Direct Observation of the Strange
b
Baryon
b
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,18E. Cheu,45F. Chevallier,13 D. K. Cho,62S. Choi,31B. Choudhary,27L. Christofek,77T. Christoudias,43S. Cihangir,50D. Claes,67C. Cle´ment,40
B. Cle´ment,18Y. Coadou,5M. Cooke,80W. E. Cooper,50M. Corcoran,80F. Couderc,17M.-C. Cousinou,14 S. Cre´pe´-Renaudin,13D. Cutts,77M. C´ wiok,29H. da Motta,2A. Das,62G. Davies,43K. De,78S. J. de Jong,34P. de Jong,33
E. De La Cruz-Burelo,64C. De Oliveira Martins,3J. D. Degenhardt,64F. De´liot,17M. Demarteau,50R. Demina,71 D. Denisov,50S. P. Denisov,38S. Desai,50H. T. Diehl,50M. Diesburg,50A. Dominguez,67H. Dong,72L. V. Dudko,37 L. Duflot,15S. R. Dugad,28D. Duggan,49A. Duperrin,14J. Dyer,65A. Dyshkant,52M. Eads,67D. Edmunds,65J. Ellison,48 V. D. Elvira,50Y. Enari,77S. Eno,61P. Ermolov,37H. Evans,54A. Evdokimov,73V. N. Evdokimov,38A. V. Ferapontov,59 T. Ferbel,71F. Fiedler,24F. Filthaut,34W. Fisher,50H. E. Fisk,50M. Ford,44M. Fortner,52H. Fox,22S. Fu,50S. Fuess,50 T. Gadfort,82C. F. Galea,34E. Gallas,50E. Galyaev,55C. Garcia,71A. Garcia-Bellido,82V. Gavrilov,36P. Gay,12W. Geist,18
D. Gele´,18C. E. Gerber,51Y. Gershtein,49D. Gillberg,5G. Ginther,71N. Gollub,40B. Go´mez,7A. Goussiou,55 P. D. Grannis,72H. Greenlee,50Z. D. Greenwood,60E. M. Gregores,4G. Grenier,19Ph. Gris,12J.-F. Grivaz,15 A. Grohsjean,24S. Gru¨nendahl,50M. W. Gru¨newald,29J. Guo,72F. Guo,72P. Gutierrez,75G. Gutierrez,50A. Haas,70 N. J. Hadley,61P. Haefner,24S. Hagopian,49J. Haley,68I. Hall,75R. E. Hall,47L. Han,6K. Hanagaki,50P. Hansson,40
K. Harder,44A. Harel,71R. Harrington,63J. M. Hauptman,57R. Hauser,65J. Hays,43T. Hebbeker,20D. Hedin,52 J. G. Hegeman,33J. M. Heinmiller,51A. P. Heinson,48U. Heintz,62C. Hensel,58K. Herner,72G. Hesketh,63 M. D. Hildreth,55R. Hirosky,81J. D. Hobbs,72B. Hoeneisen,11H. Hoeth,25M. Hohlfeld,21S. J. Hong,30R. Hooper,77
S. Hossain,75P. Houben,33Y. Hu,72Z. Hubacek,9V. Hynek,8I. Iashvili,69R. Illingworth,50A. S. Ito,50S. Jabeen,62 M. Jaffre´,15S. Jain,75K. Jakobs,22C. Jarvis,61R. Jesik,43K. Johns,45C. Johnson,70M. Johnson,50A. Jonckheere,50 P. Jonsson,43A. Juste,50D. Ka¨fer,20S. Kahn,73E. Kajfasz,14A. M. Kalinin,35J. R. Kalk,65J. M. Kalk,60S. Kappler,20 D. Karmanov,37J. Kasper,62P. Kasper,50I. Katsanos,70D. Kau,49R. Kaur,26V. Kaushik,78R. Kehoe,79S. Kermiche,14 N. Khalatyan,38A. Khanov,76A. Kharchilava,69Y. M. Kharzheev,35D. Khatidze,70H. Kim,31T. J. Kim,30M. H. Kirby,34 M. Kirsch,20B. Klima,50J. M. Kohli,26J.-P. Konrath,22M. Kopal,75V. M. Korablev,38B. Kothari,70A. V. Kozelov,38 D. Krop,54A. Kryemadhi,81T. Kuhl,23A. Kumar,69S. Kunori,61A. Kupco,10T. Kurcˇa,19J. Kvita,8F. Lacroix,12D. Lam,55
S. Lammers,70G. Landsberg,77J. Lazoflores,49P. Lebrun,19W. M. Lee,50A. Leflat,37F. Lehner,41J. Lellouch,16 V. Lesne,12J. Leveque,45P. Lewis,43J. Li,78Q. Z. Li,50L. Li,48S. M. Lietti,4J. G. R. Lima,52D. Lincoln,50J. Linnemann,65
V. V. Lipaev,38R. Lipton,50Y. Liu,6Z. Liu,5L. Lobo,43A. Lobodenko,39M. Lokajicek,10A. Lounis,18P. Love,42 H. J. Lubatti,82A. L. Lyon,50A. K. A. Maciel,2D. Mackin,80R. J. Madaras,46P. Ma¨ttig,25C. Magass,20A. Magerkurth,64
N. Makovec,15P. K. Mal,55H. B. Malbouisson,3S. Malik,67V. L. Malyshev,35H. S. Mao,50Y. Maravin,59B. Martin,13 R. McCarthy,72A. Melnitchouk,66A. Mendes,14L. Mendoza,7P. G. Mercadante,4Y. P. Merekov,35M. Merkin,37 K. W. Merritt,50J. Meyer,21A. Meyer,20M. Michaut,17T. Millet,19J. Mitrevski,70J. Molina,3R. K. Mommsen,44 N. K. Mondal,28R. W. Moore,5T. Moulik,58G. S. Muanza,19M. Mulders,50M. Mulhearn,70O. Mundal,21L. Mundim,3
B. Penning,22P. M. Perea,48K. Peters,44Y. Peters,25P. Pe´troff,15M. Petteni,43R. Piegaia,1J. Piper,65M.-A. Pleier,21 P. L. M. Podesta-Lerma,32,‡V. M. Podstavkov,50Y. Pogorelov,55M.-E. Pol,2P. Polozov,36A. Pomposˇ,75B. G. Pope,65 A. V. Popov,38C. Potter,5W. L. Prado da Silva,3H. B. Prosper,49S. Protopopescu,73J. Qian,64A. Quadt,21B. Quinn,66
A. Rakitine,42M. S. Rangel,2K. J. Rani,28K. Ranjan,27P. N. Ratoff,42P. Renkel,79S. Reucroft,63P. Rich,44 M. Rijssenbeek,72I. Ripp-Baudot,18F. Rizatdinova,76S. Robinson,43R. F. Rodrigues,3C. Royon,17A. Rozhdestvenski,35 P. Rubinov,50R. Ruchti,55G. Safronov,36G. Sajot,13A. Sa´nchez-Herna´ndez,32M. P. Sanders,16A. Santoro,3G. Savage,50
L. Sawyer,60T. Scanlon,43D. Schaile,24R. D. Schamberger,72Y. Scheglov,39H. Schellman,53P. Schieferdecker,24 T. Schliephake,25C. Schmitt,25C. Schwanenberger,44A. Schwartzman,68R. Schwienhorst,65J. Sekaric,49S. Sengupta,49 H. Severini,75E. Shabalina,51M. Shamim,59V. Shary,17A. A. Shchukin,38R. K. Shivpuri,27D. Shpakov,50V. Siccardi,18 V. Simak,9V. Sirotenko,50P. Skubic,75P. Slattery,71D. Smirnov,55R. P. Smith,50J. Snow,74G. R. Snow,67S. Snyder,73 S. So¨ldner-Rembold,44L. Sonnenschein,16A. Sopczak,42M. Sosebee,78K. Soustruznik,8M. Souza,2B. Spurlock,78
J. Stark,13J. Steele,60V. Stolin,36A. Stone,51D. A. Stoyanova,38J. Strandberg,64S. Strandberg,40M. A. Strang,69 M. Strauss,75E. Strauss,72R. Stro¨hmer,24D. Strom,53M. Strovink,46L. 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,35M. Tomoto,50T. Toole,61I. Torchiani,22T. Trefzger,23D. Tsybychev,72
B. Tuchming,17C. Tully,68P. M. Tuts,70R. Unalan,65 S. Uvarov,39L. Uvarov,39S. Uzunyan,52B. Vachon,5 P. J. van den Berg,33B. van Eijk,33R. Van Kooten,54W. M. van Leeuwen,33N. Varelas,51E. W. Varnes,45A. Vartapetian,78
I. A. Vasilyev,38M. Vaupel,25P. Verdier,19L. S. Vertogradov,35Y. Vertogradova,35M. Verzocchi,50 F. Villeneuve-Seguier,43P. Vint,43P. Vokac,9E. Von Toerne,59M. Voutilainen,67,xM. Vreeswijk,33R. Wagner,68 H. D. Wahl,49L. Wang,61M. H. L. S Wang,50J. Warchol,55G. Watts,82M. Wayne,55M. Weber,50G. Weber,23H. Weerts,65 A. Wenger,22,kN. Wermes,21M. Wetstein,61A. White,78D. Wicke,25G. W. Wilson,58S. J. Wimpenny,48M. Wobisch,60 D. R. Wood,63T. R. Wyatt,44Y. Xie,77S. Yacoob,53R. Yamada,50M. Yan,61T. Yasuda,50Y. A. Yatsunenko,35K. Yip,73 H. D. Yoo,77S. W. Youn,53J. Yu,78C. Yu,13A. Yurkewicz,72A. Zatserklyaniy,52C. Zeitnitz,25D. Zhang,50T. Zhao,82
B. Zhou,64J. Zhu,72M. Zielinski,71D. Zieminska,54A. Zieminski,54L. Zivkovic,70V. Zutshi,52and E. G. Zverev37
(D0 Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina 2
LAFEX, 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 9
Czech Technical University, Prague, Czech Republic
10Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 11
Universidad 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 16
LPNHE, IN2P3-CNRS, Universite´s Paris VI and VII, Paris, France
17DAPNIA/Service de Physique des Particules, CEA, Saclay, France 18
IPHC, 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 21
Physikalisches Institut, Universita¨t Bonn, Bonn, Germany
22Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany 23Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 24
Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany
25Fachbereich Physik, University of Wuppertal, Wuppertal, Germany 26
27Delhi University, Delhi, India
28Tata Institute of Fundamental Research, Mumbai, India 29University College Dublin, Dublin, Ireland 30Korea Detector Laboratory, Korea University, Seoul, Korea
31
SungKyunKwan University, Suwon, Korea
32CINVESTAV, Mexico City, Mexico 33
FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands
34Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands 35
Joint Institute for Nuclear Research, Dubna, Russia
36Institute for Theoretical and Experimental Physics, Moscow, Russia 37Moscow State University, Moscow, Russia
38
Institute for High Energy Physics, Protvino, Russia
39Petersburg Nuclear Physics Institute, St. Petersburg, Russia 40
Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden, and Uppsala University, Uppsala, Sweden
41
Physik 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 46
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
47California State University, Fresno, California 93740, USA 48
University of California, Riverside, California 92521, USA
49Florida State University, Tallahassee, Florida 32306, USA 50
Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
51University of Illinois at Chicago, Chicago, Illinois 60607, USA 52Northern Illinois University, DeKalb, Illinois 60115, USA
53
Northwestern University, Evanston, Illinois 60208, USA
54Indiana University, Bloomington, Indiana 47405, USA 55
University of Notre Dame, Notre Dame, Indiana 46556, USA
56Purdue University Calumet, Hammond, Indiana 46323, USA 57
Iowa 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
62
Boston University, Boston, Massachusetts 02215, USA
63Northeastern University, Boston, Massachusetts 02115, USA 64
University 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 69
State University of New York, Buffalo, New York 14260, USA
70Columbia University, New York, New York 10027, USA 71
University of Rochester, Rochester, New York 14627, USA
72State University of New York, Stony Brook, New York 11794, USA 73
Brookhaven National Laboratory, Upton, New York 11973, 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 78
University of Texas, Arlington, Texas 76019, USA
79Southern Methodist University, Dallas, Texas 75275, USA 80
Rice University, Houston, Texas 77005, USA
81University of Virginia, Charlottesville, Virginia 22901, USA 82
University of Washington, Seattle, Washington 98195, USA (Received 12 June 2007; published 3 August 2007)
We report the first direct observation of the strangebbaryon
bb. We reconstruct the decayb !
J= , with J= !, and!!p in pp collisions at
s
p
1:3 fb1 of data collected by the D0 detector, we observe15:24:4stat1:9
0:4systb candidates at a mass of5:7740:011stat 0:015systGeV. The significance of the observed signal is5:5, equiva-lent to a probability of3:3108of it arising from a background fluctuation. Normalizing to the decay
b!J= , we measure the relative ratebBb!J=
bBb!J= 0:280:09stat00::0908syst.
DOI:10.1103/PhysRevLett.99.052001 PACS numbers: 14.20.Mr, 13.30.Eg, 13.85.Ni
The quark model of hadrons [1] predicts the existence of a number of baryons containingbquarks, with a hierarch-ical structure similar to that of charmed baryons. Despite significant progress in studying b hadrons over the last decade, only the budb b baryon has been directly
observed. The
bdsb (charge conjugate states are
as-sumed throughout this Letter) is a strangebbaryon made of valence quarks from all three known generations of fermions and is expected to decay through the weak inter-action. Theoretical calculations of heavy quark effective theory [2] and nonrelativistic QCD [3] predict the
b mass
in the range5:7–5:8 GeV[4].
Experiments at the CERN LEP ee collider have reported indirect evidence of the
b baryon based on an
excess of same-sign‘ events in jets [5]. Interpreting the excess as the semi-inclusive
b !‘‘X decay,
the average lifetime of the
b is 1:4200::2824 ps[6]. In this Letter, we report the first direct observation of the
b
baryon, fully reconstructed in an exclusive decay. We observe the decay
b !J= , with J= !,
!, and !p. The analysis is based on a data sample of1:3 fb1 integrated luminosity collected in
ppcollisions atps
1:96 TeVwith the D0 detector at the
Fermilab Tevatron collider during 2002 – 2006.
The D0 detector is described in detail elsewhere [7]. The components most relevant to this analysis are the central tracking system and the muon spectrometer. The central tracking system consists of a silicon microstrip tracker (SMT) and a central fiber tracker (CFT) that are sur-rounded by a 2 T superconducting solenoid. The SMT is optimized for tracking and vertexing for the pseudorapidity region jj<3 ( lntan=2 and is the polar angle) while the CFT has coverage for jj<2. Liquid-argon and uranium calorimeters in a central and two end-cap cryostats cover the pseudorapidity region jj<4:2. The muon spectrometer is located outside the calorimeter and covers the pseudorapidity regionjj<2. It comprises a layer of drift tubes and scintillator trigger counters in front of 1.8 T iron toroids followed by two similar layers behind the toroids.
The topology of
b !J= !J= decay (see
Fig.1) is similar to that of theb!J= decay;
there-fore, the reconstruction of theJ= andand their selec-tion discussed below are guided by the strategies applied to the b lifetime measurement in D0 [8]. They are then
validated with simulated Monte Carlo (MC)
b events.
ThePYTHIAMC program [9] is used to generate
b signal
events while theEVTGENprogram [10] is used to simulate
b decays. The b mass and lifetime are set to be
5.840 GeV and 1.33 ps, respectively, as their default values in these programs. The generated events are subjected to the same reconstruction and selection programs as the data after passing through the D0 detector simulation based on theGEANTpackage [11]. MC events are reweighted using the weights determined by matching transverse momentum (pT) distributions ofJ= , proton and pion from theb!
J= !J= p decays in MC calculations to those observed in the data.
J= ! decays are reconstructed from two op-positely charged muons that have a common vertex. Muons are identified by matching tracks reconstructed in the central tracking system with either track segments in the muon spectrometer or calorimeter energies consistent with the muon trajectory. They are required to have pT>
1:5 GeV and at least one of them must be reconstructed
in each of the three muon drift tube layers. The dimuon invariant mass M is required to be in the range
2:5–3:6 GeV. In addition, events must have at least one
reconstructed primary vertex of thepp interaction. If two or more vertices are reconstructed, the one closest to the reconstructed
b vertex (see below) is used. Events
con-taining a J= candidate are reprocessed with a version of the track reconstruction algorithm that improves the effi-ciency for tracks with lowpT and high impact parameters.
Consequently, the efficiencies for K0S,, and recon-struction are significantly increased. Figure2(a)shows the
π
µ
µ
π
Λ
Ξ
Ξ
bJ/
ψ
p
FIG. 1 (color online). Schematic of the
b !J= !
J= ! p decay topology. The and
invariant mass distributions of the reconstructed can-didates (see below) before and after the reprocessing. The reprocessing increases the yield by approximately a factor of 5.5. For further analysis, J= ! candi-dates are required to have mass 2:80< M<
3:35 GeVandpT>5 GeV. The mass windows here and
below are chosen to be approximately 5 and the pT
requirement ensures that the selectedJ= candidates are above the sharp turn-on of the detector and trigger acceptances.
!p candidates are formed from two oppositely charged tracks that originate from a common vertex. The track with the higherpT is assumed to be the proton. MC studies show that this assignment gives nearly 100% cor-rect combination. The invariant mass of theppair must have a mass between 1.105 and 1.125 GeV. The two tracks are required to have a total of no more than two hits in the tracking detector before the reconstructed p vertex. Furthermore, the impact parameter significance (the im-pact parameter with respect to the event vertex divided by its uncertainty) must exceed three for both tracks and exceed four for at least one of them. These selection cuts are the same as those in Ref. [8].
The candidates are then combined with negatively charged tracks (assumed to be pions) to form! decay candidates. The pion must have an impact parameter significance greater than three. The and the pion are required to have a common vertex. For both and candidates, the distance between the event vertex and its decay vertex is required to exceed 4 times its uncertainty. Moreover, the uncertainty of the distance between the production vertex and its decay vertex (decay length) in the transverse plane (the plane perpendicular to the beam direction) must be less than 0.5 cm. These two require-ments reduce combinatoric and track mismeasurement backgrounds.
The two pions from !! p decays (right sign) have the same charge. Consequently, the com-bination (wrong sign) events form an ideal control sample for background studies. Figure2(b)compares mass distributions of the right-sign and the wrong-sign
combinations. The mass peak is evident in the distribution of the right-sign events. A pair is con-sidered to be acandidate if its mass is within the range
1:305< M<1:340 GeV.
b !J= decay candidates are formed fromJ=
and pairs that originate from a common vertex and have an opening angle in the transverse plane less than
=2 rad. The uncertainty of the proper decay length of
theJ= vertex must be less than 0.05 cm in the trans-verse plane. A total of 2308 events remains after this preselection. The wrong-sign events are subjected to the same preselection as the right-sign events. A total of 1124 wrong-sign events is selected as the control sample.
Several distinctive features of the
b !J= !
J= ! p decay are utilized to fur-ther suppress backgrounds. The wrong-sign background events from the data and MC signal
b events are used
for studying additional event selection criteria. Protons and pions from thedecays of the
b events are expected to
have higher momenta than those from most of the back-ground processes. Therefore, protons are required to have
pT>0:7 GeV. Similarly, minimum pT requirements of
0.3 and 0.2 GeV are imposed on pions from and decays, respectively. These requirements remove 91.6% of the wrong-sign background events while keeping 68.7% of the MC
b signal events. Backgrounds from
combinator-ics and other bhadrons are reduced by using topological decay information. Contamination from decays such asB !J= K!J= K0
SandB0 !J= K!
J= K0
S are suppressed by requiring the
can-didates to have decay lengths greater than 0.5 cm and
coscol>0:99, as the baryons in MC calculations have an average decay length of 4.8 cm. Here col is the angle between thedirection and the direction from the
production vertex to its decay vertex in the transverse plane. These two requirements on the reduce the background by an additional 56.4%, while removing only 1.7% of the MC signal events. The contribution from the
b baryon is estimated to be negligible. Finally, b
baryons are expected to have a sizable lifetime. To reduce prompt backgrounds, the transverse proper decay length significance of the
b candidates is required to be greater
than two. This final criterion retains 83.1% of the MC signal events but only 43.9% of the remaining background events.
In the data, 51 events with the
b candidate mass
between 5.2 and 7.0 GeV pass all selection criteria. The mass range is chosen to be wide enough to encompass masses of all known b hadrons as well as the predicted mass of the
b baryon. The candidate mass, Mb, ) (GeV)
π Λ M(
1.28 1.3 1.32 1.34 1.36
Events/(0.002 GeV) 100 200 300 400 (a) after reprocessing before reprocessing -1 DØ, 1.3 fb
) (GeV) π Λ M(
1.28 1.3 1.32 1.34 1.36
Events/(0.002 GeV) 100 200 300 400 (b) right-sign wrong-sign -1 DØ, 1.3 fb
FIG. 2. Invariant mass distributions of thepair before the
b reconstruction for (a) the right-sign combinations before and after reprocessing and (b) the right-sign and
the wrong-sign combinations after reprocessing. The
re-processing significantly increases theyield. Fits to the
post-re-processing distributions of the right-sign combination with a Gaussian signal and a first-order polynomial background yield
is calculated as M
b MJ= M
M MPDGJ= MPDG to improve the resolution. Here MJ= , M, and M are the reconstructed masses while MPDGJ= and
MPDG are taken from Ref. [1]. The distribution of
M
bis shown in Fig.3(a). A mass peak near 5.8 GeV is
apparent. A number of cross checks are performed to ensure the observed peak is not due to artifacts of the analysis: (i) The J= mass distribution of the wrong-sign events, shown in Fig. 3(b), is consistent with a flat background. (ii) The event selection is applied to the sideband events of the mass peak, requiring 1:28<
M<1:36 GeVbut excluding themass window.
Similarly, the selection is applied to the J= sideband events with 2:5< M<2:7 GeV. The high-mass sideband is not considered due to potential contami-nation from 0 events. As shown in Figs.3(c) and 3(d), no evidence of a mass peak is present for either p distribution. (iii) The possibility of a fake signal due to the residual b hadron background is investigated by applying the final
b selection to high
statistics MC samples of B!J= K!J= K0
S,
B0 !J= K0S, andb!J= . No indication of a mass
peak is observed in the reconstructedJ= mass distri-butions. (iv) The mass distributions ofJ= ,, andare investigated by relaxing the mass requirements on these particles one at a time for events both in the
b signal
region and the sidebands. The numbers of these particles determined by fitting their respective mass distribution are fully consistent with the quoted numbers of signal events plus background contributions. (v) The robustness of the observed mass peak is tested by varying selection criteria
within reasonable ranges. All studies confirm the existence of the peak at the same mass.
Interpreting the peak as
b production, candidate
masses are fitted with the hypothesis of a signal plus background model using an unbinned likelihood method. The signal and background shapes are assumed to be Gaussian and flat, respectively. The fit results in a
b
mass of 5:7740:011 GeV with a width of 0:037
0:008 GeVand a yield of15:24:4events. Unless
speci-fied, all uncertainties are statistical. Following the same procedure, a fit to the MC
b events yields a mass of
5:8390:003 GeV, in good agreement with the
5.840 GeV input mass. The fitted width of the MC mass distribution is 0:0350:002 GeV, consistent with the 0.037 GeVobtained from the data. Since the intrinsic decay width of the
b baryon in the MC calculations is
negli-gible, the width of the mass distribution is thus dominated by the detector resolution. To assess the significance of the signal, the likelihood,Lsb, of the signal plus background
fit above is first determined. The fit is then repeated using the background-only model, and a new likelihood Lb is
found. The logarithmic likelihood ratio p2 lnLsb=Lb
indicates a statistical significance of 5.5, corresponding to a probability of3:3108from background fluctuation for observing a signal that is equal to or more significant than what is seen in the data. Including systematic effects from the mass range, signal and background models, and the track momentum scale results in a minimum signifi-cance of5:3and a
b yield of15:24:4stat10::94syst. The significance can also be estimated from the numbers of candidate events and estimated background events. In the mass region of 2.5 times the fitted width centered on the fitted mass, 19 candidate events (8J= and 11J= ) are observed while 14:84:3stat1:9
0:4syst signal and
3:60:6stat0:4
1:9syst background events are estimated from the fit. The probability of backgrounds fluctuating to 19 or more events is2:2107, equivalent to a Gaussian significance of5:2.
Figure4shows distributions of the proper decay length for the 19 candidate events, the
b sideband events, and
the MC
b signal events plus estimated background
events. The distribution of the candidate events agrees well with that expected from the
b signal while the
sideband events have a lower mean proper decay length. Because of the use of lifetime information in the event selection, a
b lifetime measurement is not made in this
Letter.
Potential systematic biases on the measured
b mass
are studied for the event selection, signal and background models, and the track momentum scale. Varying cut values and using a multivariate technique of different variables for event selection leads to a maximum change of 0.020 GeV in the
b mass. Subtracting an estimated statistical
con-tribution to the change, a conservative 0:015 GeV sys-tematic uncertainty is assigned due to the event selection.
) (GeV)
-b
Ξ
M(
5.5 6 6.5 7
Events/(0.05 GeV) 0 2 4 6 8 -1
DØ, 1.3 fb
Data Fit (a) 1 2 3
4 wrong-sign -1 DØ, 1.3 fb
(b)
Events
1 2 3
4 sideband
-Ξ
(c)
Mass (GeV) 5.5 6 6.5 7 1
2 3
4 (d) J/ψ sideband
FIG. 3 (color online). (a) The M
bdistribution of theb candidates after all selection criteria. The dotted curve is an unbinned likelihood fit to the model of a constant background plus a Gaussian signal. The mass distributions
for (b) the wrong-sign background, (c) the sideband, and
(d) the J= sideband events. The mass MJ= M M
PDGJ= is plotted for (b) and (c) while the
massM M M
Using double Gaussians for the signal model, a first-order polynomial for the background model, or fixing the mass resolution to that obtained from the MC
b events all lead
to negligible changes in the mass. The mass, calculated using the world average values [1] of intermediate particle masses above, is found to have a weak dependence on the track momentum scale. This has been verified using the
b!J= and B0 !J= K0S events observed in the
data. A systematic uncertainty of0:002 GeVis assigned, corresponding to the mass difference between our mea-surement and the world average [1] for the b and B0
hadrons. Adding in quadrature, a total systematic uncer-tainty of 0:015 GeV is obtained to yield the measured
bmass:5:7740:011stat 0:015systGeV.
The
b B relative to that of the b baryon is
calculated using
b Bb !J=
b Bb!J=
b!J=
b !J=
N
b
Nb
;
whereN
b andNb are the numbers ofb andbevents
reconstructed in data. Analyzing the same data and using the similar event selection criteria and fitting procedure as the
b analysis, a yield of24030stat 12syst b
baryons is determined. The efficiencies to reconstruct the decays,
bandb, are determined by MC
simula-tion, and the efficiency ratio,b=b, is found to be
4:41:3. The uncertainty on b=b arises from
MC modeling (27%), MC statistics (10%), the reconstruc-tion of the addireconstruc-tional pion in the
b decay (7%), and the
b mass difference between data and MC calculations
(5%). The largest component, MC modeling uncertainty, is due to the difference in the efficiency ratio with and without MC reweighting. The efficiency ratio is found to be insensitive to changes in b and b production
models. Many other systematic uncertainties on the effi-ciencies themselves tend to cancel in the ratio of the efficiencies. We find a relative production ratio of0:28
0:09stat0:09
0:08syst.
In summary, in 1:3 fb1 of data collected by the D0 experiment in pp collisions at ps
1:96 TeV at the
Fermilab Tevatron collider, we have made the first direct observation of the strange bbaryon
b with a statistical
significance of 5:5. We observe the decay mode
b !
J= withJ= !,!!p. We measure the
b mass to be 5:7740:011stat
0:015syst GeV and determine itsBrelative to that
of thebto be0:280:09stat00::0908syst.
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.
*Visiting scientist from Augustana College, Sioux Falls, SD, USA.
†Visiting scientist from The University of Liverpool,
Liverpool, United Kingdom.
‡Visiting scientist from ICN-UNAM, Mexico City, Mexico. xVisiting scientist from Helsinki Institute of Physics,
Helsinki, Finland.
kVisiting scientist from Universita¨t Zu¨rich, Zu¨rich,
Switzerland.
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0
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2
4
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12
14
DØ, 1.3 fb
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Data sideband
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