of 9
Evidence for the baryonic decay
̄
B
0
D
0
Λ
̄
Λ
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
E. Grauges,
2
A. Palano,
3a,3b
G. Eigen,
4
B. Stugu,
4
D. N. Brown,
5
L. T. Kerth,
5
Yu. G. Kolomensky,
5
M. J. Lee,
5
G. Lynch,
5
H. Koch,
6
T. Schroeder,
6
C. Hearty,
7
T. S. Mattison,
7
J. A. McKenna,
7
R. Y. So,
7
A. Khan,
8
V. E. Blinov,
9a,9c
A. R. Buzykaev,
9a
V. P. Druzhinin,
9a,9b
V. B. Golubev,
9a,9b
E. A. Kravchenko,
9a,9b
A. P. Onuchin,
9a,9c
S. I. Serednyakov,
9a,9b
Yu. I. Skovpen,
9a,9b
E. P. Solodov,
9a,9b
K. Yu. Todyshev,
9a,9b
A. N. Yushkov,
9a
D. Kirkby,
10
A. J. Lankford,
10
M. Mandelkern,
10
B. Dey,
11
J. W. Gary,
11
O. Long,
11
G. M. Vitug,
11
C. Campagnari,
12
M. Franco Sevilla,
12
T. M. Hong,
12
D. Kovalskyi,
12
J. D. Richman,
12
C. A. West,
12
A. M. Eisner,
13
W. S. Lockman,
13
B. A. Schumm,
13
A. Seiden,
13
D. S. Chao,
14
C. H. Cheng,
14
B. Echenard,
14
K. T. Flood,
14
D. G. Hitlin,
14
P. Ongmongkolkul,
14
F. C. Porter,
14
R. Andreassen,
15
Z. Huard,
15
B. T. Meadows,
15
B. G. Pushpawela,
15
M. D. Sokoloff,
15
L. Sun,
15
P. C. Bloom,
16
W. T. Ford,
16
A. Gaz,
16
U. Nauenberg,
16
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17
,
W. H. Toki,
17
B. Spaan,
18
R. Schwierz,
19
D. Bernard,
20
M. Verderi,
20
S. Playfer,
21
D. Bettoni,
22a
C. Bozzi,
22a
R. Calabrese,
22a,22b
G. Cibinetto,
22a,22b
E. Fioravanti,
22a,22b
I. Garzia,
22a,22b
E. Luppi,
22a,22b
L. Piemontese,
22a
V. Santoro,
22a
R. Baldini-Ferroli,
23
A. Calcaterra,
23
R. de Sangro,
23
G. Finocchiaro,
23
S. Martellotti,
23
P. Patteri,
23
I. M. Peruzzi,
23
,
M. Piccolo,
23
M. Rama,
23
A. Zallo,
23
R. Contri,
24a,24b
E. Guido,
24a,24b
M. Lo Vetere,
24a,24b
M. R. Monge,
24a,24b
S. Passaggio,
24a
C. Patrignani,
24a,24b
E. Robutti,
24a
B. Bhuyan,
25
V. Prasad,
25
M. Morii,
26
A. Adametz,
27
U. Uwer,
27
H. M. Lacker,
28
P. D. Dauncey,
29
U. Mallik,
30
C. Chen,
31
J. Cochran,
31
W. T. Meyer,
31
S. Prell,
31
A. V. Gritsan,
32
N. Arnaud,
33
M. Davier,
33
D. Derkach,
33
G. Grosdidier,
33
F. Le Diberder,
33
A. M. Lutz,
33
B. Malaescu,
33
P. Roudeau,
33
A. Stocchi,
33
G. Wormser,
33
D. J. Lange,
34
D. M. Wright,
34
J. P. Coleman,
35
J. R. Fry,
35
E. Gabathuler,
35
D. E. Hutchcroft,
35
D. J. Payne,
35
C. Touramanis,
35
A. J. Bevan,
36
F. Di
Lodovico,
36
R. Sacco,
36
G. Cowan,
37
J. Bougher,
38
D. N. Brown,
38
C. L. Davis,
38
A. G. Denig,
39
M. Fritsch,
39
W. Gradl,
39
K. Griessinger,
39
A. Hafner,
39
E. Prencipe,
39
K. R. Schubert,
39
R. J. Barlow,
40
,
G. D. Lafferty,
40
E. Behn,
41
R. Cenci,
41
B. Hamilton,
41
A. Jawahery,
41
D. A. Roberts,
41
R. Cowan,
42
D. Dujmic,
42
G. Sciolla,
42
R. Cheaib,
43
P. M. Patel,
43
,*
S. H. Robertson,
43
P. Biassoni,
44a,44b
N. Neri,
44a
F. Palombo,
44a,44b
L. Cremaldi,
45
R. Godang,
45
P. Sonnek,
45
D. J. Summers,
45
M. Simard,
46
P. Taras,
46
G. De Nardo,
47a,47b
D. Monorchio,
47a,47b
G. Onorato,
47a,47b
C. Sciacca,
47a,47b
M. Martinelli,
48
G. Raven,
48
C. P. Jessop,
49
J. M. LoSecco,
49
K. Honscheid,
50
R. Kass,
50
J. Brau,
51
R. Frey,
51
N. B. Sinev,
51
D. Strom,
51
E. Torrence,
51
E. Feltresi,
52a,52b
M. Margoni,
52a,52b
M. Morandin,
52a
M. Posocco,
52a
M. Rotondo,
52a
G. Simi,
52a
F. Simonetto,
52a,52b
R. Stroili,
52a,52b
S. Akar,
53
E. Ben-Haim,
53
M. Bomben,
53
G. R. Bonneaud,
53
H. Briand,
53
G. Calderini,
53
J. Chauveau,
53
Ph. Leruste,
53
G. Marchiori,
53
J. Ocariz,
53
S. Sitt,
53
M. Biasini,
54a,54b
E. Manoni,
54a
S. Pacetti,
54a,54b
A. Rossi,
54a
C. Angelini,
55a,55b
G. Batignani,
55a,55b
S. Bettarini,
55a,55b
M. Carpinelli,
55a,55b
,**
G. Casarosa,
55a,55b
A. Cervelli,
55a,55b
F. Forti,
55a,55b
M. A. Giorgi,
55a,55b
A. Lusiani,
55a,55c
B. Oberhof,
55a,55b
E. Paoloni,
55a,55b
A. Perez,
55a
G. Rizzo,
55a,55b
J. J. Walsh,
55a
D. Lopes Pegna,
56
J. Olsen,
56
A. J. S. Smith,
56
R. Faccini,
57a,57b
F. Ferrarotto,
57a
F. Ferroni,
57a,57b
M. Gaspero,
57a,57b
L. Li Gioi,
57a
G. Piredda,
57a
C. Bünger,
58
O. Grünberg,
58
T. Hartmann,
58
T. Leddig,
58
C. Voß,
58
R. Waldi,
58
T. Adye,
59
E. O. Olaiya,
59
F. F. Wilson,
59
S. Emery,
60
G. Hamel de Monchenault,
60
G. Vasseur,
60
Ch. Yèche,
60
F. Anulli,
61
,
††
D. Aston,
61
D. J. Bard,
61
J. F. Benitez,
61
C. Cartaro,
61
M. R. Convery,
61
J. Dorfan,
61
G. P. Dubois-Felsmann,
61
W. Dunwoodie,
61
M. Ebert,
61
R. C. Field,
61
B. G. Fulsom,
61
A. M. Gabareen,
61
M. T. Graham,
61
C. Hast,
61
W. R. Innes,
61
P. Kim,
61
M. L. Kocian,
61
D. W. G. S. Leith,
61
P. Lewis,
61
D. Lindemann,
61
B. Lindquist,
61
S. Luitz,
61
V. Luth,
61
H. L. Lynch,
61
D. B. MacFarlane,
61
D. R. Muller,
61
H. Neal,
61
S. Nelson,
61
M. Perl,
61
T. Pulliam,
61
B. N. Ratcliff,
61
A. Roodman,
61
A. A. Salnikov,
61
R. H. Schindler,
61
A. Snyder,
61
D. Su,
61
M. K. Sullivan,
61
J. Va
vra,
61
A. P. Wagner,
61
W. F. Wang,
61
W. J. Wisniewski,
61
M. Wittgen,
61
D. H. Wright,
61
H. W. Wulsin,
61
V. Ziegler,
61
W. Park,
62
M. V. Purohit,
62
R. M. White,
62
,
‡‡
J. R. Wilson,
62
A. Randle-Conde,
63
S. J. Sekula,
63
M. Bellis,
64
P. R. Burchat,
64
T. S. Miyashita,
64
E. M. T. Puccio,
64
M. S. Alam,
65
J. A. Ernst,
65
R. Gorodeisky,
66
N. Guttman,
66
D. R. Peimer,
66
A. Soffer,
66
S. M. Spanier,
67
J. L. Ritchie,
68
A. M. Ruland,
68
R. F. Schwitters,
68
B. C. Wray,
68
J. M. Izen,
69
X. C. Lou,
69
F. Bianchi,
70a,70b
F. De Mori,
70a,70b
A. Filippi,
70a
D. Gamba,
70a,70b
S. Zambito,
70a,70b
L. Lanceri,
71a,71b
L. Vitale,
71a,71b
F. Martinez-Vidal,
72
A. Oyanguren,
72
P. Villanueva-Perez,
72
H. Ahmed,
73
J. Albert,
73
Sw. Banerjee,
73
F. U. Bernlochner,
73
H. H. F. Choi,
73
G. J. King,
73
R. Kowalewski,
73
M. J. Lewczuk,
73
T. Lueck,
73
I. M. Nugent,
73
J. M. Roney,
73
R. J. Sobie,
73
N. Tasneem,
73
T. J. Gershon,
74
P. F. Harrison,
74
T. E. Latham,
74
H. R. Band,
75
S. Dasu,
75
Y. Pan,
75
R. Prepost,
75
and S. L. Wu
75
(
B
A
B
AR
Collaboration)
1
Laboratoire d
Annecy-le-Vieux de Physique des Particules (LAPP), Université 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, Università 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
PHYSICAL REVIEW D
89,
112002 (2014)
1550-7998
=
2014
=
89(11)
=
112002(9)
112002-1
© 2014 American Physical Society
6
Ruhr Universität Bochum, Institut für Experimentalphysik 1, D-44780 Bochum, Germany
7
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
8
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
9a
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090, Russia
9b
Novosibirsk State University, Novosibirsk 630090, Russia
9c
Novosibirsk State Technical University, Novosibirsk 630092, Russia
10
University of California at Irvine, Irvine, California 92697, USA
11
University of California at Riverside, Riverside, California 92521, USA
12
University of California at Santa Barbara, Santa Barbara, California 93106, USA
13
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
14
California Institute of Technology, Pasadena, California 91125, USA
15
University of Cincinnati, Cincinnati, Ohio 45221, USA
16
University of Colorado, Boulder, Colorado 80309, USA
17
Colorado State University, Fort Collins, Colorado 80523, USA
18
Technische Universität Dortmund, Fakultät Physik, D-44221 Dortmund, Germany
19
Technische Universität Dresden, Institut für Kern- und Teilchenphysik, D-01062 Dresden, Germany
20
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
21
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
22a
INFN Sezione di Ferrara, I-44122 Ferrara, Italy
22b
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, I-44122 Ferrara, Italy
23
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
24a
INFN Sezione di Genova, I-16146 Genova, Italy
24b
Dipartimento di Fisica, Università di Genova, I-16146 Genova, Italy
25
Indian Institute of Technology Guwahati, Guwahati, Assam 781 039, India
26
Harvard University, Cambridge, Massachusetts 02138, USA
27
Universität Heidelberg, Physikalisches Institut, D-69120 Heidelberg, Germany
28
Humboldt-Universität zu Berlin, Institut für Physik, D-12489 Berlin, Germany
29
Imperial College London, London SW7 2AZ, United Kingdom
30
University of Iowa, Iowa City, Iowa 52242, USA
31
Iowa State University, Ames, Iowa 50011-3160, USA
32
Johns Hopkins University, Baltimore, Maryland 21218, USA
33
Laboratoire de l
Accélérateur Linéaire, IN2P3/CNRS et Université Paris-Sud 11,
Centre Scientifique d
Orsay, F-91898 Orsay Cedex, France
34
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
35
University of Liverpool, Liverpool L69 7ZE, United Kingdom
36
Queen Mary, University of London, London E1 4NS, United Kingdom
37
University of London, Royal Holloway and Bedford New College, Egham,
Surrey TW20 0EX, United Kingdom
38
University of Louisville, Louisville, Kentucky 40292, USA
39
Johannes Gutenberg-Universität Mainz, Institut für Kernphysik, D-55099 Mainz, Germany
40
University of Manchester, Manchester M13 9PL, United Kingdom
41
University of Maryland, College Park, Maryland 20742, USA
42
Massachusetts Institute of Technology, Laboratory for Nuclear Science,
Cambridge, Massachusetts 02139, USA
43
McGill University, Montréal, Québec, Canada H3A 2T8
44a
INFN Sezione di Milano, I-20133 Milano, Italy
44b
Dipartimento di Fisica, Università di Milano, I-20133 Milano, Italy
45
University of Mississippi, University, Mississippi 38677, USA
46
Université de Montréal, Physique des Particules, Montréal, Québec, Canada H3C 3J7
47a
INFN Sezione di Napoli, I-80126 Napoli, Italy
47b
Dipartimento di Scienze Fisiche, Università di Napoli Federico II, I-80126 Napoli, Italy
48
NIKHEF, National Institute for Nuclear Physics and High Energy Physics,
NL-1009 DB Amsterdam, The Netherlands
49
University of Notre Dame, Notre Dame, Indiana 46556, USA
50
Ohio State University, Columbus, Ohio 43210, USA
51
University of Oregon, Eugene, Oregon 97403, USA
52a
INFN Sezione di Padova, I-35131 Padova, Italy
52b
Dipartimento di Fisica, Università di Padova, I-35131 Padova, Italy
53
Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS,
Université Pierre et Marie Curie-Paris6, Université Denis Diderot-Paris7, F-75252 Paris, France
J. P. LEES
et al.
PHYSICAL REVIEW D
89,
112002 (2014)
112002-2
54a
INFN Sezione di Perugia, I-06123 Perugia, Italy
54b
Dipartimento di Fisica, Università di Perugia, I-06123 Perugia, Italy
55a
INFN Sezione di Pisa, I-56127 Pisa, Italy
55b
Dipartimento di Fisica, Università di Pisa, I-56127 Pisa, Italy
55c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
56
Princeton University, Princeton, New Jersey 08544, USA
57a
INFN Sezione di Roma, I-00185 Roma, Italy
57b
Dipartimento di Fisica, Università di Roma La Sapienza, I-00185 Roma, Italy
58
Universität Rostock, D-18051 Rostock, Germany
59
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
60
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
61
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
62
University of South Carolina, Columbia, South Carolina 29208, USA
63
Southern Methodist University, Dallas, Texas 75275, USA
64
Stanford University, Stanford, California 94305-4060, USA
65
State University of New York, Albany, New York 12222, USA
66
Tel Aviv University, School of Physics and Astronomy, Tel Aviv 69978, Israel
67
University of Tennessee, Knoxville, Tennessee 37996, USA
68
University of Texas at Austin, Austin, Texas 78712, USA
69
University of Texas at Dallas, Richardson, Texas 75083, USA
70a
INFN Sezione di Torino, I-10125 Torino, Italy
70b
Dipartimento di Fisica, Università di Torino, I-10125 Torino, Italy
71a
INFN Sezione di Trieste, I-34127 Trieste, Italy
71b
Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
72
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
73
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
74
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
75
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 27 January 2014; published 6 June 2014)
Evidence is presented for the baryonic
B
meson decay
̄
B
0
D
0
Λ
̄
Λ
based on a data sample of
471
×
10
6
B
̄
B
pairs collected with the
BABAR
detector at the PEP-II asymmetric
e
þ
e
collider located at the SLAC
National Accelerator Laboratory. The branching fraction is determined to be
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ¼
ð
9
.
8
þ
2
.
9
2
.
6

1
.
9
Þ
×
10
6
, corresponding to a significance of 3.4 standard deviations including additive
systematic uncertainties. A search for the related baryonic
B
meson decay
̄
B
0
D
0
Σ
0
̄
Λ
with
Σ
0
Λ
γ
is
performed and an upper limit
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
þ
̄
B
0
D
0
Λ
̄
Σ
0
Þ
<
3
.
1
×
10
5
is determined at 90%
confidence level.
DOI:
10.1103/PhysRevD.89.112002
PACS numbers: 13.25.Hw, 13.60.Rj, 14.20.Lq
I. INTRODUCTION
Little is known about the mechanism of baryon produc-
tion in weak decays or in the hadronization process. Baryons
are produced in
ð
6
.
8

0
.
6
Þ
%
of all
B
meson decays
[1]
.Due
to this large rate,
B
meson decays can provide important
information about baryon production. Due to the low energy
scale, perturbative quantum chromodynamics (QCD) can-
not be applied to this process. Furthermore, lattice QCD
calculations are not available. The description of baryonic
B
decays thus relies on phenomenological models.
Pole models
[2]
are a common tool used in theoretical
studies of hadronic decays. Meson pole models predict an
enhancement at low baryon-antibaryon masses. In many
three-body decays into a baryon, an antibaryon and a
meson, the baryon-antibaryon pair, can be described by a
meson pole, i.e., the decay of a virtual meson with a mass
below threshold. This leads to a steeply falling amplitude at
the threshold of the baryon-antibaryon mass and explains
the enhancement observed in decays such as
B
Λ
c
̄
p
π
[3,4]
,
B
p
̄
pK
[5
7]
, and
̄
B
0
D
0
p
̄
p
[8,9]
.
*
Deceased.
Present address: The University of Tabuk, Tabuk 71491,
Saudi Arabia.
Also at Università di Perugia, Dipartimento di Fisica, Perugia,
Italy.
§
Present address: Laboratoire de Physique Nucláire et de
Hautes Energies, IN2P3/CNRS, Paris, France.
Present address: The University of Huddersfield, Hudders-
field HD1 3DH, United Kingdom.
Present address: University of South Alabama, Mobile,
Alabama 36688, USA.
**
Also at Università di Sassari, Sassari, Italy.
††
Also at INFN Sezione di Roma, Roma, Italy.
‡‡
Present address: Universidad Técnica Federico Santa Maria,
Valparaiso, Chile 2390123.
EVIDENCE FOR THE BARYONIC DECAY
...
PHYSICAL REVIEW D
89,
112002 (2014)
112002-3
In addition to the meson pole models described above,
there are baryon pole models in which the initial state
decays through the strong interaction into a pair of baryons.
Then, one of these baryons decays via the weak interaction
into a baryon and a meson. For such baryon pole models,
no enhancement at threshold in the dibaryon invariant mass
is expected.
The decay of a
B
meson into a
D
0
meson and a pair of
baryons has been the subject of several theoretical inves-
tigations
[10,11]
. Reference
[11]
predicts the branching
fractions for
̄
B
0
D
0
Λ
̄
Λ
decays and for the sum of the
̄
B
0
D
0
Λ
̄
Σ
0
and
̄
B
0
D
0
Σ
0
̄
Λ
decays to be
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ¼ð
2

1
Þ
×
10
6
;
B
ð
̄
B
0
D
0
Λ
̄
Σ
0
þ
̄
B
0
D
0
Σ
0
̄
Λ
Þ¼ð
1
.
8

0
.
5
Þ
×
10
5
:
ð
1
Þ
It is impractical to separate the
̄
B
0
D
0
Λ
̄
Σ
0
and
̄
B
0
D
0
Σ
0
̄
Λ
decays since each leads to the final
state
Λ
̄
Λ
γ
.
As can be seen from the Feynman diagrams shown in
Fig.
1
, the only difference between the
̄
B
0
D
0
p
̄
p
decay
on the one hand and the
̄
B
0
D
0
Λ
̄
Λ
and
̄
B
0
D
0
Σ
0
̄
Λ
decays on the other hand is the replacement of a
u
̄
u
pair with an
s
̄
s
pair. In the hadronization process,
s
̄
s
-pair production is suppressed by about a factor of three
compared to
u
̄
u
-or
d
̄
d
-pair production
[12]
. Furthermore,
since both
Λ
and
Σ
0
baryons can be produced, there are four
possible final states with an
s
̄
s
pair (
Λ
̄
Λ
,
Λ
̄
Σ
0
,
Σ
0
̄
Λ
, and
Σ
0
̄
Σ
0
) compared to only one for a
u
̄
u
pair (
p
̄
p
), neglecting
the production of excited baryons. Assuming equal pro-
duction rates for these four modes and that the spin-
1
=
2
states dominate, a suppression of a factor of
12
is
expected for
̄
B
0
D
0
Λ
̄
Λ
decays compared to
̄
B
0
D
0
p
̄
p
decays, where the branching fraction of the latter
process is
B
ð
̄
B
0
D
0
p
̄
p
Þ¼ð
1
.
04

0
.
04
Þ
×
10
4
[1]
.
The branching fraction for
̄
B
0
D
0
Λ
̄
Λ
has been mea-
sured by the Belle Collaboration to be
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ¼
ð
10
.
5
þ
5
.
7
4
.
4

1
.
4
Þ
×
10
6
[13]
. There are no previous results
for the
̄
B
0
D
0
Σ
0
̄
Λ
decay mode.
II. THE
BABAR
EXPERIMENT
This analysis is based on a data sample of
429
fb
1
[14]
,
corresponding to
471
×
10
6
B
̄
B
pairs, collected with the
BABAR
detector at the PEP-II asymmetric-energy
e
þ
e
collider at the SLAC National Accelerator Laboratory at
center-of-mass energies near and equal to the
Υ
ð
4
S
Þ
mass.
The reconstruction efficiency is determined through use of
Monte Carlo (MC) simulation, based on the E
VT
G
EN
[15]
program for the event generation and the GEANT4
[16]
package for modeling of the detector response. The MC
events are generated uniformly in the
̄
B
0
D
0
Λ
̄
Λ
and
̄
B
0
D
0
Σ
0
̄
Λ
phase space.
The
BABAR
detector is described in detail elsewhere
[17,18]
. Charged particle trajectories are measured with a
five-layer double-sided silicon vertex tracker and a 40-layer
drift chamber immersed in a 1.5 T axial magnetic field.
Charged particle identification is provided by ionization
energy measurements in the tracking chambers and by
Cherenkov-radiation photons recorded with an internally
reflecting ring-imaging detector. Electrons and photons are
reconstructed with an electromagnetic calorimeter.
III. RECONSTRUCTION OF
Λ
BARYON,
D
0
MESON, AND
̄
B
0
MESON CANDIDATES
We reconstruct
Λ
baryons through the decay mode
Λ
p
π
and
D
0
mesons through the modes
D
0
K
π
þ
,
D
0
K
π
þ
π
þ
π
, and
D
0
K
π
þ
π
0
[19]
. Charged kaon
and proton candidates are required to satisfy particle
identification criteria. Charged pions are selected as
charged tracks that are not identified as a kaon or proton.
Candidate
π
0
mesons are reconstructed from two sepa-
rated energy deposits in the electromagnetic calorimeter not
associated with charged tracks. To discriminate against
neutral hadrons, the shower shape of each deposit is required
to be consistent with that of a photon
[20]
. Furthermore, we
require
E
ð
γ
1
Þ
>
0
.
125
GeV and
E
ð
γ
2
Þ
>
0
.
04
GeV, where
E
ð
γ
1
Þ
and
E
ð
γ
2
Þ
are the energies of the photon candidates,
with
E
ð
γ
1
Þ
>E
ð
γ
2
Þ
. The photon-photon invariant mass is
required to lie in the range
m
ð
γγ
Þ
½
0
.
116
;
0
.
145

GeV
=
c
2
.
The
Λ
daughters are fit to a common vertex and the
reconstructed mass is required to lie within three standard
FIG. 1 (color online). Leading-order Feynman diagrams for the decays
̄
B
0
D
0
N
̄
N
. Setting
q
¼
u
leads to the
D
0
p
̄
p
final state and
setting
q
¼
s
to the
D
0
Λ
̄
Λ
,
D
0
Σ
0
̄
Λ
,
D
0
Λ
̄
Σ
0
, and
D
0
Σ
0
̄
Σ
0
final states.
J. P. LEES
et al.
PHYSICAL REVIEW D
89,
112002 (2014)
112002-4
deviations of the nominal value
[1]
, where the standard
deviation is the mass resolution. We select
Λ
candidates by
requiring the flight significance
L
t
=
σ
L
t
to exceed 4, where
L
t
is the
Λ
flight length in the transverse plane and
σ
L
t
its
uncertainty. The
Σ
0
baryons are produced in the decay
Σ
0
Λ
γ
, and the photon is not reconstructed.
The
D
0
daughter candidates are fit to a common vertex,
and the reconstructed mass is required to lie within three
times the mass resolution from their nominal values
[1]
.
The signal-to-background ratio for
D
0
K
π
þ
π
0
is
improved by making use of the resonant substructure of
this decay, which is well known. Using results from the
E691 Collaboration
[21]
, we calculate the probability
w
Dalitz
for a
D
0
candidate to be located at a certain position in
the Dalitz plane. We require
w
Dalitz
>
0
.
02
. Figure
2
shows
the Dalitz plot distributions, based on simulation, for
candidates selected with and without the
w
Dalitz
requirement.
The
D
0
and
Λ
candidates are constrained to their
nominal masses in the reconstruction of the
̄
B
0
candidates.
We apply a fit to the entire decay chain and require the
probability for the vertex fit to be larger than 0.001.
To reduce background from
e
þ
e
q
̄
q
events with
q
¼
u
,
d
,
s
,
c
, we apply a selection on a Fisher discriminant
F
that combines the values of
j
cos
θ
Thr
j
, where
θ
Thr
is the
angle between the thrust axis of the
B
candidate and the
thrust axis formed from the remaining tracks and clusters in
the event;
j
cos
θ
z
j
, where
θ
z
is the angle between the
B
thrust axis and the beam axis;
j
cos
φ
j
, where
φ
is the angle
between the
B
momentum and the beam axis; and the
normalized second Fox Wolfram moment
[22]
. All these
quantities are defined in the center-of-mass frame. All
selection criteria are summarized in Table
I
.
IV. FIT STRATEGY
We determine the number of signal candidates with a
two-dimensional unbinned extended maximum likelihood
fit to the invariant mass
m
ð
D
0
Λ
̄
Λ
Þ
and the energy sub-
stituted mass
m
ES
. The latter is defined as
m
ES
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð
s=
2
þ
p
0
·
p
B
Þ
2
=E
2
0
j
p
j
2
B
q
;
ð
2
Þ
where
ffiffiffi
s
p
is the center-of-mass energy,
p
B
the
B
candidate
s
momentum, and
ð
E
0
;
p
0
Þ
the four-momentum vector of the
e
þ
e
system, each given in the laboratory frame. Both
m
ð
D
0
Λ
̄
Λ
Þ
and
m
ES
are centered at the
B
mass for well-
reconstructed
B
decays.
Due to the small mass difference of
76
.
9
MeV
=
c
2
[1]
between the
Λ
and
Σ
0
baryons,
̄
B
0
D
0
Σ
0
̄
Λ
decays,
where the
Σ
0
decays radiatively as
Σ
0
Λ
γ
, are a source
of background. Such events peak at the
B
mass in
m
ES
and
are slightly shifted in
m
ð
D
0
Λ
̄
Λ
Þ
with respect to
̄
B
0
D
0
Λ
̄
Λ
(Fig.
3
). We account for this decay by including an
explicit term in the likelihood function (see below), whose
yield is determined in the fit.
We divide the data sample into three subsamples
corresponding to the
D
0
decay modes. Given their different
signal-to-background ratios, we determine the number of
signal candidates in a simultaneous fit to the three inde-
pendent subsamples. We study simulated samples of signal
and background events and find no significant correlation
between
m
ES
and
m
ð
D
0
Λ
̄
Λ
Þ
. Therefore, we describe each
̄
B
0
D
0
Λ
̄
Λ
signal sample with the product of a
Novosibirsk function in
m
ES
and a sum of two Gaussian
functions
f
GG
in
m
ð
D
0
Λ
̄
Λ
Þ
. The Novosibirsk function is
defined as
f
Novo
ð
m
ES
Þ¼
exp

1
2

ln
2
½
1
þ
λα
ð
m
ES
μ
Þ
α
2
þ
α
2

;
λ
¼
sinh
ð
α
ffiffiffiffiffiffiffi
ln
4
p
Þ
=
ð
σα
ffiffiffiffiffiffiffi
ln
4
p
Þ
;
ð
3
Þ
with
μ
the mean value,
σ
the width, and
α
the tail parameter.
The decay
̄
B
0
D
0
Σ
0
̄
Λ
is described by the product of a
Novosibirsk
f
Novo
1
;
Σ
0
function in
m
ES
and a sum of another
Novosibirsk function
f
Novo
2
;
Σ
0
and a Gaussian
G
Σ
0
in
)
4
/c
2
(GeV
2
0
π
π
m
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8
2
)
4
/c
2
(GeV
2
π
K
m
0.5
1
1.5
2
2.5
3
+
ρ
-
K*(892)
0
K*(892)
+
ρ
FIG. 2 (color online). Dalitz plot for simulated
D
0
K
π
þ
π
0
events before (gray stars) and after (black crosses) the
w
Dalitz
>
0
.
02
requirement. Resonant decays are indicated.
TABLE I. Summary of selection criteria.
Selection criterion
Selected candidates
Λ
=
̄
Λ
mass
m
p
π
½
1
.
112
;
1
.
120

GeV
=
c
2
Flight significance
L
t
=
σ
L
t
>
4
D
0
K
π
mass
m
K
π
½
1
.
846
;
1
.
882

GeV
=
c
2
D
0
K
π
þ
π
þ
π
mass
m
K
πππ
½
1
.
852
;
1
.
876

GeV
=
c
2
Lateral parameter
γ
1
0
.
05
<
LAT
ð
γ
1
Þ
<
0
.
55
Lateral parameter
γ
2
LAT
ð
γ
2
Þ
>
0
.
075
Calorimeter energy
γ
1
E
ð
γ
1
Þ
>
0
.
125
GeV
Calorimeter energy
γ
2
E
ð
γ
2
Þ
>
0
.
04
GeV
π
0
mass
m
γγ
½
0
.
116
;
0
.
145

GeV
=
c
2
D
0
K
π
þ
π
0
mass
m
K
ππ
0
½
1
.
81
;
1
.
89

GeV
=
c
2
Dalitz weight
w
Dalitz
>
0
.
02
B
vertex probability
p
ð
B
Þ
>
0
.
001
Fisher discriminant
F
>
0
.
1
EVIDENCE FOR THE BARYONIC DECAY
...
PHYSICAL REVIEW D
89,
112002 (2014)
112002-5
m
ð
D
0
Λ
̄
Λ
Þ
. All parameters are determined using Monte
Carlo simulated events and are fixed in the final fit.
Background from
e
þ
e
q
̄
q
events and other
B
meson
decays is modeled by the product of an ARGUS function
[23]
in
m
ES
and a first order polynomial in
m
ð
D
0
Λ
̄
Λ
Þ
.
The full fit function is defined as
f
Fit
j
¼
f
Λ
j
þ
f
Σ
0
j
þ
f
Bkg
j
¼
f
Novo
;
Λ
j
ð
m
ES
Þ
×
f
GG
j
ð
m
ð
D
0
Λ
̄
Λ
ÞÞþ
f
Novo
1
;
Σ
0
j
ð
m
ES
Þ
×
½
f
Novo
2
;
Σ
0
j
ð
m
ð
D
0
Λ
̄
Λ
ÞÞþ
G
Σ
0
j
ð
m
ð
D
0
Λ
̄
Λ
ÞÞ
þ
f
ARGUS
j
ð
m
ES
Þ
×
f
Poly
j
ð
m
ð
D
0
Λ
̄
Λ
ÞÞ
;
ð
4
Þ
where the index
j
corresponds to the three
D
0
decay modes.
The branching fraction is determined from
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ¼
N
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
2
N
B
0
̄
B
0
×
̄
ε
×
1
B
ð
Λ
p
π
Þ
2
B
ð
D
0
X
Þ
;
ð
5
Þ
where
N
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
is the fitted signal yield,
N
B
0
̄
B
0
the
number of the
B
0
̄
B
0
pairs assuming
B
ð
Υ
ð
4
S
Þ
4
S
B
0
̄
B
0
Þ¼
0
.
5
,
̄
ε
the average reconstruction efficiency, and
B
ð
Λ
p
π
Þ
and
B
ð
D
0
X
Þ
the branching fractions for the
daughter decays of
Λ
and
D
0
, respectively. An analogous
expression holds for
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
Þ
. The average effi-
ciency
̄
ε
is defined as
N
rec
=N
gen
using signal MC events,
where
N
rec
is the number of reconstructed signal events
after all cuts and
N
gen
the number of all generated events
assuming a phase space distribution.
We perform a simultaneous fit of the three
D
0
decay
channels to obtain
N
Λ
¼
N
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
̄
ε
Λ
B
ð
D
0
X
Þ
;
N
Σ
0
¼
N
ð
̄
B
0
D
0
Σ
0
̄
Λ
Þ
̄
ε
Σ
0
B
ð
D
0
X
Þ
:
ð
6
Þ
The likelihood function is given by
L
¼
Y
j
e
ð
̄
ε
Λ
j
B
j
N
Λ
þ
N
Bkg
j
þ
̄
ε
Σ
0
j
B
j
N
Σ
0
Þ
N
ð
j
Þ
!
×
Y
N
ð
j
Þ
k
½
̄
ε
Λ
j
B
j
N
Λ
f
Λ
j
ð
m
ES
k
;m
ð
D
0
Λ
̄
Λ
Þ
k
Þ
þ
N
Bkg
j
f
Bkg
j
ð
m
ES
k
;m
ð
D
0
Λ
̄
Λ
Þ
k
Þ
þ
̄
ε
Σ
0
j
B
j
N
Σ
0
f
Σ
0
j
ð
m
ES
k
;m
ð
D
0
Λ
̄
Λ
Þ
k
Þ
;
ð
7
Þ
where
B
j
is the branching fraction for the
j
th
D
0
decay,
N
Bkg
j
the number of combinatorial background events in the
j
th subsample,
N
Λ
and
N
Σ
0
the yields of
̄
B
0
D
0
Λ
̄
Λ
and
̄
B
0
D
0
Σ
0
̄
Λ
, and
̄
ε
Λ
j
and
̄
ε
Σ
0
j
the average efficiencies for
the
j
th
D
0
decay.
V. SYSTEMATIC UNCERTAINTIES
We consider the following systematic uncertainties: the
uncertainties associated with the number of
B
̄
B
events,
the particle identification (PID) algorithm, the tracking
algorithm, the
π
0
reconstruction, the
D
0
and
Λ
branching
fractions, the efficiency correction, and the fitting algorithm.
The uncertainty associated with the number of
B
̄
B
pairs
is 0.6%. We determine the systematic uncertainty associ-
ated with the PID by applying different PID selections
and comparing the result with the nominal selection. The
difference is 0.8%, which is assigned as the PID uncer-
tainty. The systematic uncertainty associated with the
tracking algorithm depends on the number of charged
)
2
(GeV/c
ES
m
5.25
5.26
5.27
5.28
5.29
)
2
) (GeV/c
Λ
Λ
0
m(D
5.2
5.25
5.3
5.35
0
50
100
150
200
250
)
2
(GeV/c
ES
m
5.25
5.26
5.27
5.28
5.29
)
2
) (GeV/c
Λ
Λ
0
m(D
5.2
5.25
5.3
5.35
0
2
4
6
8
10
12
14
16
18
20
FIG. 3 (color online). Distributions for
̄
B
0
D
0
Λ
̄
Λ
(left) and
̄
B
0
D
0
Σ
0
̄
Λ
reconstructed as
̄
B
0
D
0
Λ
̄
Λ
(right) for the
D
0
K
π
þ
mode in simulated events.
J. P. LEES
et al.
PHYSICAL REVIEW D
89,
112002 (2014)
112002-6
tracks in the decay. We assign a systematic uncertainty of
0.9% for the
D
0
K
π
þ
and
D
0
K
π
þ
π
0
decays and
1.2% for the
D
0
K
π
þ
π
þ
π
decay. A 3% uncertainty is
assigned to account for the
π
0
reconstruction in
D
0
K
π
þ
π
0
decays. A detailed description of these detector-
related systematic uncertainties is given in Ref.
[18]
.
We rely on the known
D
0
branching fractions in our fit.
To estimate the associated systematic uncertainty we vary
each branching fraction by one standard deviation of its
uncertainty
[1]
and define the systematic uncertainty to be
the maximum deviation observed with respect to the
nominal analysis. We divide
m
ð
Λ
̄
Λ
Þ
into six bins and
determine the total reconstruction efficiency
ε
i
in each bin.
We determine the uncertainty due to the use of the average
efficiency
̄
ε
by studying
j
ε
i
̄
ε
j
=
̄
ε
as a function of
m
ð
Λ
̄
Λ
Þ
.
We average these values and take the result of 16.3%
(
D
0
K
π
þ
), 19.6% (
D
0
K
π
þ
π
0
), and 16.8%
ð
D
0
K
π
þ
π
þ
π
Þ
as our estimate of the systematic uncertainty
for the efficiency. We estimate the systematic uncertainty
due to the fit procedure by independently varying the fit
ranges of
m
ES
and
m
ð
D
0
Λ
̄
Λ
Þ
. The largest differences in the
signal yield are 3.9% for the change of the
m
ES
fit range and
2.1% for the change of the
m
ð
D
0
Λ
̄
Λ
Þ
fit range. To check
our background model, we use a second-order polynomial
in
m
ð
D
0
Λ
̄
Λ
Þ
instead of a first-order polynomial. The signal
yield changes by 1.1%. We use an ensemble of simulated
data samples reflecting our fit results to verify the stability
of the fit. We generate 1000 such samples with shapes and
yields fixed to our results and repeat the final fit. We find no
bias in the signal-yield results. All systematic uncertainties
are summarized in Table
II
.
The total systematic uncertainty, obtained by adding all
sources in quadrature, is 20.1%.
VI. RESULTS
The one-dimensional projections of the fit are shown in
Fig.
4
. We find
N
Λ
¼
1880
þ
560
500
;
N
Σ
0
¼
2870
þ
1680
1560
:
ð
8
Þ
The statistical significance is calculated as
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
log
L
0
=L
S
p
,
where
L
0
is the likelihood value for a fit without a signal
component and
L
S
is the likelihood value for the nominal fit.
The statistical significance of the combined
̄
B
0
D
0
Λ
̄
Λ
and
̄
B
0
D
0
Σ
0
̄
Λ
yields is 3.9 standard deviations (
σ
), while
those of the individual
̄
B
0
D
0
Λ
̄
Λ
and
̄
B
0
D
0
Σ
0
̄
Λ
results are
3
.
4
σ
and
1
.
2
σ
, respectively. Multiplicative
systematic uncertainties do not affect the signal significance.
Additive systematic uncertainties affecting the significance
are negligible in this analysis compared to the statistical
uncertainty. We therefore quote the statistical significance as
the global significance.
The branching fractions are
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ¼ð
9
.
8
þ
2
.
9
2
.
6

1
.
9
Þ
×
10
6
;
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
þ
̄
B
0
D
0
Λ
̄
Σ
0
Þ¼ð
15
þ
9
8

3
Þ
×
10
6
;
ð
9
Þ
where the first uncertainties represent the statistical uncer-
tainties and the second the systematic uncertainties. As a
cross-check of the method, independent fits to the three
subsamples are performed. The results of each of these fits
are consistent with each other and with the nominal
combined fit.
Since the statistical significance for
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
þ
̄
B
0
D
0
Λ
̄
Σ
0
Þ
is low, a Bayesian upper limit at the 90%
confidence level is calculated by integrating the likelihood
function,
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
þ
̄
B
0
D
0
Λ
̄
Σ
0
Þ
<
3
.
1
×
10
5
:
ð
10
Þ
To investigate the threshold dependence, we perform the
fit in bins of
m
ð
Λ
̄
Λ
Þ
and examine the resulting distribution
after accounting for the reconstruction efficiency and
D
0
branching fractions. The results are shown in Fig.
5
.No
significant enhancement in the
̄
B
0
D
0
Λ
̄
Λ
event rate is
observed at the baryon-antibaryon mass threshold within
the uncertainties, in contrast to
̄
B
0
D
0
p
̄
p
decays, which
do exhibit such an enhancement
[8]
.
We compare our results for the
̄
B
0
D
0
Λ
̄
Λ
and
̄
B
0
D
0
Σ
0
̄
Λ
branching fractions to theoretical predictions. The
result we obtain for the
̄
B
0
D
0
Σ
0
̄
Λ
branching fraction is
consistent with the prediction of
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
þ
̄
B
0
D
0
Λ
̄
Σ
0
Þ¼ð
18

5
Þ
×
10
6
from Ref.
[11]
. However, the
obtained result for the
̄
B
0
D
0
Λ
̄
Λ
branching fraction is
larger than the prediction of
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ¼ð
2

1
Þ
×
10
6
[11]
by a factor of
TABLE II. Summary of the systematic uncertainties for
̄
B
0
D
0
Λ
̄
Λ
.
Source
Relative uncertainty
Additive uncertainty
Fit procedure
4.6%
Multiplicative uncertainties
B
̄
B
counting
0.6%
Particle identification
0.8%
Tracking
D
0
K
π
þ
0.9%
D
0
K
π
þ
π
0
0.9%
D
0
K
π
þ
π
þ
π
1.2%
π
0
systematics
D
0
K
π
þ
π
0
3.0%
D
0
and
Λ
branching fractions
2.9%
Variation over phase space
D
0
K
π
þ
16.3%
D
0
K
π
þ
π
0
19.6%
D
0
K
π
þ
π
þ
π
16.8%
Total uncertainty
20.1%
EVIDENCE FOR THE BARYONIC DECAY
...
PHYSICAL REVIEW D
89,
112002 (2014)
112002-7
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
exp
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
theo
¼
4
.
9

3
.
0
:
ð
11
Þ
We further determine
B
ð
̄
B
0
D
0
Σ
0
̄
Λ
þ
̄
B
0
D
0
Λ
̄
Σ
0
Þ
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
¼
1
.
5

0
.
9
;
ð
12
Þ
which is in agreement with our assumption that all four
modes
̄
B
0
D
0
Λ
̄
Λ
,
̄
B
0
D
0
Σ
0
̄
Λ
,
̄
B
0
D
0
Λ
̄
Σ
0
, and
̄
B
0
D
0
Σ
0
̄
Σ
0
are produced at equal rates. For the ratio
of branching fractions, we find
B
ð
̄
B
0
D
0
Λ
̄
Λ
Þ
B
ð
̄
B
0
D
0
p
̄
p
Þ
¼
1
10
.
6

3
.
7
;
ð
13
Þ
using
B
ð
̄
B
0
D
0
p
̄
p
Þ¼ð
1
.
04

0
.
04
Þ
×
10
4
[1]
. This is
in agreement with the expected suppression of
1
=
12
discussed in the Introduction.
)
2
) (GeV/c
Λ
Λ
m(
2.4
2.6
2.8
3
3.2
3.4
)
2
Events / (0.2 GeV/c
0
200
400
600
800
FIG. 5 (color online). Distribution of the invariant baryon-
antibaryon mass for
D
0
-branching-fraction and efficiency-
corrected
̄
B
0
D
0
Λ
̄
Λ
signal candidates. The data points
represent the
BABAR
data and the shaded histogram indicates
phase-space-distributed simulated events, scaled to match the
area under the data.
)
2
(GeV/c
ES
m
5.2
5.21
5.22
5.23
5.24
5.25
5.26
5.27
5.28
5.29
)
2
Events / ( 3.6 MeV/c
0
2
4
6
8
10
12
14
)
2
) (GeV/c
Λ
Λ
0
m(D
5.15
5.2
5.25
5.3
5.35
5.4
)
2
Events / ( 10 MeV/c
0
1
2
3
4
5
6
7
)
2
(GeV/c
ES
m
5.2
5.21
5.22
5.23
5.24
5.25
5.26
5.27
5.28
5.29
)
2
Events / ( 3.6 MeV/c
0
10
20
30
40
50
60
70
)
2
) (GeV/c
Λ
Λ
0
m(D
5.15
5.2
5.25
5.3
5.35
5.4
)
2
Events / ( 10 MeV/c
0
2
4
6
8
10
12
14
16
18
)
2
(GeV/c
ES
m
5.2
5.21
5.22
5.23
5.24
5.25
5.26
5.27
5.28
5.29
)
2
Events / ( 3.6 MeV/c
0
10
20
30
40
50
)
2
) (GeV/c
Λ
Λ
0
m(D
5.15
5.2
5.25
5.3
5.35
5.4
)
2
Events / ( 10 MeV/c
0
2
4
6
8
10
12
14
16
FIG. 4 (color online). Results of the combined fit. The
m
ES
projection is shown for
m
ð
D
0
Λ
̄
Λ
Þ
½
5
.
15
;
5
.
31

GeV
=
c
2
and the
m
ð
D
0
Λ
̄
Λ
Þ
projection for
m
ES
½
5
.
272
;
5
.
286

GeV
=
c
2
. The solid line shows the result of the fit, the dashed curve indicates the
̄
B
0
D
0
Σ
0
̄
Λ
contribution, and the shaded histogram the combinatorial background. From top to bottom:
D
0
K
π
þ
,
D
0
K
π
þ
π
0
,
and
D
0
K
π
þ
π
þ
π
subsamples.
J. P. LEES
et al.
PHYSICAL REVIEW D
89,
112002 (2014)
112002-8