Observation of the Ω_b⁻ baryon and measurement of the properties of the Ξ_b^- and Ω_b^- baryons

Article

Reference

Observation of the Ω

baryon and measurement of the properties of the Ξ

and Ω

baryons

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al .

Abstract

We report the observation of the bottom, doubly-strange baryon Ω−b through the decay chain Ω−b→J/ψΩ−, where J/ψ→μ+μ−, Ω−→ΛK−, and Λ→pπ−, using 4.2  fb−1 of data from pp¯

collisions at s√=1.96  TeV, and recorded with the Collider Detector at Fermilab. A signal is observed whose probability of arising from a background fluctuation is 4.0×10−8, or 5.5 Gaussian standard deviations. The Ω−b mass is measured to be 6054.4±6.8(stat)±0.9(syst)   MeV/c2. The lifetime of the Ω−b baryon is measured to be 1.13+0.53−0.40(stat)±0.02(syst)   ps. In addition, for the Ξ−b baryon we measure a mass of 5790.9±2.6(stat)±0.8(syst)  MeV/c2 and a lifetime of 1.56+0.27−0.25(stat)±0.02(syst)  ps. Under the assumption that the Ξ−b and Ω−b are produced with similar kinematic distributions to the Λ0b baryon, we find σ(Ξ−b)B(Ξ−b→J/ψΞ−)σ(Λ0b)B(Λ0b→J/ψΛ)=0.167+0.037−0.025(stat)±0.012(syst) and σ(Ω−b)B(Ω−b→J/ψΩ−)σ(Λ0b)B(Λ0b→J/ψΛ)=0.045+0.017−0.012(stat)±0.004(syst) for baryons produced with transverse momentum in the range [...]

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Observation of the Ω

baryon and measurement of the properties of the Ξ

and Ω

baryons. Physical Review. D, 2009, vol. 80, no. 07, p. 072003

DOI : 10.1103/PhysRevD.80.072003

Available at:

http://archive-ouverte.unige.ch/unige:38633

Disclaimer: layout of this document may differ from the published version.

Observation of the

baryon and measurement of the properties of the

and

baryons

T. Aaltonen,²⁴J. Adelman,¹⁴T. Akimoto,⁵⁶B. A´ lvarez Gonza´lez,^12,uS. Amerio,^44b,44aD. Amidei,³⁵A. Anastassov,³⁹ A. Annovi,²⁰J. Antos,¹⁵G. Apollinari,¹⁸A. Apresyan,⁴⁹T. Arisawa,⁵⁸A. Artikov,¹⁶W. Ashmanskas,¹⁸A. Attal,⁴

A. Aurisano,⁵⁴F. Azfar,⁴³W. Badgett,¹⁸A. Barbaro-Galtieri,²⁹V. E. Barnes,⁴⁹B. A. Barnett,²⁶P. Barria,^47c,47a V. Bartsch,³¹G. Bauer,³³P.-H. Beauchemin,³⁴F. Bedeschi,^47aD. Beecher,³¹S. Behari,²⁶G. Bellettini,^47b,47aJ. Bellinger,⁶⁰

D. Benjamin,¹⁷A. Beretvas,¹⁸J. Beringer,²⁹A. Bhatti,⁵¹M. Binkley,¹⁸D. Bisello,^44b,44aI. Bizjak,^31,zR. E. Blair,² C. Blocker,⁷B. Blumenfeld,²⁶A. Bocci,¹⁷A. Bodek,⁵⁰V. Boisvert,⁵⁰G. Bolla,⁴⁹D. Bortoletto,⁴⁹J. Boudreau,⁴⁸ A. Boveia,¹¹B. Brau,^11,bA. Bridgeman,²⁵L. Brigliadori,^6b,6aC. Bromberg,³⁶E. Brubaker,¹⁴J. Budagov,¹⁶H. S. Budd,⁵⁰

S. Budd,²⁵S. Burke,¹⁸K. Burkett,¹⁸G. Busetto,^44b,44aP. Bussey,²²A. Buzatu,³⁴K. L. Byrum,²S. Cabrera,^17,w C. Calancha,³²M. Campanelli,³⁶M. Campbell,³⁵F. Canelli,^14,18 A. Canepa,⁴⁶B. Carls,²⁵D. Carlsmith,⁶⁰R. Carosi,^47a S. Carrillo,^19,oS. Carron,³⁴B. Casal,¹²M. Casarsa,¹⁸A. Castro,^6b,6aP. Catastini,^47c,47aD. Cauz,^55b,55aV. Cavaliere,^47c,47a M. Cavalli-Sforza,⁴A. Cerri,²⁹L. Cerrito,^31,qS. H. Chang,²⁸Y. C. Chen,¹M. Chertok,⁸G. Chiarelli,^47aG. Chlachidze,¹⁸ F. Chlebana,¹⁸K. Cho,²⁸D. Chokheli,¹⁶J. P. Chou,²³G. Choudalakis,³³S. H. Chuang,⁵³K. Chung,^18,pW. H. Chung,⁶⁰

Y. S. Chung,⁵⁰T. Chwalek,²⁷C. I. Ciobanu,⁴⁵M. A. Ciocci,^47c,47aA. Clark,²¹D. Clark,⁷G. Compostella,^44a M. E. Convery,¹⁸J. Conway,⁸M. Cordelli,²⁰G. Cortiana,^44b,44aC. A. Cox,⁸D. J. Cox,⁸F. Crescioli,^47b,47a C. Cuenca Almenar,^8,wJ. Cuevas,^12,uR. Culbertson,¹⁸J. C. Cully,³⁵D. Dagenhart,¹⁸M. Datta,¹⁸T. Davies,²² P. de Barbaro,⁵⁰S. De Cecco,^52aA. Deisher,²⁹G. De Lorenzo,⁴M. Dell’Orso,^47b,47aC. Deluca,⁴L. Demortier,⁵¹J. Deng,¹⁷

M. Deninno,^6aP. F. Derwent,¹⁸A. Di Canto,^47b,47aG. P. di Giovanni,⁴⁵C. Dionisi,^52b,52aB. Di Ruzza,^55b,55a J. R. Dittmann,⁵M. D’Onofrio,⁴S. Donati,^47b,47aP. Dong,⁹J. Donini,^44aT. Dorigo,^44aS. Dube,⁵³J. Efron,⁴⁰A. Elagin,⁵⁴

R. Erbacher,⁸D. Errede,²⁵S. Errede,²⁵R. Eusebi,¹⁸H. C. Fang,²⁹S. Farrington,⁴³W. T. Fedorko,¹⁴R. G. Feild,⁶¹ M. Feindt,²⁷J. P. Fernandez,³²C. Ferrazza,^47d,47aR. Field,¹⁹G. Flanagan,⁴⁹R. Forrest,⁸M. J. Frank,⁵M. Franklin,²³ J. C. Freeman,¹⁸I. Furic,¹⁹M. Gallinaro,^52aJ. Galyardt,¹³F. Garberson,¹¹J. E. Garcia,²¹A. F. Garfinkel,⁴⁹P. Garosi,^47c,47a K. Genser,¹⁸H. Gerberich,²⁵D. Gerdes,³⁵A. Gessler,²⁷S. Giagu,^52b,52aV. Giakoumopoulou,³P. Giannetti,^47aK. Gibson,⁴⁸

J. L. Gimmell,⁵⁰C. M. Ginsburg,¹⁸N. Giokaris,³M. Giordani,^55b,55aP. Giromini,²⁰M. Giunta,^47aG. Giurgiu,²⁶ V. Glagolev,¹⁶D. Glenzinski,¹⁸M. Gold,³⁸N. Goldschmidt,¹⁹A. Golossanov,¹⁸G. Gomez,¹²G. Gomez-Ceballos,³³

M. Goncharov,³³O. Gonza´lez,³²I. Gorelov,³⁸A. T. Goshaw,¹⁷K. Goulianos,⁵¹A. Gresele,^44b,44aS. Grinstein,²³ C. Grosso-Pilcher,¹⁴R. C. Group,¹⁸U. Grundler,²⁵J. Guimaraes da Costa,²³Z. Gunay-Unalan,³⁶C. Haber,²⁹K. Hahn,³³

S. R. Hahn,¹⁸E. Halkiadakis,⁵³B.-Y. Han,⁵⁰J. Y. Han,⁵⁰F. Happacher,²⁰K. Hara,⁵⁶D. Hare,⁵³M. Hare,⁵⁷S. Harper,⁴³ R. F. Harr,⁵⁹R. M. Harris,¹⁸M. Hartz,⁴⁸K. Hatakeyama,⁵¹C. Hays,⁴³M. Heck,²⁷A. Heijboer,⁴⁶J. Heinrich,⁴⁶ C. Henderson,³³M. Herndon,⁶⁰J. Heuser,²⁷S. Hewamanage,⁵D. Hidas,¹⁷C. S. Hill,^11,dD. Hirschbuehl,²⁷A. Hocker,¹⁸

S. Hou,¹M. Houlden,³⁰S.-C. Hsu,²⁹B. T. Huffman,⁴³R. E. Hughes,⁴⁰U. Husemann,⁶¹M. Hussein,³⁶J. Huston,³⁶ J. Incandela,¹¹G. Introzzi,^47aM. Iori,^52b,52aA. Ivanov,⁸E. James,¹⁸D. Jang,¹³B. Jayatilaka,¹⁷E. J. Jeon,²⁸M. K. Jha,^6a

S. Jindariani,¹⁸W. Johnson,⁸M. Jones,⁴⁹K. K. Joo,²⁸S. Y. Jun,¹³J. E. Jung,²⁸T. R. Junk,¹⁸T. Kamon,⁵⁴D. Kar,¹⁹ P. E. Karchin,⁵⁹Y. Kato,^42,nR. Kephart,¹⁸W. Ketchum,¹⁴J. Keung,⁴⁶V. Khotilovich,⁵⁴B. Kilminster,¹⁸D. H. Kim,²⁸

H. S. Kim,²⁸H. W. Kim,²⁸J. E. Kim,²⁸M. J. Kim,²⁰S. B. Kim,²⁸S. H. Kim,⁵⁶Y. K. Kim,¹⁴N. Kimura,⁵⁶L. Kirsch,⁷ S. Klimenko,¹⁹B. Knuteson,³³B. R. Ko,¹⁷K. Kondo,⁵⁸D. J. Kong,²⁸J. Konigsberg,¹⁹A. Korytov,¹⁹A. V. Kotwal,¹⁷ M. Kreps,²⁷J. Kroll,⁴⁶D. Krop,¹⁴N. Krumnack,⁵M. Kruse,¹⁷V. Krutelyov,¹¹T. Kubo,⁵⁶T. Kuhr,²⁷N. P. Kulkarni,⁵⁹

M. Kurata,⁵⁶S. Kwang,¹⁴A. T. Laasanen,⁴⁹S. Lami,^47aS. Lammel,¹⁸M. Lancaster,³¹R. L. Lander,⁸K. Lannon,^40,t A. Lath,⁵³G. Latino,^47c,47aI. Lazzizzera,^44b,44aT. LeCompte,²E. Lee,⁵⁴H. S. Lee,¹⁴S. W. Lee,^54,vS. Leone,^47a J. D. Lewis,¹⁸C.-S. Lin,²⁹J. Linacre,⁴³M. Lindgren,¹⁸E. Lipeles,⁴⁶A. Lister,⁸D. O. Litvintsev,¹⁸C. Liu,⁴⁸T. Liu,¹⁸

N. S. Lockyer,⁴⁶A. Loginov,⁶¹M. Loreti,^44b,44aL. Lovas,¹⁵D. Lucchesi,^44b,44aC. Luci,^52b,52aJ. Lueck,²⁷P. Lujan,²⁹ P. Lukens,¹⁸G. Lungu,⁵¹L. Lyons,⁴³J. Lys,²⁹R. Lysak,¹⁵D. MacQueen,³⁴R. Madrak,¹⁸K. Maeshima,¹⁸K. Makhoul,³³

T. Maki,²⁴P. Maksimovic,²⁶S. Malde,⁴³S. Malik,³¹G. Manca,^30,fA. Manousakis-Katsikakis,³F. Margaroli,⁴⁹ C. Marino,²⁷C. P. Marino,²⁵A. Martin,⁶¹V. Martin,^22,lM. Martı´nez,⁴R. Martı´nez-Balları´n,³²T. Maruyama,⁵⁶ P. Mastrandrea,^52aT. Masubuchi,⁵⁶M. Mathis,²⁶M. E. Mattson,⁵⁹P. Mazzanti,^6aK. S. McFarland,⁵⁰P. McIntyre,⁵⁴ R. McNulty,^30,kA. Mehta,³⁰P. Mehtala,²⁴A. Menzione,^47aP. Merkel,⁴⁹C. Mesropian,⁵¹T. Miao,¹⁸N. Miladinovic,⁷

R. Miller,³⁶C. Mills,²³M. Milnik,²⁷A. Mitra,¹G. Mitselmakher,¹⁹H. Miyake,⁵⁶N. Moggi,^6aM. N. Mondragon,^18,o C. S. Moon,²⁸R. Moore,¹⁸M. J. Morello,^47aJ. Morlock,²⁷P. Movilla Fernandez,¹⁸J. Mu¨lmensta¨dt,²⁹A. Mukherjee,¹⁸

Th. Muller,²⁷R. Mumford,²⁶P. Murat,¹⁸M. Mussini,^6b,6aJ. Nachtman,^18,pY. Nagai,⁵⁶A. Nagano,⁵⁶J. Naganoma,⁵⁶ K. Nakamura,⁵⁶I. Nakano,⁴¹A. Napier,⁵⁷V. Necula,¹⁷J. Nett,⁶⁰C. Neu,^46,xM. S. Neubauer,²⁵S. Neubauer,²⁷

J. Nielsen,^29,hL. Nodulman,²M. Norman,¹⁰O. Norniella,²⁵E. Nurse,³¹L. Oakes,⁴³S. H. Oh,¹⁷Y. D. Oh,²⁸I. Oksuzian,¹⁹ T. Okusawa,⁴²R. Orava,²⁴K. Osterberg,²⁴S. Pagan Griso,^44b,44aE. Palencia,¹⁸V. Papadimitriou,¹⁸A. Papaikonomou,²⁷

A. A. Paramonov,¹⁴B. Parks,⁴⁰S. Pashapour,³⁴J. Patrick,¹⁸G. Pauletta,^55b,55aM. Paulini,¹³C. Paus,³³T. Peiffer,²⁷ D. E. Pellett,⁸A. Penzo,^55aT. J. Phillips,¹⁷G. Piacentino,^47aE. Pianori,⁴⁶L. Pinera,¹⁹K. Pitts,²⁵C. Plager,⁹L. Pondrom,⁶⁰

O. Poukhov,^16,aN. Pounder,⁴³F. Prakoshyn,¹⁶A. Pronko,¹⁸J. Proudfoot,²F. Ptohos,^18,jE. Pueschel,¹³G. Punzi,^47b,47a J. Pursley,⁶⁰J. Rademacker,^43,dA. Rahaman,⁴⁸V. Ramakrishnan,⁶⁰N. Ranjan,⁴⁹I. Redondo,³²P. Renton,⁴³M. Renz,²⁷

M. Rescigno,^52aS. Richter,²⁷F. Rimondi,^6b,6aL. Ristori,^47aA. Robson,²²T. Rodrigo,¹²T. Rodriguez,⁴⁶E. Rogers,²⁵ S. Rolli,⁵⁷R. Roser,¹⁸M. Rossi,^55aR. Rossin,¹¹P. Roy,³⁴A. Ruiz,¹²J. Russ,¹³V. Rusu,¹⁸B. Rutherford,¹⁸H. Saarikko,²⁴ A. Safonov,⁵⁴W. K. Sakumoto,⁵⁰O. Salto´,⁴L. Santi,^55b,55aS. Sarkar,^52b,52aL. Sartori,^47aK. Sato,¹⁸A. Savoy-Navarro,⁴⁵

P. Schlabach,¹⁸A. Schmidt,²⁷E. E. Schmidt,¹⁸M. A. Schmidt,¹⁴M. P. Schmidt,^61,aM. Schmitt,³⁹T. Schwarz,⁸ L. Scodellaro,¹²A. Scribano,^47c,47aF. Scuri,^47aA. Sedov,⁴⁹S. Seidel,³⁸Y. Seiya,⁴²A. Semenov,¹⁶L. Sexton-Kennedy,¹⁸ F. Sforza,^47b,47aA. Sfyrla,²⁵S. Z. Shalhout,⁵⁹T. Shears,³⁰P. F. Shepard,⁴⁸M. Shimojima,^56,bS. Shiraishi,¹⁴M. Shochet,¹⁴

Y. Shon,⁶⁰I. Shreyber,³⁷P. Sinervo,³⁴A. Sisakyan,¹⁶A. J. Slaughter,¹⁸J. Slaunwhite,⁴⁰K. Sliwa,⁵⁷J. R. Smith,⁸ F. D. Snider,¹⁸R. Snihur,³⁴A. Soha,⁸S. Somalwar,⁵³V. Sorin,³⁶T. Spreitzer,³⁴P. Squillacioti,^47c,47aM. Stanitzki,⁶¹ R. St. Denis,²²B. Stelzer,³⁴O. Stelzer-Chilton,³⁴D. Stentz,³⁹J. Strologas,³⁸G. L. Strycker,³⁵J. S. Suh,²⁸A. Sukhanov,¹⁹

I. Suslov,¹⁶T. Suzuki,⁵⁶A. Taffard,^25,gR. Takashima,⁴¹Y. Takeuchi,⁵⁶R. Tanaka,⁴¹M. Tecchio,³⁵P. K. Teng,¹ K. Terashi,⁵¹R. Tesarek,¹⁸J. Thom,^18,iA. S. Thompson,²²G. A. Thompson,²⁵E. Thomson,⁴⁶P. Tipton,⁶¹ P. Ttito-Guzma´n,³²S. Tkaczyk,¹⁸D. Toback,⁵⁴S. Tokar,¹⁵K. Tollefson,³⁶T. Tomura,⁵⁶D. Tonelli,¹⁸S. Torre,²⁰ D. Torretta,¹⁸P. Totaro,^55b,55aS. Tourneur,⁴⁵M. Trovato,^47d,47aS.-Y. Tsai,¹Y. Tu,⁴⁶N. Turini,^47c,47aF. Ukegawa,⁵⁶ S. Vallecorsa,²¹N. van Remortel,^24,cA. Varganov,³⁵E. Vataga,^47d,47aF. Va´zquez,^19,oG. Velev,¹⁸C. Vellidis,³M. Vidal,³²

R. Vidal,¹⁸I. Vila,¹²R. Vilar,¹²T. Vine,³¹M. Vogel,³⁸I. Volobouev,^29,vG. Volpi,^47b,47aP. Wagner,⁴⁶R. G. Wagner,² R. L. Wagner,¹⁸W. Wagner,^27,yJ. Wagner-Kuhr,²⁷T. Wakisaka,⁴²R. Wallny,⁹S. M. Wang,¹A. Warburton,³⁴D. Waters,³¹

M. Weinberger,⁵⁴J. Weinelt,²⁷W. C. Wester III,¹⁸B. Whitehouse,⁵⁷D. Whiteson,^46,gA. B. Wicklund,²E. Wicklund,¹⁸ S. Wilbur,¹⁴G. Williams,³⁴H. H. Williams,⁴⁶P. Wilson,¹⁸B. L. Winer,⁴⁰P. Wittich,^18,iS. Wolbers,¹⁸C. Wolfe,¹⁴ T. Wright,³⁵X. Wu,²¹F. Wu¨rthwein,¹⁰S. Xie,³³A. Yagil,¹⁰K. Yamamoto,⁴²J. Yamaoka,¹⁷U. K. Yang,^14,rY. C. Yang,²⁸

W. M. Yao,²⁹G. P. Yeh,¹⁸K. Yi,^18,pJ. Yoh,¹⁸K. Yorita,⁵⁸T. Yoshida,^42,mG. B. Yu,⁵⁰I. Yu,²⁸S. S. Yu,¹⁸J. C. Yun,¹⁸ L. Zanello,^52b,52aA. Zanetti,^55aX. Zhang,²⁵Y. Zheng,^9,eand S. Zucchelli^6a

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3University of Athens, 157 71 Athens, Greece

4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

5Baylor University, Waco, Texas 76798, USA

6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy

7Brandeis University, Waltham, Massachusetts 02254, USA

8University of California, Davis, Davis, California 95616, USA

9University of California, Los Angeles, Los Angeles, California 90024, USA

10University of California, San Diego, La Jolla, California 92093, USA

11University of California, Santa Barbara, Santa Barbara, California 93106, USA

12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

15Comenius University, 842 48 Bratislava, Slovakia;

Institute of Experimental Physics, 040 01 Kosice, Slovakia

16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

17Duke University, Durham, North Carolina 27708, USA

18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

19University of Florida, Gainesville, Florida 32611, USA

20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

21University of Geneva, CH-1211 Geneva 4, Switzerland

22Glasgow University, Glasgow G12 8QQ, United Kingdom

23Harvard University, Cambridge, Massachusetts 02138, USA

24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

25University of Illinois, Urbana, Illinois 61801, USA

26The Johns Hopkins University, Baltimore, Maryland 21218, USA

27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

28Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

Sungkyunkwan University, Suwon 440-746, Korea;

Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;

Chonnam National University, Gwangju, 500-757, Korea

29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

30University of Liverpool, Liverpool L69 7ZE, United Kingdom

31University College London, London WC1E 6BT, United Kingdom

32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

34Institute of Particle Physics: McGill University, Montre´al, Que´bec, Canada H3A 2T8;

Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;

University of Toronto, Toronto, Ontario, Canada M5S 1A7;

and TRIUMF, Vancouver, British Columbia, Canada V6T 2A3

35University of Michigan, Ann Arbor, Michigan 48109, USA

36Michigan State University, East Lansing, Michigan 48824, USA

37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

38University of New Mexico, Albuquerque, New Mexico 87131, USA

39Northwestern University, Evanston, Illinois 60208, USA

40The Ohio State University, Columbus, Ohio 43210, USA

41Okayama University, Okayama 700-8530, Japan

42Osaka City University, Osaka 588, Japan

43University of Oxford, Oxford OX1 3RH, United Kingdom

44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44bUniversity of Padova, I-35131 Padova, Italy

45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

47bUniversity of Pisa, I-56127 Pisa, Italy

47cUniversity of Siena, I-56127 Pisa, Italy

aDeceased.

bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.

cVisitor from Universiteit Antwerpen, B-2610 Antwerp, Belgium.

dVisitor from University of Bristol, Bristol BS8 1TL, United Kingdom.

eVisitor from Chinese Academy of Sciences, Beijing 100864, China.

fVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.

gVisitor from University of California Irvine, Irvine, CA 92697, USA.

hVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.

iVisitor from Cornell University, Ithaca, NY 14853, USA.

jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.

kVisitor from University College Dublin, Dublin 4, Ireland.

lVisitor from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.

mVisitor from University of Fukui, Fukui City, Fukui

Prefecture, Japan 910-0017. ^zOn leave from J. Stefan Institute, Ljubljana, Slovenia.

yVisitor from Bergische Universita¨t Wuppertal, 42097 Wuppertal, Germany.

xVisitor from University of Virginia, Charlottesville, VA 22904, USA.

wVisitor from IFIC (CSIC-Universitat de Valencia), 46071 Valencia, Spain.

vVisitor from Texas Tech University, Lubbock, TX 79609, USA.

uUniversity de Oviedo, E-33007 Oviedo, Spain.

tVisitor from University of Notre Dame, Notre Dame, IN 46556, USA.

sVisitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.

rVisitor from University of Manchester, Manchester M13 9PL, England.

qVisitor from Queen Mary, University of London, London, E1 4NS, England.

pVisitor from University of Iowa, Iowa City, IA 52242, USA.

oVisitor from Universidad Iberoamericana, Mexico D.F., Mexico.

nVisitor from Kinki University, Higashi-Osaka City, Japan 577-8502.

OBSERVATION OF THE_b BARYON AND. . . PHYSICAL REVIEW D80,072003 (2009)

47dScuola Normale Superiore, I-56127 Pisa, Italy

48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

49Purdue University, West Lafayette, Indiana 47907, USA

50University of Rochester, Rochester, New York 14627, USA

51The Rockefeller University, New York, New York 10021, USA

52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy

52bSapienza Universita` di Roma, I-00185 Roma, Italy

53Rutgers University, Piscataway, New Jersey 08855, USA

54Texas A&M University, College Station, Texas 77843, USA

55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, I-33100 Udine, Italy

55bUniversity of Trieste/Udine, I-33100 Udine, Italy

56University of Tsukuba, Tsukuba, Ibaraki 305, Japan

57Tufts University, Medford, Massachusetts 02155, USA

58Waseda University, Tokyo 169, Japan

59Wayne State University, Detroit, Michigan 48201, USA

60University of Wisconsin, Madison, Wisconsin 53706, USA

61Yale University, New Haven, Connecticut 06520, USA (Received 11 May 2009; published 6 October 2009)

We report the observation of the bottom, doubly-strange baryon_b through the decay chain_b ! J=c, whereJ=c !^þ,!K, and!p, using4:2 fb¹of data fromppcollisions at ffiffiffi

ps

¼1:96 TeV, and recorded with the Collider Detector at Fermilab. A signal is observed whose probability of arising from a background fluctuation is4:010⁸, or 5.5 Gaussian standard deviations.

The_b mass is measured to be6054:46:8ðstatÞ 0:9ðsystÞMeV=c². The lifetime of the_b baryon is measured to be 1:13^þ0:53_0:40ðstatÞ 0:02ðsystÞps. In addition, for the _b baryon we measure a mass of 5790:92:6ðstatÞ 0:8ðsystÞMeV=c² and a lifetime of1:56^þ0:27_0:25ðstatÞ 0:02ðsystÞps. Under the as- sumption that the_b and_b are produced with similar kinematic distributions to the ⁰_b baryon, we find ^ð_ð^b0^ÞBðb!J=cÞ

bÞBð⁰_b!J=cÞ ¼0:167^þ0:037_0:025ðstatÞ 0:012ðsystÞ and ^ð_ð^b0^ÞBðb!J=cÞ

bÞBð⁰_b!J=cÞ ¼0:045^þ0:017_0:012ðstatÞ 0:004ðsystÞfor baryons produced with transverse momentum in the range of6–20 GeV=c.

DOI:10.1103/PhysRevD.80.072003 PACS numbers: 13.30.Eg, 13.60.Rj, 14.20.Mr

I. INTRODUCTION

Since its inception, the quark model has had great suc- cess in describing the spectroscopy of hadrons. In particu- lar, this has been the case for theDandBmesons, where all of the ground states have been observed [1]. The spectros- copy ofcbaryons also agrees well with the quark model, and a rich spectrum of baryons containing b quarks is predicted [2]. Until recently, direct observation ofbbary- ons has been limited to a single state, the⁰_b(quark content judbi) [1]. The accumulation of large data sets from the Tevatron has changed this situation, and made possible the observation of the _b (jdsbi) [3,4] and the ^ðÞ_b states ðjuubi;jddbiÞ[5].

In this paper, we report the observation of an additional heavy baryon and the measurement of its mass, lifetime, and relative production rate compared to the⁰_b produc- tion. The decay properties of this state are consistent with the weak decay of abbaryon. We interpret our result as the observation of the_b baryon (jssbi). Observation of this baryon has been previously reported by the D0 Collaboration [6]. However, the analysis presented here measures a mass of the_b to be significantly lower than Ref. [6].

This_b observation is made inppcollisions at a center of mass energy of 1.96 TeV using the Collider Detector at

Fermilab (CDF II), through the decay chain _b ! J=c^{, where} J=c !^þ, !K, and ! p. Charge conjugate modes are included implicitly.

Mass, lifetime, and production rate measurements are also reported for the_b, through the similar decay chain _b !J=c^{, where}J=c !^þ,!, and !p. The production rates of both the_b and_b are measured with respect to the ⁰_b, which is observed through the decay chain ⁰_b! J=c^{, where} J=c ! ^þ, and !p. These measurements are based on a data sample corresponding to an integrated luminosity of4:2 fb¹.

The strategy of the analysis presented here is to demon- strate the reconstruction and property measurements of the _b and _b as natural extensions of measurements that can be made on better known bhadron states obtained in the same data. All measurements made here are performed on theB⁰ !J=cKð892Þ⁰,Kð892Þ⁰ !K^þfinal state, to provide a large sample for comparison to other mea- surements. The decay modeB⁰!J=cK⁰s,K⁰s !^þis a second reference process. The K⁰s is reconstructed from tracks that are significantly displaced from the collision, similar to the final state tracks of the _b and _b. Although its properties are less well measured than those of the B⁰, the ⁰_b!J=c contributes another cross- check of this analysis, since it is a previously measured

state that contains a in its decay chain. The ⁰_b also provides the best state for comparison of relative produc- tion rates, since it is the largest sample of reconstructedb baryons.

We begin with a brief description of the detector and its simulation in Sec.II. In Sec.III, the reconstruction ofJ=c^, neutral K, hyperons, and b hadrons is described.

Section IV discusses the extraction and significance of the _b signal. In Sec. V, we present measurements of the properties of the_b and_b, which include particle masses, lifetimes, and production rates. We conclude in Sec.VI.

II. DETECTOR DESCRIPTION AND SIMULATION The CDF II detector has been described in detail else- where [7]. This analysis primarily relies upon the tracking and muon identification systems. The tracking system consists of four different detector subsystems that operate inside a 1.4 T solenoid. The first of these is a single layer of silicon detectors (L00) at a radius of1:35–1:6 cmfrom the axis of the solenoid. It measures track position in the transverse view with respect to the beam, which travels along thezdirection. A five-layer silicon detector (SVX II) surrounding L00 measures track positions at radii of 2.5 to 10.6 cm. Each of these layers provides a transverse mea- surement and a stereo measurement of 90(three layers) or 1:2(two layers) with respect to the beam direction. The outermost silicon detector lies between 19 and 30 cm radially, and provides one- or two-track position measure- ments, depending on the track pseudorapidity (), where lnðtanð=2ÞÞ, with being the angle between the particle momentum and the proton beam direction. An open-cell drift chamber (COT) completes the tracking system, and covers the radial region from 43 to 132 cm.

The COT consists of 96 sense-wire layers, arranged in 8 superlayers of 12 wires each. Four of these superlayers provide axial measurements and four provide stereo views of2.

Electromagnetic and hadronic calorimeters surround the solenoid coil. Muon candidates from the decay J=c ! ^þare identified by two sets of drift chambers located radially outside the calorimeters. The central muon cham- bers cover the pseudorapidity regionjj<0:6, and detect muons with transverse momentum p_T>1:4 GeV=c, where the transverse momentum p_T is defined as the component of the particle momentum perpendicular to the proton beam direction. A second muon system covers the region0:6<jj<1:0and detects muons with p_T >

2:0 GeV=c. Muon selection is based on matching these measurements to COT tracks, both in projected position and angle. The analysis presented here is based on events recorded with a trigger that is dedicated to the collection of aJ=c !^þ sample. The first level of the three-level trigger system requires two muon candidates with match- ing tracks in the COT and muon chamber systems. The

second level imposes the requirement that muon candi- dates have opposite charge and limits the accepted range of the opening angle. The highest level of the J=c ^trigger reconstructs the muon pair in software, and requires that the invariant mass of the pair falls within the range 2:7–4:0 GeV=c².

The mass resolution and acceptance for the bhadrons used in this analysis are studied with a Monte Carlo simu- lation that generatesbquarks according to a next-to-lead- ing-order calculation [8], and produces events containing final state hadrons by simulating b quark fragmentation [9]. The final state decay processes are simulated with the

EVTGEN [10] decay program, a value of 6:12 GeV=c² is taken for the _b mass, and all simulated b hadrons are produced without polarization. The generated events are inputted to the detector and trigger simulation based on a

GEANT3 description [11] and processed through the same reconstruction and analysis algorithms that are used for the data.

III. PARTICLE RECONSTRUCTION METHODS This analysis combines the trajectories of charged par- ticles to infer the presence of several different parent hadrons. These hadrons are distinguished by their life- times, due to their weak decay. Consequently, it is useful to define two quantities that are used frequently throughout the analysis which relate the path of weakly decaying objects to their points of origin. Both quantities are defined in the transverse view, and make use of the point of closest approach, ~r_c, of the particle trajectory to a point of origin, and the measured particle decay position, ~r_d. The first quantity used here is transverse flight distance fðhÞ, of hadronh, which is the distance a particle has traveled in the transverse view. For neutral objects, flight distance is given byfðhÞ ðr~d~rcÞ p~TðhÞ=jp~TðhÞj, wherep~TðhÞis the transverse momentum of the hadron candidate. For charged objects, the flight distance is calculated as the arc length in the transverse view from ~rcto ~rd. A comple- mentary quantity used in this analysis is transverse impact distancedðhÞ, which is the distance of the point of closest approach to the point of origin. For neutral particles, transverse impact distance is given bydðhÞ jð~r_dr~_cÞ

~

p_TðhÞj=jp~_TðhÞj. The impact distance of charged particles is simply the distance from ~r_c to the point of origin. The measurement uncertainty on impact distance, _dðhÞ, is calculated from the track parameter uncertainties and the uncertainty on the point of origin.

Several different selection criteria are employed in this analysis to identify the particles used in bhadron recon- struction. These criteria are based on the resolution or acceptance of the CDF detector. No optimization proce- dure has been used to determine the exact value of any selection requirement, since the analysis spans several final states and comparisons between optimized selection re- quirements would necessarily be model dependent.

OBSERVATION OF THE_b BARYON AND. . . PHYSICAL REVIEW D80,072003 (2009)

A.J=c Reconstruction

The analysis of the data begins with a selection of well- measuredJ=c !^þcandidates. The trigger require- ments are confirmed by selecting events that contain two oppositely charged muon candidates, each with matching COT and muon chamber tracks. Both muon tracks are required to have associated position measurements in at least three layers of the SVX II and a two-track invariant mass within80 MeV=c² of the world-average J=c ^mass [1]. This range was chosen for consistency with our earlier bhadron mass measurements [12]. The^þ mass dis- tribution obtained in these data is shown in Fig.1(a). This data sample provides approximately2:910⁷ J=c ^can- didates, measured with an average mass resolution of 20 MeV=c².

B. Neutral hadron reconstruction

The reconstruction ofK_s⁰,Kð892Þ⁰, and candidates uses all tracks with p_T>0:4 GeV=c found in the COT, that are not associated with muons in theJ=c reconstruc- tion. Pairs of oppositely charged tracks are combined to identify these neutral decay candidates, and silicon detec- tor information is not used. Candidate selection for these neutral states is based upon the mass calculated for each track pair, which is required to fall within the ranges given in TableI after the appropriate mass assignment for each track.

Candidates for K⁰s decay are chosen by assigning the pion mass to each track, and mass is measured with a resolution of 6 MeV=c². Track pairs used for Kð892Þ⁰ !K^þ candidates have both mass assign- ments examined. A broad mass selection range is chosen for the Kð892Þ⁰ signal, due to its natural width of 50 MeV=c² [1]. In the situation where both assignments fall within our selection mass range, only the combination closest to the nominal Kð892Þ⁰ mass is used. For ! p candidates, the proton (pion) mass is assigned to the track with the higher (lower) momentum. This mass as- signment is always correct for thecandidates used in this analysis because of the kinematics of decay and the lower limit in the transverse momentum acceptance of the tracking system. Backgrounds to the K⁰_s (c¼ 2:7 cm) and(c¼7:9 cm) [1] are reduced by requiring the flight distance of the K⁰s and with respect to the primary vertex (defined as the beam position in the trans- verse view) to be greater than 1.0 cm, which corresponds to 0:6. The mass distribution of the p combinations withpTðÞ>2:0 GeV=cis plotted in Fig.1(b), and con- tains approximately3:610⁶candidates.

C. Charged hyperon reconstruction

For events that contain a candidate, the remaining tracks reconstructed in the COT, again without additional silicon information, are assigned the pion or kaon mass, and or K combinations are identified that are consistent with the decay process ! or ! K. Analysis of the simulated_b events shows that the pTdistribution of thedaughters of reconstructedand decays falls steeply with increasing pTðÞ. Con- sequently, tracks with p_T as low as 0:4 GeV=care used for these reconstructions. The simulation also indicates that the p_T distribution of the K daughters from decay has a higher average value, and declines with p_T much more slowly than the p_T distribution of the pions fromordecays. A study of theK combinatorial backgrounds in two 8 MeV=c² mass ranges and centered 20 MeV=c² from the mass indicates that the back- ground track pT distribution is also steeper than the ex- pected distribution of K from decay. Therefore, pTðKÞ>1:0 GeV=c is required for our sample, which reduces the combinatorial background by 60%, FIG. 1. (a) The^þ mass distribution obtained in an inte-

grated luminosity of4:2 fb¹. The mass range used for theJ=c sample is indicated by the shaded area. (b) The p mass distribution obtained in events containingJ=c candidates. The mass range used for the sample is indicated by the shaded area.

TABLE I. Mass ranges around the nominal mass value [1]

used for thebhadron decay products.

Resonance (final state) Mass range (MeV=c²)

J=cð^þÞ 80

Kð892Þ⁰ðK^þÞ 30

K⁰sð^þÞ 20

ðpÞ 9

ðÞ 9

ðKÞ 8

while reducing the signal predicted by our Monte Carlo simulation by 25%.

An illustration of the full_b final state that is recon- structed in this analysis is shown in Fig.2. Several features of the track topology are used to reduce the background to this process. In order to obtain the best possible mass resolution for and candidates, the reconstruction requires a convergent fit of the three tracks that simulta- neously constrains thedecay products to themass, and thetrajectory to intersect with the helix of theðKÞ originating from the ðÞ candidate. In addition, the flight distance of the candidate with respect to the reconstructed decay vertex of the ðÞ candidate is required to exceed 1.0 cm. Similarly, due to the long life- times of the(c¼4:9 cm) and(c¼2:5 cm) [1], a flight distance of at least 1.0 cm (corresponding to 1:0) with respect to the primary vertex is required.

This requirement removes 75% of the background to these long-lived particles, due to prompt particle production.

Possible kinematic reflections are removed from the sample by requiring that the combinations in the sample fall outside the mass range listed in TableI when the candidateKtrack is assigned the mass of the. In some instances, the rotation of the ðKÞ helix produces a situation where twoðKÞ vertices satisfy the con- strained fit and displacement requirements. These situ- ations are resolved with the tracking measurements in the longitudinal view. The candidate with the poorer value of

probabilityPð²Þfor theðÞfit is dropped from the sample. An example of such a combination is illustrated in Fig. 2. The complexity of the ðÞ and decays allows for occasional combinations where the proper iden- tity of the three tracks is ambiguous. An example is where the ðKÞ candidate track and the candidate track from decay are interchanged, and the interchanged solution satisfies the various mass and flight distance re- quirements. A single, preferred candidate is chosen by retaining only the fit combination with the highest Pð²Þ of all the possibilities. Requiring the impact distance with respect to the primary vertex to be less than 3_dðhÞ and pTðhÞ>2:0 GeV=cresults in the combinations shown in the and K mass distributions of Fig. 3.

Approximately 41 000 and 3500 candidates are found in this data sample.

The mass distributions in Fig.3show clearand signals. However, the signal has a substantially larger combinatorial background. The kinematics of hyperon decay and the lowerp_T limit of0:4 GeV=con the decay daughter tracks force the majority of charged hyperon candidates to have p_T>1:5 GeV=c. This fact, along with the long lifetimes of the and , results in a significant fraction of the hyperon candidates having decay vertices located several centimeters radially outward from the beam position. Therefore, we are able to refine the charged hyperon reconstruction by making use of the FIG. 2. An illustration (not to scale) of the _b !J=c,

J=c !^þ,!K, and!pfinal states as seen in the view transverse to the beam direction. Five charged tracks are used to identify three decay vertices. The final fit of these track trajectories constrains the decay hadrons (J=c,, and ) to their nominal masses and the helix of theto originate from theJ=cdecay vertex. The trajectory of theKis projected back, indicated by a dotted curve, to illustrate how an alternative, incorrect intersection with thetrajectory could exist. A com- parison of the fit quality of the twoKintersections is used to choose a preferred solution.

FIG. 3. The invariant mass distributions of (a)combina- tions and (b) K combinations in events containing J=c candidates. Shaded areas indicate the mass ranges used for andcandidates. The dashed histograms in each distribution correspond to^þ (a) andK^þ (b) combinations. Additional shading in (b) correspond to sideband regions discussed in Sec.IV.

OBSERVATION OF THE_b BARYON AND. . . PHYSICAL REVIEW D80,072003 (2009)

improved determination of the trajectory that can be ob- tained by tracking these particles in the silicon detector.

The charged hyperon candidates have an additional fit performed with the three tracks that simultaneously con- strains both theandormasses of the appropriate track combinations, and provides the best possible estimate of the hyperon momentum and decay position. The result of this fit is used to define a helix that serves as the seed for an algorithm that associates silicon detector hits with the charged hyperon track. Charged hyperon candidates with track measurements in at least one layer of the silicon detector have excellent impact distance resolution (average of 60m) for the charged hyperon track. The mass dis- tributions for the subset of the inclusive and K combinations which are found in the silicon detector, and have an impact distance with respect to the primary vertex dðhÞ<3_dðhÞare shown in Fig.4. This selection provides approximately34 700 candidates and1900 candi- dates with very low combinatorial background, which allows us to confirm the mass resolutions used to select hyperons. Unfortunately, the shorter lifetime of the makes the silicon selection less efficient than it is for the . Therefore, silicon detector information on the hyperon track is used whenever it is available, but is not imposed as a requirement for the selection.

D.bhadron reconstruction

The reconstruction ofbhadron candidates uses the same method for each of the states reconstructed for this analy- sis. The Kand hyperon candidates are combined with the J=c candidates by fitting the full four-track or five-track state with constraints appropriate for each decay topology and intermediate hadron state. Specifically, the ^þ mass is constrained to the nominalJ=c mass [1], and the neutralKor hyperon candidate is constrained to originate from theJ=c decay vertex. In addition, the fits that include the charged hyperons constrain the candidate tracks to the nominalmass [1], and theandcandidates to their respective nominal masses [1]. The_b and_b mass resolutions obtained from simulated events are found to be approximately 12 MeV=c², a value that is comparable to the mass resolution obtained with the CDF II detector for otherbhadrons with aJ=c meson in the final state [12].

The selection used to reconstructbhadrons is chosen to be as generally applicable as possible, in order to minimize systematic effects in rate comparisons, and to provide confidence that the observation of _b !J=c ^{is not} an artifact of the selection. Therefore, the final samples of allbhadrons used in this analysis are selected with a small number of requirements that can be applied to any b hadron candidate. First, bhadron candidates are required to havepT>6:0 GeV=cand the neutralKor hyperon to havepT>2:0 GeV=c. ThesepT requirements restrict the sample to candidates that are within the kinematic range where our acceptance is well modeled. Mass ranges are imposed on the decay products of the K and hyperon candidates based on observed mass resolution or natural width, as listed in TableI. The promptly-produced combi- natorial background is suppressed by rejecting candidates with low proper decay time, tfðBÞMðBÞ=ðcp_TðBÞÞ, whereMðBÞis the measured mass,p_TðBÞis the transverse momentum, andfðBÞis the flight distance of thebhadron candidate measured with respect to the primary vertex.

Combinations that are inconsistent with having origi- nated from the collision are rejected by imposing an upper limit on the impact distance of the b hadron candidate measured with respect to the primary vertex (PV) dPV. Similarly, the trajectory of the decay hadron is required to originate from thebhadron decay vertex by imposing an upper limit on its impact distancedwith respect to the vertex found in the J=c fit. These two impact distance quantities are compared to their measurement uncertainties dPV andd when they are used.

IV. OBSERVATION OF THE DECAY_b !J=c The J=c mass distribution with dPV<3_dPV and d<3dis shown in Fig.5for the full sample and two different requirements ofct. The samples with actrequire- ment of 100m or greater show clear evidence of a resonance near a mass of6:05 GeV=c², with a width con- FIG. 4. The invariant mass distributions of (a)combina-

tions and (b) K combinations in events containing J=c candidates. These combinations require silicon information to be used on the hyperon track and the impact distance with respect to the primary vertex must not exceed 3 times its measurement resolution. Shaded areas indicate the mass ranges used forandcandidates. The dashed histograms in each distribution correspond to^þ andK^þcombinations.

sistent with our measurement resolution. Mass sideband regions have been defined as 8 MeV=c² wide ranges, centered 20 MeV=c² above and below the nominal mass, as indicated in Fig.3. TheJ=cKmass distribu-

tion for combinations that populate themass sideband regions is shown in Fig. 6(a). In addition, theJ=cK^þ distribution for combinations where the K^þ mass pop- ulates the signal region is shown in Fig. 6(b). No evidence of any mass resonance structure appears in either of these distributions.

The only selection criteria unique to this analysis are those used in the selection. Therefore, the quantities used in theselection were varied to provide confidence that the resonance structure centered at6:05 GeV=c²is not peculiar to the values of the selection requirements that were chosen. The first selection criterion that was varied is theKmass range used to define thesample. For the candidates that satisfy the selection used in Fig.5(c), the K mass range was opened to50 MeV=c². TheK mass distribution for combinations with aJ=cK mass in the range6:0–6:1 GeV=c²is shown in Fig.7(a). A clear indication of an signal can be seen, as expected for a real decay process. The K mass range of8 MeV=c² used in the selection was chosen to be inclusive for all likelycandidates. More restrictive mass ranges for the selection are shown in Figs.7(b) and7(c), where the K mass range is reduced to 6 and 4 MeV=c², respectively. The apparent excess ofJ=ccombinations FIG. 5. (a) The mass distribution of allJ=ccombinations.

(b) The J=c mass distribution for candidates with ct >0.

This requirement removes half of the combinations due to prompt production. (c) TheJ=c mass distribution for can- didates withct >100m. This requirement removes nearly all combinations directly produced in thepp collision.

FIG. 6. (a) The invariant mass distributions ofJ=cKcom- binations for candidates withKin thesidebands. (b) The invariant mass distributions ofJ=cK^þ combinations for can- didates withK^þ in the signal range. All other selection requirements are as in Fig.5(c).

FIG. 7. (a) The invariant mass distribution ofK combina- tions for candidates with J=cK masses in the range 6:0–6:1 GeV=c². (b) The invariant mass distribution of J=cKcombinations for candidates withKmasses within 6 MeV=c²of themass. (c) The invariant mass distributions of J=cK combinations for candidates with K masses within 4 MeV=c² of the mass. All other selection require- ments are as in Fig.5(c).

OBSERVATION OF THE_b BARYON AND. . . PHYSICAL REVIEW D80,072003 (2009)

in the 6:0–6:1 GeV=c² mass range is retained for these more restrictive requirements.

A transverse flight requirement of 1 cm is used for the selection. A lower value allows more promptly- produced background into the sample, due to our measure- ment resolution. A higher value reduces our acceptance, due to the decay of the. Two variations of the flight requirement are shown in Figs.8(a)and8(b). No striking changes in theJ=c mass distribution appear for these variations. A more restrictive flight cut can also be im- posed, which limits the sample tocandidates that are measured in the SVX II (inner radius is 2.5 cm), and provides the extremely pure sample seen in Fig. 4.

Two candidates in the 6:0–6:1 GeV=c² mass range are retained, and no others in the range expected for the_b.

ApTðKÞ>1:0 GeV=crequirement is used in the selection, to reduce the background due to tracks from fragmentation and other sources. The effect of three differ- ent selection values is shown in Fig. 9. The excess of J=c combinations in the mass range6:0–6:1 GeV=c² appears for all pTðKÞ values shown, and is probably a higher fraction of the total combinations seen for the more restrictive requirements. We conclude that the excess of J=ccombinations near6:05 GeV=c² is not an artifact of our selection process.

The mass, yield, and significance of the resonance can- didate in Fig.5(c)are obtained by performing an unbinned likelihood fit on the mass distribution of candidates. The likelihood function that is maximized has the form

L ¼Y^N

i

ðfsP^s_iþ ð1fsÞP^b_iÞ

¼Y^N

i

ðfsGðmi; m0; sm^m_i Þ þ ð1fsÞPⁿðmiÞÞ; (1) whereNis the number of candidates in the sample,P^s_iand P^b_i are the probability distribution functions for the signal and background, respectively, Gðmi; m0; sm^m_i Þ is a Gaussian distribution with average m0 and characteristic widthsm^m_i to describe the signal,miis the mass obtained for a singleJ=c^candidate,m_i is the resolution on that mass, andPⁿðm_iÞis a polynomial of ordern. The quantities obtained from the fitting procedure includef_s, the fraction of the candidates identified as signal,m0, the best average mass value,sm, a scale factor on the mass resolution, and the coefficients ofPⁿðmiÞ.

Two applications of this mass fit are used with the J=c combinations shown in Fig. 5(c). For this data sample, all background polynomials are first order and the mass resolution is fixed to12 MeV=c². The first of these fits allows the remaining parameters to vary. The second FIG. 8. (a,b) The invariant mass distribution ofJ=ccom-

binations for candidates where the transverse flight requirement of theis greater than 0.5 and 2.0 cm. (c) The invariant mass distribution ofJ=ccombinations for candidates with at least one SVX II measurement on the track. All other selection requirements are as in Fig.5(c).

FIG. 9. The invariant mass distributions ofJ=ccombina- tions for candidates with three alternative requirements for the transverse momentum of the K. (a) pTðKÞ>0:8 GeV=c. (b) pTðKÞ>1:2 GeV=c. (c)pTðKÞ>1:4 GeV=c. All other selection requirements are as in Fig.5(c).