Search for B
0s
!
decays at D0
V. M. Abazov,35B. Abbott,75M. Abolins,65B. S. Acharya,28M. Adams,51T. Adams,49E. Aguilo,5S. H. Ahn,30 M. Ahsan,59G. D. Alexeev,35G. Alkhazov,39A. Alton,64,*G. Alverson,63G. A. Alves,2M. Anastasoaie,34L. S. Ancu,34
T. Andeen,53S. Anderson,45B. Andrieu,16M. S. Anzelc,53Y. Arnoud,13M. Arov,60M. Arthaud,17A. Askew,49 B. A˚ sman,40A. C. S. Assis Jesus,3O. Atramentov,49C. Autermann,20C. Avila,7C. Ay,23F. Badaud,12A. Baden,61 L. Bagby,52B. Baldin,50D. V. Bandurin,59S. Banerjee,28P. Banerjee,28E. Barberis,63A.-F. Barfuss,14P. Bargassa,80
P. Baringer,58J. Barreto,2J. F. Bartlett,50U. Bassler,16D. Bauer,43S. Beale,5A. Bean,58M. Begalli,3M. Begel,71 C. Belanger-Champagne,40L. Bellantoni,50A. Bellavance,50J. A. Benitez,65S. B. Beri,26G. Bernardi,16R. Bernhard,22 L. Berntzon,14I. Bertram,42M. Besanc¸on,17R. Beuselinck,43V. A. Bezzubov,38P. C. Bhat,50V. Bhatnagar,26C. Biscarat,19
G. Blazey,52F. Blekman,43S. Blessing,49D. Bloch,18K. Bloom,67A. Boehnlein,50D. Boline,62T. A. Bolton,59 G. Borissov,42K. Bos,33T. Bose,77A. Brandt,78R. Brock,65G. Brooijmans,70A. Bross,50D. Brown,78N. J. Buchanan,49
D. Buchholz,53M. Buehler,81V. Buescher,21S. Burdin,42,†S. Burke,45T. H. Burnett,82C. P. Buszello,43J. M. Butler,62 P. Calfayan,24S. Calvet,14J. Cammin,71S. Caron,33W. Carvalho,3B. C. K. Casey,77N. M. Cason,55H. Castilla-Valdez,32
S. Chakrabarti,17D. Chakraborty,52K. M. Chan,55K. Chan,5A. Chandra,48F. Charles,18,††E. Cheu,45F. Chevallier,13 D. K. Cho,62S. Choi,31B. Choudhary,27L. Christofek,77T. Christoudias,43,**S. Cihangir,50D. Claes,67B. Cle´ment,18
Y. Coadou,5M. Cooke,80W. E. Cooper,50M. Corcoran,80F. Couderc,17M.-C. Cousinou,14S. Cre´pe´-Renaudin,13 D. Cutts,77M. C´ wiok,29H. da Motta,2A. Das,62G. Davies,43K. De,78S. J. de Jong,34P. de Jong,33E. De La Cruz-Burelo,64
C. De Oliveira Martins,3J. D. Degenhardt,64F. De´liot,17M. Demarteau,50R. Demina,71D. Denisov,50S. P. Denisov,38 S. Desai,50H. T. Diehl,50M. Diesburg,50A. Dominguez,67H. Dong,72L. V. Dudko,37L. Duflot,15S. R. Dugad,28 D. Duggan,49A. Duperrin,14J. Dyer,65A. Dyshkant,52M. Eads,67D. Edmunds,65J. Ellison,48V. D. Elvira,50Y. Enari,77
S. Eno,61P. Ermolov,37H. Evans,54A. Evdokimov,73V. N. Evdokimov,38A. V. Ferapontov,59T. Ferbel,71F. Fiedler,24 F. Filthaut,34W. Fisher,50H. E. Fisk,50M. Ford,44M. Fortner,52H. Fox,22S. Fu,50S. Fuess,50T. Gadfort,82C. F. Galea,34 E. Gallas,50E. Galyaev,55C. Garcia,71A. Garcia-Bellido,82V. Gavrilov,36P. Gay,12W. Geist,18D. Gele´,18C. E. Gerber,51
Y. Gershtein,49D. Gillberg,5G. Ginther,71N. Gollub,40B. Go´mez,7A. Goussiou,55P. D. Grannis,72H. Greenlee,50 Z. D. Greenwood,60E. M. Gregores,4G. Grenier,19Ph. Gris,12J.-F. Grivaz,15A. Grohsjean,24S. Gru¨nendahl,50 M. W. Gru¨newald,29J. Guo,72F. Guo,72P. Gutierrez,75G. Gutierrez,50A. Haas,70N. J. Hadley,61P. Haefner,24 S. Hagopian,49J. Haley,68I. Hall,65R. E. Hall,47L. Han,6K. Hanagaki,50P. Hansson,40K. Harder,44A. Harel,71
R. Harrington,63J. M. Hauptman,57R. Hauser,65J. Hays,43T. Hebbeker,20D. Hedin,52J. G. Hegeman,33 J. M. Heinmiller,51A. P. Heinson,48U. Heintz,62C. Hensel,58K. Herner,72G. Hesketh,63M. D. Hildreth,55R. Hirosky,81 J. D. Hobbs,72B. Hoeneisen,11H. Hoeth,25M. Hohlfeld,21S. J. Hong,30R. Hooper,77S. Hossain,75P. Houben,33Y. Hu,72
Z. Hubacek,9V. Hynek,8I. Iashvili,69R. Illingworth,50A. S. Ito,50S. Jabeen,62M. Jaffre´,15S. Jain,75K. Jakobs,22 C. Jarvis,61R. Jesik,43K. Johns,45C. Johnson,70M. Johnson,50A. Jonckheere,50P. Jonsson,43A. Juste,50D. Ka¨fer,20 S. Kahn,73E. Kajfasz,14A. M. Kalinin,35J. R. Kalk,65J. M. Kalk,60S. Kappler,20D. Karmanov,37J. Kasper,62P. Kasper,50
I. Katsanos,70D. Kau,49R. Kaur,26V. Kaushik,78R. Kehoe,79S. Kermiche,14N. Khalatyan,38A. Khanov,76 A. Kharchilava,69Y. M. Kharzheev,35D. Khatidze,70H. Kim,31T. J. Kim,30M. H. Kirby,34M. Kirsch,20B. Klima,50 J. M. Kohli,26J.-P. Konrath,22M. Kopal,75V. M. Korablev,38A. V. Kozelov,38D. Krop,54A. Kryemadhi,81T. Kuhl,23 A. Kumar,69S. Kunori,61A. Kupco,10T. Kurcˇa,19J. Kvita,8F. Lacroix,12D. Lam,55S. Lammers,70G. Landsberg,77 J. Lazoflores,49P. Lebrun,19W. M. Lee,50A. Leflat,37F. Lehner,41J. Lellouch,16J. Leveque,45P. Lewis,43J. Li,78 Q. Z. Li,50L. Li,48S. M. Lietti,4J. G. R. Lima,52D. Lincoln,50J. Linnemann,65V. V. Lipaev,38R. Lipton,50Y. Liu,6,**
Z. Liu,5L. Lobo,43A. Lobodenko,39M. Lokajicek,10A. Lounis,18P. Love,42H. J. Lubatti,82A. L. Lyon,50 A. K. A. Maciel,2D. Mackin,80R. J. Madaras,46P. Ma¨ttig,25C. Magass,20A. Magerkurth,64N. Makovec,15P. K. Mal,55
H. B. Malbouisson,3S. Malik,67V. L. Malyshev,35H. S. Mao,50Y. Maravin,59B. Martin,13R. McCarthy,72 A. Melnitchouk,66A. Mendes,14L. Mendoza,7P. G. Mercadante,4M. Merkin,37K. W. Merritt,50J. Meyer,21,xA. Meyer,20
M. Michaut,17T. Millet,19J. Mitrevski,70J. Molina,3R. K. Mommsen,44N. K. Mondal,28R. W. Moore,5T. Moulik,58 G. S. Muanza,19M. Mulders,50M. Mulhearn,70O. Mundal,21L. Mundim,3E. Nagy,14M. Naimuddin,50M. Narain,77
N. A. Naumann,34H. A. Neal,64J. P. Negret,7P. Neustroev,39H. Nilsen,22A. Nomerotski,50S. F. Novaes,4 T. Nunnemann,24V. O’Dell,50D. C. O’Neil,5G. Obrant,39C. Ochando,15D. Onoprienko,59N. Oshima,50J. Osta,55 R. Otec,9G. J. Otero y Garzo´n,51M. Owen,44P. Padley,80M. Pangilinan,77N. Parashar,56S.-J. Park,71S. K. Park,30 J. Parsons,70R. Partridge,77N. Parua,54A. Patwa,73G. Pawloski,80B. Penning,22K. Peters,44Y. Peters,25P. Pe´troff,15
M.-E. Pol,2P. Polozov,36A. Pomposˇ,75B. G. Pope,65A. V. Popov,38C. Potter,5W. L. Prado da Silva,3H. B. Prosper,49 S. Protopopescu,73J. Qian,64A. Quadt,21,xB. Quinn,66A. Rakitine,42M. S. Rangel,2K. Ranjan,27P. N. Ratoff,42
P. Renkel,79S. Reucroft,63P. Rich,44M. Rijssenbeek,72I. Ripp-Baudot,18F. Rizatdinova,76S. Robinson,43 R. F. Rodrigues,3C. Royon,17P. Rubinov,50R. Ruchti,55G. Safronov,36G. Sajot,13A. Sa´nchez-Herna´ndez,32 M. P. Sanders,16A. Santoro,3G. Savage,50L. Sawyer,60T. Scanlon,43D. Schaile,24R. D. Schamberger,72Y. Scheglov,39
H. Schellman,53P. Schieferdecker,24T. Schliephake,25C. Schwanenberger,44A. Schwartzman,68R. Schwienhorst,65 J. Sekaric,49S. Sengupta,49H. Severini,75E. Shabalina,51M. Shamim,59V. Shary,17A. A. Shchukin,38R. K. Shivpuri,27 D. Shpakov,50V. Siccardi,18V. Simak,9V. Sirotenko,50P. Skubic,75P. Slattery,71D. Smirnov,55J. Snow,74G. R. Snow,67
S. Snyder,73S. So¨ldner-Rembold,44L. Sonnenschein,16A. Sopczak,42M. Sosebee,78K. Soustruznik,8M. Souza,2 B. Spurlock,78J. Stark,13J. Steele,60V. Stolin,36A. Stone,51D. A. Stoyanova,38J. Strandberg,64S. Strandberg,40 M. A. Strang,69M. Strauss,75E. Strauss,72R. Stro¨hmer,24D. Strom,53L. Stutte,50S. Sumowidagdo,49P. Svoisky,55
A. Sznajder,3M. Talby,14P. Tamburello,45A. Tanasijczuk,1W. Taylor,5P. Telford,44J. Temple,45B. Tiller,24 F. Tissandier,12M. Titov,17V. V. Tokmenin,35T. Toole,61I. Torchiani,22T. Trefzger,23D. Tsybychev,72B. Tuchming,17
C. Tully,68P. M. Tuts,70R. Unalan,65S. Uvarov,39L. Uvarov,39S. Uzunyan,52B. Vachon,5P. J. van den Berg,33 B. van Eijk,33R. Van Kooten,54W. M. van Leeuwen,33N. Varelas,51E. W. Varnes,45I. A. Vasilyev,38M. Vaupel,25
P. Verdier,19L. S. Vertogradov,35M. Verzocchi,50F. Villeneuve-Seguier,43P. Vint,43P. Vokac,9E. Von Toerne,59 M. Voutilainen,67,kM. Vreeswijk,33R. Wagner,68H. D. Wahl,49L. Wang,61M. H. L. S Wang,50J. Warchol,55G. Watts,82
M. Wayne,55M. Weber,50G. Weber,23A. Wenger,22,{N. Wermes,21M. Wetstein,61A. White,78D. Wicke,25 G. W. Wilson,58S. J. Wimpenny,48M. Wobisch,60D. R. Wood,63T. R. Wyatt,44Y. Xie,77S. Yacoob,53R. Yamada,50 M. Yan,61T. Yasuda,50Y. A. Yatsunenko,35K. Yip,73H. D. Yoo,77S. W. Youn,53J. Yu,78A. Zatserklyaniy,52C. Zeitnitz,25
D. Zhang,50T. Zhao,82B. Zhou,64J. Zhu,72M. Zielinski,71D. Zieminska,54A. Zieminski,54L. Zivkovic,70 V. Zutshi,52and E. G. Zverev37
(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
6University of Science and Technology of China, Hefei, People’s Republic of China 7Universidad de los Andes, Bogota´, Colombia
8Center for Particle Physics, Charles University, Prague, Czech Republic 9Czech Technical University, Prague, Czech Republic
10Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 11Universidad San Francisco de Quito, Quito, Ecuador
12Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Universite´ Blaise Pascal, Clermont-Ferrand, France 13Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France
14CPPM, IN2P3-CNRS, Universite´ de la Me´diterrane´e, Marseille, France
15Laboratoire de l’Acce´le´rateur Line´aire, IN2P3-CNRS et Universite´ Paris-Sud, Orsay, France 16LPNHE, IN2P3-CNRS, Universite´s Paris VI and VII, Paris, France
17DAPNIA/Service de Physique des Particules, CEA, Saclay, France
18IPHC, Universite´ Louis Pasteur et Universite´ de Haute Alsace, CNRS, IN2P3, Strasbourg, France 19IPNL, Universite´ Lyon 1, CNRS/IN2P3, Villeurbanne, France
and Universite´ de Lyon, Lyon, France
20III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany 21Physikalisches Institut, Universita¨t Bonn, Bonn, Germany 22Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany
23Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 24Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany 25Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
26Panjab University, Chandigarh, India 27Delhi University, Delhi, India
28Tata Institute of Fundamental Research, Mumbai, India 29University College Dublin, Dublin, Ireland 30Korea Detector Laboratory, Korea University, Seoul, Korea
31SungKyunKwan University, Suwon, Korea 32CINVESTAV, Mexico City, Mexico
33FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 34Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
35Joint Institute for Nuclear Research, Dubna, Russia 36Institute for Theoretical and Experimental Physics, Moscow, Russia
37Moscow State University, Moscow, Russia 38Institute for High Energy Physics, Protvino, Russia 39Petersburg Nuclear Physics Institute, St. Petersburg, Russia
40Lund University, Lund, Sweden,
Royal Institute of Technology Stockholm, Sweden, Stockholm University, Stockholm, Sweden, a
nd Uppsala University, Uppsala, Sweden
41Physik Institut der Universita¨t Zu¨rich, Zu¨rich, Switzerland 42Lancaster University, Lancaster, United Kingdom
43Imperial College, London, United Kingdom 44University of Manchester, Manchester, United Kingdom
45University of Arizona, Tucson, Arizona 85721, USA
46Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
and University of California, Berkeley, California 94720, USA
47California State University, Fresno, California 93740, USA 48University of California, Riverside, California 92521, USA 49Florida State University, Tallahassee, Florida 32306, USA 50Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
51University of Illinois at Chicago, Chicago, Illinois 60607, USA 52Northern Illinois University, DeKalb, Illinois 60115, USA
53Northwestern University, Evanston, Illinois 60208, USA 54Indiana University, Bloomington, Indiana 47405, USA 55University of Notre Dame, Notre Dame, Indiana 46556, USA
56Purdue University Calumet, Hammond, Indiana 46323, USA 57Iowa State University, Ames, Iowa 50011, USA 58University of Kansas, Lawrence, Kansas 66045, USA 59Kansas State University, Manhattan, Kansas 66506, USA 60Louisiana Tech University, Ruston, Louisiana 71272, USA 61University of Maryland, College Park, Maryland 20742, USA
62Boston University, Boston, Massachusetts 02215, USA 63Northeastern University, Boston, Massachusetts 02115, USA
64University of Michigan, Ann Arbor, Michigan 48109, USA 65Michigan State University, East Lansing, Michigan 48824, USA
66University of Mississippi, University, Mississippi 38677, USA 67University of Nebraska, Lincoln, Nebraska 68588, USA 68Princeton University, Princeton, New Jersey 08544, USA 69State University of New York, Buffalo, New York 14260, USA
70Columbia University, New York, New York 10027, USA 71University of Rochester, Rochester, New York 14627, USA 72State University of New York, Stony Brook, New York 11794, USA
73Brookhaven National Laboratory, Upton, New York 11973, USA
††Deceased
**Fermilab International Fellow
{
Visitor from Universita¨t Zu¨rich, Zu¨rich, Switzerland
kVisitor from Helsinki Institute of Physics, Helsinki, Finland
x
Visitor from II. Physikalisches Institut, Georg-August-University Go¨ttingen, Germany
‡Visitor from ICN-UNAM, Mexico City, Mexico
†Visitor from The University of Liverpool, Liverpool, United Kingdom
*Visitor from Augustana College, Sioux Falls, South Dakota, USA
74Langston University, Langston, Oklahoma 73050, USA 75University of Oklahoma, Norman, Oklahoma 73019, USA 76Oklahoma State University, Stillwater, Oklahoma 74078, USA
77Brown University, Providence, Rhode Island 02912, USA 78University of Texas, Arlington, Texas 76019, USA 79Southern Methodist University, Dallas, Texas 75275, USA
80Rice University, Houston, Texas 77005, USA 81University of Virginia, Charlottesville, Virginia 22901, USA
82University of Washington, Seattle, Washington 98195, USA
(Received 28 July 2007; published 5 November 2007)
We report results from a search for the decay B0
s ! using 1:3 fb1 of p pcollisions at s
p 1:96 TeV collected by the D0 experiment at the Fermilab Tevatron Collider. We find two candidate events, consistent with the expected background of 1:24 0:99, and set an upper limit on the branching fraction
ofBB0
s! < 1:2 107 at the 95% C.L.
DOI:10.1103/PhysRevD.76.092001 PACS numbers: 13.20.He, 12.15.Mm, 12.60.Jv, 13.85.Qk
The branching fractionBB0
s! is predicted to be 3:4 0:5 109[1] within the standard model (SM), where the decay occurs through helicity and Cabibbo-Kobayashi-Maskawa quark-mixing matrix (CKM)-suppressed processes involving multiple electroweak bo-son exchanges. In supersymmetric (SUSY) models, inter-actions with neutral Higgs bosons can enhance the branching ratio by several orders of magnitude if the value of tan, the ratio of vacuum expectation values for the two neutral CP-even Higgs fields, is high [2–6]. Large en-hancements to BB0
s! are possible in SUSY models with R-parity violating couplings even if tan is low [7]. Improvements to the limit on BB0
s! will constrain the parameter space of such models. The best published experimental bound isBB0
s ! < 2:0 107 at the 95% C.L. [8]. The analysis reported in this paper used 1:3 fb1of p pcollisions collected by the D0 experiment at the Fermilab Tevatron. It supersedes our previous result [9] based on a 240 pb1subsample of the data.
The D0 detector [10] features a three layer muon system [11] with each layer consisting of a scintillator plane and a three or four plane drift chamber, providing coverage for
< j2j, where lntan=2 , and is the polar
angle with respect to the beam line. Muon backgrounds are low due to shielding from 1.8 T iron toroids located between the first and second muon detector layers, and from a 6 –10 interaction length deep uranium/liquid-argon calorimeter located in front of the first layer. Charged particles are detected in the inner central tracking system, which consists of a silicon microstrip tracker (SMT) and a central fiber tracker (CFT), both located within a 2 T superconducting solenoidal magnet. The CFT has eight thin coaxial barrels, each supporting two doublets of over-lapping scintillating fibers of 0.835 mm diameter, one doublet being parallel to the beam axis, and the other alternating by 3. The SMT has four layers of double
sided detectors divided into six longitudinal sections inter-spersed with 16 radial disks. Each layer has a side with strips parallel to the beam axis; two layers have a 2 stereo side, and two layers have a 90 side. Typical strip pitch is 50–80 m.
Events were recorded using a set of single muon trig-gers, dimuon trigtrig-gers, and triggers that selected p p inter-actions based on energy depositions in the calorimeter.
B0
s! [12] candidates were formed from pairs of oppositely charged muons. Each muon was required to have transverse momentum pT> 2:5 GeV, and to have
hits in at least two layers of the muon system, four layers of the CFT, and three layers of the SMT. The B0
scandidate was required to have pT > 5 GeV. There is a large back-ground due primarily to muons from the decay of pions, kaons, and b- or c- flavored hadrons. The B0
s !
signal is characterized by the long lifetime of the B0s, which results in an observable distance between the point at which the B0
s is produced (the primary vertex) and the point at which it decays. The distance from the primary vertex to the B0
s vertex in the transverse plane (LT) was required to have an uncertainty LT< 0:015 cm and a
significance LT=LT> 12. The average LT for signal
events passing the pT requirement is 0:1 cm. Typically
LT is between 0.002 and 0.009 cm for both signal and
background. The angle between the projections onto the transverse plane of the B0
s momentum and the displace-ment from the primary vertex to the B0
svertex was required to be less than 15. The distance of closest approach of each muon to the primary vertex in the transverse plane was calculated, along with the corresponding uncertainty
and significance =. The smaller of the two signifi-cances, min=, was required to be greater than 2.8. This removes a class of events in which one of the tracks is consistent with originating from the primary vertex. A constrained fit was applied, enforcing the conditions that the tracks making up the B0
dimensional B0
strajectory pass through the primary vertex. The fit probability P2 is the fraction of the area of the 2
distribution that lies below the 2 value returned by the
constrained fit. It was required to be at least 0.01.
To further suppress the background, a likelihood ratio test was applied. Five variables were incorporated:
(1) isolation, defined as pB T=pBT
P
pT where pBT is the transverse momentum of the B0
s system, and Pp
T is the scalar sum of the transverse momenta of all other tracks within a cone of R < 1 around the B0 s system, where R 2 2 p and
is the azimuthal angle (2) P2
(3) LT=LT
(4) min=
(5) m, the mass of the dimuon system.
The likelihood ratio was approximated as r Q5i1Si=Bi where Siis the probability distribution of the ith variable for the signal, and Biis the distribution for the background. The discriminant D5 r=1 r takes a value between
zero (background-like) and one (signal-like). Figure 1 shows the distributions of Si and Bi for isolation and for functions of LT=LT, min=, and P
2. The
func-tions map the quantities into the range zero to one. They are given by f1LT=LT 1 exp0:057LT=LT
12 , f2min= 1 exp0:093min= 2:8 , and f3P2 P2 0:01=0:99. In Fig. 1,
the signal and background events satisfy all of the prese-lection cuts defined earlier except for the cut on LT sig-nificance. To increase the statistics, the LT significance cut was relaxed from 12 to five. The signal distributions Siare given by the histograms in Fig.1. These distributions are
Isolation
0
0.2
0.4 0.6 0.8
1
Probability/0.02
0
0.02
0.04
0.06
0.08
-1DØ, L=1.3 fb
(a)
)
Lσ
/
T(L
1f
0
0.2
0.4 0.6 0.8
1
Probability/0.02
0
0.02
0.04
0.06
(b)
)]
δσ
/
δ
[min(
2f
0
0.2
0.4 0.6 0.8
1
Probability/0.02
0
0.02
0.04
0.06
0.08
(c)
)]
2χ
[p(
3f
0
0.2
0.4 0.6 0.8
1
Probability/0.02
0
0.05
0.1
0.15
(d)
FIG. 1. Signal and background distributions for four of the variables used in the likelihood ratio test. The smooth curves are
parametrizations of the sideband data (points), representing the background. The sideband distribution in (a) is parametrized as a Gaussian function. In (b), (c), and (d), the sideband distributions are parametrized as the sum of two exponential functions. The signal distributions (histograms) are from MC.
the result of Monte Carlo (MC) simulations using the
PYTHIA event generator [13] interfaced with the EVTGEN
decay package [14], followed by full GEANT v3.15 [15] modeling of the detector response. The simulation was tuned to reproduce the momentum resolution and scale, the trigger efficiency, and the B meson pT distribution observed in data. The MC events were processed with the same event reconstruction used for the data. The back-ground distributions Bi are given by parametrizations of the sideband data, shown in Fig. 1. The sideband data consist of candidates having a dimuon invariant mass
mbetween 4.5 and 7.0 GeV excluding the signal region. The signal region is between 4.972 and 5.717 GeV, ap-proximately 3 standard deviations around the mean of the Gaussian m distribution in the signal MC. The sideband isolation distribution was fit to a Gaussian func-tion, and the other three sideband distributions were fit to the sum of two exponential functions. The m distribu-tion of the background was approximated to be flat when computing the likelihood ratio. The distribution of D5for
signal and background is shown in Fig.2. Final candidates were required to have mwithin the signal region and to satisfy D5> 0:949. This threshold was chosen to optimize
the expected 95% C.L. upper bound onBB0
s! . Two candidates pass the final selection.
An important feature of the background is seen in Fig.3, which shows the distribution of m after various cuts, beginning with the LT significance cut and ending with
D4> 0:949. The discriminant D4 was calculated in the same way as D5except that the variable mwas omitted, thereby simulating the effect of a cut on D5without biasing the mdistribution toward the B0
smass. Two components are evident in the distributions: a steeply falling component
in the low mass region and a gradually falling component whose slope diminishes as the cuts tighten. This structure was studied using b bevents generated withPYTHIA, which reproduced the main features of the data. The contributions from particles misidentified as muons and other sources of real muons are small. The gradually falling component consists of events in which the two muons arise from the decay of separate b quarks, while the steeply falling com-ponent consists of events in which the two muons arise from decay of the same b quark, via sequential decay b !
c followed by c ! s or from b ! 0X with 0!
. Higher mass 0states may also contribute in the data. Because the same-b processes result from a single b quark, they have a better chance of producing a dimuon system that forms a common vertex and points back to the primary vertex than do the separate-b processes.
The expected number of background events in the final candidate sample was estimated using events from the data in the low and high sidebands, together with the assump-tion that the background consists of same-b events having an exponential mass distribution and separate-b events having a flat mass distribution. This model of the shape of the backgrounds fits the sideband regions well and accurately predicts the number of events in the signal region, see Fig.3. The slope of the exponential was taken from the fit in Fig.3(d). The fits in Figs.3(c)and3(d)are consistent with a flat distribution for separate-b events. The separate-b distribution might still decrease gradually with mass after a cut on D4, but the slope is not well constrained by the statistics in the high sideband, and to neglect it is conservative in its effect on the branching fraction limit. Given the number of events in the low sideband and the slope of the exponential, the expected contribution of same-b events to the high sideband is negligible. The estimated background from separate-b events isPiPi w where the sum is over all events in the high sideband. The variable w is the expected number of separate-b events in the signal region per separate-b event in the high sideband, determined from the range of the signal region, the range of the high sideband region, and the shape of the mass distri-bution for separate-b events. The variable Piis the proba-bility for a separate-b event to pass the cut D5> 0:949
given that it falls within the signal region and has the specific value of D4 observed for the ith event in the high sideband. This probability was determined by inte-grating over the possible mass values in the signal region. Likewise, the background from same-b events was com-puted using the corresponding sum over events in the low sideband. However, the low sideband contains separate-b events as well as same-b events. As a result, the low sideband sum is an overestimate of the same-b back-ground. The contribution due to separate-b events in the low sideband was estimated using the high sideband data and subtracted. The total estimated background is 1:24 0:99 0:08 events, where the first uncertainty is statistical
5 D 0 0.2 0.4 0.6 0.8 1 Events/0.02 1 10 -1 DØ, L=1.3 fb
FIG. 2. The distribution of discriminant D5 for signal (solid
histogram) and background (dashed histogram). The background distribution is derived from events in the sidebands, folded over
possible values of m in the signal region. The signal
and the second is due to the uncertainty in the shape of the
mdistribution.
The branching fraction was obtained by normalizing to the number of B! J= K! Kcandidates
ob-served in the data. Bcandidates were formed in a similar fashion to the B0
scandidates, but with the addition of a third track, which was assumed to be a kaon and required to have
pT> 1:0 GeV. The three tracks had to form a common vertex, and the two muons had to have a mass near the J= mass. As with B0
scandidates, the muon pair was required to have min= > 2:8. The B system had to pass the same pT, angle, LT, LT=LT, and P
2 cuts as the B0
s system. Finally, the Bsystem was required to have D4>
0:949. The number of B decays nB 2016 55stat 45syst was determined from the fit to the reconstructed mass distribution shown in Fig.4.
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5
7
Entries/0.03 GeV
1
10
210
(a)
-1DØ, L=1.3 fb
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5
7
Entries/0.03 GeV
1
10
210
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5 7
Entries/0.03 GeV
1
10
210
(b)
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5 7
Entries/0.03 GeV
1
10
210
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5
7
Entries/0.03 GeV
1
10
210
(c)
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5
7
Entries/0.03 GeV
1
10
210
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5 7
Entries/0.03 GeV
-210
-110
1
10
210
(d)
) [GeV]
-µ
+µ
Mass(
4
4.5 5 5.5 6 6.5 7
Entries/0.03 GeV
-210
-110
1
10
210
FIG. 3 (color online). The dimuon mass distribution at different stages in a sequence of cuts: (a) after LT> 12LT, (b) after
min= > 2:8 and P2 > 0:01, (c) after D4> 0:5, and (d) after D4> 0:949. In (a) and (b) the histograms are fit to the sum of two
exponential functions, while in (c) and (d) they are fit to the sum of an exponential and a constant. The signal region (shaded) was excluded in the fits. In (d), three entries are included in the signal region: the entry near the upper bound of the signal region has a value
of D4close to the threshold and fails the D5cut; the other two entries are the final candidates.
K) [GeV] µ µ Mass( 5 5.1 5.2 5.3 5.4 5.5 5.6 Entries/0.03 GeV 0 100 200 300 400 500 600 DØ, L=1.3 fb-1
FIG. 4. Mass distribution of B candidates. The background
distribution is parametrized as a parabola and the signal distri-bution as a Gaussian function.
The branching fraction is related to nBby BB0 s ! nB0 s nB B B0 s f b ! B f b ! B0s BB! J= K BJ= ! ; (1)
which is obtained by eliminating the integrated luminosity and b quark production cross section from the expressions for the Band B0
syields. The quantity nB0
sis the number of B0
s ! decays observed in the data. The efficiencies
B and B0
s are, respectively, the fractions of B ! J= K ! K decays and B0
s! decays that are observed in the MC. The ratio B=
B0
s is 0:172
0:015, where the sources of uncertainty include the dimuon mass resolution and scale, the shape of the discriminant distribution, trigger efficiency, MC statistics, and the shape of the pT distribution for B0s and B. The B meson pro-duction ratio was calculated to be f f b!Bb!B0
s 3:86 0:54
from the production fractions of Refs. [16,17] and the correlation coefficient from Ref. [17]. The branching fractions BB! J= K 1:008 0:035 103
and BJ= ! 0:0593 0:0006 are from Ref. [16]. The product of the factors multiplying nB0
s on
the right-hand side of Eq. (1) is therefore k BB0
s !
=nB0
s 1:97 0:34 10
8, often called the
single event sensitivity. The contributions of the various sources of uncertainty to the relative uncertainty in k are listed in TableI.
Uncertainties due to differences between the data and MC largely cancel in the ratio B=
B0
s, although not
completely. For instance, muons from B0
s ! decay mostly have higher pTthan muons from B! J= K ! K decay, in which the energy is shared among three particles. The resulting effect on the efficiency of the trigger and muon pT cuts depends on the pT distribu-tion of the parent B mesons, and the shape of this distri-bution is the dominant source of uncertainty in B=
B0
s.
The extra track in Bdecays together with better tracking and vertexing in the MC than in the data result in an overestimate of B=
B0
s and a slight worsening of the
limit. The uncertainty due to modeling of the first four likelihood variables was estimated to be 3% based on a
comparison between B data and MC. The uncertainties due to the mass resolution (0.7%) and scale (1.3%) were estimated by comparing the 1S ! mass distri-bution in data and MC. Other uncertainties in B=
B0
s are
MC statistics (2.4%) and trigger efficiency (0.7%). Given two candidates observed in the data, an upper limit on nB0
s was computed taking into account the
ex-pected background and uncertainties using a Bayesian method. The resulting upper limit on the branching fraction is BB0
s! < 1:2 1079:4 108 at the 95% (90%) C.L. The expected 95% C.L. limit is 0:97 107. This result improves upon the best previously published upper bound for this branching fraction [8].
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); Science and Technology Facilities Council (United Kingdom); MSMT and GACR (Czech Republic); CRC Program, CFI, NSERC and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); CAS and CNSF (China); Alexander von Humboldt Foundation; and the Marie Curie Program.
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TABLE I. Sources of uncertainty and their contributions to the
relative uncertainty in the single event sensitivity k.
Source k=k Mass resolution 0.007 Mass scale 0.013 Discriminant distribution 0.030 Trigger efficiency 0.007 MC statistics 0.024 Bmeson pTspectrum 0.080 f b ! B=f b ! B0 s 0.140 BB! J= K 0.035 BJ= ! 0.010 Bfit (stat) 0.027 Bfit (syst) 0.022 Combined 0.17
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