Search for the rare decay B
0s
!
with the D0 detector
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 9 April 2006; published 17 August 2006)
*On leave from IEP SAS Kosice, Slovakia.
†Visitor from Helsinki Institute of Physics, Helsinki, Finland. SEARCH FOR THE RARE DECAY B0
We present a search for the flavor-changing neutral current decay B0
s ! using about 0:45 fb1 of data collected in p p collisions at ps 1:96 TeV with the D0 detector at the Fermilab Tevatron Collider. We find an upper limit on the branching ratio of this decay normalized to B0
s ! J= of
BB0
s!
BB0
s!J= < 4:4 10
3 at the 95% C.L. Using the central value of the world average branching fraction of B0
s ! J= , the limit corresponds to BB0s! < 4:1 106 at the 95% C.L., the most stringent upper bound to date.
DOI:10.1103/PhysRevD.74.031107 PACS numbers: 13.20.He, 12.15.Mm, 14.40.Nd
The investigation of rare flavor-changing neutral current (FCNC) B meson decays has received special attention in the past since this opens up the possibility of precision tests of the flavor structure of the standard model (SM). In the SM, FCNC decays are absent at tree level but proceed at higher order through electroweak penguin and box dia-grams. FCNC decays are sensitive to new physics, since decay amplitudes involving new particles interfere with SM amplitudes. Although inclusive FCNC decays like
B ! Xs‘‘ or B ! Xsare theoretically easier to
cal-culate, exclusive decays with one hadron in the final state are experimentally easier to study. For instance, the exclu-sive decays B0
d! K
‘‘and B! K‘‘have been already measured at B-factories [1,2] and were found to be consistent with the SM within the present experimental uncertainties. Related to the same quark-level transition of
b ! s‘‘ is the corresponding exclusive FCNC decay
B0s ! in the B0smeson system. An observation of
this decay or experimental upper limit on its rate will yield additional important information on the flavor dynamics of FCNC decays.
Within the SM, the decay rate for the B0
s!
decay, neglecting the interference effects with the much stronger B0
s! J= and B0s ! 2S resonance decays,
is predicted to be of the order of 1:6 106[3] with about 30% uncertainty due to poorly known form factors. The interference effects with the B0
s resonance decay
ampli-tudes are large, with their expected magnitude depending on the exact modeling of the charmonium states [4]. To separate experimentally the FCNC-mediated process B0
s !
, one has to restrict the invariant mass of the final state lepton pair to be outside the charmonium resonances. Presently, the only existing experimental bound on the
B0
s ! decay is given by CDF from the analysis
of Run I data [5]. CDF sets an upper limit at the 95% C.L. ofBB0
s! < 6:7 105.
In this Letter, we report on a new experimental limit of the decay B0
s! , that is an order of magnitude
more stringent than the existing limit. The mesons are reconstructed through their KK decay mode assuming the two tracks forming the candidate to be kaons. The invariant mass of the two muons in the final state is required to be outside the charmonium resonances. The events in our search are normalized to resonant decay
B0s ! J= events. Using the B0s! J= mode as the
normalization channel has the advantage that the efficien-cies to detect the system in signal and normal-ization events are similar, and systematic effects tend to cancel.
The search uses a data set corresponding to approxi-mately 0:45 fb1 of p p collisions at ps 1:96 TeV re-corded by the D0 detector operating at the Fermilab Tevatron Collider. The D0 detector is described in detail elsewhere [6]. The main elements relevant for this analysis are the central tracking and muon detector systems. The central tracking system consists of a silicon microstrip tracker (SMT) and a central fiber tracker (CFT), both located within a 2 T superconducting solenoidal magnet. The muon detector, which is located outside the calorime-ter, consists of a layer of tracking detectors and scintillation trigger counters in front of 1.8 T toroidal magnets, fol-lowed by two more similar layers after the toroids, allow-ing for efficient muon detection out to pseudorapidity () of 2:2.
Dimuon triggers were used in the data selection for this analysis. A trigger simulation was used to estimate the trigger efficiency for the signal and normalization samples. These efficiencies were also checked with samples of J= events collected with single muon triggers [7]. The trigger is almost fully efficient for muons above 5 GeV=c already at the first level.
The event preselection starts with a loose selection of
B0
s! candidates. These candidates are identified
by requiring exactly two muons fulfilling quality cuts on the number of hits in the muon system and the two addi-tional charged particle tracks to form a good vertex. The reconstructed invariant mass of the B0
scandidate should be
within 4:4 < m
< 6:2 GeV=c2.
We then require the invariant mass of the two muons to be within 0:5 < m< 4:4 GeV=c2. In this mass re-gion, the J= ! and 2S!
reso-nances are excluded to discriminate against dominant resonant decays by rejecting the mass region 2:72 <
m
< 4:06 GeV=c2. The J= mass resolution in data
is given by a Gaussian distribution with 75 MeV=c2. The rejected mass region then covers 5 wide windows around the resonance masses.
The 2=d:o:f: of the two-muon vertex is required to be less than 10. The tracks that are matched to each muon are
required to have at least three (four) measurements in the SMT (CFT) and the transverse momentum of each of the muons (pT) is required to be greater than 2:5 GeV=c with jj < 2:0 to be well inside the fiducial tracking and muon detector acceptances. In order to select well-measured secondary vertices, we define the two-dimensional decay length Lxy in the plane transverse to the beamline, and
require its uncertainty Lxyto be less than 0.15 mm. Lxyis calculated as Lxy ~lvtx ~pBT pB T , where pB T is the transverse
momentum of the candidate B0
s, and ~lvtx represents the
vector pointing from the primary vertex to the secondary vertex. The uncertainty on the transverse decay length,
Lxy, is calculated by taking into account the uncertainties
in both the primary and secondary vertex positions. The primary vertex itself is found for each event using a beam-spot constrained fit as described in Ref. [8].
Next, the number of B0
s! candidates is further
reduced by requiring pB
T> 5 GeV=c and asking the B0s
candidate vertex to have 2< 36 with 5 d.o.f. The two tracks forming the candidate are further required to have
pT> 0:7 GeV=c and their invariant mass within the range
1:008 < m< 1:032 GeV=c2. The successive cuts and the
remaining candidates surviving each cut are shown in TableI.
We apply the same selection for the resonant B0
s !
J= candidates except that the invariant mass of the muon pair is now required to be within 250 MeV=c2 of the J= mass.
For the final event selection, we require the candidate events to satisfy additional criteria. The long lifetime of the
B0
s mesons allows us to reject the random combinatoric
background. For this purpose we use the decay length significance Lxy=Lxy as one of the discriminating
varia-bles, since it gives better discriminating power than the transverse decay length alone.
The fragmentation characteristics of the b quark are such that most of its momentum is carried by the B hadron. Thus the number of extra tracks near the B0
s candidate
tends to be small. Therefore the second discriminant is an isolation variable, I, of the muon and kaon pairs, defined as: I j ~p j j ~pj P trackiB piR < 1 : (1)
Here,PtrackiBpiis the scalar sum over all tracks excluding
the muon and kaon pairs within a cone of R < 1 around the momentum vector ~p of the B0
s candidate
where R p2 2. The final discriminating variable used is the pointing angle , defined as the angle between the momentum vector ~p of the B0
s
candidate and the vector ~lvtx between the primary and secondary vertices. This requirement ensures consistency between the direction of the decay vertex and the momen-tum vector of the B0
s candidate.
We generate signal Monte Carlo (MC) events for the decay B0
s ! using a decay model which includes
the NNLO improved Wilson coefficients [9] for the short-distance part. The form factors obtained from QCD light-cone sum rules are taken from Ref. [10]. These form factors were originally determined for B ! K transitions and were compared with experimental measurements of the branching fraction BB0
d! K
‘‘ in Ref. [9]. Recently, new form factors for the B0
s! transition,
obtained from the light-cone QCD sum rules, were pub-lished [11]. The difference between the form factors in Ref. [9] and those in Ref. [11] reaches 20% for m
<
1 GeV=c2, while elsewhere it remains well below 10%. The analysis is carried out based on signal MC events in the B0
s mass region and on data events in regions outside
the experimental signal window defined as 4:51 <
m
< 6:13 GeV=c2. A 44 MeV=c2 mass shift in the
mass region of interest is introduced to calibrate the D0 tracker.
In order to avoid biasing the analysis procedure, data candidates in the signal mass region are not examined until completion of the analysis, and events in the sideband regions around the B0
smass are used instead. The expected
mass resolution for B0
s ! in the MC is
75 MeV=c2. The start (end) of the upper (lower) sideband was chosen such that it is at least 270 MeV=c2away from the B0
s mass. The widths of the sidebands used for
back-ground estimation are chosen to be 540 MeV=c2each. The size of the blind signal region is 225 MeV=c2 which corresponds to a 3 region around the B0
s mass. To
determine the final limit on the branching fraction, we use a smaller mass region of 2:5.
A random-grid search [12] was used to find simulta-neously the optimal values of the discriminants by max-imizing the figure of merit [13] P sig=a=2 Nback
p .
TABLE I. Number of candidate events surviving the cuts in data used in the preselection analysis.
Selection criteria Value # candidates
Good B0
s vertex 1555320
Mass region (GeV=c2) 0:5 < m
< 4:4 530892 excl. J= , 2S Muon quality 276875 2=d:o:f: of vertex <10 127509 Muon pT GeV=c >2:5 73555 Muon jj <2:0 72350 Tracking hits CFT > 3, SMT > 2 58012 Lxy(mm) <0:15 54752 B0 s candidate pTGeV=c >5:0 54399 B0 s2 vertex <36 53195 Kaon pT GeV=c >0:7 9639 mass (GeV=c2) 1:008 < m < 1:032 2602 SEARCH FOR THE RARE DECAY B0
Here, sig is the reconstruction efficiency of the signal events relative to the preselection (estimated using MC), and Nback is the expected number of background events interpolated from the sidebands. The constant a is the number of standard deviations corresponding to the con-fidence level at which the signal hypothesis is tested. This constant a was set to 2.0, corresponding to about the 95% C.L. After optimization, we find the following values for the discriminating variables: Lxy=Lxy> 10:3, I > 0:72,
and < 0:1 rad.
The total signal efficiency relative to preselection of the three discriminating cuts is 54 3% where the uncer-tainty is statistical only. After a linear interpolation of the sideband population for the whole data sample into the mass window signal region, we obtain an expected number of 1:6 0:4 background events with statistical uncertainty only.
Upon examining the data in the mass region, zero can-didate events are observed in the signal region, consistent with the background events as estimated from sidebands. Figure1shows the remaining events populating the lower and upper sidebands. The Poisson probability of observing zero events for an expected background of 1:6 0:4 is p 0:22.
In the absence of an apparent signal, a limit on the branching fraction BB0
s ! can be computed
by normalizing the upper limit on the number of events in the B0
s signal region to the number of reconstructed
B0 s ! J= events: BB0 s ! BB0 s ! J= Nul NB0 s J= BJ= ! ; (2) where Nul is the upper limit on the number of signal decays, estimated from the number of observed events and expected background events, and NB0
s is the observed
number of B0
s ! J= events. The measured branching
fractions are BJ= ! 5:88 0:10 102 and BB0
s ! J= 9:3 3:3 104 [14]. The
global efficiencies of the signal and normalization channels
are and J= respectively, and include all event selection cuts and the acceptance relative to the entire di-muon mass region. They are determined from MC yielding an efficiency ratio of J= =
2:80 0:21,
where the uncertainty is due to MC statistics. Applying no cut around the charmonium resonances the efficiency ratio would be J= = 0 1:06 0:07. In order to avoid large uncertainties associated with the poorly known branching fraction of B0
s ! J= , we normalize the limit
of B0
s! relative toBB0s ! J= as shown by
Eq. (2).
The same cuts are applied to the B0
s ! J=
candi-dates. The contamination of muon pairs from the nonreso-nant decay in the resonant normalization region
J= ! is negligible. We therefore constrain the
two muons to have an invariant mass equal to the J= mass [14] when calculating the KK invariant mass. The mass spectrum of the reconstructed B0
s ! J= is
shown in Fig. 2. A fit using a Gaussian function for the signal and a second order polynomial for the background yields 73 10 4 B0
s candidates, where the first
uncer-tainty is due to statistics and the second represents the systematic uncertainty which is estimated by varying the fit range as well as the background and signal shape hypotheses.
The different sources of relative uncertainty that enter into the limit calculation ofB of B0
s! are given
in Table II. The largest uncertainty, 25%, is due to the background interpolation into the signal region and is based on the statistical uncertainty of the fit integral. The uncertainty on the number of observed B0
s ! J= events
in the normalization channel is 14.8%. The pT distribution of the B0
s in data is on average
slightly higher than that from MC. Therefore, MC events for the signal and normalization channels have been re-weighted accordingly and an additional uncertainty of 3.7% is applied. The CP-even signal MC events are gen-erated with a B0
slifetime of 1.44 ps [15]. To account for a
possible efficiency difference related with the shorter life-time of the CP-even B0
s, the signal MC events are weighted
according to the combined world average CP-even lifetime [16]. The efficiency difference is estimated to be 8% which ] 2 ) [GeV/c K + K -µ + µ Invariant mass ( 5 5.2 5.4 5.6 5.8 2 Events / 25 MeV/c 0 10 20 30 DØ B0s→ J/ψφ 4 ± 10 ± = 73 s 0 B N
FIG. 2. The invariant mass distribution of the normalization channel B0
s! J= after all selection criteria. ] 2 ) [GeV/c K + K -µ + µ Invariant mass ( 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2 2 Events / 5 MeV/c 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.82 Signal region Sideband 1 Sideband 2 DØ
FIG. 1 (color online). The invariant mass distribution after optimized requirements on the discriminating variables. The solid line shows the sidebands background interpolation into the signal region.
is taken as an additional systematic uncertainty. The sta-tistical uncertainty on the efficiency ratio J= =
is
found to be 7.5%. The signal efficiency obtained from MC is based on the input for the NNLO Wilson coefficients and form factors of Ref. [9]. We do not include any theoretical uncertainty in our systematics uncertainty estimation. The statistical and systematic uncertainties can be included in the limit calculation by integrating over probability func-tions that parameterize the uncertainties. We use a pre-scription [17] where we construct a frequentist confidence interval with the Feldman and Cousins [18] ordering scheme for the MC integration. The background is mod-eled as a Gaussian distribution with its mean value equal to the expected number of background events and its standard deviation equal to the background uncertainty. Including the statistical and systematic uncertainties, the Feldman and Cousins (FC) limit is
BB0
s !
BB0
s! J=
< 4:43:5 103
at the 95% (90%) C.L. respectively [19]. Taking a
Bayesian approach [20] with a flat prior and the uncertain-ties treated as Gaussian distributions in the integration, we find an upper limit of BB0
s ! =BB0s!
J= < 7:45:6 103 at the 95% (90%) C.L.,
respectively.
Since we have fewer events observed than expected, we also quote the sensitivity of our search. Assuming there is only background, we calculate for each possible value of observation a 95% C.L. upper limit weighted by the Poisson probability of occurrence. Including the statistical and systematical uncertainties, our sensitivity is given by hBB0
s ! i=BB0s ! J= 1:11:2 102
at the 95% C.L. using the FC (Bayesian) approaches, respectively.
Using only the central value of the world average branching fraction [14] of BB0
s ! J= 9:3
3:3 104, the FC limit corresponds to BB0
s!
< 4:13:2 106 at the 95% (90%) C.L. re-spectively. This is presently the most stringent upper bound and can be compared with the SM calculation ofBB0
s!
1:6 106of Ref. [3].
We thank the staffs at Fermilab and collaborating insti-tutions, 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.
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Source Relative Uncertainty [%]
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SEARCH FOR THE RARE DECAY B0
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