Observation of the Doubly Strange b Baryon
bV. M. Abazov,36B. Abbott,75M. Abolins,65B. S. Acharya,29M. Adams,51T. Adams,49E. Aguilo,6M. Ahsan,59 G. D. Alexeev,36G. Alkhazov,40A. Alton,64,*G. Alverson,63G. A. Alves,2M. Anastasoaie,35L. S. Ancu,35T. Andeen,53
B. Andrieu,17M. S. Anzelc,53M. Aoki,50Y. Arnoud,14M. Arov,60M. Arthaud,18A. Askew,49B. A˚ sman,41 A. C. S. Assis Jesus,3O. Atramentov,49C. Avila,8F. Badaud,13L. Bagby,50B. Baldin,50D. V. Bandurin,59P. Banerjee,29
S. Banerjee,29E. Barberis,63A.-F. Barfuss,15P. Bargassa,80P. Baringer,58J. Barreto,2J. F. Bartlett,50U. Bassler,18 D. Bauer,43S. Beale,6A. Bean,58M. Begalli,3M. Begel,73C. Belanger-Champagne,41L. Bellantoni,50A. Bellavance,50
J. A. Benitez,65S. B. Beri,27G. Bernardi,17R. Bernhard,23I. Bertram,42M. Besanc¸on,18R. Beuselinck,43 V. A. Bezzubov,39P. C. Bhat,50V. Bhatnagar,27C. Biscarat,20G. Blazey,52F. Blekman,43S. Blessing,49K. Bloom,67
A. Boehnlein,50D. Boline,62T. A. Bolton,59E. E. Boos,38G. Borissov,42T. Bose,77A. Brandt,78R. Brock,65 G. Brooijmans,70A. Bross,50D. Brown,81X. B. Bu,7N. J. Buchanan,49D. Buchholz,53M. Buehler,81V. Buescher,22 V. Bunichev,38S. Burdin,42,†T. H. Burnett,82C. P. Buszello,43J. M. Butler,62P. Calfayan,25S. Calvet,16J. Cammin,71 E. Carrera,49W. Carvalho,3B. C. K. Casey,50H. Castilla-Valdez,33S. Chakrabarti,18D. Chakraborty,52K. M. Chan,55 A. Chandra,48E. Cheu,45F. Chevallier,14D. K. Cho,62S. Choi,32B. Choudhary,28L. Christofek,77T. Christoudias,43 S. Cihangir,50D. Claes,67J. Clutter,58M. Cooke,50W. E. Cooper,50M. Corcoran,80F. Couderc,18M.-C. Cousinou,15 S. Cre´pe´-Renaudin,14V. Cuplov,59D. Cutts,77M. C´ wiok,30H. da Motta,2A. Das,45G. Davies,43K. De,78S. J. de Jong,35
E. De La Cruz-Burelo,33C. De Oliveira Martins,3K. DeVaughan,67J. D. Degenhardt,64F. De´liot,18M. Demarteau,50 R. Demina,71D. Denisov,50S. P. Denisov,39S. Desai,50H. T. Diehl,50M. Diesburg,50A. Dominguez,67H. Dong,72
T. Dorland,82A. Dubey,28L. V. Dudko,38L. Duflot,16S. R. Dugad,29D. Duggan,49A. Duperrin,15J. Dyer,65 A. Dyshkant,52M. Eads,67D. Edmunds,65J. Ellison,48V. D. Elvira,50Y. Enari,77S. Eno,61P. Ermolov,38,††H. Evans,54 A. Evdokimov,73V. N. Evdokimov,39A. V. Ferapontov,59T. Ferbel,71F. Fiedler,24F. Filthaut,35W. Fisher,50H. E. Fisk,50 M. Fortner,52H. Fox,42S. Fu,50S. Fuess,50T. Gadfort,70C. F. Galea,35C. Garcia,71A. Garcia-Bellido,71V. Gavrilov,37 P. Gay,13W. Geist,19W. Geng,15,65C. E. Gerber,51Y. Gershtein,49D. Gillberg,6G. Ginther,71N. Gollub,41B. Go´mez,8
A. Goussiou,82P. D. Grannis,72H. Greenlee,50Z. D. Greenwood,60E. M. Gregores,4G. Grenier,20Ph. Gris,13 J.-F. Grivaz,16A. Grohsjean,25S. Gru¨nendahl,50M. W. Gru¨newald,30F. Guo,72J. Guo,72G. Gutierrez,50P. Gutierrez,75 A. Haas,70N. J. Hadley,61P. Haefner,25S. Hagopian,49J. Haley,68I. Hall,65R. E. Hall,47L. Han,7K. Harder,44A. Harel,71
J. M. Hauptman,57J. Hays,43T. Hebbeker,21D. Hedin,52J. G. Hegeman,34A. P. Heinson,48U. Heintz,62C. Hensel,22,‡ K. Herner,72G. Hesketh,63M. D. Hildreth,55R. Hirosky,81J. D. Hobbs,72B. Hoeneisen,12H. Hoeth,26M. Hohlfeld,22 S. Hossain,75P. Houben,34Y. Hu,72Z. Hubacek,10V. Hynek,9I. Iashvili,69R. Illingworth,50A. S. Ito,50S. Jabeen,62
M. Jaffre´,16S. Jain,75K. Jakobs,23C. Jarvis,61R. Jesik,43K. Johns,45C. Johnson,70M. Johnson,50D. Johnston,67 A. Jonckheere,50P. Jonsson,43A. Juste,50E. Kajfasz,15J. M. Kalk,60D. Karmanov,38P. A. Kasper,50I. Katsanos,70 D. Kau,49V. Kaushik,78R. Kehoe,79S. Kermiche,15N. Khalatyan,50A. Khanov,76A. Kharchilava,69Y. M. Kharzheev,36 D. Khatidze,70T. J. Kim,31M. H. Kirby,53M. Kirsch,21B. Klima,50J. M. Kohli,27E. V. Komissarov,36,††J.-P. Konrath,23 A. V. Kozelov,39J. Kraus,65T. Kuhl,24A. Kumar,69A. Kupco,11T. Kurcˇa,20V. A. Kuzmin,38J. Kvita,9F. Lacroix,13 D. Lam,55S. Lammers,70G. Landsberg,77P. Lebrun,20W. M. Lee,50A. Leflat,38J. Lellouch,17J. Li,78,††L. Li,48Q. Z. Li,50
S. M. Lietti,5J. K. Lim,31J. G. R. Lima,52D. Lincoln,50J. Linnemann,65V. V. Lipaev,39R. Lipton,50Y. Liu,7Z. Liu,6 A. Lobodenko,40M. Lokajicek,11P. Love,42H. J. Lubatti,82R. Luna,3A. L. Lyon,50A. K. A. Maciel,2D. Mackin,80 R. J. Madaras,46P. Ma¨ttig,26C. Magass,21A. Magerkurth,64P. K. Mal,82H. B. Malbouisson,3S. Malik,67V. L. Malyshev,36
Y. Maravin,59B. Martin,14R. McCarthy,72A. Melnitchouk,66L. Mendoza,8P. G. Mercadante,5Y. P. Merekov,36 M. Merkin,38K. W. Merritt,50A. Meyer,21J. Meyer,22,‡J. Mitrevski,70R. K. Mommsen,44N. K. Mondal,29R. W. Moore,6
T. Moulik,58G. S. Muanza,20M. Mulhearn,70O. Mundal,22L. Mundim,3E. Nagy,15M. Naimuddin,50M. Narain,77 N. A. Naumann,35H. A. Neal,64J. P. Negret,8P. Neustroev,40H. Nilsen,23H. Nogima,3S. F. Novaes,5T. Nunnemann,25 V. O’Dell,50D. C. O’Neil,6G. Obrant,40C. Ochando,16D. Onoprienko,59J. Orduna,33N. Oshima,50N. Osman,43J. Osta,55 R. Otec,10G. J. Otero y Garzo´n,50M. Owen,44P. Padley,80M. Pangilinan,77N. Parashar,56S.-J. Park,22,‡S. K. Park,31 J. Parsons,70R. Partridge,77N. Parua,54A. Patwa,73G. Pawloski,80B. Penning,23M. Perfilov,38K. Peters,44Y. Peters,26
P. Pe´troff,16M. Petteni,43R. Piegaia,1J. Piper,65M.-A. Pleier,22P. L. M. Podesta-Lerma,33,xV. M. Podstavkov,50 Y. Pogorelov,55M.-E. Pol,2P. Polozov,37B. G. Pope,65A. V. Popov,39C. Potter,6W. L. Prado da Silva,3H. B. Prosper,49
S. Protopopescu,73J. Qian,64A. Quadt,22,‡B. Quinn,66A. Rakitine,42M. S. Rangel,2K. Ranjan,28P. N. Ratoff,42 P. Renkel,79P. Rich,44J. Rieger,54M. Rijssenbeek,72I. Ripp-Baudot,19F. Rizatdinova,76S. Robinson,43R. F. Rodrigues,3
M. Rominsky,75C. Royon,18A. Rozhdestvenski,36P. Rubinov,50R. Ruchti,55G. Safronov,37G. Sajot,14 A. Sa´nchez-Herna´ndez,33M. P. Sanders,17B. Sanghi,50G. Savage,50L. Sawyer,60T. Scanlon,43D. Schaile,25 R. D. Schamberger,72Y. Scheglov,40H. Schellman,53T. Schliephake,26S. Schlobohm,82C. Schwanenberger,44
A. Schwartzman,68R. Schwienhorst,65J. Sekaric,49H. Severini,75E. Shabalina,51M. Shamim,59V. Shary,18 A. A. Shchukin,39R. K. Shivpuri,28V. Siccardi,19V. Simak,10V. Sirotenko,50P. Skubic,75P. Slattery,71D. Smirnov,55
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D. Zieminska,54A. Zieminski,54,††L. Zivkovic,70V. Zutshi,52and E. G. Zverev38 (D0 Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil
3
Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
4Universidade Federal do ABC, Santo Andre´, Brazil
5Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil 6University of Alberta, Edmonton, Alberta, Canada,
Simon Fraser University, Burnaby, British Columbia, Canada, York University, Toronto, Ontario, Canada,
and McGill University, Montreal, Quebec, Canada
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 12
Universidad San Francisco de Quito, Quito, Ecuador
13LPC, Universite´ Blaise Pascal, CNRS/IN2P3, Clermont, France
14LPSC, Universite´ Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France 15CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France
16LAL, Universite´ Paris-Sud, IN2P3/CNRS, Orsay, France 17LPNHE, IN2P3/CNRS, Universite´s Paris VI and VII, Paris, France
18CEA, Irfu, SPP, Saclay, France
19IPHC, Universite´ Louis Pasteur, CNRS/IN2P3, Strasbourg, France
20IPNL, Universite´ Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universite´ de Lyon, Lyon, France 21III. Physikalisches Institut A, RWTH Aachen University, 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 28
Delhi University, Delhi, India
29Tata Institute of Fundamental Research, Mumbai, 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
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 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 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
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 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
80
Rice University, Houston, Texas 77005, USA
81University of Virginia, Charlottesville, Virginia 22901, USA 82University of Washington, Seattle, Washington 98195, USA
(Received 29 August 2008; published 5 December 2008)
We report the observation of the doubly strangeb baryon b in the decay channelb ! J=c , withJ=c ! þand! K! ðpÞK, inp p collisions atpffiffiffis¼ 1:96 TeV. Using approxi-mately1:3 fb1 of data collected with the D0 detector at the Fermilab Tevatron Collider, we observe 17:8 4:9ðstatÞ 0:8ðsystÞ
b signal events at a mass of6:165 0:010ðstatÞ 0:013ðsystÞ GeV. The
significance of the observed signal is5:4, corresponding to a probability of 6:7 108of it arising from a background fluctuation.
The baryon, composed of three strange quarks, has played an important historical role in our understanding of the basic structure of matter. Its discovery in 1964 [1] at a mass predicted from SU(3) symmetry breaking was a great success for the theory [2]. ThebðbssÞ (charge conjugate states are assumed throughout this Letter) is a predicted heavy cousin of thewith ab quark replacing one of the three strange quarks. While the has JP¼ 3=2þ, the ground stateb is expected to have JP¼ 1=2þ, a mass between 5.94–6.12 GeV and a lifetime such that 0:55 < ð
bÞ=ðB0Þ < 1:10 [3].
In this Letter, we report the first observation of theb baryon, fully reconstructed from its decayb ! J=c, with J=c ! þ, ! K, and ! p. The analysis is based on a data sample of 1:3 fb1 collected inp p collisions atpffiffiffis¼ 1:96 TeV with the D0 detector [4] at the Fermilab Tevatron Collider. The detector compo-nents 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) inside a 2 Tesla superconduct-ing solenoid. The SMT is optimized for tracksuperconduct-ing and vertexing over the pseudorapidity region jj < 3 while the CFT has coverage for jj < 2. A liquid argon and uranium calorimeter provides coverage up to jj < 4:2. The muon spectrometer coversjj < 2.
The b ! J=c! J=cK! J=cpK de-cay topology is similar to that of the b ! J=c ! J=c ! J=cpdecay with in place of
andKin place of. Consequently, the reconstruction of theJ=c and and their selection discussed below follow closely the analysis that led to the first direct observation of the b baryon [5]. However, in this analysis we use a multivariate selection for the owing to the smaller signal to background ratio compared to that for the in theb analysis. We use thePYTHIAMonte Carlo (MC) program [6] to generateb and theEVTGENprogram [7] to simulateb decays. Theb mass and lifetime are set to be 6.052 GeV and 1.54 ps, respectively. The generated events are subjected to a GEANT [8] based D0 detector simulation, and to the same reconstruction and selection programs as the data. We optimize theselection using MCb events for the signal and a sample ofJ=cðKþÞ data (referred to below as wrong-sign events) for the background, while keeping the J=c data blinded. Once all selection criteria have been determined, we apply them to theJ=cdata.
We begin the event selection by reconstructingJ=c ! þ candidates from two oppositely charged muons
with transverse momentum (pT) greater than 1.5 GeV that are compatible with being from a common vertex. Muons are identified by tracks reconstructed in the central tracking system that are matched with either track seg-ments in the muon spectrometer or calorimeter energy deposits consistent with a minimum ionizing particle.
Events must have a well-reconstructedp p interaction point that we take to be theb production vertex and aJ=c ! þ candidate in the mass window 2:75 < M
<
3:40 GeV. Events with J=c candidates are reprocessed with a version of the track reconstruction algorithm that increases the efficiency for tracks with low pT and high impact parameters.
We form ! p candidates from two oppositely charged particles, each withpT> 0:2 GeV, that are con-sistent with having originated from a common vertex. The two tracks are required to have a total of no more than two hits in the tracking system before the reconstructed p vertex. The impact parameter significance (the impact parameter with respect to thep p interaction point divided by its uncertainty) must exceed four for at least one of the tracks and three for the other. The track with the higherpT is assumed to be the proton. MC studies show that this assignment leads to the correct combination nearly 100% of the time. Furthermore, we require the transverse decay length to be greater than 4 times its uncertainty and the proper decay length to exceed 10 times its uncer-tainty, where the transverse decay length is the distance between the production and decay vertices in the transverse plane while the proper decay length is the transverse decay length corrected by the Lorentz boost calculated from pTðÞ. candidates must have reconstructed masses
be-tween 1.108 and 1.126 GeV.
We combine candidates with negatively charged par-ticles (assumed to be kaons) to form ! K decay candidates. The and the kaon are required to have a common vertex. Thecandidates must have transverse decay length significances greater than four and the un-certainties of the proper decay lengths less than 0.5 cm. These two requirements reduce backgrounds from combi-natorics and mismeasured tracks. The baryon has a mass of 1.322 GeV [9] and decays into. If the kaon mass is assigned to the pion, this decay could be a major background for ! K. To eliminate this back-ground, we remove candidates with mass less than 1.34 GeV. Figure1(a) shows the mass distribution of the reconstructed ! K candidates after these selec-tions. The distribution of wrong-sign Kþ events is also shown. An excess of events above the background around the expected mass of 1.672 GeV [9] is visible in the distribution of the right-signK events.
To further enhance thesignal over the combinatorial background, kinematic variables associated with daughter particle momenta, vertices, and track qualities are com-bined using boosted decision trees (BDT) [10,11]. The K mass distribution after the BDT selection is shown
in Fig. 1(b). The BDT selection retains 87% of the signal while rejecting 89% of the background. The en-hanced mass peak is evident in the distribution. A K pair is considered to be a candidate if its mass
To selectb ! J=ccandidates, we develop selec-tion criteria using the MCb events as the signal and the data wrong-sign events as the background. The background events are formed by combining J=c candidates with Kþ pairs with mass between 1.662 and 1.682 GeV. We
formb ! J=cdecay candidates fromJ=c and pairs that are consistent with being from a common vertex. We require the uncertainty of theb proper decay length to be less than 0.03 cm and impose a minimumpT cut of 6 GeV on theb candidates. Finally,J=cand daugh-ters from theb decays are expected to be boosted in the direction of the b; therefore, we require the opening angle in the transverse plane between the J=c and the to be less than=2.
We then apply the above selections to the right-sign events in the data to search for the b baryon in the mass window between 5.6 and 7.0 GeV. This range is chosen since 5.624 GeV is the mass of the lightest b baryon, the b, and the upper limit of 7.0 GeV is nearly 1 GeV higher than the predictedb mass [11]. We calcu-late theb candidate mass using the formula MðbÞ¼ MðJ=cÞMðþÞMðKÞþ ^MðJ=cÞþ ^MðÞ.
Here MðJ=cÞ, MðþÞ, and MðKÞ are the re-constructed masses while ^MðJ=cÞ and ^MðÞ are taken from Ref. [9]. This calculation improves the mass resolu-tion of the MCb events from 0.080 GeV to 0.034 GeV. In the mass search window, we observe 79 candidates in the data with the mass distribution shown in Fig. 2(a). An excess of events near 6.2 GeV is apparent. No such struc-ture, however, is seen in the corresponding mass distribu-tion [Fig.2(b)] of the 30 wrong-sign events.
Assuming the excess is due to theb production, we fit
b candidate masses with the hypothesis of a Gaussian
signal plus a flat background using an unbinned likelihood method. We fix the Gaussian width to 0.034 GeV, the width of the MCb signal. The fit gives anb mass of6:165
0:010ðstatÞ GeV and a yield of 17:8 4:9ðstatÞ signal events. To assess the significance of the excess, we first determine the likelihood Lsþb of the signal plus back-ground fit above and then repeat the fit with only the background contribution to find a new likelihood Lb. The logarithmic likelihood ratiopffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 lnðLsþb=LbÞyields a statistical significance of5:4, equivalent to a probability of 6:7 108 that the background could fluctuate with a significance equal to or greater than what is observed. Fitted yields for positively and negatively charged candi-dates are 6:2 3:1ðstatÞ þb and 12:0 3:9ðstatÞ b, respectively.
Various checks have been performed to ensure that the observed resonance is genuine. (1) We apply the event se-lection to data events in the sidebands of the reconstructed and resonances separately. The J=cðpÞKmass
distributions of these sideband events are shown in Figs. 2(c) and 2(d). No significant excess is present in either distribution. (2) We investigate the possibility of a false signal due to residual b hadron backgrounds by applying the finalb selection to MCB! J=cK! J=cK0
S, b ! J=c, and b! J=c samples
with equivalent integrated luminosities significantly greater than that of the analyzed data. No indication of any resonance is observed in the reconstructed J=c mass distribution. (3) We check the mass distributions of theb decay products. Forb candidates within a3 mass window around the observed peak, we relax the mass requirements on theand candidates and perform a fit to each mass distribution. The numbers of the and candidates from the fits are consistent with the observed number of b signal events. (4) We replace the BDT selection with individual cuts on the most important vari-ables according to the BDT optimization and confirm the existence of a peak with a comparable event yield but a
) (GeV) b − Ω M( 5.8 6 6.2 6.4 6.6 6.8 7 Events/(0.04 GeV) 0 2 4 6 8 10 12 14 D0 −1 1.3 fb Data Fit (a) Events/(0.04 GeV) Mass (GeV) 2 4 6 8 D0 −1 1.3 fb (b) wrong−sign 5.8 6 6.2 2 4 6 8 (c) sidebands − Ω 5.8 6 6.2 6.4 6.6 6.8 7 2 4 6 8 (d) Λ sidebands
FIG. 2 (color online). (a) TheMðbÞ distribution of the b 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 the wrong-sign background (b), thesideband events (c), and the sideband events (d). K) (GeV) Λ M( 1.6 1.65 1.7 1.75 1.8 Events/(0.005 GeV) 0 100 200 300 400 500 Right−sign Wrong−sign −1 D0, 1.3 fb (a) K) (GeV) Λ M( 1.6 1.65 1.7 1.75 1.8 Events/(0.005 GeV) 0 20 40 60 80 100 120 140 Right−sign Wrong−sign −1 D0, 1.3 fb (b)
FIG. 1 (color online). The invariant mass distribution of the K pair before (a) and after (b) the BDT selection. Solid circles are from the right-signKevents while the histogram is from the wrong-signKþ events without any additional normaliza-tion.
higher background at a mass consistent with that observed using the BDT. (5) We test the robustness of the peak by varying selections such as the veto, and mass windows, transverse decay requirements, BDT selec-tion, and the requirement onpTðbÞ. All the above studies confirm the existence of the resonance.
Potential sources of systematic uncertainties on the measured b mass include event selection, signal and background models, and momentum scale. Varying the selection criteria and applying a set of cuts on individual kinematic variables lead to a maximum change of 0.012 GeV in the fitted mass. Using a linear function as the background model results in negligible change in the mass. Varying the Gaussian width in the signal model between 0.028 and 0.040 GeV changes the fitted mass by at most 0.003 GeV. When a tighter selection is applied to enhance signal over background, we can float the width of the signal model in the fit. This leads to a mass shift of 0.002 GeV and a fitted signal width of0:033 0:010 GeV, consistent with the MC expectation. To study the effect of the track momentum scale uncertainty on the measured
b mass, we reconstruct the higher statisticsb! J=c
decays and measure thebmass for different minimumpT requirements on the b daughter particles. We compare these measurements to the world average value of theb mass [9] and take the maximum deviation of 0.004 GeV as a systematic uncertainty. Adding in quadrature, we get a total systematic uncertainty of 0.013 GeV to obtain a mea-sured b mass: 6:165 0:010ðstatÞ 0:013ðsystÞ GeV. Similarly, we estimate the systematic uncertainty on the
b yield by varying the signal and background models in
the fit. The observed maximum change of 0.8 is assigned as the systematic uncertainty on the yield:17:8 4:9ðstatÞ 0:8ðsystÞ. In all these studies, the signal significance re-mains above5.
Figure 3 shows the distribution of the proper decay length of the b candidates observed in the 3 mass
window around the fittedb mass. The distribution of the MC b signal plus the data background events is also shown. The background distribution is modeled using events in the b sidebands of 5.8–6.0 and 6.4–6.6 GeV. Despite the low statistics, the data distribution contains significantly more positive than negative decay lengths as expected and consistent with a weakly decayingb hadron. We calculate theb production rate relative to that of the b [5]. The selection efficiency ratio ðb ! J=cÞ=ð
b ! J=cÞ is found to be 1:5
0:2ðstatÞ assuming inclusive and decays. The
higher efficiency for theb is due primarily to a harder pT spectrum of the kaon from the decay than that of
the pion from the decay and a shorter lifetime of the compared to the. By using the reported
b events
[5] and the observedb yield here, we estimate R ¼fðb ! bÞ fðb ! bÞ Brð b ! J=cÞ Brð b ! J=cÞ
to be R ¼ 0:80 0:32ðstatÞþ0:140:22ðsystÞ. Here fðb ! bÞ andfðb ! bÞ are the fractions of b quarks that hadronize to form b andb, respectively. The systematic uncer-tainty includes contributions from the signal yields as well as the efficiency ratio. Usingðb ! J=cÞ=ðb ! J=cÞ ¼ 9:8 [12], the central values of ð
bÞ ¼
1:42þ0:28
0:24 ps [9], the R value above, and ðbÞ in the
range of 0.83–1.67 ps [3], we obtain fðb ! bÞ=fðb !
bÞ 0:07–0:14.
In summary, by analyzing1:3 fb1of data collected by the D0 experiment inp p collisions at pffiffiffis¼ 1:96 TeV at the Fermilab Tevatron Collider, we have made the first observation of the doubly strangeb baryon b in the fully reconstructed decay mode b ! J=c with J=c ! þ, ! K and ! p. We measure the
b mass to be 6:165 0:010ðstatÞ 0:013ðsystÞ GeV.
The significance of the observed signal is greater than5. We thank the staffs at Fermilab and collaborating insti-tutions, and acknowledge support from the DOE and NSF (U.S.); CEA and CNRS/IN2P3 (France); FASI, Rosatom, and RFBR (Russia); 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); STFC (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); and the Alexander von Humboldt Foundation (Germany).
*Visitor from Augustana College, Sioux Falls, SD, USA. †Visitor from The University of Liverpool, Liverpool, U.K. Proper decay length (cm)
−0.1 0 0.1 0.2 0.3 0.4 Events/(0.05 cm) 1 10 2 10 D0 −1
1.3 fb DataMC signal + data bkgd
FIG. 3 (color online). The distribution of the proper decay length of theb candidates in the3 mass window around the observed peak along with the expected distribution from the MCb signal with a lifetime 1.54 ps plus the data background events.
‡Visitor from II. Physikalisches Institut, Georg-August-University, Go¨ttingen, Germany.
xVisitor from ECFM, Universidad Autonoma de Sinaloa, Culiaca´n, Mexico.
kVisitor from Helsinki Institute of Physics, Helsinki, Finland.
{Visitor from Universita¨t Bern, Bern, Switzerland.
**Visitor from Universita¨t Zu¨rich, Zu¨rich, Switzerland. ††Deceased.
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