Measurements of ${\cal B}(\bar{B}^0 \to Λ_{c}^{+} \bar{p})$ and ${\cal B}(B^- \to Λ_{c}^{+} \bar{p} π^-)$ and Studies of $Λ_{c}^{+}π^-$ Resonances

0.43 MB·

by  The BABAR Collaboration

BABAR-PUB-08/016
SLAC-PUB-13341
arXiv:0807.4974
Measurements of B(B0 → Λ+p) and B(B− → Λ+pπ−) and Studies of Λ+π−
c c c
Resonances
B. Aubert,1 M. Bona,1 Y. Karyotakis,1 J. P. Lees,1 V. Poireau,1 E. Prencipe,1 X. Prudent,1 V. Tisserand,1
J. Garra Tico,2 E. Grauges,2 L. Lopezab,3 A. Palanoab,3 M. Pappagalloab,3 G. Eigen,4 B. Stugu,4 L. Sun,4
G. S. Abrams,5 M. Battaglia,5 D. N. Brown,5 R. N. Cahn,5 R. G. Jacobsen,5 L. T. Kerth,5 Yu. G. Kolomensky,5
9 G. Kukartsev,5 G. Lynch,5 I. L. Osipenkov,5 M. T. Ronan,5,∗ K. Tackmann,5 T. Tanabe,5 C. M. Hawkes,6 N. Soni,6
0 A. T. Watson,6 H. Koch,7 T. Schroeder,7 D. Walker,8 D. J. Asgeirsson,9 T. Cuhadar-Donszelmann,9 B. G. Fulsom,9
0
C. Hearty,9 T. S. Mattison,9 J. A. McKenna,9 M. Barrett,10 A. Khan,10 L. Teodorescu,10 V. E. Blinov,11
2
A. D. Bukin,11 A. R. Buzykaev,11 V. P. Druzhinin,11 V. B. Golubev,11 A. P. Onuchin,11 S. I. Serednyakov,11
n
Yu. I. Skovpen,11 E. P. Solodov,11 K. Yu. Todyshev,11 M. Bondioli,12 S. Curry,12 I. Eschrich,12 D. Kirkby,12
a
J A. J. Lankford,12 P. Lund,12 M. Mandelkern,12 E. C. Martin,12 D. P. Stoker,12 S. Abachi,13 C. Buchanan,13
7 J. W. Gary,14 F. Liu,14 O. Long,14 B. C. Shen,14,∗ G. M. Vitug,14 Z. Yasin,14 L. Zhang,14 V. Sharma,15
C. Campagnari,16 T. M. Hong,16 D. Kovalskyi,16 M. A. Mazur,16 J. D. Richman,16 T. W. Beck,17 A. M. Eisner,17
]
x C. J. Flacco,17 C. A. Heusch,17 J. Kroseberg,17 W. S. Lockman,17 T. Schalk,17 B. A. Schumm,17 A. Seiden,17
e L. Wang,17 M. G. Wilson,17 L. O. Winstrom,17 C. H. Cheng,18 D. A. Doll,18 B. Echenard,18 F. Fang,18
-
p D. G. Hitlin,18 I. Narsky,18 T. Piatenko,18 F. C. Porter,18 R. Andreassen,19 G. Mancinelli,19 B. T. Meadows,19
e K. Mishra,19 M. D. Sokoloff,19 F. Blanc,20 P. C. Bloom,20 W. T. Ford,20 A. Gaz,20 J. F. Hirschauer,20 A. Kreisel,20
h
M. Nagel,20 U. Nauenberg,20 J. G. Smith,20 K. A. Ulmer,20 S. R. Wagner,20 R. Ayad,21,† A. Soffer,21,‡
[
W. H. Toki,21 R. J. Wilson,21 D. D. Altenburg,22 E. Feltresi,22 A. Hauke,22 H. Jasper,22 M. Karbach,22 J. Merkel,22
2
A. Petzold,22 B. Spaan,22 K. Wacker,22 M. J. Kobel,23 W. F. Mader,23 R. Nogowski,23 K. R. Schubert,23
v
4 R. Schwierz,23 J. E. Sundermann,23 A. Volk,23 D. Bernard,24 G. R. Bonneaud,24 E. Latour,24 Ch. Thiebaux,24
7 M. Verderi,24 P. J. Clark,25 W. Gradl,25 S. Playfer,25 J. E. Watson,25 M. Andreottiab,26 D. Bettonia,26 C. Bozzia,26
9 R. Calabreseab,26 A. Cecchiab,26 G. Cibinettoab,26 P. Franchiniab,26 E. Luppiab,26 M. Negriniab,26 A. Petrellaab,26
4
. L. Piemontesea,26 V. Santoroab,26 R. Baldini-Ferroli,27 A. Calcaterra,27 R. de Sangro,27 G. Finocchiaro,27
7
S. Pacetti,27 P. Patteri,27 I. M. Peruzzi,27,§ M. Piccolo,27 M. Rama,27 A. Zallo,27 A. Buzzoa,28 R. Contriab,28
0
8 M. Lo Vetereab,28 M. M. Macria,28 M. R. Mongeab,28 S. Passaggioa,28 C. Patrignaniab,28 E. Robuttia,28
0 A. Santroniab,28 S. Tosiab,28 K. S. Chaisanguanthum,29 M. Morii,29 R. S. Dubitzky,30 J. Marks,30 S. Schenk,30
v: U. Uwer,30 V. Klose,31 H. M. Lacker,31 G. De Nardoab,32 L. Listaa,32 D. Monorchioab,32 G. Onoratoab,32
i C. Sciaccaab,32 D. J. Bard,33 P. D. Dauncey,33 J. A. Nash,33 W. Panduro Vazquez,33 M. Tibbetts,33 P. K. Behera,34
X
X. Chai,34 M. J. Charles,34 U. Mallik,34 J. Cochran,35 H. B. Crawley,35 L. Dong,35 W. T. Meyer,35 S. Prell,35
r
a E. I. Rosenberg,35 A. E. Rubin,35 Y. Y. Gao,36 A. V. Gritsan,36 Z. J. Guo,36 C. K. Lae,36 A. G. Denig,37
M. Fritsch,37 G. Schott,37 N. Arnaud,38 J. B´equilleux,38 A. D’Orazio,38 M. Davier,38 J. Firmino da Costa,38
G. Grosdidier,38 A. Ho¨cker,38 V. Lepeltier,38 F. Le Diberder,38 A. M. Lutz,38 S. Pruvot,38 P. Roudeau,38
M. H. Schune,38 J. Serrano,38 V. Sordini,38,¶ A. Stocchi,38 G. Wormser,38 D. J. Lange,39 D. M. Wright,39
I. Bingham,40 J. P. Burke,40 C. A. Chavez,40 J. R. Fry,40 E. Gabathuler,40 R. Gamet,40 D. E. Hutchcroft,40
D. J. Payne,40 C. Touramanis,40 A. J. Bevan,41 K. A. George,41 F. Di Lodovico,41 R. Sacco,41 M. Sigamani,41
G. Cowan,42 H. U. Flaecher,42 D. A. Hopkins,42 S. Paramesvaran,42 F. Salvatore,42 A. C. Wren,42 D. N. Brown,43
C. L. Davis,43 K. E. Alwyn,44 N. R. Barlow,44 R. J. Barlow,44 Y. M. Chia,44 C. L. Edgar,44 G. D. Lafferty,44
T. J. West,44 J. I. Yi,44 J. Anderson,45 C. Chen,45 A. Jawahery,45 D. A. Roberts,45 G. Simi,45 J. M. Tuggle,45
C. Dallapiccola,46 S. S. Hertzbach,46 X. Li,46 E. Salvati,46 S. Saremi,46 R. Cowan,47 D. Dujmic,47
P. H. Fisher,47 K. Koeneke,47 G. Sciolla,47 M. Spitznagel,47 F. Taylor,47 R. K. Yamamoto,47 M. Zhao,47
S. E. Mclachlin,48,∗ P. M. Patel,48 S. H. Robertson,48 A. Lazzaroab,49 V. Lombardoa,49 F. Palomboab,49
J. M. Bauer,50 L. Cremaldi,50 V. Eschenburg,50 R. Godang,50,∗∗ R. Kroeger,50 D. A. Sanders,50 D. J. Summers,50
H. W. Zhao,50 M. Simard,51 P. Taras,51 F. B. Viaud,51 H. Nicholson,52 M. A. Baak,53 G. Raven,53 H. L. Snoek,53
C. P. Jessop,54 K. J. Knoepfel,54 J. M. LoSecco,54 W. F. Wang,54 G. Benelli,55 L. A. Corwin,55 K. Honscheid,55
2
H. Kagan,55 R. Kass,55 J. P. Morris,55 A. M. Rahimi,55 J. J. Regensburger,55 S. J. Sekula,55 Q. K. Wong,55
N. L. Blount,56 J. Brau,56 R. Frey,56 O. Igonkina,56 J. A. Kolb,56 M. Lu,56 R. Rahmat,56 N. B. Sinev,56 D. Strom,56
J. Strube,56 E. Torrence,56 G. Castelliab,57 N. Gagliardiab,57 M. Margoniab,57 M. Morandina,57 M. Posoccoa,57
M. Rotondoa,57 F. Simonettoab,57 R. Stroiliab,57 C. Vociab,57 P. del Amo Sanchez,58 E. Ben-Haim,58 H. Briand,58
G. Calderini,58 J. Chauveau,58 P. David,58 L. Del Buono,58 O. Hamon,58 Ph. Leruste,58 J. Ocariz,58 A. Perez,58
J. Prendki,58 L. Gladney,59 M. Biasiniab,60 R. Covarelliab,60 E. Manoniab,60 C. Angeliniab,61 G. Batignaniab,61
S. Bettariniab,61 M. Carpinelliab,61,†† A. Cervelliab,61 F. Fortiab,61 M. A. Giorgiab,61 A. Lusianiac,61
G. Marchioriab,61 M. Morgantiab,61 N. Neriab,61 E. Paoloniab,61 G. Rizzoab,61 J. J. Walsha,61 J. Biesiada,62
D. Lopes Pegna,62 C. Lu,62 J. Olsen,62 A. J. S. Smith,62 A. V. Telnov,62 F. Anullia,63 E. Baracchiniab,63
G. Cavotoa,63 D. del Reab,63 E. Di Marcoab,63 R. Facciniab,63 F. Ferrarottoa,63 F. Ferroniab,63 M. Gasperoab,63
P. D. Jacksona,63 L. Li Gioia,63 M. A. Mazzonia,63 S. Morgantia,63 G. Pireddaa,63 F. Polciab,63 F. Rengaab,63
C. Voenaa,63 M. Ebert,64 T. Hartmann,64 H. Schr¨oder,64 R. Waldi,64 T. Adye,65 B. Franek,65 E. O. Olaiya,65
W. Roethel,65 F. F. Wilson,65 S. Emery,66 M. Escalier,66 L. Esteve,66 A. Gaidot,66 S. F. Ganzhur,66
G. Hamel de Monchenault,66 W. Kozanecki,66 G. Vasseur,66 Ch. Y`eche,66 M. Zito,66 X. R. Chen,67 H. Liu,67
W. Park,67 M. V. Purohit,67 R. M. White,67 J. R. Wilson,67 M. T. Allen,68 D. Aston,68 R. Bartoldus,68
P. Bechtle,68 J. F. Benitez,68 R. Cenci,68 J. P. Coleman,68 M. R. Convery,68 J. C. Dingfelder,68 J. Dorfan,68
G. P. Dubois-Felsmann,68 W. Dunwoodie,68 R. C. Field,68 A. M. Gabareen,68 S. J. Gowdy,68 M. T. Graham,68
P. Grenier,68 C. Hast,68 W. R. Innes,68 J. Kaminski,68 M. H. Kelsey,68 H. Kim,68 P. Kim,68 M. L. Kocian,68
D. W. G. S. Leith,68 S. Li,68 B. Lindquist,68 S. Luitz,68 V. Luth,68 H. L. Lynch,68 D. B. MacFarlane,68
H. Marsiske,68 R. Messner,68 D. R. Muller,68 H. Neal,68 S. Nelson,68 C. P. O’Grady,68 I. Ofte,68 A. Perazzo,68
M. Perl,68 B. N. Ratcliff,68 A. Roodman,68 A. A. Salnikov,68 R. H. Schindler,68 J. Schwiening,68 A. Snyder,68
D. Su,68 M. K. Sullivan,68 K. Suzuki,68 S. K. Swain,68 J. M. Thompson,68 J. Va’vra,68 A. P. Wagner,68
M. Weaver,68 C. A. West,68 W. J. Wisniewski,68 M. Wittgen,68 D. H. Wright,68 H. W. Wulsin,68 A. K. Yarritu,68
K. Yi,68 C. C. Young,68 V. Ziegler,68 P. R. Burchat,69 A. J. Edwards,69 S. A. Majewski,69 T. S. Miyashita,69
B. A. Petersen,69 L. Wilden,69 S. Ahmed,70 M. S. Alam,70 R. Bula,70 J. A. Ernst,70 B. Pan,70 M. A. Saeed,70
S. B. Zain,70 S. M. Spanier,71 B. J. Wogsland,71 R. Eckmann,72 J. L. Ritchie,72 A. M. Ruland,72 C. J. Schilling,72
R. F. Schwitters,72 B. W. Drummond,73 J. M. Izen,73 X. C. Lou,73 F. Bianchiab,74 D. Gambaab,74 M. Pelliccioniab,74
M. Bombenab,75 L. Bosisioab,75 C. Cartaroab,75 G. Della Riccaab,75 L. Lanceriab,75 L. Vitaleab,75 V. Azzolini,76
N. Lopez-March,76 F. Martinez-Vidal,76 D. A. Milanes,76 A. Oyanguren,76 J. Albert,77 Sw. Banerjee,77
B. Bhuyan,77 H. H. F. Choi,77 K. Hamano,77 R. Kowalewski,77 M. J. Lewczuk,77 I. M. Nugent,77 J. M. Roney,77
R. J. Sobie,77 T. J. Gershon,78 P. F. Harrison,78 J. Ilic,78 T. E. Latham,78 G. B. Mohanty,78 H. R. Band,79
X. Chen,79 S. Dasu,79 K. T. Flood,79 Y. Pan,79 M. Pierini,79 R. Prepost,79 C. O. Vuosalo,79 and S. L. Wu79
(The BABAR Collaboration)
1Laboratoire de Physique des Particules, IN2P3/CNRS et Universit´e de Savoie, F-74941 Annecy-Le-Vieux, France
2Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain
3INFN Sezione di Baria; Dipartmento di Fisica, Universit`a di Barib, I-70126 Bari, Italy
4University of Bergen, Institute of Physics, N-5007 Bergen, Norway
5Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
6University of Birmingham, Birmingham, B15 2TT, United Kingdom
7Ruhr Universit¨at Bochum, Institut fu¨r Experimentalphysik 1, D-44780 Bochum, Germany
8University of Bristol, Bristol BS8 1TL, United Kingdom
9University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
10Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
11Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
12University of California at Irvine, Irvine, California 92697, USA
13University of California at Los Angeles, Los Angeles, California 90024, USA
14University of California at Riverside, Riverside, California 92521, USA
15University of California at San Diego, La Jolla, California 92093, USA
16University of California at Santa Barbara, Santa Barbara, California 93106, USA
17University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
18California Institute of Technology, Pasadena, California 91125, USA
19University of Cincinnati, Cincinnati, Ohio 45221, USA
20University of Colorado, Boulder, Colorado 80309, USA
21Colorado State University, Fort Collins, Colorado 80523, USA
22Technische Universit¨at Dortmund, Fakult¨at Physik, D-44221 Dortmund, Germany
3
23Technische Universit¨at Dresden, Institut fu¨r Kern- und Teilchenphysik, D-01062 Dresden, Germany
24Laboratoire Leprince-Ringuet, CNRS/IN2P3, Ecole Polytechnique, F-91128 Palaiseau, France
25University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
26INFN Sezione di Ferraraa; Dipartimento di Fisica, Universit`a di Ferrarab, I-44100 Ferrara, Italy
27INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
28INFN Sezione di Genovaa; Dipartimento di Fisica, Universit`a di Genovab, I-16146 Genova, Italy
29Harvard University, Cambridge, Massachusetts 02138, USA
30Universit¨at Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany
31Humboldt-Universit¨at zu Berlin, Institut fu¨r Physik, Newtonstr. 15, D-12489 Berlin, Germany
32INFN Sezione di Napolia; Dipartimento di Scienze Fisiche,
Universit`a di Napoli Federico IIb, I-80126 Napoli, Italy
33Imperial College London, London, SW7 2AZ, United Kingdom
34University of Iowa, Iowa City, Iowa 52242, USA
35Iowa State University, Ames, Iowa 50011-3160, USA
36Johns Hopkins University, Baltimore, Maryland 21218, USA
37Universit¨at Karlsruhe, Institut fu¨r Experimentelle Kernphysik, D-76021 Karlsruhe, Germany
38Laboratoire de l’Acc´el´erateur Lin´eaire, IN2P3/CNRS et Universit´e Paris-Sud 11,
Centre Scientifique d’Orsay, B. P. 34, F-91898 ORSAY Cedex, France
39Lawrence Livermore National Laboratory, Livermore, California 94550, USA
40University of Liverpool, Liverpool L69 7ZE, United Kingdom
41Queen Mary, University of London, E1 4NS, United Kingdom
42University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
43University of Louisville, Louisville, Kentucky 40292, USA
44University of Manchester, Manchester M13 9PL, United Kingdom
45University of Maryland, College Park, Maryland 20742, USA
46University of Massachusetts, Amherst, Massachusetts 01003, USA
47Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
48McGill University, Montr´eal, Qu´ebec, Canada H3A 2T8
49INFN Sezione di Milanoa; Dipartimento di Fisica, Universit`a di Milanob, I-20133 Milano, Italy
50University of Mississippi, University, Mississippi 38677, USA
51Universit´e de Montr´eal, Physique des Particules, Montr´eal, Qu´ebec, Canada H3C 3J7
52Mount Holyoke College, South Hadley, Massachusetts 01075, USA
53NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
54University of Notre Dame, Notre Dame, Indiana 46556, USA
55Ohio State University, Columbus, Ohio 43210, USA
56University of Oregon, Eugene, Oregon 97403, USA
57INFN Sezione di Padovaa; Dipartimento di Fisica, Universit`a di Padovab, I-35131 Padova, Italy
58Laboratoire de Physique Nucl´eaire et de Hautes Energies,
IN2P3/CNRS, Universit´e Pierre et Marie Curie-Paris6,
Universit´e Denis Diderot-Paris7, F-75252 Paris, France
59University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
60INFN Sezione di Perugiaa; Dipartimento di Fisica, Universit`a di Perugiab, I-06100 Perugia, Italy
61INFN Sezione di Pisaa; Dipartimento di Fisica,
Universit`a di Pisab; Scuola Normale Superiore di Pisac, I-56127 Pisa, Italy
62Princeton University, Princeton, New Jersey 08544, USA
63INFN Sezione di Romaa; Dipartimento di Fisica,
Universit`a di Roma La Sapienzab, I-00185 Roma, Italy
64Universit¨at Rostock, D-18051 Rostock, Germany
65Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
66DSM/Dapnia, CEA/Saclay, F-91191 Gif-sur-Yvette, France
67University of South Carolina, Columbia, South Carolina 29208, USA
68Stanford Linear Accelerator Center, Stanford, California 94309, USA
69Stanford University, Stanford, California 94305-4060, USA
70State University of New York, Albany, New York 12222, USA
71University of Tennessee, Knoxville, Tennessee 37996, USA
72University of Texas at Austin, Austin, Texas 78712, USA
73University of Texas at Dallas, Richardson, Texas 75083, USA
74INFN Sezione di Torinoa; Dipartimento di Fisica Sperimentale, Universit`a di Torinob, I-10125 Torino, Italy
75INFN Sezione di Triestea; Dipartimento di Fisica, Universit`a di Triesteb, I-34127 Trieste, Italy
76IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
77University of Victoria, Victoria, British Columbia, Canada V8W 3P6
78Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
79University of Wisconsin, Madison, Wisconsin 53706, USA
4
We present an investigation of the decays B0 → Λ+p and B− → Λ+pπ− based on 383×106
c c
Υ(4S) → BB decays recorded with the BABAR detector. We measure the branching fractions of
these decays; their ratio is B(B− →Λ+pπ−)/B(B0 →Λ+p)=15.4±1.8±0.3. The B− →Λ+pπ−
c c c
process exhibits an enhancement at the Λ+p threshold and is a laboratory for searches for excited
c
charmbaryonstates. WeobservetheresonantdecaysB− →Σc(2455)0pandB− →Σc(2800)0pbut
seenoevidenceforB− →Σc(2520)0p. Thisisthefirstobservation ofthedecayB− →Σc(2800)0p;
however, the mass of the observed excited Σ0 state is (2846±8±10)MeV/c2, which is somewhat
c
inconsistentwithpreviousmeasurements. Finally,weexaminetheangulardistributionoftheB− →
Σc(2455)0p decays and measure the spin of the Σc(2455)0 baryon to be 1/2, as predicted by the
quarkmodel.
PACSnumbers: 13.25.Hw,13.60.Rj,14.20.Lq
INTRODUCTION cay proceeds through an intermediate b-flavored baryon
state,whichthendecaysweaklyintooneofthefinalstate
Baryonic decays of B mesons, which contain a heavy baryons [14, 15]. However, it is not clear that the pole
bottom quark and a light up or down quark, provide a model is reliable for baryon poles, and the predictions
laboratory for a range of particle physics investigations: given in the literature vary significantly. Perhaps the
trendsindecayratesandbaryonproductionmechanisms; most satisfying theoretical interpretation of baryonic B
searches for exotic states such as pentaquarks and glue- decay rates is the qualitative one proposed by Hou and
balls [1, 2]; searches for excited baryon resonances; ex- Soniin 2001[16],who arguethatB decaysarefavoredif
amination of the angular distributions of B-meson de- thebaryonandantibaryoninthefinal-stateconfiguration
cay products to determine baryon spins; and measure- are close together in phase space. A consequence is that
ments of radiative baryonic B decays that could be sen- decay rates to two-body baryon-antibaryon final states
sitivetonewphysicsthroughflavor-changingneutralcur- aresuppressedrelativetoratesofthree-bodyfinalstates
rents [3, 4]. The latter measurements rely on improving containing the same baryon-antibaryon system plus an
our theoretical understanding of baryonic B decays in additionalmeson. Inthethree-bodycase,thebaryonand
general [5, 6]. antibaryoncanbeinthefavoredconfiguration—closeto-
TheinclusivebranchingfractionforbaryonicB decays gether in phase space—rather than back-to-back as in
is (6.8 0.6)%[7], and manyexclusive baryonicB decay the two-body case.
modes±have been observed [8]. If we order the measured Inthispaper,weinvestigatethedecaysB0 →Λ+c pand
decays by Q-value: B− →Λ+c pπ− [17]. We investigatebaryonproductionin
B decays by comparing the two-body (B0 Λ+p) and
→ c
Q=m m , (1) three-body (B− Λ+pπ−) decay rates directly. The
B − f dynamics of the b→aryocn-antibaryon(Λ+p) system in the
Xf c
three-bodydecayprovideinsightintobaryonproduction
where mf is the mass of each daughter in the final state mechanisms. Additionally,theB− Λ+pπ− systemisa
of the B decay, we find that for each type of baryonic laboratoryforstudyingexcitedbary→onsctatesandisused
B decay,the branchingfractionsdecreaseasthe Q-value to measure the spin of the Σ (2455)0. This is the first
c
increases. Thesmallestmeasuredbranchingfractionisof measurement of the spin of this state.
theorder10−6,whichalsocorrespondstoourexperimen-
tal sensitivity for measuring these branching fractions.
Potentially interesting B-meson decays such as B pp, BABAR DETECTOR AND DATA SAMPLE
→
B ΛΛ, and B Λ+Λ− have not yet been seen.
→ → c c
Theoretical approaches to calculating baryonic B de-
Themeasurementspresentedinthispaperarebasedon
cays include pole models [9, 10], diquark models [11], 383 106 Υ(4S) BB decays recordedwith the BABAR
and QCD sum rules [12, 13]. Recently, theoretical cal- dete×ctor [18] at t→he PEP-II e+e− asymmetric-energy B
culations have focused on pole models, where the B de-
Factory at the Stanford Linear Accelerator Center. At
the interaction point, 9-GeV electrons collide with 3.1-
GeV positrons at the Υ(4S) resonance with a center-of-
mass energy of 10.58GeV/c2.
∗Deceased
†NowatTempleUniversity,Philadelphia,PA19122,USA Charged particle trajectories are measured by a five-
‡NowatTelAvivUniversity,TelAviv,69978,Israel layer silicon vertex tracker (SVT) and a 40-layer drift
§AlsowithUniversita`diPerugia,DipartimentodiFisica,Perugia, chamber(DCH)immersedina1.5-Taxialmagneticfield.
Italy Charged particle identification is provided by ionization
¶AlsowithUniversita`diRomaLaSapienza,I-00185Roma,Italy
∗∗NowatUniversityofSouthAlabama,Mobile,AL36688, USA energy (dE/dx) measurements in the SVT and DCH
††AlsowithUniversita`diSassari,Sassari,Italy along with Cherenkov radiation detection by an inter-
5
j
nally reflecting ring-imaging detector (DRC). L is definedas p cosθ , where θ is the angle with
j i i| i| i
Exclusive B-meson decays are simulated with the respectto the B candidatethrustaxisoftheithcharged
Monte Carlo (MC) event generator EvtGen [19]. Back- particle or neutrPal cluster in the rest of the event and p
i
ground continuum MC samples (e+e− qq, where is its momentum. The optimal maximum value of the
q = u,d,s,c) are simulated using Jets→et7.4 [20] to Fisherdiscriminantis chosenseparatelyforeachΛ+ and
c
model generic hadronization processes. BackgroundMC B decay mode.
samples of e+e− B+B− and B0B0 are based on simu- Kinematic properties of B-meson pair production at
lations of many e→xclusive B decays (also using EvtGen). the Υ(4S) provide further background discrimination.
The largesamples of simulatedevents are generatedand We define a pair of observables, m and m , that are
m r
propagatedthrough a detailed detector simulation using uncorrelated and exploit these constraints:
the GEANT4 simulation package [21].
2
mm = qe+e− −qˆΛ+cp(π−) and
r (2)
(cid:16) (cid:17)
CANDIDATE SELECTION 2
m = q m .
r Λ+cp(π−) − B
r
We select candidates that are kinematically consistent (cid:16) (cid:17)
The variable m is based onthe apparentrecoilmass of
with B0 Λ+p andB− Λ+pπ−. For the decaymode m
B0 Λ+→p,wec reconstruc→tΛ+ccandidatesinthepK−π+, the unreconstructed B meson in the event, where qe+e−
pK→0,pKc0π+π−,andΛπ+ deccaymodes,requiringthein- is the four-momentum of the e+e− system and qˆΛ+cp(π−)
S S is the four-momentum of the reconstructed B candidate
variantmassofeachΛ+candidatetobewithin10MeV/c2
c after applying a mass constraint. The variable m is the
oftheworldaveragevalue[8]. ForB− Λ+pπ−,wealso r
→ c difference between the unconstrained mass of the recon-
reconstructΛ+ candidatesintheΛπ+π−π+ decaymode,
c structed B candidate and m , the world average value
andrequirealloftheΛ+ candidatestohaveaninvariant B
c of the mass of the B meson [8]. Signal events peak at
mass within 12MeV/c2 of the world averagevalue.
m in m and 0 in m . This set of variables was first
Thep,K,andπcandidatesmustbewell-reconstructed B m r
used in [24] and is chosen as an uncorrelated alternative
inthe DCH andareidentified with likelihood-basedpar-
to ∆E = E∗ 1√s and the energy-substituted mass
ticleselectorsusinginformationfromtheSVT,DCH,and B − 2
DRC. mES = 14s−p∗B2 (where s = qe2+e− and the asterisk
The K0 candidates are reconstructed from two oppo- denotes qthe e+e− rest frame), which exhibit a 30%
S
∼
sitelychargedpioncandidatesthatcomefromacommon correlationfor B− Λ+pπ−.
→ c
vertex; Λ candidates are formed by combining a proton The event selection criteria are optimized based on
candidatewithanoppositelychargedpioncandidatethat studies of sideband data (in the region 0.10 < m <
r
comes from a common vertex. The invariant mass of 0.20GeV/c2)andsimulatedsignalMCsamples. Thedata
each K0 and Λ candidate must be within 10MeV/c2 of in a signal region (approximately 2σ wide in m and
S m
±
theworldaveragevalue[8]andtheflightsignificance(de- m ) were blinded until the selection criteria were de-
r
finedastheflightdistancefromtheΛ+vertexinthex y termined and the signal extraction procedure was spec-
c −
plane divided by the measurement uncertainty) must be ified and validated. B candidates that satisfy m >
m
greater than 2. The mass of eachK0 and Λ candidate is 5.121GeV/c2 and m < 0.10GeV/c2 are used in the
S r
| |
then constrained to the world averagevalue [8]. maximum likelihood fit.
A mass constraint is applied to all of the Λ+ candi-
c
dates,andallΛ+daughtertracksmustcomefromacom-
c
mon vertex. The Λ+ candidates are then combined with BACKGROUNDS
c
an antiproton to form a B0 Λ+p candidate, or with
→ c
anantiprotonandapiontoformaB− Λ+pπ− candi- The primarysourceofbackgroundforB0 Λ+p can-
→ c → c
date. ThedaughtersofeachBcandidatemustcomefrom didates is continuum e+e− qq events. Backgrounds
→
a common vertex, and the candidate with the largestχ2 due to decays such as B− Λ+pπ−, B0 Λ+pπ0, and
→ c → c
probability in each event is selected. B− Σ0p,Σ0 Λ+π− are rejected by the criterion
→ c c → c
Additional background suppression is provided by in- m <0.10GeV/c2.
r
| |
formation about the topology of the events. A Fisher Approximately equal amounts of continuum e+e−
→
discriminant [22] is constructed based on the absolute qq and e+e− BB events make up the background
→
value of the cosine of the angle of the B candidate mo- for B− Λ+pπ− events. Again, the requirement
→ c
mentumvectorwithrespecttothebeamaxisinthee+e− m <0.10GeV/c2rejectsmostofthecontributionsfrom
r
| |
center-of-mass(CM) frame, the absolute value ofthe co- such decays as B0 Λ+pπ+π− and B− Λ+pπ−π0.
→ c → c
sineoftheanglebetweentheBcandidatethrustaxis[23] Approximately 1% of the background in the fit region
and the thrust axis of the rest of the event in the e+e− is due to these four-body events, but they do not peak
CM frame, and the moments L and L . The quantity in m and m . A small peaking background is present
0 2 m r
6
from B0 Σ+p,Σ+ Λ+π0 events, especially when
the π0 ha→s lowcmomcen→tum.cBased on a branching frac- TABLE I: Detection efficiency for B0 → Λ+cp signal events,
determined from signal Monte Carlo samples and separated
tion measurement of the isospin partner decay (B−
Σc(2455)0p)=(3.7±0.7±0.4±1.0)×10−5 [25B], whe→re fboyrΛB+c0d→ecΛay+cmp(oBde−. →ThΛe+cnupπm−b)e,rsΛc+cor→resfplo,nwdhetorethfleiesffiacgieinvecny
the uncertainties are statistical, systematic, and the un- final state. The efficiencies quoted for the B− → Λ+pπ−
c
certainty due to (Λ+ pK−π+), respectively, we ex- decaysare averaged across phase space.
B c →
pect 11.5 2.5 peaking background events in the signal
±
regionforB− Λ+pπ−, Λ+ pK−π+. Acorrectionis Efficiency for Λ+ →f
→ c c → c l
appliedand a systematic uncertaintyis assignedto com- f B0→Λ+p B− →Λ+pπ−
l c c
pensate for these events. pK−π+ 22.9% 15.4%
pK0 21.6% 14.3%
S
pK0π+π− 9.6% 5.6%
S
DETECTION EFFICIENCY
Λπ+ 17.2% 11.6%
Λπ+π−π+ – 4.0%
The detection efficiencies for B0 Λ+p and B−
→ c →
Λ+pπ−signaleventsaredeterminedfromsignalMCsam-
c
ples with 175,000to over 1,600,000events in each sam-
ple, depending on the Λ+ decay mode. To account for
c lar bins than the traditional set of Dalitz variables. The
inaccuracies in the simulation of the detector, each MC
m bins are narrower near the kinematic limits where
event is assigned a weight based on each daughter par- Λcπ
the efficiency changes more rapidly and are centered on
ticle’s momentum and angle. These weights are deter-
expected resonances. For B− Λ+pπ−, Λ+ pK−π+
minedfromstudiescomparinglargepuresamplesofpro- → c c →
near cosθ =0, the efficiency varies from approximately
tons, kaons, and pions in MC samples and data. Small h
13% at low m , to 16% in the central m region, to
corrections (0.4 1.6%) are also applied to account for Λcπ Λcπ
trackinginefficie−nciesduetothedisplacedK0 andΛver- 8% at high mΛcπ. The efficiency is fairly uniform with
S respect to cosθ , except at cosθ 1 and low m ,
tices. Thesecorrectionsdependonthe K0 andΛdaugh- h h ∼ Λcπ
S where it drops to 7.4%. The other Λ+ decay modes ex-
tertrajectories’transversemomentumandangle,andthe c
hibit similar variations in efficiency.
distancebetweenthebeamspotandthedisplacedvertex.
The detection efficiency (ε ) for B0 Λ+p signal
l → c
events in each Λ+ decay mode (l) is determined from
c
the number of signal events extracted from an extended
unbinned maximum likelihood fit to signal MC events. SIGNAL EXTRACTION
These events pass the same selection criteria as applied
to data. The fit is performed in two dimensions, mm To extract the number of signal events in data, a two-
andmr. Theprobabilitydistributionfunction(PDF)for dimensional (mm vs. mr) extended unbinned maximum
the background consists of a threshold function [26] in likelihood fit is performed simultaneously across Λ+ de-
c
mm multiplied by a first-order polynomial in mr; this is cay modes. B0 Λ+p candidates and B− Λ+pπ−
the same as the background PDF used in the fit to the candidates are fit→sepacrately. → c
B0 Λ+p data. The signal PDF consists of a Gaussian
→ c The background PDF for each fit is a threshold func-
in m multiplied by a modified asymmetric Gaussian
m tion [26] in m multiplied by a first-orderpolynomial in
withatailparameterinm . Thedetectionefficienciesin m
r m . The shape parameter (~s ) of the threshold func-
each Λ+ decay mode are summarized in Table I. r bkg
c tion is free but is common to all of the Λ+ decay modes.
The detection efficiency for B− Λ+pπ− signal c
→ c The slope a of the first-order polynomial is allowed to
events ineachΛ+ decay mode varies considerablyacross
c vary independently for each Λ+ decay mode.
the Dalitz plane of the three-body decay. For reference, c
The signal PDF is a single Gaussian distribution in
we quote the average efficiencies in Table I, but we ap-
m multiplied by a single Gaussian distribution in m
ply a more sophisticated treatment to these events. We m r
for B0 Λ+p and multiplied by a double Gaussian dis-
parameterize the physical Dalitz region using the vari- → c
habelliecsitcyosanθhgleanθdh tishedeΛfi+cneπd−aisntvhaeriaanntglmeabsest,wmeeΛncπt.heTπh−e tsruiffibuctieionnt tino mderscfroirbeBt−he→siΛgn+caplπP−D.FAfsoirngBle0 G→auΛs+csipanbeis-
and the p in the B− rest frame. The quantity cosθ can cause of the small number of expected signal events. All
h
ofthe shapeparametersofthe signalPDF(~s )arefree
be expressed in terms of Lorentz-invariant products of sig
butaresharedamongtheΛ+ decaymodes. Separatesig-
four-vectors. We divide the kinematic region into rea- c
nal(N ) andbackground(N ) yields are extracted
sonably sized bins that are uniform in cosθ (0.2 units sig,l bkg,l
h
wide) and nonuniform in mΛcπ (60−200MeV/c2 wide). for each Λ+c decay mode l.
This choice of variables is more conducive to rectangu- The total likelihood is the product of the likelihoods
7
of the covariance matrix V are calculated as follows:
TABLE II: Signal yields from simultaneous fits (across Λ+
c
decay modes) to B0 →Λ+cp and B− →Λ+cpπ− candidates. ∂2( ln ) N f (~y )f (~y )
V−1 = − L = n e j e , (5)
Nsig nj ∂Nn∂Nj Xe=1 Nk=s1Nkfk(~ye) 2
Mode B0 →Λ+p B− →Λ+pπ−
c c (cid:16) (cid:17)
P
pK−π+ 90 ± 11 991 ± 45 where the sum is over the N candidates. Note that in
pK0 10 ± 4 165 ± 15 the calculationof the covariancematrix, the data is refit
S
pK0π+π− 14 ± 5 86 ± 14 to the same simultaneous PDF described above, except
S
Λπ+ 3 ± 3 114 ± 13 that all fit parameters other than the yields are fixed to
Λπ+π−π+ – 88 ± 13 the values from the original fit.
We use these lot weights to generate a signal or
s
P
backgrounddistributionforanyquantitythatis notcor-
related with m or m . The lot formalism is easily
m r s
P
for each Λ+ decay mode: extended to incorporate an efficiency correction for each
c
candidate. Each candidate is assigned a weight of 1/ε,
= (~y ;N ,N ,~s ,~s ,a ). (3) where the efficiency ε for an event is determined by its
tot l l sig,l bkg,l sig bkg l
L L location in the cosθ vs. m plane.
l h Λcπ
Y The branching fraction for B0 Λ+p for Λ+ decay
→ c c
Thesymbol~y representsthevariablesusedinthe2-Dfit, mode l is calculated as follows:
m ,m .
m r
{ }
Thefullsimultaneousfitisvalidatedusingindependent (B0 Λ+p) =
B → c l
samplesofsignalMCeventstosimulatesignaleventsand
N
sig,l
toy MC samples (generated from the background MC , (6)
N ε (Λ+ pK−π+)
sample distribution) to represent background events in BB × l × Rl × B c →
the fit region. For both B0 Λ+p and B− Λ+pπ−,
→ c → c whereN isthenumberofBB eventsinthedatasam-
we perform fits to 100 combined MC samples and find BB
pleand istheratioofΛ+ branchingfractionfordecay
that the fit is robust and the results are unbiased. Rl c
mode l to (Λ+ pK−π+), taking care to include the
Theresultsofthe2-Dfitstodataareshowninprojec- K0 π+πB− acnd→Λ pπ− branching fractions where
tions of m and m for each Λ+ decay mode. Figure 1 S → →
m r c applicable.
shows the result of the fit to B0 Λ+p candidates and
→ c InordertodeterminethebranchingfractionforB−
Fig. 2 shows the resultof the fit to B− Λ+pπ− candi- →
dates. The signal yields from the fits ar→e sumc marized in Λ+c pπ−,wetaketheproductofthesPlotweightandeffi-
ciency weight for each candidate and sum over all of the
Table II.
candidates in the fit region. We simplify the notationby
using W to denote the value of the signal lot weight
s i s
P
for event i and include a 1% correction for the peaking
BRANCHING FRACTION MEASUREMENTS backgrounddue to B0 Σ+p,Σ+ Λ+π0:
→ c c → c
Forthe three-bodymode B− Λ+pπ−,the efficiency (B− Λ+pπ−) =
→ c B → c l
variation across the Dalitz plane requires a correction
W
s i
for each signal event in order to extract the branching 0.99
× ε
fraction for this mode. We use the lot method [27] to (cid:18) i i (cid:19)l . (7)
sP N P(Λ+ pK−π+)
calculate a weight for each event e based on the 2-D fit BB × Rl × B c →
to the variables ~y. We have N = 2 species (signal and
s The contribution from the peaking background is esti-
background) for each Λ+ decay mode and define f as
c j,k mated using the Λ+ pK−π+ decay mode. Since the
the signal (j,k = 1) or background (j,k = 2) PDF. The c →
overall branching fraction for the peaking background
lot weights are calculated as
sP contribution is the same regardless of Λ+ decay mode,
c
it is applied as a proportional correction. The measure-
sPn(~ye)= PNjNk==ss11VNnkjffkj((~y~yee)), (4) mΛ+cendtsecfaoyr mB(oBde0 →areΛs+cump)maanrdizBed(Bin−T→abΛle+cIpIIπ.−) for each
TheBLUE(BestLinearUnbiasedEstimate)technique
where (~y ) is the lPot weight for species n, V is is used as described in Ref. [28] to combine the corre-
s n e s
the covaPriance matrix foPr signal and background yields, lated branching fraction measurements for different Λ+
c
f (~y )isthevalueofPDFf forevente,and~y isthe decay modes. The purpose of the method is to obtain
j,k e j,k e
m andm valueforevente. Theelementsoftheinverse anestimatexˆthatis alinearcombinationoftindividual
m r
8
(a) (b)
6600
3300
4400 2200
2200 1100
88
(c) (d)
) 1100 ) 66

2c 2c
V/ V/ 44
e 55 e
G G 22

8 8
0 01100
0 1155 0
. (e) . (f)
0 0

( (
/ 1100 /

s s 55
t t
n n
e 55 e
v v
E E
1100 88
(g) (h)
66
55 44
22
00 00
5.15 5.20 5.25 5.30 --00..1100 --00..0055 00..0000 00..0055 00..1100
mm ((GGeeVV//cc22)) mm ((GGeeVV//cc22))
mm rr
FIG. 1: Projections of mm (left) and mr (right) in data for B0 → Λ+cp candidates, separated by Λ+c decay mode: (a,b) are
Λ+c →pK−π+,(c,d)areΛ+c →pKS0,(e,f)areΛ+c →pKS0π+π−,and(g,h)areΛ+c →Λπ+. Themm projections (a,c,e,g)are
for |mr|<0.030GeV/c2 and themr projections (b,d,f,h) are for mm>5.27GeV/c2. The solid curvescorrespond to thePDF
from the simultaneous 2-D fit to candidates for the four Λ+ decay modes, and the dashed curves represent the background
c
component of thePDF.
measurements (x ), is unbiased, and has the minimum betweenmeasurements l andl′). The errormatricesadd
l
possible variance σˆ2. The estimate xˆ is defined by linearly, so we define E = E +E . E includes
stat syst stat
the uncertainties in the fit yields and the correlations
xˆ= αlxl. (8) betweenyieldsfromthesimultaneousfitresult. Esyst in-
l cludes the systematic uncertainties that are described in
X
the next section. Overall multiplicative constants (N
The condition α =1 ensures that the method is un- BB
l l and (Λ+ pK−π+)) that are common to all the mea-
biased. Each coefficient αl is a constant weight for mea- B c →
P surementsandtheiruncertaintiesarenotincludedinthe
surement x and is not necessarily positive. The set of
l
BLUE method.
coefficientsα(avectorwithtelements)isdeterminedby
E−1U The solutions for α are
α= , (9)
U E−1U
T
where U is a t-component vector whose elements are all
1 (U is its transpose) and E is the (t t) error matrix.
T
The diagonalelements of E are the ind×ividual variances, B0 →Λ+c p:
σ2. The off-diagonal elements are the covariances be-
l αT = 0.757 0.128 0.019 0.096 , (10)
tween measurements (rσlσl′, where r is the correlation
(cid:16) (cid:17)
9
(a) (b)
660000 330000
440000 220000
220000 110000
110000 (c) 6600 (d)
4400
5500
2200
) )

2c 2c
/ /
V V
e 8800 (e) e (f)
G G 4400
8 6600 8
0 0
0 4400 0
. . 2200
0 0
( 2200 (

/ /

s s
t t
n 8800 n
e (g) e (h)
v v 4400
E 6600 E
4400
2200
2200
110000
(i) (j)
4400
5500
2200
00 00
5.15 5.20 5.25 5.30 --00..1100 --00..0055 00..0000 00..0055 00..1100
mm ((GGeeVV//cc22)) mm ((GGeeVV//cc22))
mm rr
FIG.2: Projections of mm (left) andmr (right) indatafor B− →Λ+cpπ− candidates, separated byΛ+c decaymode: (a,b) are
Λ+ →pK−π+, (c,d) are Λ+ →pK0, (e,f) are Λ+ →pK0π+π−, (g,h) are Λ+ →Λπ+, and (i,j) are Λ+ →Λπ+π−π+. The
c c S c S c c
mm projections (a,c,e,g,i) are for |mr| < 0.030GeV/c2 and the mr projections (b,d,f,h,j) are for mm > 5.27GeV/c2. The
solid curves correspond to the PDF from the simultaneous 2-D fit to candidates for the five Λ+ decay modes, and the dashed
c
curvesrepresent thebackground component of thePDF.
and We calculate the variance of xˆ
B− Λ+pπ− :
→ c σˆ2 =α Eα. (12)
T
αT = 0.913 0.043 0.003 0.029 0.018 , (11)
−
(cid:16) (cid:17)
where α is the transpose of α. The order of the co- Since the error matrices add linearly, we quote separate
T
efficients corresponds to the order of Λ+ decay modes statistical and systematic uncertainties. The statisti-
c
presented in Table III. cal and systematic uncertainties in N are added in
BB
We calculate the best estimate xˆ according to Eqn. 8 quadrature with the statistical and systematic σˆ results,
and divide this quantity by N and (Λ+ pK−π+). respectively. The combined branching fraction measure-
BB B c →
10
ments are thus The systematic uncertainty for charged particle iden-
tification is a measure of how well the corrections ap-
(B0 Λ+p)= plied to the events in the signal MC sample for the
B → c
(1.89 0.21 0.06 0.49) 10−5, efficiency determination describe the B0 → Λ+c p and
± ± ± × B− Λ+pπ− decay modes. The corrections are deter-
B(B− →Λ+cpπ−)= mine→d frocm control MC and data samples (a Λ pπ−
→
(3.38 0.12 0.12 0.88) 10−4, (13) sample for protons and D∗+ D0π+,D0 K−π+
± ± ± × → →
samples for pions and kaons). The efficiency as a func-
where the uncertainties are statistical, systematic, and tion of momentum and angle for the B0 Λ+p and
theWuenucseerttahientsyamineBp(rΛoc+ced→urpeKto−dπe+te)r,mreinspeetchteivberlya.nching sBa−mp→lesΛa+cgpreπe−tosiwgnitahlinM(C0.8sam3p.l5e)s%anfodrtdhieff→ecroennttcroralnMgeCs
ratio B(B− →Λ+c pπ−)/B(B0 →Λ+c p): of momentum and angle. −
Systematic uncertainties due to the fit procedure are
(B− Λ+pπ−)
B → c =15.4 1.8 0.3. (14) also considered. The dominant fit uncertainty is due
(B0 Λ+p) ± ±
c to the threshold function parameter in the background
B →
PDF. The fit validation study showed that this param-
In the branching ratio, many of the systematic uncer-
tainties, including the dominant (Λ+ pK−π+) un- eter must be shared among Λ+c decay modes to ensure
B c → fit robustness. We allowed this parameter to vary inde-
certainty, cancel.
pendently among Λ+ decay modes and repeated the fit
c
to data; the difference between the fit results is taken as
SYSTEMATIC UNCERTAINTIES IN THE a systematic uncertainty of (1 24)% depending on the
−
BRANCHING FRACTIONS Λ+ decay mode. A peaking background due to misre-
c
constructed B0 Σ+p,Σ+ Λ+π0 events is present
→ c c → c
The uncertainties in the (B0 Λ+p) and (B− at the level of 1.0%. We assign a systematic uncertainty
Λ+c pπ−)measurementsaredBomina→tedbcytheunBcertain→ty of0.5%to accountfor the uncertainty inB(B0 →Σc+p).
itnhethuencΛer+cta→intipeKs i−nπt+hebΛra+ncbhrianngchfriancgtiroant,ioasn(dcotmhepnarbedy wTehevnaroymtinhaislebnydpo0i.n5tMineVth/ce2fi,tretosumltimngisin52a89s.y0sMteemV/act2ic;
c ±
to Λ+ pK−π+) [8]. uncertainty of (0.2 1.5)%.
c → −
The systematic uncertainties for each Λ+ decay mode
c
are summarized in Tables IV and V. The systematic
uncertainty in the number of BB events is 1.1%. Λ+c p THRESHOLD ENHANCEMENT
The uncertainty in the efficiency determination is due
to MC statistics, charged particle tracking, and particle The kinematic features and resonances in B−
→
identification. ForB0 Λ+p,theuncertaintyduetoMC Λ+pπ− are investigated through examination of the 2-D
→ c c
statisticsis(0.4 0.6)%. ForB− Λ+pπ−,wecalculate Dalitz plot (m2 vs. m2 ) and its projections (the res-
− → c pπ Λcπ
the uncertainty due to MC statistics by independently onanceswill be discussedin the next section). Using the
varying the number of reconstructed signal MC events lot formalism, we project the events in the m ,m
s m r
P { }
in each cosθ ,m bin according to a Poissondistribu- fit regionusing the signal lot weights and background
h Λcπ sP
tion (ensuring that the data events in the same bin are lot weights along with the efficiency corrections. This
s
P
correlated). The resulting uncertainty is (0.6 3.1)%. methodallowsustoprojectonlythefeaturesofthesignal
−
Thetrackingefficiencysystematicuncertaintiesarede- events,whiletakingtheefficiencyvariationsintoaccount.
termined from two separate studies. In the first study, Figure3showsthe lotweightsform2 vs.m2 . Note
sP pπ Λcπ
τ+τ− candidates are selected, in which one τ candidate that the negative bins are suppressed in the 2-D Dalitz
decaysto leptons andthe other decaysto morethan one plot.
hadronplus aneutrino. Eventsareselectedif onelepton We project the events in the fit region onto the m
Λcp
andatleasttwochargedhadronsarereconstructed. The axis with signal lot weights and efficiency corrections
s
P
efficiency is then measured for reconstructing the third to study the enhancement at threshold in the baryon-
chargedparticle for the hadronicdecay. Fromthis study antibaryon mass distribution. This enhancement can be
there is a (0.38 0.45)% uncertainty in the tracking ef- seen in B− Λ+pπ− decays as a peak in m near
− → c Λcp
ficiency per charged particle. In the second method, a the kinematic threshold, m0 = 3224.8MeV/c2. We di-
Λcp
charged particle trajectory is reconstructed in the SVT, videthenormalizedm distributionbytheexpectation
Λcp
andthenthe efficiencyfor findingthe correspondingtra- from three-body phase-space; the resulting distribution
jectory in the DCH is measured. For the latter study, is shown in Figure 4. An enhancement is clearly visi-
the uncertainties range from 0.21% to 1.18% depending ble near threshold. The observationofthis enhancement
on the Λ+ decay mode. The systematic uncertainties is consistent with baryon-antibaryonthreshold enhance-
c
determined in the two studies are added in quadrature. ments as seen in other decay modes such as B ppK,
→