Measurement of the masses and widths of the bottom baryons 

Measurement of the masses and widths of the bottom baryons

_b

and

_b

T. Aaltonen,21B. A´ lvarez Gonza´lez,9,aaS. Amerio,40aD. Amidei,32A. Anastassov,15,yA. Annovi,17J. Antos,12 G. Apollinari,15J. A. Appel,15T. Arisawa,54A. Artikov,13J. Asaadi,49W. Ashmanskas,15B. Auerbach,57A. Aurisano,49

F. Azfar,39W. Badgett,15T. Bae,25A. Barbaro-Galtieri,26V. E. Barnes,44B. A. Barnett,23P. Barria,42c,42aP. Bartos,12 M. Bauce,40b,40aF. Bedeschi,42aS. Behari,23G. Bellettini,42b,42aJ. Bellinger,56D. Benjamin,14A. Beretvas,15A. Bhatti,46

D. Bisello,40b,40aI. Bizjak,28K. R. Bland,5B. Blumenfeld,23A. Bocci,14A. Bodek,45D. Bortoletto,44J. Boudreau,43 A. Boveia,11L. Brigliadori,6b,6aC. Bromberg,33E. Brucken,21J. Budagov,13H. S. Budd,45K. Burkett,15G. Busetto,40b,40a

P. Bussey,19A. Buzatu,31A. Calamba,10C. Calancha,29S. Camarda,4M. Campanelli,28M. Campbell,32F. Canelli,11,15 B. Carls,22D. Carlsmith,56R. Carosi,42aS. Carrillo,16,nS. Carron,15B. Casal,9,lM. Casarsa,50aA. Castro,6b,6a P. Catastini,20D. Cauz,50aV. Cavaliere,22M. Cavalli-Sforza,4A. Cerri,26,gL. Cerrito,28,tY. C. Chen,1M. Chertok,7

G. Chiarelli,42aG. Chlachidze,15F. Chlebana,15K. Cho,25D. Chokheli,13W. H. Chung,56Y. S. Chung,45 M. A. Ciocci,42c,42aA. Clark,18C. Clarke,55G. Compostella,40b,40aM. E. Convery,15J. Conway,7M. Corbo,15 M. Cordelli,17C. A. Cox,7D. J. Cox,7F. Crescioli,42b,42aJ. Cuevas,9,aaR. Culbertson,15D. Dagenhart,15N. d’Ascenzo,15,x

M. Datta,15P. de Barbaro,45M. Dell’Orso,42b,42aL. Demortier,46M. Deninno,6aF. Devoto,21M. d’Errico,40b,40a A. Di Canto,42b,42aB. Di Ruzza,15J. R. Dittmann,5M. D’Onofrio,27S. Donati,42b,42aP. Dong,15M. Dorigo,50aT. Dorigo,40a K. Ebina,54A. Elagin,49A. Eppig,32R. Erbacher,7S. Errede,22N. Ershaidat,15,ffR. Eusebi,49S. Farrington,39M. Feindt,24 J. P. Fernandez,29R. Field,16G. Flanagan,15,vR. Forrest,7M. J. Frank,5M. Franklin,20J. C. Freeman,15Y. Funakoshi,54 I. Furic,16M. Gallinaro,46J. E. Garcia,18A. F. Garfinkel,44P. Garosi,42c,42aH. Gerberich,22E. Gerchtein,15S. Giagu,47a

V. Giakoumopoulou,3P. Giannetti,42aK. Gibson,43C. M. Ginsburg,15N. Giokaris,3P. Giromini,17G. Giurgiu,23 V. Glagolev,13D. Glenzinski,15M. Gold,35D. Goldin,49N. Goldschmidt,16A. Golossanov,15G. Gomez,9 G. Gomez-Ceballos,30M. Goncharov,30O. Gonza´lez,29I. Gorelov,35A. T. Goshaw,14K. Goulianos,46S. Grinstein,4

C. Grosso-Pilcher,11R. C. Group,53,15J. Guimaraes da Costa,20S. R. Hahn,15E. Halkiadakis,48A. Hamaguchi,38 J. Y. Han,45F. Happacher,17K. Hara,51D. Hare,48M. Hare,52R. F. Harr,55K. Hatakeyama,5C. Hays,39M. Heck,24

J. Heinrich,41M. Herndon,56S. Hewamanage,5A. Hocker,15W. Hopkins,15,hD. Horn,24S. Hou,1R. E. Hughes,36 M. Hurwitz,11U. Husemann,57N. Hussain,31M. Hussein,33J. Huston,33G. Introzzi,42aM. Iori,47b,47aA. Ivanov,7,q E. James,15D. Jang,10B. Jayatilaka,14E. J. Jeon,25S. Jindariani,15M. Jones,44K. K. Joo,25S. Y. Jun,10T. R. Junk,15 T. Kamon,25,49P. E. Karchin,55A. Kasmi,5Y. Kato,38,pW. Ketchum,11J. Keung,41V. Khotilovich,49B. Kilminster,15 D. H. Kim,25H. S. Kim,25J. E. Kim,25M. J. Kim,17S. B. Kim,25S. H. Kim,51Y. K. Kim,11Y. J. Kim,25N. Kimura,54 M. Kirby,15S. Klimenko,16K. Knoepfel,15K. Kondo,54,aD. J. Kong,25J. Konigsberg,16A. V. Kotwal,14M. Kreps,24 J. Kroll,41D. Krop,11M. Kruse,14V. Krutelyov,49,dT. Kuhr,24M. Kurata,51S. Kwang,11A. T. Laasanen,44L. Labarga,29,cc

S. Lami,42aS. Lammel,15M. Lancaster,28R. L. Lander,7K. Lannon,36,zA. Lath,48G. Latino,42c,42aT. LeCompte,2 E. Lee,49H. S. Lee,11,rJ. S. Lee,25S. W. Lee,49,ddS. Leo,42b,42aS. Leone,42aJ. D. Lewis,15A. Limosani,14,uC.-J. Lin,26

M. Lindgren,15E. Lipeles,41A. Lister,18D. O. Litvintsev,15C. Liu,43H. Liu,53Q. Liu,44T. Liu,15S. Lockwitz,57 A. Loginov,57D. Lucchesi,40b,40aJ. Lueck,24P. Lujan,26P. Lukens,15G. Lungu,46J. Lys,26R. Lysak,12,fR. Madrak,15 K. Maeshima,15P. Maestro,42c,42aS. Malik,46G. Manca,27,bA. Manousakis-Katsikakis,3F. Margaroli,47aC. Marino,24 M. Martı´nez,4P. Mastrandrea,47aK. Matera,22M. E. Mattson,55A. Mazzacane,15P. Mazzanti,6aK. S. McFarland,45

P. McIntyre,49R. McNulty,27,kA. Mehta,27P. Mehtala,21C. Mesropian,46T. Miao,15D. Mietlicki,32A. Mitra,1 H. Miyake,51S. Moed,15N. Moggi,6aM. N. Mondragon,15,nC. S. Moon,25R. Moore,15M. J. Morello,42d,42aJ. Morlock,24

P. Movilla Fernandez,15A. Mukherjee,15Th. Muller,24P. Murat,15M. Mussini,6b,6aJ. Nachtman,15,oY. Nagai,51 J. Naganoma,54I. Nakano,37A. Napier,52J. Nett,49C. Neu,53M. S. Neubauer,22J. Nielsen,26,eL. Nodulman,2S. Y. Noh,25

O. Norniella,22L. Oakes,39S. H. Oh,14Y. D. Oh,25I. Oksuzian,53T. Okusawa,38R. Orava,21L. Ortolan,4 S. Pagan Griso,40b,40aC. Pagliarone,50aE. Palencia,9,gV. Papadimitriou,15A. A. Paramonov,2J. Patrick,15 G. Pauletta,50b,50aM. Paulini,10C. Paus,30D. E. Pellett,7A. Penzo,50aT. J. Phillips,14G. Piacentino,42aE. Pianori,41

J. Pilot,36K. Pitts,22C. Plager,8L. Pondrom,56S. Poprocki,15,hK. Potamianos,44F. Prokoshin,13,eeA. Pranko,26 F. Ptohos,17,iG. Punzi,42b,42aA. Rahaman,43V. Ramakrishnan,56N. Ranjan,44I. Redondo,29P. Renton,39M. Rescigno,47a

T. Riddick,28F. Rimondi,6b,6aL. Ristori,42a,15A. Robson,19T. Rodrigo,9T. Rodriguez,41E. Rogers,22S. Rolli,52,j R. Roser,15F. Ruffini,42c,42aA. Ruiz,9J. Russ,10V. Rusu,15A. Safonov,49W. K. Sakumoto,45Y. Sakurai,54L. Santi,50b,50a

K. Sato,51V. Saveliev,15,xA. Savoy-Navarro,15,bbP. Schlabach,15A. Schmidt,24E. E. Schmidt,15T. Schwarz,15 L. Scodellaro,9A. Scribano,42c,42aF. Scuri,42aS. Seidel,35Y. Seiya,38A. Semenov,13F. Sforza,42c,42aS. Z. Shalhout,7

K. Sliwa,52 J. R. Smith,7 F. D. Snider,15 A. Soha,15 V. Sorin,4 H. Song,43 P. Squillacioti,42c,42a M. Stancari,15 R. St. Denis,19 B. Stelzer,31 O. Stelzer-Chilton,31 D. Stentz,15,y J. Strologas,35 G. L. Strycker,32 Y. Sudo,51 A. Sukhanov,15 I. Suslov,13 K. Takemasa,51 Y. Takeuchi,51 J. Tang,11 M. Tecchio,32 P. K. Teng,1 J. Thom,15,h J. Thome,10 G. A. Thompson,22 E. Thomson,41 D. Toback,49 S. Tokar,12 K. Tollefson,33 T. Tomura,51 D. Tonelli,15 S. Torre,17 D. Torretta,15 P. Totaro,40a M. Trovato,42d,42a F. Ukegawa,51 S. Uozumi,25 A. Varganov,32 F. Va´zquez,16,n

G. Velev,15 C. Vellidis,15 M. Vidal,44 I. Vila,9 R. Vilar,9 J. Viza´n,9 M. Vogel,35 G. Volpi,17 P. Wagner,41 R. L. Wagner,15 T. Wakisaka,38 R. Wallny,8 S. M. Wang,1 A. Warburton,31 D. Waters,28 W. C. Wester III,15 D. Whiteson,41,cA. B. Wicklund,2E. Wicklund,15S. Wilbur,11F. Wick,24H. H. Williams,41J. S. Wilson,36P. Wilson,15 B. L. Winer,36 P. Wittich,15,h S. Wolbers,15H. Wolfe,36 T. Wright,32 X. Wu,18 Z. Wu,5 K. Yamamoto,38D. Yamato,38 T. Yang,15 U. K. Yang,11,sY. C. Yang,25 W.-M. Yao,26 G. P. Yeh,15 K. Yi,15,o J. Yoh,15 K. Yorita,54 T. Yoshida,38,m

G. B. Yu,14 I. Yu,25 S. S. Yu,15 J. C. Yun,15 A. Zanetti,50a Y. Zeng,14 C. Zhou,14 and S. Zucchelli6b,6a (CDF Collaboration)

1_{Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China} 2_{Argonne National Laboratory, Argonne, Illinois 60439, USA}

3_{University of Athens, 157 71 Athens, Greece}

4_{Institut de Fisica d’Altes Energies, ICREA, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain} 5_{Baylor University, Waco, Texas 76798, USA}

6a_{Istituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy} 6b_{University of Bologna, I-40127 Bologna, Italy}

7_{University of California, Davis, Davis, California 95616, USA} 8_{University of California, Los Angeles, Los Angeles, California 90024, USA} 9_{Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain}

10_{Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA} 11_{Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA}

12_{Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia} 13_{Joint Institute for Nuclear Research, RU-141980 Dubna, Russia}

14_{Duke University, Durham, North Carolina 27708, USA} 15

Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

16_{University of Florida, Gainesville, Florida 32611, USA}

17_{Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy} 18_{University of Geneva, CH-1211 Geneva 4, Switzerland}

19_{Glasgow University, Glasgow G12 8QQ, United Kingdom} 20_{Harvard University, Cambridge, Massachusetts 02138, USA}

21_{Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics,}

FIN-00014, Helsinki, Finland

22_{University of Illinois, Urbana, Illinois 61801, USA} 23_{The Johns Hopkins University, Baltimore, Maryland 21218, USA}

24_{Institut fu¨r Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany} 25_{Center 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;

Chonbuk National University, Jeonju 561-756, Korea

26_{Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA} 27_{University of Liverpool, Liverpool L69 7ZE, United Kingdom}

28_{University College London, London WC1E 6BT, United Kingdom}

29_{Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain} 30_{Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA}

31_{Institute 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

32_{University of Michigan, Ann Arbor, Michigan 48109, USA} 33_{Michigan State University, East Lansing, Michigan 48824, USA}

34_{Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia}

35_{University of New Mexico, Albuquerque, New Mexico 87131, USA} 36_{The Ohio State University, Columbus, Ohio 43210, USA}

37_{Okayama University, Okayama 700-8530, Japan} 38_{Osaka City University, Osaka 588, Japan} 39_{University of Oxford, Oxford OX1 3RH, United Kingdom}

40a_{Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy} 40b_{University of Padova, I-35131 Padova, Italy}

41_{University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA} 42a

Istituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

42b_{University of Pisa, I-56127 Pisa, Italy} 42c_{University of Siena, I-56127 Pisa, Italy} 42d_{Scuola Normale Superiore, I-56127 Pisa, Italy} 43_{University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA}

44_{Purdue University, West Lafayette, Indiana 47907, USA} 45_{University of Rochester, Rochester, New York 14627, USA} 46_{The Rockefeller University, New York, New York 10065, USA}

47a_{Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy} 47b_{Sapienza Universita` di Roma, I-00185 Roma, Italy}

48_{Rutgers University, Piscataway, New Jersey 08855, USA} 49_{Texas A&M University, College Station, Texas 77843, USA} 50a_{Istituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy}

50b_{University of Udine, I-33100 Udine, Italy} 51_{University of Tsukuba, Tsukuba, Ibaraki 305, Japan} 52_{Tufts University, Medford, Massachusetts 02155, USA} 53

University of Virginia, Charlottesville, Virginia 22906, USA

54_{Waseda University, Tokyo 169, Japan}

55_{Wayne State University, Detroit, Michigan 48201, USA} 56_{University of Wisconsin, Madison, Wisconsin 53706, USA}

57_{Yale University, New Haven, Connecticut 06520, USA}

(Received 14 December 2011; published 16 May 2012)

a_Deceased.

b_{Visitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.} c_{Visitor from University of California Irvine, Irvine, CA 92697, USA.}

d_{Visitor from University of California Santa Barbara, Santa Barbara, CA 93106, USA.} e_{Visitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.}

f_{Visitor from Institute of Physics, Academy of Sciences of the Czech Republic, Czech Republic.} g_{Visitor from CERN, CH-1211 Geneva, Switzerland.}

h_{Visitor from Cornell University, Ithaca, NY 14853, USA.} i_{Visitor from University of Cyprus, Nicosia CY-1678, Cyprus.}

j_{Visitor from Office of Science, U.S. Department of Energy, WA, DC 20585, USA.} k_{Visitor from University College Dublin, Dublin 4, Ireland.}

l_{Visitor from ETH, 8092 Zurich, Switzerland.}

m_{Visitor from University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017.} n_{Visitor from Universidad Iberoamericana, Mexico D.F., Mexico.}

o_{Visitor from University of Iowa, Iowa City, IA 52242, USA.} p_{Visitor from Kinki University, Higashi-Osaka City, Japan 577-8502.} q

Visitor from Kansas State University, Manhattan, KS 66506, USA. r_{Visitor from Korea University, Seoul, 136-713, Korea.}

s_{Visitor from University of Manchester, Manchester M13 9PL, United Kingdom.} t_{Visitor from Queen Mary, University of London, London, E1 4NS, United Kingdom.} u_{Visitor from University of Melbourne, Victoria 3010, Australia.}

v_{Visitor from Muons, Inc., Batavia, IL 60510, USA.}

w_{Visitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.} x_{Visitor from National Research Nuclear University, Moscow, Russia.} y_{Visitor from Northwestern University, Evanston, IL 60208, USA.} z_{Visitor from University of Notre Dame, Notre Dame, IN 46556, USA.} aa_{Visitor from Universidad de Oviedo, E-33007 Oviedo, Spain.}

bb_{Visitor from CNRS-IN2P3, Paris, F-75205 France.}

cc_{Visitor from Universidad Auto´noma de Madrid, Cantoblanco, 28049, Madrid.} dd_{Visitor from Texas Tech University, Lubbock, TX 79609, USA.}

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Using data from p
p collisions at pffiffiffis¼ 1:96 TeV recorded by the CDF II detector at the Fermilab Tevatron, we present improved measurements of the masses and first measurements of natural widths of the four bottom baryon resonance states þ_b, 
þ_b and _b, 
_b . These states are fully reconstructed in their decay modes to 0

b
 where 0b! þc
with þc ! pK
þ. The analysis is based on a data

sample corresponding to an integrated luminosity of 6:0 fb1collected by an online event selection based on tracks displaced from the p
p interaction point.

DOI:10.1103/PhysRevD.85.092011 PACS numbers: 14.20.Mr, 13.30.Eg, 14.65.Fy

I. INTRODUCTION

Baryons with a heavy quark Q as the ‘‘nucleus’’ and a light diquark q1q2 as the two orbiting ‘‘electrons’’ can be

viewed as the ‘‘helium atoms’’ of quantum chromodynam-ics (QCD). The heavy quark in the baryon may be used as a probe of confinement that allows the study of nonperturba-tive QCD in a different regime from that of the light baryons.

Remarkable achievements in the theory of heavy quark hadrons were made when it was realized that a single heavy quark Q with mass mQ QCD in the heavy hadron HQ

can be considered as a static color source in the hadron’s rest frame [1]. Based on this conjecture, the light diquark properties of the charm baryon þcðcÞ and its bottom

partner 0

bðbÞ can be related by an approximate SUð2Þ

symmetry with c $ b quark exchange. Another symmetry emerges because the spin of the heavy quark S_Qdecouples from the gluon field. Models exploiting these heavy quark symmetries are collectively identified as heavy quark ef-fective theories (HQET) [2,3].

As the spin S_qqof a light diquark (plus a gluon field) and the spin SQ of a heavy quark are decoupled in HQET,

heavy baryons can be described by the quantum numbers SQ, mQ, Sqq, mqq. The total spins of the S-wave (no orbital

excitation) baryon multiplets can be expressed as the sum ~

J ¼ ~SQþ ~Sqq. Then the singlet 0b baryon, with quark

content b½ud according to HQET, has spin of the heavy quark SP

b ¼12

þ_{. Its flavor antisymmetric ½ud diquark has}

spin SP_½ud¼ 0þ[4]. Under these conditions the b quark and the ½ud diquark make the lowest-lying singlet ground state JP¼1₂þ. The partner of the 0_bbaryon in the strange quark sector is the 0 _{baryon. The other two states }

b and 
b

with quark content and spin of the flavor symmetric fqqg diquark Sfqqg¼ 1þ, constitute two isospin I ¼ 1 triplets

with total spin JP ¼1₂þand JP¼3₂þ[4]. These states are the lowest-lying S-wave states that can decay to the singlet 0

b via strong processes involving soft pion emission—

provided sufficient phase space is available. The band 
b

particles are classified as bottom baryon resonant states. The partners of the ð
Þ_b states [5] in the strange quark sector are ð
Þbaryon resonances, though the JP_¼1

2 þ _

states are light enough to decay only weakly or radiatively, and only the JP_¼3

2

þ_{states ð1385Þ decay strongly via the}

0
mode [6].

Some recent HQET calculations for bottom baryons are available in Ref. [7]. The mass spectra of single heavy quark baryons calculated with HQET in combined expan-sions in 1=m_Q and 1=N_c, with N_c defined as a number of colors, are presented in Ref. [8]. In the potential quark model, the mass differences mð_QÞ  mð_QÞ and mð
_QÞ  mðQÞ are largely due to hyperfine splittings,

hence the mass differences scale as 1=mQ. Some recent

predictions based on potential quark models are found in Refs. [9,10]. There are striking patterns in the masses and mass differences of known hadrons. Some of these regu-larities can be understood from known general properties of the interactions of quarks, without specifying the explicit form of the Hamiltonian. Following this approach, the authors of Ref. [11] use semi-empirical mass formulas to predict the spectra of c and b baryons. The nonperturbative formalism of QCD sum rules has been applied within HQET to calculate the mass spectra of the heavy baryons _Qand _Q[12]. Lattice nonrelativistic QCD calculations for bottom baryons [13] have been quite successful, though the uncertainties are typically large and exceed the uncer-tainties of the experimental measurements.

The mass splittings between members of the I ¼ 1 iso-spin triplets ð
Þ_b arise from a combination of the intrinsic quark mass difference mðdÞ > mðuÞ and the electromag-netic interactions between quarks [10,14]. Because of electromagnetic effects and the d quark being heavier than the u quark, the ð
Þ_b states (with composition bfddg i.e. all quarks with negative electric charge) are expected to be heavier than the ð
Þþ_b states whose compo-sition is bfuug [15]. No previous experimental measure-ments of isospin mass splitting of bottom baryons are available.

The description of strong decays of baryon resonances is a difficult theoretical task [16]. Only a few calculations [4,17,18] of the ð
Þ_b natural widths are available. The widths are predicted in the range 4:5–13:5 MeV=c2 for ðb;12

þ_{Þ, and the range 8:5–18:0 MeV=c}2_{for ð}
b;32

þ_Þ.

Until recently, direct observation of b baryons has been limited to the 0

b reconstructed in its weak decays to

J=c0 _{and }þ

c
 [6]. The substantially enlarged

experi-mental data sets delivered by the Tevatron allow significant advances in the spectroscopy of heavy quark baryon states. The resonance ð
Þ_b states were discovered by CDF [19]. The charged bottom strange _b baryon was observed and

measured [20–22] by both the CDF and D0 Collaborations. Later, D0 reported the first observation of the bottom doubly-strange particle _b [23]. Subsequently, the CDF Collaboration confirmed the signal and measured the mass of the _b baryon [22]. Lastly, the neutral partner of _b, the bottom strange baryon 0

b, was reported for the first

time by CDF [24]. Precise measurements of the masses and natural widths of baryon resonances in the charm sector, specifically the ð
Þ0c , ð
Þþþc , and 
þc , were recently

reported by the CDF Collaboration [25].

This study follows the first observation of the ð
Þ_b states using 1:1 fb1 [19]. We confirm the observation of those states using a larger data sample, improve the measurement technique, and add new measurements of properties of the ð
Þ_b resonances. In the present analysis, the masses of the ð
Þþ_b and ð
Þ_b states are determined independently, with no input from theory assumptions, differing from the pre-vious CDF analysis [19]. Using an enlarged data sample of 6 fb1, we extract the direct mass measurements with smaller statistical and systematic uncertainties than previ-ously. First, measurements of the natural widths of the JP¼3₂þ and JP¼1₂þ states are presented. Based on the new mass measurements, we determine the isospin mass splitting for the b and 
bisospin I ¼ 1 triplets.

Section II provides a brief description of the CDF II detector, the online event selection (trigger) important for this analysis, and the detector simulation. In Sec. IIIthe data selection, analysis requirements, and reconstruction of the signal candidates are described. SectionIV discusses the fit model of the final spectra and summarizes the fit results. In Sec.V we estimate the significance of signals extracted from the fits. The systematic uncertainties are discussed in Sec.VI. We present a summary of the mea-surements and conclusions in Sec.VII.

II. THE CDF II DETECTOR AND SIMULATION The component of the CDF II detector [26] most rele-vant to this analysis is the charged particle tracking system. The tracking system operates in a uniform axial magnetic field of 1.4 T generated by a superconducting solenoidal magnet.

The CDF II detector uses a cylindrical coordinate system with z axis along the nominal proton beam line, radius r measured from the beam line and  defined as an azimu-thal angle. The transverse plane (r, ) is perpendicular to the z axis. The polar angle  is measured from the z axis. The impact parameter of a charged particle track d0 is

defined as the distance of closest approach of the particle track to the primary vertex in the transverse plane. Transverse momentum pT is the component of the

parti-cle’s momentum projected onto the transverse plane. Pseudorapidity is defined as    lnðtanð=2ÞÞ.

The inner tracking system comprises three silicon de-tectors: layer 00 (L00), the silicon vertex detector (SVX II)

and the intermediate silicon layers (ISL) [27–30]. The innermost part, the L00 detector, is a layer of single-sided radiation tolerant silicon sensors mounted directly on the beam pipe at a radius of 1.35–1.6 cm from the proton beam line. It provides only an r   measurement and enhances the impact parameter resolution. Outside this, the five double-sided layers of SVX II provide up to 10 track position measurements. Each of the layers provides an r   measurement, while three return a measurement along z, and the other two return a measurement along a direction oriented at 1:2to the z axis. The SVX II spans the radii between 2.5 and 10.6 cm and covers the pseudorapidity range jj < 2:0. The SVX II detector provides a vertex resolution of approximately 15 m in the transverse plane and 70 m along the z axis. A fine track impact parameter resolution _d0 ’ 35 m is achieved, where the _d0 in-cludes an approximate 28 m contribution from the actual transverse size of the beam spot. The outermost silicon subdetector, ISL, consists of double-sided layers at radii 20 to 28 cm, providing two or four hits per track depending on the track pseudorapidity within the range jj < 2:0 instru-mented by the ISL.

A large open cell cylindrical drift chamber, the central outer tracker (COT) [31], completes the CDF detector tracking system. 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 at 2. The active volume of the COT spans the radial region from 43.4 to 132.3 cm. The pseudorapid-ity range jj < 1:0 is covered for tracks passing through all layers of the COT, while for the range out to 1:0 < jj < 2:0, tracks pass through less than the full 96 layers. The trajectory of COT tracks is extrapolated into the SVX II detector, and the tracks are refitted with additional silicon hits consistent with the track extrapolation. The two addi-tional layers of the ISL help to link tracks in the COT to hits in the SVX II. The combined track transverse momen-tum resolution is ðpTÞ=pT’ 0:07%pT ½GeV=c1.

The analysis presented here is based on events recorded with a three-tiered trigger system configured to collect large data samples of heavy hadrons decaying through multibody hadronic channels. We refer to this as the dis-placed two-track trigger. We use two configurations of this trigger, the ‘‘low-pT’’ and the ‘‘medium-pT’’ selections.

At level 1, the trigger uses information from the hardware extremely fast tracker [32]. The ‘‘low-pT ’’ configuration

of the displaced two-track trigger requires two tracks in the COT with p_T> 2:0 GeV=c for each track, and with an opening angle of jj < 90 between the tracks in the transverse plane. Additionally the track pair scalar sum must satisfy pT1þ pT2> 4:0 GeV=c. The corresponding criteria imposed in the ‘‘medium-pT ’’ configuration are

pT> 2:0 GeV=c for each track, opening angle jj <

135, and pT1þ pT2> 5:5 GeV=c. The level 2 silicon vertex trigger (SVT) [33,34] associates the track pair

from the extremely fast tracker with hits in the SVX II detector and recognizes both tracks using a large look-up table of hit patterns. The SVT repeats the level 1 p_Tcriteria and limits the opening angle to 2< jj < 90. Only in the case of the medium-p_Tconfiguration are the charges of the tracks required to be of opposite sign. Crucially, the SVT imposes a requirement on the transverse impact parameter of each track to be 0:12 < d0< 1 mm, given

the excellent resolution provided by SVX II. Finally, the distance in the transverse plane between the beam axis and the intersection point of the two tracks projected onto their total transverse momentum is required to be Lxy> 200 m.

The level 3 software trigger uses a full reconstruction of the event with all detector information and confirms the criteria applied at level 2. The trigger criteria applied to the d0 of

each track in the pair and to Lxypreferentially select decays

of long-lived heavy hadrons over prompt background, en-suring that the data sample is enriched with b hadrons.

The mass resolution on the ð
Þ_b resonances is predicted with a Monte Carlo simulation that generates b quarks according to a next-to-leading order calculation [35] and produces events containing final state hadrons by simulating b quark fragmentation [36]. Mass values of 5807:8 MeV=c2 _{for }

b and 5829:0 MeV=c2 for 
b [19]

are used in the Monte Carlo generator. Final state decay processes are simulated with the EVTGEN [37] program, and all simulated b hadrons are produced without polar-ization. The generated events are input to the detector and trigger simulation based on GEANT3 [38] and processed

through the same reconstruction and analysis algorithms as are used on the data.

III. DATA SAMPLE AND EVENT SELECTION This analysis is based on data equivalent to 6:0 fb1 of pp collisions collected with the displaced two-track trigger
between March 2002 and February 2010. We study ð
Þ_b resonances in the exclusive strong decay mode ð
Þ_b ! 0

b


s, where the low momentum pion
s is produced

near kinematic threshold [39]. The 0_b decays to þc
b

with a prompt pion
_b produced in the weak decay. This is followed by the weak decay þc ! pK
þ.

To reconstruct the parent baryons, the tracks of charged particles are combined in a kinematic fit to form candi-dates. No particle identification is used in this analysis. The following two complementary quantities defined in the plane transverse to the beam line and relating the decay path of baryons to their points of origin are used: the proper decay time of the baryon candidate h expressed in length units ctðhÞ, and the impact parameter d₀ðhÞ. Specifically, the decay length is defined as

ctðhÞ ¼ LxyðhÞ

MðhÞc

p_TðhÞ; (1)

where L_xyðhÞ is expressed in length units and defined as the projection onto ~pTðhÞ of the vector connecting the primary

vertex to the heavy baryon decay vertex in the transverse plane. The transverse impact parameter d0ðhÞ of the

can-didate is defined analogous to the one of a charged particle track. An event-specific primary interaction vertex is used in the calculation of the ctðhÞ and d₀ðhÞ quantities. The measurement uncertainties ct and d0 originate from the track parameter uncertainties and the uncertainty on the primary vertex.

A. Reconstruction of the 0_bcandidates

The analysis begins with reconstruction of the þc !

pK
þdecay by fitting three tracks to a common vertex. The invariant mass of the þ_c candidate is required to be within 18 MeV=c2 _{of the world-average }þ

c mass [6].

The momentum vector of the þc candidate is then

extrapolated to intersect with a fourth pion track, the
_b candidate, to form the 0_b! þc
b candidate vertex. The

0

bvertex is subjected to a three-dimensional kinematic fit

with the þc candidate mass constrained to its

world-average value [6]. The probability of the constrained 0 b

vertex fit must exceed 0.01%. Standard quality require-ments are applied to each track, and only tracks with p_T> 400 MeV=c are used. All tracks are refitted using pion, kaon and proton mass hypotheses to properly correct for the differences in multiple scattering and ionization energy loss. At least two tracks among the p, K,
þ, and
_b candidates are required to fulfill the level 2 (SVT) trigger requirements.

To suppress prompt backgrounds from the primary in-teraction, the decay vertex of the 0

b is required to be

distinct from the primary vertex. To achieve this, cuts on ctð0

bÞ and its significance ctð0bÞ=ct are applied. We

require the þ_c vertex to be close to the 0

b vertex by

applying cuts on ctðþcÞ where the corresponding quantity

L_xyðþ_cÞ is calculated with respect to the 0

b vertex. The

requirement ctðþcÞ > 150 m reduces contributions

from þc baryons directly produced in p
p interaction and

from random combination of tracks faking þ_c candidates which may have negative ctðþcÞ values. The other

restric-tion, ctðþcÞ < 250 m, aims at reducing contributions

from
B0_{! D}þ
 _{decays, followed by D}þ_{! K}
þ
þ

decays. The requirements take into account ct resolution effects and exploit the much shorter þ_c lifetime compared to the Dþ [19,40]. To reduce combinatorial background and contributions from partially reconstructed decays, we ask 0

bcandidates to point to the primary vertex by

requir-ing the impact parameter d0ð0bÞ not to exceed 80 m. The

choice of analysis requirements to identify 0_b! þc
b

candidates is made using an optimization based on the experimental data only. The figure of merit S=pffiffiffiffiffiffiffiffiffiffiffiffiffiS þ B is used during the optimization, where S is the 0

bsignal and

B is the background under the signal, respectively. At every step of the optimization procedure, both quantities are obtained from fits of the þc
b invariant mass spectrum

and are determined from the corresponding numbers of candidates fit within 3 of the 0

b signal peak. TableI

summarizes the resulting 0_banalysis requirements. Figure 1 shows a prominent 0

b signal in the  þ c
b

invariant mass distribution, reconstructed using the opti-mized criteria. A binned maximum-likelihood fit finds a signal of approximately 16300 candidates at the expected 0

bmass, with a signal to background ratio around 1.8. The

fit model describing the invariant mass distribution comprises the Gaussian 0_b ! þc
b signal on top of a

background shaped by several contributions. Random four-track combinations dominating the right sideband are mod-eled with an exponentially decreasing function. Coherent sources populate the left sideband and leak under the signal. These include reconstructed B mesons that pass the 0

b !  þ

c
b selection criteria, partially reconstructed

0

b decays, and fully reconstructed 0b decays other than

þc
b (e.g. 0b! þcK). Shapes representing the

physi-cal background sources are derived from Monte Carlo simulations. Their normalizations are constrained to branching ratios that are either measured (for B meson decays, reconstructed within the same þc
b sample) or

theoretically predicted (for 0

bdecays) [19,40].

B. Reconstruction of
ð
Þ_b candidates

To reconstruct the ð
Þ_b ! 0_b
s candidates, each

þc
b candidate with invariant mass within the 0b signal

region, 5:561–5:677 GeV=c2, is combined with one of the tracks remaining in the event with transverse momentum down to 200 MeV=c. The 0_b mass range covers 3 standard deviations as determined by a fit to the signal peak of Fig.1. To increase the efficiency for reconstructing ð
Þ_b decays near the kinematic threshold, the quality criteria applied to soft pion tracks are loosened in com-parison with tracks used for the 0

bcandidates. The basic

COT and SVX II hit requirements are imposed on
s

tracks, and only tracks with a valid track fit and error matrix are accepted.

Random combinations of 0_bsignal candidates with
s

tracks constitute the dominant background to the ð
Þ_b ! 0

b


s signal. The remaining backgrounds are random

combinations of soft tracks with B mesons reconstructed as 0

bbaryons, and combinatorial background events [19].

To reduce the background level, a kinematic fit is applied to the resulting combinations of 0

b candidates and soft

pion tracks
_s to constrain them to originate from a common point. Furthermore, since the bottom baryon resonance originates and decays at the primary vertex, the soft pion track is required to point back to the primary vertex by requiring an impact parameter significance, d0ð
sÞ=d0, smaller than 3. The transverse momentum of the soft pion is required to be smaller than the
_b transverse momentum. As we already require pTð
bÞ >

1:5 GeV=c (TableI) the condition imposed on the soft pion p_Tis fully efficient. The ð
Þ_b candidate selection require-ments are summarized in TableII.

IV. DETERMINATION OF RESONANCE PROPERTIES

The analysis of the ð
Þ_b mass distributions is performed using the Q value

Q ¼ mð0_b
sÞ  mð0bÞ  m
; (2)

where m
is the known charged pion mass [6] and mð0bÞ is

the reconstructed þ_c
_b mass. The mass resolution of the 0_bsignal and most of the systematic uncertainties cancel

] 2 ) [GeV/c b -π + c Λ Mass( 5 5.2 5.4 5.6 5.8 6 6.2 6.4 2

candidates per 20 MeV/c

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Data Signal + Background Background only

FIG. 1 (color online). Invariant mass distribution of 0

b!

þc
b candidates with the projection of a mass fit overlaid.

TABLE I. Analysis requirements for 0

b ! 

þ

c
b

reconstruc-tion. The quantity ctðþc 0bÞ is defined analogously to

Eq. (1) as the þc proper time where LxyðþcÞ is calculated

with respect to the 0 b vertex. Quantity Requirement ctð0 bÞ >200 m ctð0 bÞ=ct >12:0 d0ð0bÞ <80 m ctðþc 0bÞ >  150 m ctðþc 0bÞ <250 m pTð
bÞ >1:5 GeV=c pTð0bÞ >4:0 GeV=c Probð2 3DÞ of 0b vertex fit >0:01%

TABLE II. ð
Þ_b candidate selection requirements.

Quantity Requirement mðþc
bÞ 2 ð5:561; 5:677Þ GeV=c2 d0ð
sÞ <0:1 cm pTð
sÞ >200 MeV=c d0ð
sÞ=d0 <3:0 pTð
sÞ <pTð
bÞ pTð ð
Þ b Þ >4:0 GeV=c

in the mass difference spectrum. The _b and 
_b signals are reconstructed as two narrow structures in the Q-value spectrum. The properties, yields, and significance of the resonance candidates are obtained by performing unbinned maximum-likelihood fits on the Q-value spectra.

The shapes of the ð
Þ_b resonances are each modeled with a nonrelativistic Breit-Wigner function. Since the soft pion in ð
Þ_b strong decay modes is emitted in a P wave, the width of the Breit-Wigner function is modified as follows [41]: ðQ; Q₀; ₀Þ ¼ ₀ _p

s p
0
_s ₃ ; (3)

where Q0is the Q value at the resonance pole, p

s and p
0
s are the momenta of the soft pion in the ð
Þ_b rest frame, off and on the resonance pole, respectively, and 0 is the

corrected width. The soft pion momenta are calculated based on two-body decay kinematics [6]. Both Q0and 0

are floating fit parameters.

The Breit-Wigner function is convoluted with the detec-tor resolution, which is described by a narrow core Gaussian plus a broad Gaussian. Their widths _nand _w and relative weights g_n and ð1  g_nÞ are calculated from the CDF full Monte Carlo simulation. Numerical convolu-tion is necessary because the modified width depends on the mass. The effects of imperfect modeling in the simu-lation are discussed with the systematic uncertainties in Sec.VI.

We use a kinematically motivated model for the back-ground, described by a second order polynomial modulated with a threshold square rootlike term,

B GðQ; mT; C; b1; b2Þ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðQ þ m
Þ2_{ m}2 T q  P2_{ðQ; C; b} 1; b2Þ; (4)

where C, b1, and b2 are the second orderP2 polynomial

coefficients and mT is a threshold fixed to 0:140 GeV=c2,

the mass of the pion.

The full model for the Q-value spectra of all isospin partner states ð
Þþ_b and ð
Þ_b describes two narrow struc-tures on top of a smooth background with a threshold. The negative logarithm of the extended likelihood function (NLL) is minimized over the unbinned set of Q values observed for N candidates in data:

lnðLÞ ¼ XN

k¼1

lnðN₁S₁þ N₂S₂þ N_bBGÞ

þ ðN₁þ N₂þ N_bÞ  N lnðN₁þ N₂þ N_bÞ: (5) Independent likelihood functions are used for ð
Þþ_b and ð
Þ_b candidates. The Q-value spectrum is fit over the range 0:003–0:210 GeV=c2_{. The effect of this choice is}

discussed in Sec. VI. The probability density functions (PDF) in Eq. (5) are defined as follows:

(i) S_i¼ SðQ; Qi

0; i0; in; gin; iwÞ is the normalized

convolution of a Breit-Wigner and a double Gaussian responsible for the þ_bð_bÞ (i ¼ 1) or 
þ_b ð
_b Þ (i ¼ 2) signals. Here Qi₀ is the floating pole mass and i

0 is the floating natural width. The

detector’s Gaussian resolution parameters i n, iw

and gi

n are set from the Monte Carlo data. A

domi-nant with gn 70% relative weight narrow core n

of about 1:2 MeV=c2 _{is set for the }þ bð

 bÞ and

about 1:4 MeV=c2 for 
þ_b ð
_b Þ. A broad compo-nent _wof about 2:9 MeV=c2_{is set for the }þ

bð  bÞ

and about 3:8 MeV=c2 _{for }
þ b ð

 b Þ.

(ii) N_i is the floating yield of the þ_bð_bÞ (i ¼ 1) or 
þ_b ð
_b Þ (i ¼ 2).

(iii) BG ¼ BGðQ; m_T; C; b₁; b₂Þ is the PDF corre-sponding to the background form in Eq. (4). (iv) Nb is the floating yield of the background

contri-bution. The sum of fitted yields, N1þ N2þ Nb, is

the Poisson mean value of the total number of candidates N for the particular species þ_b, 
þ_b or _b, 
_b corresponding to isospin triplets _b and 
_b.

The total number of floating parameters in the fit per each pair of isospin partners is nine.

Extensive tests on several thousand statistical trials show that the likelihood fit yields unbiased estimates with proper uncertainties.

The experimental ð
Þ_b and ð
Þþ_b Q-value distributions, each fitted with the unbinned likelihoods described above, are shown in Fig. 2. The projection of the corresponding likelihood fit is superimposed on each graph. The Q-value distributions show clear signals of _b, 
_b and þ_b, 
þ_b , respectively. The pull distributions are shown in the bottom plots of both figures and are calculated as the residuals of the histogram with respect to the corresponding likelihood fit projection normalized by the data uncertainty. Both pull distributions are evenly distributed around zero with fluc-tuations of 2, approximately. The fit results are given in TableIII.

V. SIGNAL SIGNIFICANCE

The significance of the signals is determined using a log-likelihood ratio statistic [42,43], 2 lnðL₀=L₁Þ. We define hypothesis H₁ corresponding to the presence of _b, 
_b or þ_b, 
þ_b signals on top of the background. The H1 hypothesis is described by the likelihood L1; see

Eq. (5). The various null hypotheses, each identified with H0 and nested toH1 correspond to a few different less

complex scenarios described by the likelihood L₀. The likelihood ratio is used as a 2 _{variable to derive p values}

for observing a deviation as large as is in our data or larger, assumingH0is true. The number of degrees of freedom of

the 2 equals the difference Ndofin the number degrees

of freedom between theH₁ andH₀ hypotheses in each case. We consider the following types ofH₀ to estimate the significance of the two-peak signal structure and of individual peaks of the observed ð
Þ_b and ð
Þþ_b states:

(i) A single enhancement is observed anywhere in the fit range. The corresponding likelihoodL0 includes

only a single peak PDF on top of the background form in Eq. (4), the same as for the L1. The

dif-ference in the number of degrees of freedom is N_dof¼ 3. The width ₀ floats in the fit over the wide range 1–70 MeV=c2_{. The position of the}

en-hancement Q₀ is allowed to be anywhere within the default fit range. We test the case in which the observed two narrow structures could be an artifact of a wide bump where a few bins fluctuated down to the background level.

(ii) The signal 
_b is observed but the bis interpreted

as background. We impose a loose requirement on the existence of the second peak, 
_bfixing only the width of 
_b to the expected theoretical value of

12 MeV=c2 _{[17]. We let the fitter find the }
b

posi-tion within the default fit range. The number of free parameters is changed by 4.

(iii) The signal bis observed but the 
bis interpreted

as background. This null hypothesis is similar to the previous one. The width of the _b is fixed to 7 MeV=c2 _[17].

(iv) Neither the _bnor the 
_bis observed, and theH₀ hypothesis is the default background model used in L1. We consider the case in which the smooth

background fluctuates to two narrow structures cor-responding to theH1hypothesis. The difference in

the number of degrees of freedom is 6.

In addition to all the cases considered above, we introduce an additional case in which theH1hypothesis corresponds

to any single wide enhancement considered in (i) while the H0hypothesis is the default background considered in (iv).

This special test determines the significance of the single enhancement with respect to pure background.

TableIVsummarizes the results of these tests. The null hypothesis most likely to resemble our signal is a broad single enhancement fluctuating to the two narrow struc-tures. The results of this study establish conclusively the ð
Þ_b and ð
Þþ_b signals with significance of 6 or higher.

VI. SYSTEMATIC UNCERTAINTIES

The systematic uncertainties considered in our analysis are the following:

(i) The uncertainty due to the CDF tracker momentum scale. ] 2 ) [GeV/c π b Λ → b Σ Q( 0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 2

candidates per 3 MeV/c

0 50 100 150 200 250 300 -b

Σ

Data Signal + Background Background only 0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 σ (data-fit)/ -2.5 -1.5 -0.5 0.5 1.5 2.5 ] 2 ) [GeV/c π b Λ → b Σ Q( 0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 2

candidates per 3 MeV/c

0 50 100 150 200 250 300 350 + b

Σ

Data Signal + Background Background only 0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 σ (data-fit)/ -2.5 -1.5 -0.5 0.5 1.5 2.5

FIG. 2 (color online). The left (right) plot shows the Q-value spectrum for ð
Þ_b (ð
Þþ_b ) candidates with the projection of the corresponding unbinned likelihood fit superimposed. The Q value is defined in Eq. (2). The pull distribution of each fit is shown in the bottom of the corresponding plot.

TABLE III. Summary of the results of the fits to the Q ¼ Mð0

b

_{Þ  Mð}0

bÞ  m
spectra. The statistical uncertainties

are returned by the unbinned maximum-likelihood fits. State Q0value, MeV=c2 Natural width, 0, MeV=c2 Yield _b 56:2þ0:6_0:5 4:9þ3:1_2:1 340þ90_70 
_b 75:8  0:6 7:5þ2:2_1:8 540þ90_80 þ_b 52:1þ0:9_0:8 9:7þ3:8_2:8 470þ110_90 
þ_b 72:8  0:7 11:5þ2:7_2:2 800þ110_100

(ii) The uncertainty due to the resolution model (see Sec. IV) described by the sum of two Gaussians. This source is expected to dominate the systematic uncertainties on width measurements.

(iii) The choice of background model.

(iv) An uncertainty due to the choice of Q-value fit range.

To calibrate the tracker momentum scale, the energy loss in the material of CDF tracking detectors and the strength of the magnetic field must be determined. Both effects are calibrated and analyzed in detail using high statistics samples of J=c, cð2SÞ, ð1SÞ, Z0 _{reconstructed in their}

þ decay modes as well as D0 _{! K}
þ_, c_{ð2SÞ !}

J=cð! þÞ
þ
[44,45]. The corresponding correc-tions are taken into account by tracking algorithms. Any systematic uncertainties on these corrections are largely negligible in the ð
Þ_b Q-value measurements. The uncer-tainties on the measured mass differences due to the momentum scale are estimated from the deviations be-tween Q₀ values observed in similar decays reconstructed in CDF data and the known Q₀ values [6]. The reference modes are þþ_c ! þ_c
þ_s, 0

c ! þc
s, 
þc !

þ_c
þ_s
_s, and D
þ! D0
þ

s. The linear extrapolation

of the measured offsets as a function of Q0 towards the

ð
Þ_b kinematic regime is taken as the mass-scale uncer-tainty. The determined systematic uncertainty on the momentum scale covers also any residual charge-dependence of the scale. For the mass difference Q₀, the systematic uncertainty due to a possible imperfect align-ment of the detector is negligible [44].

Following the method used in Ref. [46], the D
þ! D0_{ð! K}
þ_Þ
þ

s signal peak in the mass difference

dis-tribution mðD
þÞ  mðD0_{Þ has been reconstructed in}

sev-eral bins of soft pion transverse momentum pTð
sÞ starting

with 200 MeV=c as in the data. Each signal distribution is subjected to an unbinned maximum-likelihood fit with the sum of a Breit-Wigner function convoluted with a double Gaussian function to describe the detector resolution. The background under the D
þ signals is described by an empirical function [47,48]. For each of the pTð
sÞ bins,

the fit determines the D
þ width, which never exceeds 0:2 MeV=c2_{. Because the D}
þ _{natural width is much}

smaller than the tracking resolution, the value of 0:2 MeV=c2 is assigned as a systematic uncertainty on the measured ð
Þ_b natural width due to the momentum scale of the CDF tracker.

Unless otherwise specified, the systematic uncertainties discussed below are evaluated for the measurable quantities Q0and 0by generation of statistical trials. In each trial, the

sample is generated according to the PDF (see TableIII) with the nuisance parameters modified by the uncertainty with respect to the default set of parameters. Then the sample is subjected to the unbinned maximum-likelihood fit twice, with the default PDF and with the PDF of the modified nuisance parameter set. The fit results are compared on a trial-by-trial basis, and their difference is computed. The systematic uncertainty is found from the mean of a Gaussian fit of the distribution of the computed differences. The statistical uncertainties on the resolution model parameters due to the finite size of the Monte Carlo data-sets introduce a systematic uncertainty. Variations of the double Gaussian widths _n and _w and the weight g_n within their statistical uncertainties returned from the fits of Monte Carlo spectra are propagated into the measurable quantities using the statistical trials.

The CDF tracking simulation does not reproduce with perfect accuracy the tracking resolutions, especially for soft tracks at the kinematic threshold of ð
Þ_b decays. To estimate this contribution, we use the D
meson decay as the reference mode reconstructed down to p_Tð
_sÞ ¼ 200 MeV=c in the observed and simulated samples. We compare the mass resolution of the reference signal found in data with the one predicted by Monte Carlo simulation. The comparison is made independently for D
þ! D0
þ s

and D
!
D0


s states, as a function of soft pion pT

using early data (Period 1) and late data (Period 2). Figure 3shows the comparisons of the narrow core reso-lution between the data and Monte Carlo both for D
þ(left plot) and D
 (right plot). The resolution is stable as a function of data-taking time.

TABLE IV. Statistical significances of the observed signals against various null hypotheses. Nis the calculated number of Gaussian standard deviations based on Probð2Þ.

H0 States N H1

Any single wide enhancement ð
Þ_b 6.7 Two narrow structures ð
Þþ_b 6.1

No structures ð
Þ_b 10.7 Any single wide enhancement

ð
Þþ_b 13.2

No b, with 
b, 

ð
Þ

b 7.6 Two narrow structures

02¼ 12 MeV=c2 

ð
Þþ

b 7.9

No 
_b, with b, 

ð
Þ

b 10.0 Two narrow structures

01¼ 7 MeV=c2 

ð
Þþ

b 12.5

No structures ð
Þ_b 12.4 Two narrow structures

ð
Þþ_b 14.3

The CDF Monte Carlo simulation typically underesti-mates the D
 resolutions in the experimental data: nðdataÞ & 1:25n (Monte Carlo). Similar relations are

found for the broad component of the resolution: wðdataÞ & 1:40w (Monte Carlo). These factors are

used as the sources of the systematic uncertainties. The resolution extracted for the D
 is systematically smaller than for the D
þby at most 20% for nand by at most 40%

for w. The Monte Carlo predictions for n and w are

decreased by these latter factors to estimate the other bounds of the systematic uncertainties. In both cases the conservative approach is taken.

To find the systematic uncertainty associated with the choice of background shape, we change our background PDF to the one used for the D
 mass difference spectra [47,48] and compare with the default background PDF.

The uncertainty associated with the fit range is estimated by varying the default low edge down to 0:0015 GeV=c2

and up to 0:006 GeV=c2_{. The fit results are slightly}

sensi-tive to the choice of the low edge and any observed biases are assigned as another systematic uncertainty.

The final systematic uncertainties are listed in TableV. VII. RESULTS AND CONCLUSIONS

The analysis results are arranged in TableVI. From the measured ð
Þ_b Q values, we extract the absolute masses using the known value of the
 mass [6] and the CDF 0

b mass measurement, mð0bÞ ¼ 5619:7  1:2ðstatÞ 

1:2ðsystÞ MeV=c2_{, as obtained in an independent sample}

[44]. The 0

bstatistical and systematic uncertainties contribute

to the systematic uncertainty on the ð
Þ_b absolute masses. Using the measured Q values, we extract the isospin mass splittings for the isotriplets of the JP_¼1

2

þ _{and J}P_¼3 2 þ

states. The statistical uncertainties on the Q measurements of the corresponding charge states are added in quadrature.

) [GeV/c] soft π ( T p 0.5 1 1.5 ) narrow σ Ratio ( 0.8 1 1.2 (data, Period 1) σ (MC simulation) / σ (data, Period 2) σ (data, Period 1) / σ + π 0 D → *+ D ) [GeV/c] soft π ( T p 0.5 1 1.5 ) narrow σ Ratio ( 0.8 1 1.2 (data, Period 1) σ (MC simulation) / σ (data, Period 2) σ (data, Period 1) / σ -π 0 D → *-D

FIG. 3 (color online). The left (right) plot shows the ratio of the widths of the narrow component of the D
þ(D
) mass resolution for data and simulation (circles) and for different subsamples of data (triangles) as a function of the transverse momentum of the soft pion. The last bin on every plot corresponds to a statistics integrated above 1:0 GeV=c.

TABLE V. Summary of the systematic uncertainties listed in the following order: mass scale, resolution, choice of background model, and fit range. The total systematic uncertainty is obtained by adding all the associated uncertainties in quadrature. The last column shows the percentage of the total systematic uncertainty relative to its central value.

Measurable quantity Scale Resolution Background Fit range Total Percentage

Qð_bÞ [MeV=c2_{] þ0:06 þ0:04 þ0:02 þ0:07 þ0:1} 0:38 0:07 0:04 0:03 0:39 0:7 ð_bÞ [MeV=c2_{] þ0:20 þ0:85 þ0:50 þ0:50 þ1:13 þ23} 0:20 0:87 0:50 0:51 1:14 23 Qð
_b Þ [MeV=c2_{] þ0:06 þ0:06 þ0:02 þ0:09 þ0:1} 0:56 0:08 0:06 0:09 0:58 0:8 ð
_b Þ [MeV=c2_{] þ0:20 þ0:65 þ0:30 þ0:50 þ0:89 þ12} 0:20 0:96 0:30 0:90 1:36 18 Qðþ_bÞ [MeV=c2_{] þ0:07 þ0:05 þ0:02 þ0:09 þ0:2} 0:35 0:12 0:05 0:03 0:38 0:7 ðþbÞ [MeV=c2] þ0:20 þ0:94 þ0:40 þ0:50 þ1:16 þ12 0:20 0:90 0:40 0:51 1:12 12 Qð
þ_b Þ [MeV=c2_{] þ0:06 þ0:10 þ0:02 þ0:12 þ0:2} 0:52 0:13 0:10 0:09 0:55 0:8 ð
þ_b Þ [MeV=c2_{] þ0:20 þ0:64 þ0:50 þ0:50 þ0:97 þ8:5} 0:20 1:01 0:50 0:90 1:46 13

We assume that the correlated systematic uncertainties due to mass scale, fit bias due to choice of fit range, and imperfect Monte Carlo description of the resolution are completely canceled in the isospin mass splittings. The uncertainties due to background choice are added in quadrature.

In conclusion, we have measured the masses and widths of the ð
Þ_b baryons using a sample of approximately 16 300 0

bcandidates reconstructed in their 0b!  þ c


mode corresponding to 6 fb1of CDF data.

The first observation [19] of the ð
Þ_b bottom baryons has been confirmed with every individual signal recon-structed with a significance well in excess of six Gaussian standard deviations.

The statistical precision on the direct mass differences is improved by a factor of 2 over the previous measurement [19]. The measurements are in good agreement with the previous results and supersede them.

The isospin mass splittings within the I ¼ 1 triplets of the band 
bstates have been extracted for the first time.

The ð
Þ_b states have higher masses than their ð
Þþ_b partners, following a pattern common to most of the known isospin multiplets [15]. This measurement favors the phe-nomenological explanation of this ordering as due to the higher masses of the d quark with respect to the u quark and the larger electromagnetic contribution due to electro-static Coulomb forces between quarks in ð
Þ_b states than

in ð
Þþ_b ones. The difference in the measured isospin mass splittings between the 
_b and b isotriplets supports the

theoretical estimate of Ref. [10]. The natural widths of the _b and 
_b states have been measured for the first time. The measurements are in agreement with theoretical expectations.

ACKNOWLEDGMENTS

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions. This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean World Class University Program, the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovacio´n, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; the Academy of Finland; and the Australian Research Council (ARC).

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