Measurement of the branching fraction and
polarization in
B
0
!
p
B. Aubert,
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
M. Martinelli,
3a,3b
A. Palano,
3a,3b
M. Pappagallo,
3a,3b
G. Eigen,
4
B. Stugu,
4
L. Sun,
4
M. Battaglia,
5
D. N. Brown,
5
L. T. Kerth,
5
Yu. G. Kolomensky,
5
G. Lynch,
5
I. L. Osipenkov,
5
K. Tackmann,
5
T. Tanabe,
5
C. M. Hawkes,
6
N. Soni,
6
A. T. Watson,
6
H. Koch,
7
T. Schroeder,
7
D. J. Asgeirsson,
8
B. G. Fulsom,
8
C. Hearty,
8
T. S. Mattison,
8
J. A. McKenna,
8
M. Barrett,
9
A. Khan,
9
A. Randle-Conde,
9
V. E. Blinov,
10
A. D. Bukin,
10
A. R. Buzykaev,
10
V. P. Druzhinin,
10
V. B. Golubev,
10
A. P. Onuchin,
10
S. I. Serednyakov,
10
Yu. I. Skovpen,
10
E. P. Solodov,
10
K. Yu. Todyshev,
10
M. Bondioli,
11
S. Curry,
11
I. Eschrich,
11
D. Kirkby,
11
A. J. Lankford,
11
P. Lund,
11
M. Mandelkern,
11
E. C. Martin,
11
J. Schultz,
11
D. P. Stoker,
11
H. Atmacan,
12
J. W. Gary,
12
F. Liu,
12
O. Long,
12
G. M. Vitug,
12
Z. Yasin,
12
L. Zhang,
12
V. Sharma,
13
C. Campagnari,
14
T. M. Hong,
14
D. Kovalskyi,
14
M. A. Mazur,
14
J. D. Richman,
14
T. W. Beck,
15
A. M. Eisner,
15
C. A. Heusch,
15
J. Kroseberg,
15
W. S. Lockman,
15
A. J. Martinez,
15
T. Schalk,
15
B. A. Schumm,
15
A. Seiden,
15
L. Wang,
15
L. O. Winstrom,
15
C. H. Cheng,
16
D. A. Doll,
16
B. Echenard,
16
F. Fang,
16
D. G. Hitlin,
16
I. Narsky,
16
T. Piatenko,
16
F. C. Porter,
16
R. Andreassen,
17
G. Mancinelli,
17
B. T. Meadows,
17
K. Mishra,
17
M. D. Sokoloff,
17
P. C. Bloom,
18
W. T. Ford,
18
A. Gaz,
18
J. F. Hirschauer,
18
M. Nagel,
18
U. Nauenberg,
18
J. G. Smith,
18
S. R. Wagner,
18
R. Ayad,
19,
†
W. H. Toki,
19
R. J. Wilson,
19
E. Feltresi,
20
A. Hauke,
20
H. Jasper,
20
T. M. Karbach,
20
J. Merkel,
20
A. Petzold,
20
B. Spaan,
20
K. Wacker,
20
M. J. Kobel,
21
R. Nogowski,
21
K. R. Schubert,
21
R. Schwierz,
21
A. Volk,
21
D. Bernard,
22
E. Latour,
22
M. Verderi,
22
P. J. Clark,
23
S. Playfer,
23
J. E. Watson,
23
M. Andreotti,
24a,24b
D. Bettoni,
24a
C. Bozzi,
24a
R. Calabrese,
24a,24b
A. Cecchi,
24a,24b
G. Cibinetto,
24a,24b
E. Fioravanti,
24a,24b
P. Franchini,
24a,24b
E. Luppi,
24a,24b
M. Munerato,
24a,24b
M. Negrini,
24a,24b
A. Petrella,
24a,24b
L. Piemontese,
24a
V. Santoro,
24a,24b
R. Baldini-Ferroli,
25
A. Calcaterra,
25
R. de Sangro,
25
G. Finocchiaro,
25
S. Pacetti,
25
P. Patteri,
25
I. M. Peruzzi,
25,
‡
M. Piccolo,
25
M. Rama,
25
A. Zallo,
25
R. Contri,
26a,26b
E. Guido,
26a,26b
M. Lo Vetere,
26a,26b
M. R. Monge,
26a,26b
S. Passaggio,
26a
C. Patrignani,
26a,26b
E. Robutti,
26a
S. Tosi,
26a,26b
K. S. Chaisanguanthum,
27
M. Morii,
27
A. Adametz,
28
J. Marks,
28
S. Schenk,
28
U. Uwer,
28
F. U. Bernlochner,
29
V. Klose,
29
H. M. Lacker,
29
D. J. Bard,
30
P. D. Dauncey,
30
M. Tibbetts,
30
P. K. Behera,
31
M. J. Charles,
31
U. Mallik,
31
J. Cochran,
32
H. B. Crawley,
32
L. Dong,
32
V. Eyges,
32
W. T. Meyer,
32
S. Prell,
32
E. I. Rosenberg,
32
A. E. Rubin,
32
Y. Y. Gao,
33
A. V. Gritsan,
33
Z. J. Guo,
33
N. Arnaud,
34
J. Be
́
quilleux,
34
A. D’Orazio,
34
M. Davier,
34
D. Derkach,
34
J. Firmino da Costa,
34
G. Grosdidier,
34
F. Le Diberder,
34
V. Lepeltier,
34
A. M. Lutz,
34
B. Malaescu,
34
S. Pruvot,
34
P. Roudeau,
34
M. H. Schune,
34
J. Serrano,
34
V. Sordini,
34,
x
A. Stocchi,
34
G. Wormser,
34
D. J. Lange,
35
D. M. Wright,
35
I. Bingham,
36
J. P. Burke,
36
C. A. Chavez,
36
J. R. Fry,
36
E. Gabathuler,
36
R. Gamet,
36
D. E. Hutchcroft,
36
D. J. Payne,
36
C. Touramanis,
36
A. J. Bevan,
37
C. K. Clarke,
37
F. Di Lodovico,
37
R. Sacco,
37
M. Sigamani,
37
G. Cowan,
38
S. Paramesvaran,
38
A. C. Wren,
38
D. N. Brown,
39
C. L. Davis,
39
A. G. Denig,
40
M. Fritsch,
40
W. Gradl,
40
A. Hafner,
40
K. E. Alwyn,
41
D. Bailey,
41
R. J. Barlow,
41
G. Jackson,
41
G. D. Lafferty,
41
T. J. West,
41
J. I. Yi,
41
J. Anderson,
42
C. Chen,
42
A. Jawahery,
42
D. A. Roberts,
42
G. Simi,
42
J. M. Tuggle,
42
C. Dallapiccola,
43
E. Salvati,
43
S. Saremi,
43
R. Cowan,
44
D. Dujmic,
44
P. H. Fisher,
44
S. W. Henderson,
44
G. Sciolla,
44
M. Spitznagel,
44
R. K. Yamamoto,
44
M. Zhao,
44
P. M. Patel,
45
S. H. Robertson,
45
M. Schram,
45
A. Lazzaro,
46a,46b
V. Lombardo,
46a
F. Palombo,
46a,46b
S. Stracka,
46a,46b
J. M. Bauer,
47
L. Cremaldi,
47
R. Godang,
47,
k
R. Kroeger,
47
P. Sonnek,
47
D. J. Summers,
47
H. W. Zhao,
47
M. Simard,
48
P. Taras,
48
H. Nicholson,
49
G. De Nardo,
50a,50b
L. Lista,
50a
D. Monorchio,
50a,50b
G. Onorato,
50a,50b
C. Sciacca,
50a,50b
G. Raven,
51
H. L. Snoek,
51
C. P. Jessop,
52
K. J. Knoepfel,
52
J. M. LoSecco,
52
W. F. Wang,
52
L. A. Corwin,
53
K. Honscheid,
53
H. Kagan,
53
R. Kass,
53
J. P. Morris,
53
A. M. Rahimi,
53
J. J. Regensburger,
53
S. J. Sekula,
53
Q. K. Wong,
53
N. L. Blount,
54
J. Brau,
54
R. Frey,
54
O. Igonkina,
54
J. A. Kolb,
54
M. Lu,
54
R. Rahmat,
54
N. B. Sinev,
54
D. Strom,
54
J. Strube,
54
E. Torrence,
54
G. Castelli,
55a,55b
N. Gagliardi,
55a,55b
M. Margoni,
55a,55b
M. Morandin,
55a
M. Posocco,
55a
M. Rotondo,
55a
F. Simonetto,
55a,55b
R. Stroili,
55a,55b
C. Voci,
55a,55b
P. del Amo Sanchez,
56
E. Ben-Haim,
56
G. R. Bonneaud,
56
H. Briand,
56
J. Chauveau,
56
O. Hamon,
56
Ph. Leruste,
56
G. Marchiori,
56
J. Ocariz,
56
A. Perez,
56
J. Prendki,
56
S. Sitt,
56
L. Gladney,
57
M. Biasini,
58a,58b
E. Manoni,
58a,58b
C. Angelini,
59a,59b
G. Batignani,
59a,59b
S. Bettarini,
59a,59b
G. Calderini,
59a,59b,
{
M. Carpinelli,
59a,59b,
**
A. Cervelli,
59a,59b
F. Forti,
59a,59b
M. A. Giorgi,
59a,59b
A. Lusiani,
59a,59c
M. Morganti,
59a,59b
N. Neri,
59a,59b
E. Paoloni,
59a,59b
G. Rizzo,
59a,59b
J. J. Walsh,
59a
D. Lopes Pegna,
60
C. Lu,
60
J. Olsen,
60
A. J. S. Smith,
60
A. V. Telnov,
60
F. Anulli,
61a
E. Baracchini,
61a,61b
G. Cavoto,
61a
R. Faccini,
61a,61b
F. Ferrarotto,
61a
F. Ferroni,
61a,61b
M. Gaspero,
61a,61b
P. D. Jackson,
61a
L. Li Gioi,
61a
M. A. Mazzoni,
61a
S. Morganti,
61a
G. Piredda,
61a
F. Renga,
61a,61b
C. Voena,
61a
M. Ebert,
62
T. Hartmann,
62
H. Schro
̈
der,
62
R. Waldi,
62
T. Adye,
63
B. Franek,
63
E. O. Olaiya,
63
F. F. Wilson,
63
S. Emery,
64
L. Esteve,
64
G. Hamel de Monchenault,
64
W. Kozanecki,
64
G. Vasseur,
64
Ch. Ye
`
che,
64
M. Zito,
64
M. T. Allen,
65
D. Aston,
65
R. Bartoldus,
65
J. F. Benitez,
65
R. Cenci,
65
J. P. Coleman,
65
PHYSICAL REVIEW D
79,
112009 (2009)
1550-7998
=
2009
=
79(11)
=
112009(10)
112009-1
Ó
2009 The American Physical Society
M. R. Convery,
65
J. C. Dingfelder,
65
J. Dorfan,
65
G. P. Dubois-Felsmann,
65
W. Dunwoodie,
65
R. C. Field,
65
M. Franco Sevilla,
65
A. M. Gabareen,
65
M. T. Graham,
65
P. Grenier,
65
C. Hast,
65
W. R. Innes,
65
J. Kaminski,
65
M. H. Kelsey,
65
H. Kim,
65
P. Kim,
65
M. L. Kocian,
65
D. W. G. S. Leith,
65
S. Li,
65
B. Lindquist,
65
S. Luitz,
65
V. Luth,
65
H. L. Lynch,
65
D. B. MacFarlane,
65
H. Marsiske,
65
R. Messner,
65,
*
D. R. Muller,
65
H. Neal,
65
S. Nelson,
65
C. P. O’Grady,
65
I. Ofte,
65
M. Perl,
65
B. N. Ratcliff,
65
A. Roodman,
65
A. A. Salnikov,
65
R. H. Schindler,
65
J. Schwiening,
65
A. Snyder,
65
D. Su,
65
M. K. Sullivan,
65
K. Suzuki,
65
S. K. Swain,
65
J. M. Thompson,
65
J. Va’vra,
65
A. P. Wagner,
65
M. Weaver,
65
C. A. West,
65
W. J. Wisniewski,
65
M. Wittgen,
65
D. H. Wright,
65
H. W. Wulsin,
65
A. K. Yarritu,
65
C. C. Young,
65
V. Ziegler,
65
X. R. Chen,
66
H. Liu,
66
W. Park,
66
M. V. Purohit,
66
R. M. White,
66
J. R. Wilson,
66
P. R. Burchat,
67
A. J. Edwards,
67
T. S. Miyashita,
67
S. Ahmed,
68
M. S. Alam,
68
J. A. Ernst,
68
B. Pan,
68
M. A. Saeed,
68
S. B. Zain,
68
A. Soffer,
69
S. M. Spanier,
70
B. J. Wogsland,
70
R. Eckmann,
71
J. L. Ritchie,
71
A. M. Ruland,
71
C. J. Schilling,
71
R. F. Schwitters,
71
B. C. Wray,
71
B. W. Drummond,
72
J. M. Izen,
72
X. C. Lou,
72
F. Bianchi,
73a,73b
D. Gamba,
73a,73b
M. Pelliccioni,
73a,73b
M. Bomben,
74a,74b
L. Bosisio,
74a,74b
C. Cartaro,
74a,74b
G. Della Ricca,
74a,74b
L. Lanceri,
74a,74b
L. Vitale,
74a,74b
V. Azzolini,
75
N. Lopez-March,
75
F. Martinez-Vidal,
75
D. A. Milanes,
75
A. Oyanguren,
75
J. Albert,
76
Sw. Banerjee,
76
B. Bhuyan,
76
H. H. F. Choi,
76
K. Hamano,
76
G. J. King,
76
R. Kowalewski,
76
M. J. Lewczuk,
76
I. M. Nugent,
76
J. M. Roney,
76
R. J. Sobie,
76
T. J. Gershon,
77
P. F. Harrison,
77
J. Ilic,
77
T. E. Latham,
77
G. B. Mohanty,
77
E. M. T. Puccio,
77
H. R. Band,
78
X. Chen,
78
S. Dasu,
78
K. T. Flood,
78
Y. Pan,
78
R. Prepost,
78
C. O. Vuosalo,
78
and S. L. Wu
78
(
B
A
B
AR
Collaboration)
1
Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP), Universite
́
de Savoie, CNRS/IN2P3, F-74941 Annecy-Le-Vieux,
France
2
Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain
3a
INFN Sezione di Bari, I-70126 Bari, Italy
3b
Dipartimento di Fisica, Universita
`
di Bari, I-70126 Bari, Italy
4
University of Bergen, Institute of Physics, N-5007 Bergen, Norway
5
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
6
University of Birmingham, Birmingham, B15 2TT, United Kingdom
7
Ruhr Universita
̈
t Bochum, Institut fu
̈
r Experimentalphysik 1, D-44780 Bochum, Germany
8
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
9
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
10
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
11
University of California at Irvine, Irvine, California 92697, USA
12
University of California at Riverside, Riverside, California 92521, USA
13
University of California at San Diego, La Jolla, California 92093, USA
14
University of California at Santa Barbara, Santa Barbara, California 93106, USA
15
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
16
California Institute of Technology, Pasadena, California 91125, USA
17
University of Cincinnati, Cincinnati, Ohio 45221, USA
18
University of Colorado, Boulder, Colorado 80309, USA
19
Colorado State University, Fort Collins, Colorado 80523, USA
20
Technische Universita
̈
t Dortmund, Fakulta
̈
t Physik, D-44221 Dortmund, Germany
21
Technische Universita
̈
t Dresden, Institut fu
̈
r Kernund Teilchenphysik, D-01062 Dresden, Germany
22
Laboratoire Leprince-Ringuet, CNRS/IN2P3, Ecole Polytechnique, F-91128 Palaiseau, France
23
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
24a
INFN Sezione di Ferrara, I-44100 Ferrara, Italy
24b
Dipartimento di Fisica, Universita
`
di Ferrara, I-44100 Ferrara, Italy
25
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
26a
INFN Sezione di Genova, I-16146 Genova, Italy
26b
Dipartimento di Fisica, Universita
`
di Genova, I-16146 Genova, Italy
27
Harvard University, Cambridge, Massachusetts 02138, USA
28
Universita
̈
t Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany
29
Humboldt-Universita
̈
t zu Berlin, Institut fu
̈
r Physik, Newtonstr. 15, D-12489 Berlin, Germany
30
Imperial College London, London, SW7 2AZ, United Kingdom
31
University of Iowa, Iowa City, Iowa 52242, USA
32
Iowa State University, Ames, Iowa 50011-3160, USA
33
Johns Hopkins University, Baltimore, Maryland 21218, USA
B. AUBERT
et al.
PHYSICAL REVIEW D
79,
112009 (2009)
112009-2
34
Laboratoire de l’Acce
́
le
́
rateur Line
́
aire, IN2P3/CNRS et Universite
́
Paris-Sud 11, Centre Scientifique d’Orsay,
B. P. 34, F-91898 Orsay Cedex, France
35
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
36
University of Liverpool, Liverpool L69 7ZE, United Kingdom
37
Queen Mary, University of London, London, E1 4NS, United Kingdom
38
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
39
University of Louisville, Louisville, Kentucky 40292, USA
40
Johannes Gutenberg-Universita
̈
t Mainz, Institut fu
̈
r Kernphysik, D-55099 Mainz, Germany
41
University of Manchester, Manchester M13 9PL, United Kingdom
42
University of Maryland, College Park, Maryland 20742, USA
43
University of Massachusetts, Amherst, Massachusetts 01003, USA
44
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
45
McGill University, Montre
́
al, Que
́
bec, Canada H3A 2T8
46a
INFN Sezione di Milano, I-20133 Milano, Italy
46b
Dipartimento di Fisica, Universita
`
di Milano, I-20133 Milano, Italy
47
University of Mississippi, University, Mississippi 38677, USA
48
Universite
́
de Montre
́
al, Physique des Particules, Montre
́
al, Que
́
bec, Canada H3C 3J7
49
Mount Holyoke College, South Hadley, Massachusetts 01075, USA
50a
INFN Sezione di Napoli, I-80126 Napoli, Italy
50b
Dipartimento di Scienze Fisiche, Universita
`
di Napoli Federico II, I-80126 Napoli, Italy
51
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
52
University of Notre Dame, Notre Dame, Indiana 46556, USA
53
Ohio State University, Columbus, Ohio 43210, USA
54
University of Oregon, Eugene, Oregon 97403, USA
55a
INFN Sezione di Padova, I-35131 Padova, Italy
55b
Dipartimento di Fisica, Universita
`
di Padova, I-35131 Padova, Italy
56
Laboratoire de Physique Nucle
́
aire et de Hautes Energies, IN2P3/CNRS, Universite
́
Pierre et Marie Curie-Paris6,
Universite
́
Denis Diderot-Paris7, F-75252 Paris, France
57
University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
58a
INFN Sezione di Perugia, I-06100 Perugia, Italy
58b
Dipartimento di Fisica, Universita
`
di Perugia, I-06100 Perugia, Italy
59a
INFN Sezione di Pisa, I-56127 Pisa, Italy
59b
Dipartimento di Fisica, Universita
`
di Pisa, I-56127 Pisa, Italy
59c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
60
Princeton University, Princeton, New Jersey 08544, USA
61a
INFN Sezione di Roma, I-00185 Roma, Italy
61b
Dipartimento di Fisica, Universita
`
di Roma La Sapienza, I-00185 Roma, Italy
62
Universita
̈
t Rostock, D-18051 Rostock, Germany
63
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
64
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
65
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
66
University of South Carolina, Columbia, South Carolina 29208, USA
67
Stanford University, Stanford, California 94305-4060, USA
68
State University of New York, Albany, New York 12222, USA
69
Tel Aviv University, School of Physics and Astronomy, Tel Aviv, 69978, Israel
70
University of Tennessee, Knoxville, Tennessee 37996, USA
71
University of Texas at Austin, Austin, Texas 78712, USA
72
University of Texas at Dallas, Richardson, Texas 75083, USA
73a
INFN Sezione di Torino, I-10125 Torino, Italy
73b
Dipartimento di Fisica Sperimentale, Universita
`
di Torino, I-10125 Torino, Italy
74a
INFN Sezione di Trieste, I-34127 Trieste, Italy
**
Also with Universita
`
di Sassari, Sassari, Italy.
{
Also with Laboratoire de Physique Nucle
́
aire et de Hautes Energies, IN2P3/CNRS, Universite
́
Pierre et Marie Curie-Paris6,
Universite
́
Denis Diderot-Paris7, F-75252 Paris, France.
k
Now at University of South Alabama, Mobile, Alabama 36688, USA.
x
Also with Universita
`
di Roma La Sapienza, I-00185 Roma, Italy.
‡
Also with Universita
`
di Perugia, Dipartimento di Fisica, Perugia, Italy.
†
Now at Temple University, Philadelphia, Pennsylvania 19122, USA.
*
Deceased.
MEASUREMENT OF THE BRANCHING FRACTION AND
...
PHYSICAL REVIEW D
79,
112009 (2009)
112009-3
74b
Dipartimento di Fisica, Universita
`
di Trieste, I-34127 Trieste, Italy
75
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
76
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
77
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
78
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 30 April 2009; published 19 June 2009)
We present a measurement of the
B
0
!
p
branching fraction performed using the
BABAR
detector
at the PEP-II asymmetric
e
þ
e
collider. Based on a sample of
467
10
6
B
B
pairs we measure
B
ð
B
0
!
p
Þ¼½
3
:
07
0
:
31
ð
stat
Þ
0
:
23
ð
syst
Þ
10
6
. The measured differential spectrum as a function of
the dibaryon invariant mass
m
ð
p
Þ
shows a near-threshold enhancement similar to that observed in other
baryonic
B
decays. We study the
polarization as a function of
energy in the
B
0
rest frame (
E
) and
compare it with theoretical expectations of fully longitudinally right-polarized
at large
E
.
DOI:
10.1103/PhysRevD.79.112009
PACS numbers: 13.25.Hw, 13.60.Rj
I. INTRODUCTION
Observations of charmless three-body baryonic
B
de-
cays have been reported recently by both the Belle and
BABAR
collaborations [
1
–
3
]. A common feature of these
decay modes is the peaking of the baryon-antibaryon mass
spectrum near threshold. This feature has stimulated con-
siderable interest among theorists as a key element in the
explanation of the unexpectedly high branching fractions
for these decays [
4
,
5
].
In the standard model, the
B
0
!
p
decay proceeds
through tree level
b
!
u
and penguin
b
!
s
amplitudes. It
is of interest to study the structure of the decay amplitude
in the Dalitz plane to test theoretical expectations. The
weak decay
!
p
þ
is spin self-analyzing. Since the
s
quark carries the
spin, the V-A transition
b
!
s
leads to
the expectation that the
is fully longitudinally right-
polarized at large
energy in the
B
0
rest frame [
6
]. This
channel may also be used to search for direct
CP
violation.
II. DATA SET AND SELECTION
The data sample consists of
467
10
6
B
B
pairs, corre-
sponding to an integrated luminosity of
426 fb
1
, col-
lected at the
ð
4
S
Þ
resonance with the
BABAR
detector.
The detector is described in detail elsewhere [
7
]. Charged-
particle trajectories are measured in a tracking system
consisting of a five-layer double-sided silicon vertex
tracker (SVT) and a 40-layer central drift chamber
(DCH), both operating in a 1.5-T axial magnetic field. A
ring-imaging Cherenkov detector (DIRC) is used for
charged-particle identification. A CsI(Tl) electromagnetic
calorimeter (EMC) is used to detect and identify photons
and electrons, while muons and hadrons are identified in
the instrumented flux return of the magnet (IFR). A
BABAR
detector Monte Carlo simulation based on GEANT4 [
8
]is
used to optimize selection criteria and determine selection
efficiencies.
We reconstruct
candidates in the
!
p
decay
mode as combinations of oppositely charged tracks, assign
the proton and pion mass hypotheses, and fit to a common
vertex [
9
]. Combinations with invariant mass in the range
1
:
111
–
1
:
121 GeV
=c
2
are refit requiring the track pairs to
originate from a common vertex and constraining the mass
to the world-average
mass [
10
]. Candidate
B
0
mesons
are formed by combining
candidates with two additional
oppositely charged tracks, each with momentum transverse
to the beam greater than
50 MeV
=c
.
Measurements of the average energy loss (
dE=dx
) in the
tracking devices, the angle of the Cherenkov cone in the
DIRC, and energies deposited in the EMC and IFR are
combined to give a likelihood estimator
L
for a track to be
consistent with a given particle hypothesis
. We require
that the
-decay proton candidates satisfy the particle-
identification criteria
L
p
=L
K
>
0
:
33
and
L
p
=L
>
1
to
discriminate from kaons and pions, respectively. The can-
didate protons, which are assumed to originate from the
B
0
decay vertex, are analyzed with a selection algorithm based
on bagged decision trees [
11
] which provide efficient
particle discrimination, retaining 96.4% of the signal can-
didates and 17.8% of the background. The candidate pions
from the
B
0
vertex are required to pass a similar selection
algorithm, tuned to discriminate pions, that retains 98.8%
of the signal and 66.8% of the background. A Kalman fit
[
12
] to the full decay sequence is used to reconstruct the
B
0
vertex using the position of the beam spot and the total
beam energy as kinematic constraints. Only candidates
with a fit probability
P
vtx
>
10
6
are considered, a require-
ment that retains 94.4% of the signal and 58.2% of the
background.
The primary background arises from light-quark contin-
uum events
e
þ
e
!
q
q
(
q
¼
u; d; s; c
), which are charac-
terized by collimation of final-state particles with respect
to the quark direction, in contrast to the more spherical
B
B
events. Exploiting this shape difference, we increase the
signal significance using event-shape variables computed
from the center-of-mass (CM) momenta of charged and
neutral particles in the event. For each event, we combine
the sphericity [
13
], the angle between the
B
0
thrust axis and
detector longitudinal axis, and the zeroth and second-order
Legendre polynomial moments [
14
] of the tracks not asso-
B. AUBERT
et al.
PHYSICAL REVIEW D
79,
112009 (2009)
112009-4
ciated with the reconstructed
B
candidate, into a Fisher
discriminant [
15
], where the coefficients are chosen to
optimize the separation between signal and continuum-
background Monte Carlo samples. We find that the selec-
tion using the optimal cut on the Fisher discriminant retains
72% of the candidates from the signal Monte Carlo sample
and 8% from the continuum-background Monte Carlo
sample.
To further reduce the combinatoric background, we take
advantage of the long mean lifetime of
particles and
require that the separation of the
and
B
0
vertices, divided
by its measurement error, computed on a per-candidate
basis by the fit procedure, exceeds 20. This criterion is
optimized on Monte Carlo events and is effective in reject-
ing 42% of combinatoric background that survives all other
selection requirements, while retaining 90% of the signal
candidates. The only sizable
B
0
background is from the
process
B
0
!
c
p
!
p
, which we suppress by re-
moving candidates with an invariant mass
m
ð
Þ
within
5 standard deviations (
20 MeV
=c
2
) of the nominal
c
mass [
10
].
The kinematic constraints on
B
0
mesons produced at
the
ð
4
S
Þ
allow further background discrimination from
the variables
m
ES
and
E
. We define
m
ES
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð
s
2
þ
~
p
i
~
p
B
Þ
2
=E
2
i
~
p
2
B
q
, where
ð
E
i
;
~
p
i
Þ
is the four mo-
mentum of the initial
e
þ
e
system and
~
p
B
is the momen-
tum of the reconstructed
B
0
candidate, both measured in
the laboratory frame, and
s
is the square of the total energy
in the
e
þ
e
center-of-mass frame. We define
E
¼
E
B
ffiffi
s
p
2
, where
E
B
is the
B
0
energy in the
e
þ
e
center-of-mass
frame. Signal candidates have
m
ES
close to the
B
0
mass and
E
near zero. Candidates satisfying
j
E
j
<
100 MeV
and
5
:
20
<m
ES
<
5
:
29 GeV
=c
2
are used in the maximum-
likelihood fitting process.
III. BRANCHING FRACTION
We measure the branching fraction with a maximum-
likelihood fit on the
m
ES
-
E
observables of reconstructed
B
0
candidates. The
s
P
lot
technique [
16
] is then used to
determine the
m
ð
p
Þ
distribution and, after correcting for
the nonuniform reconstruction efficiency, measure the
m
ð
p
Þ
-dependent differential branching fraction.
We consider as signal candidates only reconstructed
B
0
candidates in which all particles are correctly assigned in
the decay chain. By self-cross-feed, we refer to events in
which
B
0
mesons decay to
p
and are reconstructed as
signal candidates in which one or more particles are not
correctly assigned in the decay chain. An example of such
a misreconstruction is where the protons from the signal
B
0
and
decays are interchanged. We define the probability
density function (PDF) in the
E
-
m
ES
plane as the sum of
signal, self-cross-feed, and background components. The
likelihood function is given by
L
¼
1
N
!
e
ð
N
S
þ
N
scf
þ
N
B
Þ
Y
N
e
¼
1
f
N
S
P
S
ð
y
e
Þþ
N
scf
P
scf
ð
y
e
Þ
þ
N
B
P
B
ð
y
e
Þg
;
(1)
where
y
e
¼ð
m
ES
;e
;
E
e
Þ
, the product is over the
N
fitted
candidates with
N
S
and
N
B
representing the numbers of
signal and background events, and
N
scf
N
S
f
scf
repre-
senting the self-cross-feed contribution. The three
P
func-
tions are taken as products of one-dimensional
E
and
m
ES
PDFs. We are justified in this simplification by the small
correlation between these two variables in our Monte Carlo
sample. The
m
ES
PDF is taken as a sum of two Gaussians
for the signal and an ARGUS function [
17
] for the back-
ground. The
E
PDF is taken as a sum of two Gaussians
for the signal and a first-order polynomial for the back-
ground. Finally, the self-cross-feed contribution shows a
peaking component that is modeled as the product of a sum
of two Gaussians in
E
, and a single Gaussian in
m
ES
.We
determine
f
scf
¼
0
:
006
and the other parameters that char-
acterize this background from fits to simulated events.
We fit the means of the narrow
E
and
m
ES
signal
Gaussians, the coefficient in the exponential of the Argus
function, the linear coefficient of the
E
background
distribution, and the event yields
N
S
and
N
B
. The means
of the wide Gaussians are determined by applying
Monte Carlo-determined offsets to the means of the narrow
ones, such that only an overall shift of the fixed PDF shape
is allowed. All other parameters used in the likelihood
definition are fixed to values determined from fits to
Monte Carlo-simulated events.
Once the maximum-likelihood fit provides the best es-
timates of the PDF parameters, we use the
s
P
lot
technique
to reconstruct the efficiency-corrected
m
ð
p
Þ
distribution
and measure the branching fraction. The PDF is used to
compute the
s
-weight for the
n
th component of event
e
as
s
P
n
ð
y
e
Þ¼
P
n
c
j
¼
1
V
nj
P
j
ð
y
e
Þ
P
n
c
k
¼
1
N
k
P
k
ð
y
e
Þ
;
(2)
where the indices
n
,
j
, and
k
run over the
n
c
¼
3
signal,
background, and self-cross-feed components. The symbol
V
nj
is the covariance matrix of the event yields as mea-
sured from the fit to the data sample. An important property
of the
s
P
lot
is that the sum of the
s
-weights for the signal
or background component equals the corresponding num-
ber of fitted signal or background events. We have demon-
strated with simulated experiments that the
s
P
lot
is an
unbiased and nearly optimal estimator of the
m
ð
p
Þ
dis-
tribution. To retrieve the efficiency-corrected number of
signal events in a given
m
ð
p
Þ
bin
J
we use the
s
-weight
sum
~
N
S;J
¼
X
e
2
J
s
P
S
ð
y
e
Þ
"
ð
x
e
Þ
;
(3)
where the per-event efficiency
"
ð
x
e
Þ
depends on the posi-
MEASUREMENT OF THE BRANCHING FRACTION AND
...
PHYSICAL REVIEW D
79,
112009 (2009)
112009-5
tion
x
e
¼ð
m
p
;
cos
Þ
in the square Dalitz plane. Here
is the angle between the momenta of the pion and the
candidate in the
p
rest frame, and the efficiency is
determined over a
20
20
grid in the Dalitz plane, using
fully reconstructed signal-Monte Carlo events. The error
½
~
N
S;J
in
~
N
S;J
is given by
2
½
~
N
S;J
¼
X
e
2
J
s
P
S
ð
y
e
Þ
"
ð
x
e
Þ
2
:
(4)
An estimate of the efficiency-corrected number of signal
events in the sample is given by the sum of the efficiency-
corrected
s
-weights, or
~
N
S
¼
X
J
~
N
S;J
;
(5)
and the branching fraction is obtained from
B
ð
B
0
!
p
Þ¼
~
N
S
N
B
B
B
ð
!
p
Þ
;
(6)
where
N
B
B
is the total number of
B
B
pairs and
B
ð
!
p
Þ¼
0
:
639
0
:
005
[
10
]. Using a collection of
Monte Carlo pseudoexperiments, in which signal candi-
dates, generated and reconstructed with a complete detec-
tor simulation, were mixed with background candidates,
generated according to the background PDF, we confirm
that this procedure provides a measurement of the branch-
ing fraction with negligible biases and accurate errors.
We can measure the
CP
-violating branching-fraction
asymmetry by tagging the flavor of the
B
0
(
B
0
) meson
with the charge of its daughter proton (antiproton). We
repeat the maximum-likelihood fit described above includ-
ing the partial rate asymmetry
A
¼
B
ð
B
0
!
p
þ
Þ
B
ð
B
0
!
p
Þ
B
ð
B
0
!
p
þ
Þþ
B
ð
B
0
!
p
Þ
(7)
as a free parameter. We reduce the effect of systematic
differences in particle-identification efficiencies between
protons and antiprotons, and between positive and negative
pions, by performing the fit on a sample of reconstructed
candidates, where protons and pions that originate from the
decay satisfy the same particle-identification criteria as
those imposed on the protons and pions that originate from
the
B
0
vertex.
IV.
POLARIZATION MEASUREMENT
We study the three orthogonal components of the polar-
ization of
candidates reconstructed in the
B
0
!
p
decay as a function of
E
, the
energy in the
B
0
rest frame
[
6
]. The distribution of the helicity angle
H
for the
decay is given by
1
d
d
cos
H
¼
1
2
½
1
þ
P
ð
E
Þ
cos
H
;
(8)
where
H
is the angle between the antiproton direction, in
the
rest frame, and either (1)
^
L
, the unit vector in the
direction of the
in the
B
0
rest frame; (2)
^
T
, the unit vector
along the direction of the cross product between the mo-
menta, in the
B
0
rest frame, of the proton and the
;or
(3)
^
N
¼
^
L
^
T
. The symbol
P
ð
E
Þ
is the component of the
polarization in the
^
L
,
^
T
,or
^
N
direction as a function of
E
, and
is the
decay-asymmetry parameter [
10
].
CP
conservation in
B
0
!
p
decays implies that
P
½
L;N
;
B
0
!
p
ð
E
Þ¼
P
½
L;N
;B
0
!
p
ð
E
Þ
;
(9)
while the product
P
T
changes sign under
CP
conjuga-
tion. We use these relations to fit the
B
0
and
B
0
candidate
samples together.
We use a maximum-likelihood fit in
m
ES
,
E
,
E
, and
cos
H
to measure the polarization as a function of
E
along each of the three axes defined above. We divide the
E
range into three bins with boundaries 1.10, 1.53, 1.80,
and 2.40 GeV, chosen in order to have similar numbers of
signal events in each bin. We define a PDF as the sum of
signal and background components. The likelihood is
L
¼
1
N
!
Y
3
k
¼
1
e
ð
N
k;S
þ
N
k;B
Þ
Y
N
k
e
¼
1
½
N
k;S
P
0
k;S
ð
z
e
Þ
P
S
ð
y
e
Þ
þ
N
k;B
P
0
k;B
ð
z
e
Þ
P
B
ð
y
e
Þ
;
(10)
where we have divided the observables into two sets
y
e
¼
ð
m
ES
;
E
Þ
and
z
e
¼ð
cos
H
;E
Þ
, and the products are over
the three bins in
E
and over the
N
k
events that populate
the
k
th bin, where
N
k;S
and
N
k;B
represent the numbers of
fitted signal and background events. The
P
S;B
ð
y
e
Þ
PDFs are
the same functions used in the branching-fraction measure-
ment. However the self-cross-feed component is not in-
cluded since it corresponds to a negligible fraction of the
signal events. For the
k
th bin in
E
, the signal (
cos
H
,
E
)
PDF is written as the product of the differential branching
fraction of Eq. (
8
), times the signal-reconstruction effi-
ciency
ð
H
;E
Þ
:
P
0
k;S
ð
H
;E
Þ¼
1
2
ð
H
;E
Þ½
1
þf
P
g
k
cos
H
;
(11)
where the
f
P
g
k
are fit parameters. The signal-selection
efficiency is measured with a sample of reconstructed
signal-Monte Carlo events that pass the same selection
criteria as those used to define the data sample. We bin
the signal efficiency in
20
20
rectangular boxes that
cover the allowed region of the
E
-
cos
H
plane (Fig.
1
).
The background
H
distribution is modeled as a linear
combination of Chebyshev polynomials up to fourth order.
The four coefficients that define the linear combination are
fitted independently for each of the three bins in
E
.We
study the
H
distribution of background events using can-
didates in the sideband region
m
ES
<
5
:
27 GeV
=c
2
, and
B. AUBERT
et al.
PHYSICAL REVIEW D
79,
112009 (2009)
112009-6
find it to be nearly independent of
m
ES
. We consider this
insensitivity as an indication that the shape of the back-
ground
H
distribution is the same for events in and out of
the signal region.
We have confirmed that this PDF representation does not
bias the polarization measurement by performing pseu-
doexperiments in which signal candidates, generated and
reconstructed with a complete detector simulation, were
mixed with background candidates generated according
to the observed helicity distribution in the
m
ES
<
5
:
27 GeV
=c
2
sideband. The number of signal and back-
ground candidates are chosen to match the characteristics
of the data.
V. SYSTEMATIC UNCERTAINTIES
Systematic uncertainties in the branching-fraction mea-
surement are listed in Table
I
and classified as overall
uncertainties, uncertainties associated with event selection,
and uncertainties associated with fitting the event distribu-
tion. We study the uncertainty due to tracking efficiency by
comparing data and Monte Carlo for a sample of
-pair
events, in which one
decays to one charged track and the
other
decays to three charged tracks. We separately study
the tracking efficiency of
decay products using an in-
clusive sample of
!
p
candidates and estimate an
overall tracking reconstruction efficiency of 2.4%. The
uncertainty associated with particle-identification (PID)
selection criteria is estimated as 1.4% by comparing data
and Monte Carlo identification efficiencies for protons
from
!
p
decays and pions from
K
0
S
!
decays.
The limited signal-Monte Carlo sample available to mea-
sure the reconstruction efficiency over the Dalitz plane
results in an additional 0.4% contribution. The uncertainty
in the number of
B
B
pairs in the data sample accounts for a
1.1% contribution, while the assumption of a 50% ratio of
B
0
B
0
to
B
B
at the
ð
4
S
Þ
gives an additional 3.2% contri-
bution, computed from the difference between 50% and the
current measured value
ð
48
:
4
0
:
6
Þ
%
[
10
].
Uncertainties associated with event-selection require-
ments on the Fisher discriminant and vertex fit probability
are estimated by comparing data and Monte Carlo-
selection efficiencies for a sample of
B
0
!
J=
c
K
0
S
candi-
dates. We use an inclusive sample of
!
p
candidates
to estimate uncertainties associated with the efficiencies of
the flight-length significance and
-mass requirements.
The application of the requirement on the reconstructed
m
ð
Þ
invariant mass to suppress
B
0
!
c
p
background
FIG. 1. Reconstruction efficiency measured on the
Monte Carlo signal sample, as a function of
cos
H
and
E
.In
this plot
H
is the angle between the antiproton direction, in the
rest frame, and
^
L
, the unit vector in the direction of the
in
the
B
0
rest frame.
TABLE I. Systematic uncertainties on the branching-fraction measurement. ‘‘Total’’ is the
sum in quadrature of all the individual contributions.
Source
Uncertainty (%)
Overall
Tracking efficiency
2.4
PID efficiency
1.4
MC statistics
0.4
B
B
counting
1.1
B
0
B
0
=B
B
fraction
3.2
!
p
branching fraction
0.8
Event selection requirements
Event shape
1.0
Fit probability
1.0
flight length
2.8
mass
2.4
c
veto
0.5
Fit procedure
Likelihood parameters
3.9
E
resolution
1.7
Self cross-feed fraction
0.8
s
P
lot
bias
0.6
Total
7.4
MEASUREMENT OF THE BRANCHING FRACTION AND
...
PHYSICAL REVIEW D
79,
112009 (2009)
112009-7