B
A
B
AR
-PUB-14/004
SLAC-PUB-16118
Observation of the baryonic decay
B
0
→
Λ
+
c
pK
−
K
+
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
E. Grauges,
2
A. Palano
ab
,
3
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
abc
,
9
A. R. Buzykaev
a
,
9
V. P. Druzhinin
ab
,
9
V. B. Golubev
ab
,
9
E. A. Kravchenko
ab
,
9
A. P. Onuchin
abc
,
9
S. I. Serednyakov
ab
,
9
Yu. I. Skovpen
ab
,
9
E. P. Solodov
ab
,
9
K. Yu. Todyshev
ab
,
9
A. J. Lankford,
10
M. Mandelkern,
10
B. Dey,
11
J. W. Gary,
11
O. Long,
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
W. Panduro Vazquez,
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
T. S. Miyashita,
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
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17,
∗
W. H. Toki,
17
B. Spaan,
18
D. Bernard,
19
M. Verderi,
19
S. Playfer,
20
D. Bettoni
a
,
21
C. Bozzi
a
,
21
R. Calabrese
ab
,
21
G. Cibinetto
ab
,
21
E. Fioravanti
ab
,
21
I. Garzia
ab
,
21
E. Luppi
ab
,
21
L. Piemontese
a
,
21
V. Santoro
a
,
21
A. Calcaterra,
22
R. de Sangro,
22
G. Finocchiaro,
22
S. Martellotti,
22
P. Patteri,
22
I. M. Peruzzi,
22,
†
M. Piccolo,
22
M. Rama,
22
A. Zallo,
22
R. Contri
ab
,
23
M. Lo Vetere
ab
,
23
M. R. Monge
ab
,
23
S. Passaggio
a
,
23
C. Patrignani
ab
,
23
E. Robutti
a
,
23
B. Bhuyan,
24
V. Prasad,
24
A. Adametz,
25
U. Uwer,
25
H. M. Lacker,
26
P. D. Dauncey,
27
U. Mallik,
28
C. Chen,
29
J. Cochran,
29
S. Prell,
29
H. Ahmed,
30
A. V. Gritsan,
31
N. Arnaud,
32
M. Davier,
32
D. Derkach,
32
G. Grosdidier,
32
F. Le Diberder,
32
A. M. Lutz,
32
B. Malaescu,
32,
‡
P. Roudeau,
32
A. Stocchi,
32
G. Wormser,
32
D. J. Lange,
33
D. M. Wright,
33
J. P. Coleman,
34
J. R. Fry,
34
E. Gabathuler,
34
D. E. Hutchcroft,
34
D. J. Payne,
34
C. Touramanis,
34
A. J. Bevan,
35
F. Di Lodovico,
35
R. Sacco,
35
G. Cowan,
36
J. Bougher,
37
D. N. Brown,
37
C. L. Davis,
37
A. G. Denig,
38
M. Fritsch,
38
W. Gradl,
38
K. Griessinger,
38
A. Hafner,
38
K. R. Schubert,
38
R. J. Barlow,
39,
§
G. D. Lafferty,
39
R. Cenci,
40
B. Hamilton,
40
A. Jawahery,
40
D. A. Roberts,
40
R. Cowan,
41
G. Sciolla,
41
R. Cheaib,
42
P. M. Patel,
42,
¶
S. H. Robertson,
42
N. Neri
a
,
43
F. Palombo
ab
,
43
L. Cremaldi,
44
R. Godang,
44,
∗∗
P. Sonnek,
44
D. J. Summers,
44
M. Simard,
45
P. Taras,
45
G. De Nardo
ab
,
46
G. Onorato
ab
,
46
C. Sciacca
ab
,
46
M. Martinelli,
47
G. Raven,
47
C. P. Jessop,
48
J. M. LoSecco,
48
K. Honscheid,
49
R. Kass,
49
E. Feltresi
ab
,
50
M. Margoni
ab
,
50
M. Morandin
a
,
50
M. Posocco
a
,
50
M. Rotondo
a
,
50
G. Simi
ab
,
50
F. Simonetto
ab
,
50
R. Stroili
ab
,
50
S. Akar,
51
E. Ben-Haim,
51
M. Bomben,
51
G. R. Bonneaud,
51
H. Briand,
51
G. Calderini,
51
J. Chauveau,
51
Ph. Leruste,
51
G. Marchiori,
51
J. Ocariz,
51
M. Biasini
ab
,
52
E. Manoni
a
,
52
S. Pacetti
ab
,
52
A. Rossi
a
,
52
C. Angelini
ab
,
53
G. Batignani
ab
,
53
S. Bettarini
ab
,
53
M. Carpinelli
ab
,
53,
††
G. Casarosa
ab
,
53
A. Cervelli
ab
,
53
M. Chrzaszcz
ab
,
53
F. Forti
ab
,
53
M. A. Giorgi
ab
,
53
A. Lusiani
ac
,
53
B. Oberhof
ab
,
53
E. Paoloni
ab
,
53
A. Perez
a
,
53
G. Rizzo
ab
,
53
J. J. Walsh
a
,
53
D. Lopes Pegna,
54
J. Olsen,
54
A. J. S. Smith,
54
R. Faccini
ab
,
55
F. Ferrarotto
a
,
55
F. Ferroni
ab
,
55
M. Gaspero
ab
,
55
L. Li Gioi
a
,
55
A. Pilloni
ab
,
55
G. Piredda
a
,
55
C. Bünger,
56
S. Dittrich,
56
O. Grünberg,
56
M. Hess,
56
T. Leddig,
56
C. Voß,
56
R. Waldi,
56
T. Adye,
57
E. O. Olaiya,
57
F. F. Wilson,
57
S. Emery,
58
G. Vasseur,
58
F. Anulli,
59,
‡‡
D. Aston,
59
D. J. Bard,
59
C. Cartaro,
59
M. R. Convery,
59
J. Dorfan,
59
G. P. Dubois-Felsmann,
59
W. Dunwoodie,
59
M. Ebert,
59
R. C. Field,
59
B. G. Fulsom,
59
M. T. Graham,
59
C. Hast,
59
W. R. Innes,
59
P. Kim,
59
D. W. G. S. Leith,
59
P. Lewis,
59
D. Lindemann,
59
S. Luitz,
59
V. Luth,
59
H. L. Lynch,
59
D. B. MacFarlane,
59
D. R. Muller,
59
H. Neal,
59
M. Perl,
59
T. Pulliam,
59
B. N. Ratcliff,
59
A. Roodman,
59
A. A. Salnikov,
59
R. H. Schindler,
59
A. Snyder,
59
D. Su,
59
M. K. Sullivan,
59
J. Va’vra,
59
W. J. Wisniewski,
59
H. W. Wulsin,
59
M. V. Purohit,
60
R. M. White,
60,
§§
J. R. Wilson,
60
A. Randle-Conde,
61
S. J. Sekula,
61
M. Bellis,
62
P. R. Burchat,
62
E. M. T. Puccio,
62
M. S. Alam,
63
J. A. Ernst,
63
R. Gorodeisky,
64
N. Guttman,
64
D. R. Peimer,
64
A. Soffer,
64
S. M. Spanier,
65
J. L. Ritchie,
66
A. M. Ruland,
66
R. F. Schwitters,
66
B. C. Wray,
66
J. M. Izen,
67
X. C. Lou,
67
F. Bianchi
ab
,
68
F. De Mori
ab
,
68
A. Filippi
a
,
68
D. Gamba
ab
,
68
L. Lanceri
ab
,
69
L. Vitale
ab
,
69
F. Martinez-Vidal,
70
A. Oyanguren,
70
P. Villanueva-Perez,
70
J. Albert,
71
Sw. Banerjee,
71
A. Beaulieu,
71
F. U. Bernlochner,
71
H. H. F. Choi,
71
G. J. King,
71
R. Kowalewski,
71
M. J. Lewczuk,
71
T. Lueck,
71
I. M. Nugent,
71
J. M. Roney,
71
R. J. Sobie,
71
N. Tasneem,
71
T. J. Gershon,
72
P. F. Harrison,
72
T. E. Latham,
72
H. R. Band,
73
S. Dasu,
73
Y. Pan,
73
R. Prepost,
73
and S. L. Wu
73
(The
B
A
B
AR
Collaboration)
arXiv:1410.3644v1 [hep-ex] 14 Oct 2014
2
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
3
INFN Sezione di Bari
a
; Dipartimento di Fisica, Università di Bari
b
, 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
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
9
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090
a
,
Novosibirsk State University, Novosibirsk 630090
b
,
Novosibirsk State Technical University, Novosibirsk 630092
c
, 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
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
20
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
21
INFN Sezione di Ferrara
a
; Dipartimento di Fisica e Scienze della Terra, Università di Ferrara
b
, I-44122 Ferrara, Italy
22
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
23
INFN Sezione di Genova
a
; Dipartimento di Fisica, Università di Genova
b
, I-16146 Genova, Italy
24
Indian Institute of Technology Guwahati, Guwahati, Assam, 781 039, India
25
Universität Heidelberg, Physikalisches Institut, D-69120 Heidelberg, Germany
26
Humboldt-Universität zu Berlin, Institut für Physik, D-12489 Berlin, Germany
27
Imperial College London, London, SW7 2AZ, United Kingdom
28
University of Iowa, Iowa City, Iowa 52242, USA
29
Iowa State University, Ames, Iowa 50011-3160, USA
30
Physics Department, Jazan University, Jazan 22822, Kingdom of Saudia Arabia
31
Johns Hopkins University, Baltimore, Maryland 21218, USA
32
Laboratoire de l’Accélérateur Linéaire, IN2P3/CNRS et Université Paris-Sud 11,
Centre Scientifique d’Orsay, F-91898 Orsay Cedex, France
33
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
34
University of Liverpool, Liverpool L69 7ZE, United Kingdom
35
Queen Mary, University of London, London, E1 4NS, United Kingdom
36
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
37
University of Louisville, Louisville, Kentucky 40292, USA
38
Johannes Gutenberg-Universität Mainz, Institut für Kernphysik, D-55099 Mainz, Germany
39
University of Manchester, Manchester M13 9PL, United Kingdom
40
University of Maryland, College Park, Maryland 20742, USA
41
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
42
McGill University, Montréal, Québec, Canada H3A 2T8
43
INFN Sezione di Milano
a
; Dipartimento di Fisica, Università di Milano
b
, I-20133 Milano, Italy
44
University of Mississippi, University, Mississippi 38677, USA
45
Université de Montréal, Physique des Particules, Montréal, Québec, Canada H3C 3J7
46
INFN Sezione di Napoli
a
; Dipartimento di Scienze Fisiche,
Università di Napoli Federico II
b
, I-80126 Napoli, Italy
47
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
48
University of Notre Dame, Notre Dame, Indiana 46556, USA
49
Ohio State University, Columbus, Ohio 43210, USA
50
INFN Sezione di Padova
a
; Dipartimento di Fisica, Università di Padova
b
, I-35131 Padova, Italy
51
Laboratoire de Physique Nucléaire et de Hautes Energies,
IN2P3/CNRS, Université Pierre et Marie Curie-Paris 6,
Université Denis Diderot-Paris 7, F-75252 Paris, France
52
INFN Sezione di Perugia
a
; Dipartimento di Fisica, Università di Perugia
b
, I-06123 Perugia, Italy
53
INFN Sezione di Pisa
a
; Dipartimento di Fisica,
Università di Pisa
b
; Scuola Normale Superiore di Pisa
c
, I-56127 Pisa, Italy
54
Princeton University, Princeton, New Jersey 08544, USA
55
INFN Sezione di Roma
a
; Dipartimento di Fisica,
Università di Roma La Sapienza
b
, I-00185 Roma, Italy
3
56
Universität Rostock, D-18051 Rostock, Germany
57
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
58
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
59
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
60
University of South Carolina, Columbia, South Carolina 29208, USA
61
Southern Methodist University, Dallas, Texas 75275, USA
62
Stanford University, Stanford, California 94305-4060, USA
63
State University of New York, Albany, New York 12222, USA
64
Tel Aviv University, School of Physics and Astronomy, Tel Aviv, 69978, Israel
65
University of Tennessee, Knoxville, Tennessee 37996, USA
66
University of Texas at Austin, Austin, Texas 78712, USA
67
University of Texas at Dallas, Richardson, Texas 75083, USA
68
INFN Sezione di Torino
a
; Dipartimento di Fisica, Università di Torino
b
, I-10125 Torino, Italy
69
INFN Sezione di Trieste
a
; Dipartimento di Fisica, Università di Trieste
b
, I-34127 Trieste, Italy
70
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
71
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
72
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
73
University of Wisconsin, Madison, Wisconsin 53706, USA
We report the observation of the baryonic decay
B
0
→
Λ
+
c
pK
−
K
+
using a data sample of
471
×
10
6
B
B
pairs produced in
e
+
e
−
annihilations at
√
s
= 10
.
58 GeV
. This data sample was recorded
with the
B
A
B
AR
detector at the PEP-II storage ring at SLAC. We find
B
(
B
0
→
Λ
+
c
pK
−
K
+
)
=
(
2
.
5
±
0
.
4
(
stat
)
±
0
.
2
(
syst
)
±
0
.
6
B
(
Λ
+
c
)
)
×
10
−
5
, where the uncertainties are statistical, systematic,
and due to the uncertainty of the
Λ
+
c
→
pK
−
π
+
branching fraction, respectively. The result has a
significance corresponding to 5.0 standard deviations, including all uncertainties. For the resonant
decay
B
0
→
Λ
+
c
pφ
, we determine the upper limit
B
(
B
0
→
Λ
+
c
pφ
)
<
1
.
2
×
10
−
5
at
90%
confidence
level.
PACS numbers: 13.25.Hw, 13.60.Rj, 14.20.Lq
About
7%
of all
B
mesons decay into final states with
baryons [1]. Measurements of the branching fractions for
baryonic
B
decays and studies of the decay dynamics,
e.g., the fraction of resonant subchannels or the possi-
ble enhancement in the production rate at the baryon-
antibaryon threshold seen in some reactions [2, 3], can
provide detailed information that can be used to test phe-
nomenological models [4–6]. Studying baryonic
B
decays
can also allow a better understanding of the mechanism
of these decays and, more generally, of the baryon pro-
duction process.
In this paper we present a measurement of the branch-
ing fraction for the decay
B
0
→
Λ
+
c
pK
−
K
+
[7]. No
experimental results are currently available for this de-
cay mode. However, the related decay
B
0
→
Λ
+
c
pπ
−
π
+
has been observed with a branching fraction
B
(
B
0
→
Λ
+
c
pπ
−
π
+
) = (1
.
17
±
0
.
23)
×
10
−
3
[1]. The main difference
between the decay presented here and
B
0
→
Λ
+
c
pπ
−
π
+
is that there are fewer kinematically accessible resonant
subchannels for
B
0
→
Λ
+
c
pK
−
K
+
. The heavier mass of
the
s
quark suggests a suppression factor of about
1
/
3
[8], which is consistent with the observed suppression of
B
0
→
D
0
Λ
Λ
relative to
B
0
→
D
0
p
p
[9]. However, the
B
0
→
Λ
+
c
pK
−
K
+
and
B
0
→
Λ
+
c
pπ
−
π
+
decay processes
are described by different Feynman diagrams and this
simple expectation might not hold.
The analysis is based on an integrated luminosity of
429
fb
−
1
[10] of data collected at a center-of-mass energy
equivalent to the
Υ
(4
S
)
mass,
√
s
= 10
.
58 GeV
, with
the
B
A
B
AR
detector at the PEP-II asymmetric-energy
e
+
e
−
collider at SLAC, corresponding to
471
×
10
6
B
B
pairs. Trajectories of charged particles are measured
with a five-layer double-sided silicon vertex tracker
and a 40-layer drift chamber, operating in the
1
.
5
T
magnetic field of a superconducting solenoid. Ionization
energy loss measurements in the tracking chambers and
information from an internally reflecting ring-imaging
detector provide charged-particle identification [11].
The
B
A
B
AR
detector is described in detail elsewhere
[12, 13]. Monte Carlo (MC) simulations of events are
used to study background processes and to determine
signal efficiencies. The simulations are based on the
E
vtGen [14] event generator, with the
G
eant4 [15] suite of
programs used to describe the detector and its response.
The
B
0
→
Λ
+
c
pK
−
K
+
and
Λ
+
c
→
pK
−
π
+
final states
are generated according to four-body and three-body
phase space, respectively.
We reconstruct
Λ
+
c
baryons in the decay mode
Λ
+
c
→
pK
−
π
+
. For the
B
meson reconstruction, we combine
the
Λ
+
c
candidate with identified
p
,
K
−
, and
K
+
can-
didates and fit the decay tree to a common vertex con-
straining the
Λ
+
c
candidate to its nominal mass. We re-
quire the
χ
2
probability of the fit to exceed
0
.
001
.
We determine the number of signal candidates with a
two-dimensional unbinned extended maximum likelihood
4
)
2
(GeV/c
ES
m
5.265
5.27
5.275
5.28
5.285
5.29
)
2
(GeV/c
B
m
5.2
5.25
5.3
5.35
5.4
0
50
100
150
200
250
300
350
FIG. 1: The
m
B
versus
m
ES
distribution for correctly recon-
structed simulated signal events.
fit to the
B
meson candidate invariant mass,
m
B
, and the
energy-substituted mass,
m
ES
, defined as
m
ES
=
√
(
s/
2 +
~p
B
·
~p
0
E
0
)
2
−
~p
2
B
,
(1)
where the
B
momentum vector,
~p
B
, and the four-
momentum vector of the
e
+
e
−
system,
(
E
0
,~p
0
)
, are
measured in the laboratory frame. For correctly recon-
structed
B
decays,
m
B
and
m
ES
are centered at the nom-
inal
B
mass. No significant correlation is found between
m
B
and
m
ES
in simulated signal (Fig. 1) and back-
ground events. For signal events, the shape of the
m
ES
distributions is described by the sum
f
2
G
of two Gaus-
sian functions, as is the
m
B
distribution. The means,
widths, and relative weights in the four Gaussians are
determined using simulated events, and are fixed in the
final fit. Background from other
B
meson decays and
continuum events
(
e
+
e
−
→
q
q, q
=
u,
,
.
s,
) ̧ is modeled
using an ARGUS function [16],
f
ARGUS
, for
m
ES
and a
first-order polynomial,
f
poly
, for
m
B
.
To suppress combinatorial background, we require the
Λ
+
c
candidate mass to lie within
10 MeV
/c
2
of the nomi-
nal mass, corresponding to a deviation from the nominal
mass of no more than about 2 standard deviations of the
expected
Λ
+
c
mass resolution.
The fit function is defined as
f
fit
=
N
sig
·S
(
m
ES
,m
B
) +
N
bkg
·B
(
m
ES
,m
B
)
=
N
sig
·
f
2G
(
m
ES
)
·
f
2G
(
m
B
)
+
N
bkg
·
f
ARGUS
(
m
ES
)
·
f
poly
(
m
B
)
,
(2)
where
N
sig
and
N
bkg
are the number of signal and back-
ground events, respectively, with
S
and
B
the correspond-
ing probability density functions (PDFs). The extended
)
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
5.3
)
2
Events / ( 0.002 GeV/c
0
5
10
15
20
25
30
35
40
(a)
)
2
(GeV/c
B
m
5.2
5.25
5.3
5.35
5.4
5.45
5.5
5.55
)
2
Events / ( 7 MeV/c
0
10
20
30
40
50
(b)
FIG. 2: Data (points with statistical uncertainties) and pro-
jections of the maximum likelihood fit (solid curves) for
B
0
→
Λ
+
c
pK
−
K
+
candidates. The dashed curves show the projec-
tions of the PDF for background events. (a) Results for
m
ES
,
with the requirement
5
.
26 GeV
/c
2
≤
m
B
≤
5
.
30 GeV
/c
2
. (b)
Results for
m
B
, with the requirement
5
.
275 GeV
/c
2
≤
m
ES
≤
5
.
285 GeV
/c
2
.
likelihood function is:
L
(
N
sig
,N
bkg
) =
e
−
(
N
sig
+
N
bkg
)
N
!
N
∏
i
=1
[
N
sig
S
i
(
m
ES
i
,m
Bi
)
+
N
bkg
B
i
(
m
ES
i
,m
Bi
)]
,
(3)
where
i
denotes the
i
th candidate and
N
is the total
number of events in the fit region. The fit region is de-
fined by the intervals
5
.
2 GeV
/c
2
< m
B
<
5
.
55 GeV
/c
2
and
5
.
2 GeV
/c
2
< m
ES
<
5
.
3 GeV
/c
2
.
Figure 2 shows the one-dimensional projections of the
fit results onto the
m
ES
and
m
B
axes in comparison with
the data. Clear signal peaks at the
B
meson mass are
visible. We find
N
sig
= 66
±
12
, where the uncertainty
is statistical only. The statistical significance
S
of the
signal is determined from the ratio of the likelihood val-
ues for the best-fit signal hypothesis,
L
sig
, and the best
5
fit with no signal included,
L
0
,
S
=
√
−
2 ln(
L
0
/L
sig
)
,
corresponding to
5
.
4
standard deviations.
The efficiency to reconstruct signal events depends
on the baryon-antibaryon invariant mass. Therefore
to determine the
B
0
→
Λ
+
c
pK
−
K
+
branching frac-
tion, we divide the data into two regions. Region I is
defined as
3
.
225 GeV
/c
2
< m
Λ
+
c
p
≤
3
.
475 GeV
/c
2
and
region II as
3
.
475 GeV
/c
2
< m
Λ
+
c
p
≤
4
.
225 GeV
/c
2
.
The results are shown in Table I. To determine an
upper limit on the branching fraction for the decay
B
0
→
Λ
+
c
pφ
, we do not divide the events into regions
of
m
Λ
+
c
p
. Instead we use only events in the
φ
signal
region, which we denote as region III, defined by
1
.
005 GeV
/c
2
< m
KK
<
1
.
034 GeV
/c
2
.
TABLE I: Number of observed signal events,
N
sig
, and sig-
nal efficiency,
, for
B
0
→
Λ
+
c
pK
−
K
+
decays. The re-
gions are defined by the following invariant mass ranges –
I:
3
.
225 GeV
/c
2
< m
Λ
+
c
p
≤
3
.
475 GeV
/c
2
, II:
3
.
475 GeV
/c
2
<
m
Λ
+
c
p
≤
4
.
225 GeV
/c
2
.
Region
N
sig
I
37
.
7
±
8
.
0 (10
.
93
±
0
.
08)%
II
28
.
2
±
8
.
4 (11
.
47
±
0
.
07)%
We consider systematic uncertainties associated with
the initial number of
B
B
pairs, the tracking efficiency,
the particle identification efficiency, the limited number
of MC events, the description of the background, and the
description of the signal (Table II).
The uncertainty for the number of
B
B
pairs is
0
.
6%
[10]. We determine the systematic uncertainty for the
charged-particle reconstruction to be
1
.
3%
, and for the
charged-particle identification (ID) to be
5
.
6%
. The un-
certainty for the charged-particle identification is evalu-
ated by adding the uncertainty of the identification for
each particle in quadrature. For the kaon the uncertainty
is
5
.
6%
, for the proton
0
.
7%
, and for the pion
0
.
2%
. The
information on the detector-related uncertainties is de-
scribed in Ref. [13]. The statistical uncertainty associ-
ated with the MC sample is
0
.
4%
. The systematic un-
certainties arising from the fit procedure are determined
by changing the background description for
m
B
from a
first-order polynomial to a second-order polynomial, and
by changing the fit ranges in
m
ES
and
m
B
while using
a first-order polynomial for
m
B
(
7
.
0%
). Changing the
signal description for
m
B
and
m
ES
from a sum of two
Gaussian functions with fixed shape parameters to a sin-
gle Gaussian function whose parameters are determined
in the maximum likelihood fit leads to an uncertainty of
3
.
1%
. The total systematic uncertainty is
9
.
6%
, obtained
by adding all contributions in quadrature.
The
26%
uncertainty of the
Λ
+
c
branching fraction is
listed as a third uncertainty, separate from the statistical
and systematic components. In order to be consistent
with prior branching fraction measurements of baryonic
B
decays we use the current value for
B
(
Λ
+
c
→
pK
−
π
+
)
[1], and do not incorporate the recent measurement by
Belle [17].
Only additive systematic uncertainties, i.e., uncer-
tainties influencing the signal and background yields
differently, affect the significance of the signal. The
significance of the
B
0
→
Λ
+
c
pK
−
K
+
signal taking into
account the additive systematic uncertainties is
5
.
0
standard deviations.
To determine the branching fraction, we use the fol-
lowing relation:
B
(
B
0
→
Λ
+
c
pK
−
K
+
) =
1
B
(
Λ
+
c
→
pK
−
π
+
)
·
1
N
B
·
(
N
sigI
I
+
N
sigII
II
)
.
(4)
Here,
N
B
= (471
±
3)
×
10
6
is the initial number of
B
B
events [10]. We assume equal production of
B
0
B
0
and
B
+
B
−
pairs. The
Λ
+
c
branching fraction is
B
(
Λ
+
c
→
pK
−
π
+
) = (5
.
0
±
1
.
3)%
[1], and
N
sigI
,
N
sigII
, and
I
,
II
are the numbers of signal events and the efficiencies in
the two regions of the baryon-antibaryon invariant mass.
We obtain:
B
(
B
0
→
Λ
+
c
pK
−
K
+
)
=
(
2
.
5
±
0
.
4
(
stat
)
±
0
.
2
(
syst
)
±
0
.
6
(
Λ
+
c
)
)
×
10
−
5
.
(5)
Eliminating the uncertainty of the
Λ
+
c
branching fraction,
the result is
B
(
B
0
→
Λ
+
c
pK
−
K
+
)
=
(
2
.
5
±
0
.
4
(
stat
)
±
0
.
2
(
syst
)
)
×
10
−
5
×
0
.
050
B
(
Λ
+
c
→
pK
−
π
+
)
.
(6)
This result is a factor of
47
smaller than the
B
0
→
Λ
+
c
pπ
−
π
+
branching fraction.
All Feynman diagram contributions for
B
0
→
Λ
+
c
pK
−
K
+
lead to Feynman diagram contributions for
B
0
→
Λ
+
c
pπ
−
π
+
through replacement of the
s
s
pair in
TABLE II: Summary of the systematic uncertainties for
B
0
→
Λ
+
c
pK
−
K
+
.
Source
Relative uncertainty
Multiplicative uncertainties:
B
B
counting
0
.
6%
Track reconstruction
1
.
3%
Charged particle ID
5
.
6%
MC sample size
0
.
4%
Additive uncertainties:
Background description
7
.
0%
Signal description
3
.
1%
Total
9
.
6%
6
the final state with a
d
d
pair. The expectation from
hadronization models for these common processes is that
the
B
0
→
Λ
+
c
pπ
−
π
+
and
B
0
→
Λ
+
c
pK
−
K
+
branching
fractions should differ by a factor of 3. The expected
B
0
→
Λ
+
c
pπ
−
π
+
branching fraction arising from these
common processes is about
7
.
5
×
10
−
5
, representing only
6
.
4%
of the observed
B
0
→
Λ
+
c
pπ
−
π
+
branching fraction
[1]. The remaining contributions arise from other Feyn-
man diagrams, notably diagrams with external
W
boson
emission (operator product expansion operator 1 [18]),
which are not allowed for
B
0
→
Λ
+
c
pK
−
K
+
. Moreover,
B
0
→
Λ
+
c
pπ
−
π
+
decays receive a large contribution from
resonant subchannels. These differences likely explain
why we find the
B
0
→
Λ
+
c
pK
−
K
+
and
B
0
→
Λ
+
c
pπ
−
π
+
branching fractions to differ more than the naive factor
of 3.
We perform a fit in intervals of
m
(
Λ
+
c
p
)
to determine
the dependence of the number of signal events on the
baryon-antibaryon invariant mass. The lower limit of
the mass range is given by the kinematic threshold for
Λ
+
c
p
production, while the upper limit corresponds to the
threshold
K
−
K
+
mass with the
K
−
K
+
system at rest
in the
B
0
rest frame. The results are shown in Fig. 3(a).
The trend of the data is consistent with a small thresh-
old enhancement, but the result is not statistically sig-
nificant. The fit results for the intervals I and II in
m
Λ
+
c
p
and the detection efficiencies for these regions are shown
in Table I.
We also perform fits in intervals of
m
(
K
−
K
+
)
. As can
be seen in Fig. 3(b), the data deviate from the phase
space expectation near threshold, in the region of the
φ
meson resonance. The events, in region III, include
contributions from
B
0
→
Λ
+
c
pK
−
K
+
and
B
0
→
Λ
+
c
pφ
.
The number of events in region III is used to determine a
Bayesian upper limit at
90%
confidence level (CL) for the
decay
B
0
→
Λ
+
c
pφ
by integrating the likelihood function.
This upper limit is estimated to be 17 events. The effi-
ciency for
B
0
→
Λ
+
c
pφ
decays is
(12
.
04
±
0
.
06)%
. Using
the result
B
(
φ
→
K
+
K
−
) = (48
.
9
±
0
.
5)%
[1], we obtain
B
(
B
0
→
Λ
+
c
pφ
)
<
1
.
2
×
10
−
5
.
(7)
In summary, we observe the baryonic decay
B
0
→
Λ
+
c
pK
−
K
+
with a significance of
5
.
0
stan-
dard deviations including statistical and systematic
uncertainties, and determine the branching fraction to
be
(
2
.
5
±
0
.
4
(
stat
)
±
0
.
2
(
syst
)
±
0
.
6
B
(
Λ
+
c
)
)
×
10
−
5
. The
uncertainties are statistical, systematic, and due to the
uncertainty in the
Λ
+
c
→
pK
−
π
+
branching fraction,
respectively. We obtain an upper limit of
1
.
2
×
10
−
5
at
90%
confidence level for the resonant decay
B
0
→
Λ
+
c
pφ
.
We are grateful for the extraordinary contributions of
our PEP-II colleagues in achieving the excellent luminos-
ity and machine conditions that have made this work pos-
sible. The success of this project also relies critically on
)
2
(GeV/c
)
p
+
c
Λ
m(
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
4.1
4.2
)
2
Events / (1 GeV/c
0
50
100
150
200
250
(a)
I
II
)
2
(GeV/c
)
+
K
-
m(K
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
)
2
Events / (1 GeV/c
0
50
100
150
200
250
300
350
400
(b)
III
FIG. 3: (a) Baryon-antibaryon invariant mass signal distri-
bution and (b) kaon-kaon invariant mass signal distribution
for data (points with statistical uncertainties) compared to
distributions for simulated
B
0
→
Λ
+
c
pK
−
K
+
decays gener-
ated according to four-body phase space (shaded histogram),
scaled to the same number of events as in data. Regions I, II,
and III are indicated in the figure and described in the text.
the expertise and dedication of the computing organiza-
tions that support
B
A
B
AR
. The collaborating institutions
wish to thank SLAC for its support and the kind hospital-
ity extended to them. This work is supported by the US
Department of Energy and National Science Foundation,
the Natural Sciences and Engineering Research Council
(Canada), the Commissariat à l’Energie Atomique and
Institut National de Physique Nucléaire et de Physique
des Particules (France), the Bundesministerium für Bil-
dung und Forschung and Deutsche Forschungsgemein-
schaft (Germany), the Istituto Nazionale di Fisica Nu-
cleare (Italy), the Foundation for Fundamental Research
on Matter (The Netherlands), the Research Council of
Norway, the Ministry of Education and Science of the
Russian Federation, Ministerio de Ciencia e Innovación
(Spain), and the Science and Technology Facilities Coun-
cil (United Kingdom). Individuals have received support
from the Marie-Curie IEF program (European Union),
7
the A. P. Sloan Foundation (USA), and the Binational
Science Foundation (USA-Israel).
∗
Now at the University of Tabuk, Tabuk 71491, Saudi
Arabia
†
Also with Università di Perugia, Dipartimento di Fisica,
Perugia, Italy
‡
Now at Laboratoire de Physique Nucléaire et de Hautes
Energies, IN2P3/CNRS, Paris, France
§
Now at the University of Huddersfield, Huddersfield HD1
3DH, UK
¶
Deceased
∗∗
Now at University of South Alabama, Mobile, Alabama
36688, USA
††
Also with Università di Sassari, Sassari, Italy
‡‡
Also with INFN Sezione di Roma, Roma, Italy
§§
Now at Universidad Técnica Federico Santa Maria, Val-
paraiso, Chile 2390123
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