55G.Pope,42O.Poukhov,11 F.Prakoshyn,11T.Pratt,27 A.Pronko,14J.Proudfoot,2 F.Ptohos,15G.Punzi,41 J.Rademacker,38A.Rakitine,29S.Rappoccio,18F.Ratnikov,47H.Ray,30A.Reichold,38 V.Rekovic,33P.Renton,38M.Rescigno,46 F.Rimondi,3K.Rinnert,23L.Ristori,41 M.Riveline,50W.J.Robertson,12 A.Robson,38T.Rodrigo,8S.Rolli,53L.Rosenson,29 R.Roser,13R.Rossin,39C.Rott,43J.Russ,9A.Ruiz,8D.Ryan,53H.Saarikko,19A.Safonov,5 R.St.Denis,17W.K.Sakumoto,44D.Saltzberg,6C.Sanchez,35A.Sansoni,15L.Santi,51 S.Sarkar,46K.Sato,52P.Savard,50 A.Savoy-Navarro,13P.Schemitz,23P.Schlabach,13
E.E.Schmidt,13M.P.Schmidt,56M.Schmitt,34G.Scho?eld,5L.Scodellaro,39A.Scribano,41
F.Scuri,41A.Sedov,43S.Seidel,33Y.Seiya,52F.Semeria,3L.Sexton-Kennedy,13I.S?ligoi,15 M.D.Shapiro,26T.Shears,27P.F.Shepard,42M.Shimojima,52M.Shochet,10Y.Shon,55 A.Sidoti,41M.Siket,1A.Sill,49P.Sinervo,50A.Sisakyan,11A.Skiba,23A.J.Slaughter,13 K.Sliwa,53J.R.Smith,5 F.D.Snider,13R.Snihur,28S.V.Somalwar,47J.Spalding,13 M.Spezziga,49L.Spiegel,13 F.Spinella,41M.Spiropulu,10H.Stadie,23 B.Stelzer,50 O.Stelzer-Chilton,50J.Strologas,21D.Stuart,7A.Sukhanov,14K.Sumorok,29H.Sun,53 T.Suzuki,52 A.Ta?ard,21S.F.Takach,30H.Takano,52R.Takashima,20Y.Takeuchi,52 K.Takikawa,52P.Tamburello,12M.Tanaka,2R.Tanaka,36B.Tannenbaum,6N.Tanimoto,36 S.Tapprogge,19M.Tecchio,30P.K.Teng,1K.Terashi,45R.J.Tesarek,13S.Tether,29 J.Thom,13A.S.Thompson,17E.Thomson,35R.Thurman-Keup,2P.Tipton,44V.Tiwari,9 https://www.sodocs.net/doc/6f2088848.html,aczyk,13 D.Toback,48K.Tollefson,31 D.Tonelli,41M.T¨o nnesmann,31S.Torre,41 D.Torretta,13W.Trischuk,50J.Tseng,29R.Tsuchiya,54S.Tsuno,52 D.Tsybychev,14 N.Turini,41M.Turner,27 https://www.sodocs.net/doc/6f2088848.html,egawa,52T.Unverhau,17S.Uozumi,52 https://www.sodocs.net/doc/6f2088848.html,ynin,40 L.Vacavant,26T.Vaiciulis,44 A.Varganov,30 E.Vataga,41S.Vejcik III,13
G.Velev,13 G.Veramendi,26T.Vickey,21R.Vidal,13I.Vila,8R.Vilar,8I.Volobouev,26M.von der Mey,6 R.G.Wagner,2R.L.Wagner,13W.Wagner,23N.Wallace,47T.Walter,23Z.Wan,47 M.J.Wang,1S.M.Wang,14B.Ward,17S.Waschke,17D.Waters,28T.Watts,47M.Weber,26
W.Wester,13B.Whitehouse,53A.B.Wicklund,2E.Wicklund,13T.Wilkes,5H.H.Williams,40 P.Wilson,13 B.L.Winer,35P.Wittich,40S.Wolbers,13M.Wolter,53M.Worcester,6 S.Worm,47T.Wright,30X.Wu,16F.W¨u rthwein,29A.Wyatt,28A.Yagil,13T.Yamashita,36 K.Yamamoto,37U.K.Yang,10W.Yao,26G.P.Yeh,13K.Yi,22J.Yoh,13P.Yoon,44 K.Yorita,54T.Yoshida,37I.Yu,25S.Yu,40Z.Yu,56J.C.Yun,13L.Zanello,46A.Zanetti,51 I.Zaw,18F.Zetti,41J.Zhou,47A.Zsenei,16and S.Zucchelli,3
(CDF Collaboration)
1
Institute of Physics,Academia Sinica,Taipei,Taiwan11529,Republic of China
2
Argonne National Laboratory,Argonne,Illinois60439
3
Istituto Nazionale di Fisica Nucleare,University of Bologna,I-40127Bologna,Italy
4
Brandeis University,Waltham,Massachusetts02254
5
University of California at Davis,Davis,California95616
6
University of California at Los Angeles,Los Angeles,California90024
7
University of California at Santa Barbara,Santa Barbara,California93106
8
Instituto de Fisica de Cantabria,CSIC-University of Cantabria,39005Santander,Spain
9
Carnegie Mellon University,Pittsburgh,Pennsylvania15213
10
Enrico Fermi Institute,University of Chicago,Chicago,Illinois60637
11
Joint Institute for Nuclear Research,RU-141980Dubna,Russia
12
Duke University,Durham,North Carolina27708
13
Fermi National Accelerator Laboratory,Batavia,Illinois60510
14
University of Florida,Gainesville,Florida32611
15
Laboratori Nazionali di Frascati,Istituto Nazionale di Fisica Nucleare,I-00044Frascati,Italy
16
University of Geneva,CH-1211Geneva4,Switzerland
17
Glasgow University,Glasgow G128QQ,United Kingdom
18
Harvard University,Cambridge,Massachusetts02138
19
The Helsinki Group:Helsinki Institute of Physics;and Division of High Energy Physics,Department of Physical
Sciences,University of Helsinki,FIN-00014Helsinki,Finland
20
Hiroshima University,Higashi-Hiroshima724,Japan
21
University of Illinois,Urbana,Illinois61801
22
The Johns Hopkins University,Baltimore,Maryland21218
23
Institut f¨u r Experimentelle Kernphysik,Universit¨a t Karlsruhe,76128Karlsruhe,Germany
24
High Energy Accelerator Research Organization(KEK),Tsukuba,Ibaraki305,Japan
25
Center for High Energy Physics:Kyungpook National University,Taegu702-701;Seoul National University,Seoul 151-742;and SungKyunKwan University,Suwon440-746;Korea
26
Ernest Orlando Lawrence Berkeley National Laboratory,Berkeley,California94720
27
University of Liverpool,Liverpool L697ZE,United Kingdom
28
University College London,London WC1E6BT,United Kingdom
29
Massachusetts Institute of Technology,Cambridge,Massachusetts02139
30
University of Michigan,Ann Arbor,Michigan48109
31
Michigan State University,East Lansing,Michigan48824
32
Institution for Theoretical and Experimental Physics,ITEP,Moscow117259,Russia
33
University of New Mexico,Albuquerque,New Mexico87131
34
Northwestern University,Evanston,Illinois60208
35
The Ohio State University,Columbus,Ohio43210
36
Okayama University,Okayama700-8530,Japan
37
Osaka City University,Osaka588,Japan
38
University of Oxford,Oxford OX13RH,United Kingdom
39
Universit′a di Padova,Istituto Nazionale di Fisica Nucleare,Sezione di Padova-Trento,I-35131Padova,Italy
40
University of Pennsylvania,Philadelphia,Pennsylvania19104
41
Istituto Nazionale di Fisica Nucleare,University and Scuola Normale Superiore of Pisa,I-56100Pisa,Italy
42
University of Pittsburgh,Pittsburgh,Pennsylvania15260
43
Purdue University,West Lafayette,Indiana47907
44
University of Rochester,Rochester,New York14627
45
The Rockefeller University,New York,New York10021
46
Instituto Nazionale de Fisica Nucleare,Sezione di Roma,University di Roma I,“La Sapienza,”I-00185Roma,Italy
47
Rutgers University,Piscataway,New Jersey08855
48
Texas A&M University,College Station,Texas77843
49
Texas Tech University,Lubbock,Texas79409
50
Institute of Particle Physics,University of Toronto,Toronto M5S1A7,Canada
51
Istituto Nazionale di Fisica Nucleare,Universities of Trieste and Udine,Italy
52
University of Tsukuba,Tsukuba,Ibaraki305,Japan
53
Tufts University,Medford,Massachusetts02155
54
Waseda University,Tokyo169,Japan
55
University of Wisconsin,Madison,Wisconsin53706
56
Yale University,New Haven,Connecticut06520
We report on a search for the?avor-changing neutral current decay D0→μ+μ?
in p s=1.96TeV using65pb?1of data collected by the CDF II
experiment at the Fermilab Tevatron Collider.A displaced-track trigger se-
lects long-lived D0candidates in the D0→μ+μ?search channel,the kine-
matically similar D0→π+π?channel used for normalization,the Cabbibo-
favored D0→K?π+channel used to optimize the selection criteria in an un-
biased manner,and their charge conjugates.Finding no signal events in the
D0→μ+μ?search window,we set an upper limit on the branching fraction
B(D0→μ+μ?)≤2.5×10?6(3.3×10?6)at the90%(95%)con?dence level.
PACS numbers:13.20.Fc,14.40.Lb
The?avor-changing neutral current(FCNC)decay D0→μ+μ?[1]is highly suppressed in the Standard Model(SM)by the nearly exact Glashow-Iliopoulos-Maiani(GIM)[2]cancel-lation.Observation of this decay at a rate signi?cantly exceeding the SM expectation would indicate the presence of non-SM particles or couplings.In the context of the SM,Burdman et al.[3]calculate the branching fraction to be B(D0→μ+μ?)≈10?18from short-distance processes,increasing to B(D0→μ+μ?)≈10?13when long-distance processes are included. This prediction is many orders of magnitude beyond the reach of the present generation of experiments,whose most stringent published limits are4.1×10?6from BEATRICE[4]and 4.2×10?6from E771[5]at the90%con?dence level.Thus,a large,unexplored region exists in which to search for new physics.
Burdman et al.consider the e?ects on D0→μ+μ?from a number of extensions to the Standard Model:R-parity violating SUSY,multiple Higgs doublets,extra fermions,extra dimensions,and extended technicolor.They?nd that the D0→μ+μ?branching ratio can be enhanced by orders of magnitude to the range of10?8to10?10in these scenarios,and in the case of R-parity violating SUSY,roughly to the level of the existing experimental limit. Similar enhancements can occur in K and B-decays,but charm decays provide a unique laboratory to search for new physics couplings in the up-quark sector.
This search uses a65pb?1data sample recorded by the upgraded Collider Detector at Fermilab(CDF II)at the Tevatron p s=1.96TeV between February2002 and January2003.The components of the CDF II detector pertinent to this analysis are described brie?y below.Detailed descriptions can be found elsewhere[6].CDF uses a cylindrical coordinate system in whichφis the azimuthal angle,r is the radius from the nominal beamline,and z points in the proton beam direction and is zero at the center of the detector.The transverse plane is the plane perpendicular to the z axis.The pseudorapidity ηis de?ned asη≡tanh?1(cosθ),whereθis the polar angle measured from the z axis.A silicon microstrip detector(SVX II)[7]and a cylindrical drift chamber(COT)[8]immersed in a1.4T solenoidal magnetic?eld track charged particles in the range|η|<1.0.The SVX II provides up to?ve r-φposition measurements,each of roughly15μm precision, at radii between2.5and10.6cm.The COT has96measurement layers,between40cm and137cm in radius,organized into alternating axial and±2?stereo superlayers.The solenoid covers r<150cm,and electromagnetic and hadronic calorimetry occupy the region between150and350cm in radius.Four layers of planar drift chambers(CMU)[9] outside the hadron calorimeter cover the region|η|<0.6and detect muons of transverse momentum p T>1.4GeV/c penetrating the5absorption lengths of calorimeter material.
The D0decays used in this analysis are selected with a three-level trigger system.At the?rst level,charged tracks are reconstructed in the COT transverse plane by a hardware processor(XFT)[10].The trigger requires two oppositely charged tracks with reconstructed transverse momenta p T≥2GeV/c and p T1+p T2≥5.5GeV/c.At the second level,the
Silicon Vertex Tracker(SVT)[11]associates SVX II position measurements with XFT tracks. The impact parameter of the track,d0,with respect to the beamline,is measured with50μm resolution,which includes a～30μm contribution from the transverse beam size.Requiring two tracks with120μm≤|d0|≤1.0mm selects a sample enriched in heavy?avor.The two trigger tracks must have an opening angle satisfying2?≤|?φ|≤90?and be consistent with the decay of a particle traveling a transverse distance L xy>200μm from the beamline.At the third level,a computing farm performs complete event reconstruction.The sample of ～105D?-tagged two-body D0decays selected by the trigger is used to estimate backgrounds, to optimize selection requirements,and to normalize the sensitivity of the search from the data sample itself.
The D0→μ+μ?branching ratio,or upper limit,is determined using B(D0→μ+μ?)≤B(D0→π+π?)N(μμ)?(μμ)a(ππ)
and kept candidates in the range1.840GeV/c2 additional pion track(πs)with the D0candidate and requiring the mass di?erence M pair+πs?M pair to lie in the range144MeV/c2to147MeV/c2.Theπs track was not required to be CMU?ducial or to be a trigger track,but it had to have the Cabbibo-favored charge for the Kπdecay. The ratio?(ππ)/?(μμ)was determined from the muon identi?cation e?ciency and the pion reconstruction e?ciency,measured in other analyses,as follows.From a sample of J/ψ→μ+μ?decays collected by a trigger requiring one identi?ed muon and one SVT track,the CMU identi?cation e?ciency for the unbiased muon was measured o?ine as a function of its transverse momentum p T.We convoluted the e?ciency spectrum with the p T spectrum of pions from D0→π+π?and determined the e?ective dimuon identi?cation e?ciency to be?(μμ)=0.800±https://www.sodocs.net/doc/6f2088848.html,ing a detailed GEANT[13]detector simulation,the pion reconstruction e?ciency was found to be95±1%,yielding?(ππ)=0.90±0.02,where the ine?ciency arises primarily from hadronic interactions with detector https://www.sodocs.net/doc/6f2088848.html,bining these values we?nd?(ππ)/?(μμ)=1.13±https://www.sodocs.net/doc/6f2088848.html,ing the same detector simulation,we?nd the acceptance ratio a(ππ)/a(μμ)=0.96±0.02. The number of D0→π+π?decays,N(ππ),was determined by?tting the peak in the ππinvariant mass spectrum.We performed a binnedχ2?t with Gaussian signal plus linear background,as shown in Fig.1.Both the mean and width of the Gaussian were free parameters in the?t.N(ππ)is the integral of the Gaussian over the±22MeV/c2mass window around1.862GeV/c2. The background to D0→μ+μ?was taken as the sum of two contributions having di?erent mass spectra:a peaked contribution from D0→π+π?decays in which both pions are misidenti?ed as muons,and a relatively?at background due to all other sources.The ?at background was estimated from the number ofμμcandidates in a high mass sideband spanning the range1.90GeV/c2 events scaled by the ratio of the width of the signal region to the sideband region,44/150. The misidenti?cation background was estimated from the number of D0→π+π?events reconstructed with theμμmass assignment and lying in the±22MeV/c2signal window (shaded area falling between the arrows in Fig.1)times the square of the probability for a pion to be misidenti?ed as a muon.Theπ-misidenti?cation probability was determined from the sample of D?-tagged D0→K?π+events.The averageπ-misidenti?cation probability is1.3±0.1%. Three additional selection requirements were imposed.To remove instances in which the two D0decay daughters extrapolate to the same region of the CMU,potentially correlating the muon identi?cation of the two tracks,we cut on the azimuthal angle?φCMU between their projections into the CMU.To suppress combinatoric backgrounds,we cut on the impact parameter with respect to the beamline,d xy,of the reconstructed D0trajectory.Further, we cut on the transverse decay length of the D0candidate,L xy.The values of these cuts were optimized as described below. We determined the optimal cut values by maximizing a?gure of merit given by √ S/(1.5+ B for large B and it behaves as S as the esti-mated background approaches zero.The constant in the denominator is chosen to favor cuts that maximize the discovery reach at3σsigni?cance.To estimate S in the optimization, we used the D0→π+π?sample.To estimate the misidenti?cation component of B,we used a sample of D0→K?π+decays in which both tracks were found to be misidenti?ed as muons.To estimate all remaining contributions to B,we used the subset of the high-mass ππsideband sample in which one track was identi?ed as a muon and the other was not. Note that the events used to estimate B in the optimization are distinct from the events used in the?nal background estimate for the result.The resulting selection requirements are:|?φCMU|>0.085rad,|d xy|<150μm,and L xy<0.45cm.When applied to the samples used for optimization these cuts remove approximately58%of the background events and 12%of the signal events. Using the optimized selection requirements,5.0±2.2events remain in the high mass sideband,yielding1.6±0.7expected from the?at component of the background.The number of D0→π+π?events falling in the signal window when reconstructed using the muon mass,multiplied by the square of the1.3%π-misidenti?cation probability,yields 0.22±0.02expected misidenti?cation events.The total expected background is1.8±0.7 events.The number of events in the normalization mode is N(ππ)=1412±54(Fig.1). Using this background estimate and normalization,the90%con?dence level sensitivity[15] is4.4×10?6. We apply the optimized selection requirements to the signal region of theμμsample and?nd no events remaining,as displayed in Fig.2.Conservatively taking the number of background events to equal zero,the90%(95%)con?dence level upper limit on the number of D0→μ+μ?events is2.3(3.0).Using Eq.(1)we?nd an upper limit on the branching fraction of B(D0→μ+μ?)≤2.5×10?6(3.3×10?6)at the90%(95%)con?dence level. The uncertainties on N(ππ),?(ππ)/?(μμ),a(ππ)/a(μμ),and B(D0→π+π?)are incor-porated into the limit using the prescription of Cousins and Highland[16].However,all of the uncertainties are smaller than5%and have a negligible e?ect on the limit. In summary,we have searched for the FCNC decay D0→μ+μ?,using the new displaced-track trigger of the CDF II experiment.This is the?rst result from CDF in the?eld of rare charm decays.To minimize bias in the event selection,a blinded search was performed. To minimize dependence on Monte Carlo simulation,most of the needed quantities were determined directly from the data.No events were observed and we set an upper limit on the branching ratio of B(D0→μ+μ?)≤2.5×10?6(3.3×10?6)(2) at the90%(95%)con?dence level.This result improves on the best limits published to date. Acknowledgments We thank the Fermilab sta?and the technical sta?s 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 fuer Bildung and 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;and the Comision Interministerial de Ciencia y Tecnologia,Spain. [1]Throughout this paper inclusion of charge conjugate modes is implicit. [2]S.L.Glashow,J.Iliopoulos,and L.Maiani,Phys.Rev.D2,1285(1970). 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[16]R.D.Cousins and V.L.Highland,Nucl.Instrum.Meth.A320,331(1992). 0100 200 300 M(ππ) (GeV/c 2) e v e n t s p e r 5 M e V /c 2FIG.1:The mass distribution o f candidate D 0→π+π?events.The D 0→μ+μ?branchin g ratio was normalized to the kinematically similar mode D 0→π+π?.The arrows indicate the ±22MeV /c 2mass window used for the signal.The curve is a ?t over the range 1.82to 1.90GeV /c 2wit h Gaussian signal plus linear background.The shaded Gaussian represents the e?ect of recon-structing the events with a μ+μ?mass assignment.The large K πsignal below 1.82GeV /c 2is kinematically separate from the region of interest.The distribution of events in the region above the D 0mass is roughly ?at. 01 2 M(μμ) (GeV/c 2) e v e n t s p e r 5 M e V /c 2FIG.2:The mass distribution o f candidate D 0→μ+μ?events.No events remain in the D 0mass region satisfyin g the event requirements.The events in the hig h mass sideband were used to estimate the background from all sources other than misidenti?cation of D 0→π+π?.