Measurement of the
B
0
!
J=
Branching Fraction
B. Aubert,
1
D. Boutigny,
1
J.-M. Gaillard,
1
A. Hicheur,
1
Y. Karyotakis,
1
J. P. Lees,
1
P. Robbe,
1
V. Tisserand,
1
A. Zghiche,
1
A. Palano,
2
A. Pompili,
2
J. C. Chen,
3
N. D. Qi ,
3
G. Rong,
3
P. Wang,
3
Y. S. Zhu,
3
G. Eigen,
4
I. Ofte,
4
B. Stugu,
4
G. S. Abrams,
5
A. W. Borgland,
5
A. B. Breon,
5
D. N. Brow n ,
5
J. Button-Shafer,
5
R. N. Cahn,
5
E. Charles,
5
M. S. Gill,
5
A.V. Gritsan,
5
Y. Groysman,
5
R. G. Jacobsen,
5
R.W. Kadel,
5
J. Kadyk,
5
L. T. Ker t h ,
5
Yu. G. Kolomensky,
5
J. F. K ra l ,
5
C. LeClerc,
5
M. E. L ev i ,
5
G. Lynch,
5
L. M. Mi r,
5
P. J. Oddone,
5
T. J. Orimoto,
5
M. Pripstein,
5
N. A. Ro e ,
5
A. Romosan,
5
M. T. Ronan,
5
V. G. Shelkov,
5
A. V. Tel nov,
5
W. A . We n z e l ,
5
T. J. Harrison,
6
C. M. Hawkes,
6
D. J. Knowles,
6
S.W. O’Neale,
6
R. C. Penny,
6
A. T. Watson,
6
N. K. Wat son ,
6
T. Deppermann,
7
K. Goetzen,
7
H. Koch,
7
B. Lewandowski,
7
K. Peters,
7
H. Schmuecker,
7
M. Steinke,
7
N. R. Barlow,
8
W. Bhimji,
8
J. T. Boyd,
8
N. Chevalier,
8
P. J. Clark,
8
W. N. Cottingham,
8
C. Mackay,
8
F. F. Wilson,
8
K. Abe,
9
C. Hearty,
9
T. S. Mattison,
9
J. A. McKenna,
9
D. Thiessen,
9
S. Jolly,
10
A. K. McKemey,
10
V. E . B l i n o v ,
11
A. D. Bu k i n ,
11
A. R. Buzykaev,
11
V. B. Golubev,
11
V. N. Ivanchenko,
11
A. A. Korol,
11
E. A. Kravchenko,
11
A. P. Onuchin,
11
S. I. Serednyakov,
11
Yu. I. Skovpen,
11
A. N. Yushkov,
11
D. Best,
12
M. Chao,
12
D. Kirkby,
12
A. J. Lankford,
12
M. Mandelkern,
12
S. McMahon,
12
D. P. Stoker,
12
K. Arisaka,
13
C. Buchanan,
13
S. Chun,
13
D. B. MacFarlane,
14
S. Prell,
14
Sh. Rahatlou,
14
G. Raven ,
14
V. Sharma,
14
J.W. Berryhill,
15
C. Campagnari,
15
B. Dahmes,
15
P. A. Hart,
15
N. Kuznetsova,
15
S. L. L ev y,
15
O. Long,
15
A. Lu,
15
M. A. Ma z u r,
15
J. D. Richman,
15
W. Verkerke,
15
J. Beringer,
16
A. M. Eisner,
16
M. Grothe,
16
C. A. Heusch,
16
W. S. Lockman,
16
T. P u l l i a m ,
16
T. Schalk,
16
R. E. Schmitz,
16
B. A. Schumm,
16
A. Seiden,
16
M. Tu r r i ,
16
W. Wa l k o w i a k ,
16
D. C. Williams,
16
M. G. Wilson,
16
E. Chen,
17
G. P. Dubois-Felsmann,
17
A. Dvoretskii,
17
D. G. H it l i n ,
17
F. C. Porter,
17
A. Ryd,
17
A. Samuel,
17
S. Yang,
17
S. Jayatilleke,
18
G. Mancinelli,
18
B. T. Meadows,
18
M. D. Sokoloff,
18
T. B a r i l l a r i ,
19
P. B l o o m ,
19
W. T. Ford,
19
U. Nauenberg,
19
A. Olivas,
19
P. Rankin,
19
J. Roy,
19
J. G. Smith,
19
W. C . v a n H o e k ,
19
L. Zhang,
19
J. Blouw,
20
J. L. Harton,
20
M. Krishnamurthy,
20
A. Soffer,
20
W. H . To k i ,
20
R. J. Wilson,
20
J. Zhang,
20
D. Altenburg,
21
T. Brandt,
21
J. Brose,
21
T. Colberg,
21
M. Dickopp,
21
R. S. Dubitzky,
21
A. Hauke,
21
E. Maly,
21
R. Mu
̈
ller-Pfefferkorn,
21
S. O t t o ,
21
K. R. Schubert,
21
R. Schwierz,
21
B. Spaan,
21
L. Wilden,
21
D. Bernard,
22
G. R. Bonneaud,
22
F. Brochard,
22
J. Cohen-Tanugi,
22
S. Ferrag,
22
S. T’Jampens,
22
Ch. Thiebaux,
22
G. Vasileiadis,
22
M. Verderi,
22
A. Anjomshoaa,
23
R. Bernet,
23
A. K ha n ,
23
D. L av i n ,
23
F. Muheim,
23
S. Playfer,
23
J. E. Swain,
23
J. Tinslay,
23
M. Falbo,
24
C. Borean,
25
C. Bozzi,
25
L. Piemontese,
25
A. Sa r t i ,
25
E. Treadwell,
26
F. Anulli,
27,
*
R. Baldini-Ferroli,
27
A. Calcaterra,
27
R. de Sangro,
27
D. Falciai,
27
G. Finocchiaro,
27
P. Patteri,
27
I. M. Peruzzi,
27,
*
M. Piccolo,
27
A. Z a l lo ,
27
S. Bagnasco,
28
A. Buzzo,
28
R. Contri,
28
G. Crosetti,
28
M. Lo Vetere,
28
M. Macri,
28
M. R. Monge,
28
S. Passaggio,
28
F. C. Pastore,
28
C. Patrignani,
28
E. Robutti,
28
A. Santroni,
28
S. Tosi,
28
M. Morii,
29
R. Bartoldus,
30
G. J. Grenier,
30
U. Mallik,
30
J. Cochran,
31
H. B. Crawley,
31
J. L a m sa ,
31
W. T. Meyer,
31
E. I. Rosenberg,
31
J. Yi ,
31
M. Davier,
32
G. Grosdidier,
32
A. Ho
̈
cker,
32
H. M. Lacker,
32
S. Laplace,
32
F. Le Diberder,
32
V. Lepeltier,
32
A. M. Lutz,
32
T. C. Petersen,
32
S. Plaszczynski,
32
M. H. Schune,
32
L. Tantot,
32
S. Trincaz-Duvoid,
32
G. Wormser,
32
R. M. Bionta,
33
V. Brigljevic
́
,
33
D. J. Lange,
33
M. Mugge,
33
K. van Bibber,
33
D. M. Wright,
33
A. J. Beva n ,
34
J. R. F r y,
34
E. Gabathuler,
34
R. Gamet,
34
M. George,
34
M. Kay,
34
D. J. Payne,
34
R. J. Sloane,
34
C. Touramanis,
34
M. L. Aspinwall,
35
D. A. Bowerman,
35
P. D. Dauncey,
35
U. Egede,
35
I. Eschrich,
35
G. W. Morton,
35
J. A. Nash,
35
P. Sanders,
35
D. Smith,
35
G. P. Taylor,
35
J. J. Back,
36
G. Bellodi,
36
P. Dixon,
36
P. F. Harrison,
36
R. J. L. Potter,
36
H.W. Shorthouse,
36
P. Strother,
36
P. B. Vidal,
36
G. Cowan,
37
H. U. Flaecher,
37
S. George,
37
M. G. Green,
37
A. Kur up ,
37
C. E. Marker,
37
T. R. McMahon,
37
S. Riccia rdi,
37
F. Salvatore,
37
G. Vaitsas,
37
M. A. Winter,
37
D. Brown,
38
C. L. Davis,
38
J. Allison,
39
R. J. Barlow,
39
A. C. Forti,
39
F. Jackson,
39
G. D. Lafferty,
39
N. Savvas,
39
J. H. Weatherall,
39
J. C. Williams,
39
A. Farbin,
40
A. Jawahery,
40
V. L i l l a r d ,
40
D. A. Roberts,
40
J. R. Schieck,
40
G. Blaylock,
41
C. Dallapiccola,
41
K. T. Flood,
41
S. S. Hertzbach,
41
R. Kofler,
41
V. B. Koptchev,
41
T. B. Moore,
41
H. Staengle,
41
S. Wi l lo cq ,
41
B. Brau,
42
R. Cowan,
42
G. Sciolla,
42
F. Taylor,
42
R. K. Yamamoto,
42
M. Milek,
43
P. M. Patel,
43
F. Palombo,
44
J. M. Bauer,
45
L. Cremaldi,
45
V. Eschenburg,
45
R. Kroeger,
45
J. Reidy,
45
D. A. Sanders,
45
D. J. Summers,
45
C. Hast,
46
P. Taras,
46
H. Nicholson,
47
C. Ca r t a ro ,
48
N. Cava l lo ,
48
G. De Nardo,
48
F. Fabozzi,
48
C. Gatto,
48
L. Lista,
48
P. Paolucci,
48
D. Piccolo,
48
C. Sciacca,
48
J. M. LoSecco,
49
J. R. G. Alsmiller,
50
T. A. Gabriel,
50
J. Brau,
51
R. F rey,
51
M. Iwasaki,
51
C. T. Potter,
51
N. B. Sinev,
51
D. Strom,
51
E. Torrence,
51
F. Colecchia,
52
A. Dorigo,
52
F. Galeazzi,
52
M. Margoni,
52
M. Morandin,
52
M. Posocco,
52
M. Rotondo,
52
F. Simonetto,
52
R. Stroili,
52
C. Voci,
52
M. Benayoun,
53
H. Briand,
53
J. Chauveau,
53
P. D a v i d ,
53
Ch. de la Vaissie
`
re,
53
L. Del Buono,
53
O. Hamon,
53
Ph. Leruste,
53
J. Ocariz,
53
M. Pivk,
53
L. Roos,
53
J. St a rk ,
53
P. F. Manfredi,
54
V. Re,
54
V. Speziali,
54
L. Gladney,
55
Q. H. G uo ,
55
J. Panetta,
55
C. Angelini,
56
G. Batignani,
56
S. Bettarini,
56
M. Bondioli,
56
F. Bucci,
56
G. Calderini,
56
E. Campagna,
56
M. Carpinelli,
56
F. F o r t i ,
56
M. A. Giorgi,
56
A. Lusiani,
56
G. Marchiori,
56
PHYSICAL REVIEW LETTERS
week ending
7 MARCH 2003
V
OLUME
90, N
UMBER
9
091801-1
0031-9007
=
03
=
90(9)
=
091801(7)$20.00
2003 The American Physical Society
091801-1
F. Martinez-Vidal,
56
M. Morganti,
56
N. Neri,
56
E. Paoloni,
56
M. Rama,
56
G. R i z zo ,
56
F. Sandrelli,
56
G. Triggiani,
56
J. Walsh,
56
M. Ha i re ,
57
D. Judd,
57
K. Paick,
57
L. Turnbull,
57
D. E. Wagoner,
57
J. Albert,
58
P. Elmer,
58
C. Lu,
58
V. M i f t a k o v ,
58
J. Olsen,
58
S. F. Schaffner,
58
A. J. S. Smith,
58
A. Tumanov,
58
E.W. Varnes,
58
F. Bellini,
59
G. Cavoto,
58,59
D. del Re,
14 ,59
R. Faccini,
14,59
F. Ferrarotto,
59
F. Ferroni,
59
E. Leonardi,
59
M. A. Mazzoni,
59
S. Morganti,
59
G. Piredda,
59
F. Safai Tehrani,
59
M. Serra,
59
C. Voena,
59
S. Christ,
60
G. Wagner,
60
R. Wa ld i ,
60
T. Adye,
61
N. De G ro ot ,
61
B. Franek,
61
N. I. Geddes,
61
G. P. Gopal,
61
S. M. Xella,
61
R. Aleksan,
62
S. Emery,
62
A. Gaidot,
62
P.-F. Giraud,
62
G. Hamel de Monchenault,
62
W. Kozanecki,
62
M. Langer,
62
G.W. London,
62
B. Mayer,
62
G. Schott,
62
B. Serfass,
62
G. Vasseur,
62
Ch. Yeche,
62
M. Z it o ,
62
M.V. Purohit,
63
A.W. Weidemann,
63
F. X. Yumiceva,
63
I. Adam,
64
D. A st on ,
64
N. Berger,
64
A. M. Boyarski,
64
M. R. Convery,
64
D. P. Coupal,
64
D. Dong,
64
J. Dorfan,
64
W. Dunwoodie,
64
R. C. Field,
64
T. Glanzman,
64
S. J. Gowdy,
64
E. Grauges,
64
T. Haas,
64
T. Hadig,
64
V. Halyo,
64
T. Himel,
64
T. H r y n’ o v a ,
64
M. E. Huffer,
64
W. R. Innes,
64
C. P. Jessop,
64
M. H. Kelsey,
64
P. K i m ,
64
M. L. Kocian,
64
U. Langenegger,
64
D. W. G. S. L eit h ,
64
S. Luitz,
64
V. Luth,
64
H. L. Ly nch ,
64
H. Marsiske,
64
S. Menke,
64
R. Messner,
64
D. R. Muller,
64
C. P. O’Grady,
64
V. E. Ozcan,
64
A. Perazzo,
64
M. Perl,
64
S. Petrak,
64
B. N. Ratcliff,
64
S. H. Robertson,
64
A. Roodman,
64
A. A. Salnikov,
64
T. Schietinger,
64
R. H. Schindler,
64
J. Schwiening,
64
G. Simi,
64
A. Snyder,
64
A. Soha,
64
S. M. Spanier,
64
J. Stelzer,
64
D. Su,
64
M. K. Sullivan,
64
H. A. Tanaka,
64
J. Va’vra,
64
S. R. Wagner,
64
M. Weaver,
64
A. J. R. Weinstein,
64
W. J. Wisniewski,
64
D. H. Wright,
64
C. C. Young,
64
P. R. Burchat,
65
C. H. Cheng,
65
T. I. Meyer,
65
C. Roat,
65
R. Henderson,
66
W. Bugg,
67
H. Cohn,
67
J. M. Izen,
68
I. Kitayama,
68
X. C. Lou,
68
F. Bianchi,
69
M. Bona,
69
D. Gamba,
69
L. Bosisio,
70
G. Della Ricca,
70
S. Dittongo,
70
L. Lanceri,
70
P. Poropat,
70
L. Vitale,
70
G. Vuagnin,
70
R. S. Pa nv i n i ,
71
S.W. Banerjee,
72
C. M. Brown,
72
D. Fortin,
72
P. D. Jackson,
72
R. Kowalewski,
72
J. M. Roney,
72
H. R. Band,
73
S. Dasu,
73
M. Datta,
73
A. M. Eichenbaum,
73
H. Hu,
73
J. R. Johnson,
73
R. Liu,
73
F. Di Lodovico,
73
A. Mohapatra,
73
Y. P a n ,
73
R. Prepost,
73
I. J. Scott,
73
S. J. Sekula,
73
J. H. von Wimmersperg-Toeller,
73
J. Wu,
73
S. L. Wu,
73
Z. Yu,
73
and H. Neal
74
(
BA BAR
Collaboration)
1
Laboratoire de Physique des Particules, F-74941 Annecy-le-Vieux, France
2
Universita
`
di Bari, Dipartimento di Fisica and INFN, I-70126 Bari, Italy
3
Institute of High Energy Physics, Beijing 100039, China
4
University of Bergen, Institute of Physics, N-5007 Bergen, Norway
5
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720
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 Bristol, Bristol BS8 1TL, United Kingdom
9
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
10
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
11
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
12
University of California at Irvine, Irvine, California 92697
13
University of California at Los Angeles, Los Angeles, California 90024
14
University of California at San Diego, La Jolla, California 92093
15
University of California at Santa Barbara, Santa Barbara, California 93106
16
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064
17
California Institute of Technology, Pasadena, California 91125
18
University of Cincinnati, Cincinnati, Ohio 45221
19
University of Colorado, Boulder, Colorado 80309
20
Colorado State University, Fort Collins, Colorado 80523
21
Technische Universita
̈
t Dresden, Institut fu
̈
r Kern- und Teilchenphysik, D-01062 Dresden, Germany
22
Ecole Polytechnique, LLR, F-91128 Palaiseau, France
23
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
24
Elon University, Elon University, North Carolina 27244-2010
25
Universita
`
di Ferrara, Dipartimento di Fisica and INFN, I-44100 Ferrara, Italy
26
Florida A&M University, Tallahassee, Florida 32307
27
Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati, Italy
28
Universita
`
di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy
29
Harvard University, Cambridge, Massachusetts 02138
30
University of Iowa, Iowa City, Iowa 52242
31
Iowa State University, Ames, Iowa 50011-3160
32
Laboratoire de l’Acce
́
le
́
rateur Line
́
aire, F-91898 Orsay, France
PHYSICAL REVIEW LETTERS
week ending
7 MARCH 2003
V
OLUME
90, N
UMBER
9
091801-2
091801-2
33
Lawrence Livermore National Laboratory, Livermore, California 94550
34
University of Liverpool, Liverpool L69 3BX, United Kingdom
35
University of London, Imperial College, London SW7 2BW, United Kingdom
36
Queen Mary, University of 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
39
University of Manchester, Manchester M13 9PL, United Kingdom
40
University of Maryland, College Park, Maryland 20742
41
University of Massachusetts, Amherst, Massachusetts 01003
42
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139
43
McGill University, Montre
́
al, Quebec, Canada H3A 2T8
44
Universita
`
di Milano, Dipartimento di Fisica and INFN, I-20133 Milano, Italy
45
University of Mississippi, University, Mississippi 38677
46
Universite
́
de Montre
́
al, Laboratoire Rene
́
J. A. L e
́
vesque, Montre
́
al, Quebec, Canada H3C 3J7
47
Mount Holyoke College, South Hadley, Massachusetts 01075
48
Universita
`
di Napoli Federico II, Dipartimento di Scienze Fisiche and INFN, I-80126, Napoli, Italy
49
University of Notre Dame, Notre Dame, Indiana 46556
50
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
51
University of Oregon, Eugene, Oregon 97403
52
Universita
`
di Padova, Dipartimento di Fisica and INFN, I-35131 Padova, Italy
53
Universite
́
s Paris VI et VII, Lab de Physique Nucle
́
aire H. E., F-75252 Paris, France
54
Universita
`
di Pavia, Dipartimento di Elettronica and INFN, I-27100 Pavia, Italy
55
University of Pennsylvania, Philadelphia, Pennsylvania 19104
56
Universita
`
di Pisa, Scuola Normale Superiore and INFN, I-56010 Pisa, Italy
57
Prairie View A&M University, Prairie View, Texas 77446
58
Princeton University, Princeton, New Jersey 08544
59
Universita
`
di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy
60
Universita
̈
t Rostock, D-18051 Rostock, Germany
61
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
62
DAPNIA, Commissariat a
`
l’Energie Atomique/Saclay, F-91191 Gif-sur-Yvette, France
63
University of South Carolina, Columbia, South Carolina 29208
64
Stanford Linear Accelerator Center, Stanford, California 94309
65
Stanford University, Stanford, California 94305-4060
66
TRIUMF, Vancouver, British Columbia, Canada V6T 2A3
67
University of Tennessee, Knoxville, Tennessee 37996
68
University of Texas at Dallas, Richardson, Texas 75083
69
Universita
`
di Torino, Dipartimento di Fisica Sperimentale and INFN, I-10125 Torino, Italy
70
Universita
`
di Trieste, Dipartimento di Fisica and INFN, I-34127 Trieste, Italy
71
Vanderbilt University, Nashville, Tennessee 37235
72
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
73
University of Wisconsin, Madison, Wisconsin 53706
74
Yale University, New Haven, Connecticut 06511
(Received 7 September 2002; published 5 March 2003)
We present a measurement of the branching fraction for the decay of the neutral
B
meson into
the final state
J=
. The data set contains approximately
56
10
6
B
B
pairs produced at the
4
S
resonance and recorded with the
BA BAR
detector at the PEP-II asymmetric-energy
e
e
storage
ring. The result of this analysis is
B
B
0
!
J=
4
:
6
0
:
7
0
:
6
10
5
, where the first
error is statistical and the second is systematic. In addition, we measure
B
B
0
!
J=
0
1
:
6
0
:
6
0
:
4
10
5
.
DOI: 10.1103/PhysRevLett.90.091801
PACS numbers: 13.25.Hw, 11.30.Er, 12.15.Hh
In the standard model, the decay
B
0
!
J=
0
can give
rise to
CP
-violating asymmetries (directly and through
B
0
-
B
0
mixing) [1]. Therefore, it is interesting to study the
decay mode
B
0
!
J=
to understand the
J=
0
component in the final state. Since these decays are
Cabibbo and color suppressed, they could be sensitive to
non-standard-model processes contributing, for example,
through penguin amplitudes. Large non-standard-model
effects could cause the branching fraction to differ sig-
nificantly from the standard model prediction of
B
B
0
!
J=
4
:
8
0
:
8
10
5
[2]. This decay mode
has not previously been observed. CLEO quotes an upper
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limit of
B
B
0
!
J=
0
<
2
:
5
10
4
at the 90% con-
fidence level [4]. Here we present the first measurement of
B
B
0
!
J=
.
The data used in the present analysis were collected at
the PEP-II storage ring with the
BA BAR
detector, de-
scribed in detail elsewhere [5]. Charged particles are
detected, and their momenta measured, with a 40-layer
drift chamber (DCH) and a five-layer silicon vertex
tracker (SVT), both operating in a 1.5 T solenoidal
magnetic field. Surrounding the DCH is a detector of
internally reflected Cherenkov radiation (DIRC), and
outside this is a CsI(Tl) electromagnetic calorimeter
(EMC). The iron flux return of the solenoid is instru-
mented with resistive plate chambers (IFR). The data
sample used for the analysis contains approximately
56
10
6
B
B
pairs, corresponding to a luminosity of
51
:
7fb
1
recorded near the
4
S
resonance. An additional
6
:
4fb
1
, recorded approximately 40 MeV below the
4
S
peak, were used to study continuum backgrounds.
Events containing
B
B
pairs are selected based on track
multiplicity and event topology [6]. At least three tracks
are required to originate near the nominal beam spot,
with polar angle in the range
0
:
41
<
lab
<
2
:
54 rad
,
transverse momentum greater than
100 MeV
=c
, and a
minimum number of DCH hits used in the track fit. To
reduce continuum background the ratio of the second to
zeroth Fox-Wolfram moment,
R
2
H
2
=H
0
, is required to
be less than 0.5. The sum of charged and neutral energy
must be greater than 4.5 GeV in the laboratory frame. The
primary vertex of the event must be within 0.5 cm of the
average measured position of the interaction point in
the plane transverse to the beam line.
The
J=
is reconstructed in the
e
e
and
final
states. Electron candidates must satisfy the requirement
that the ratio of calorimeter energy to track momentum
lies in the range
0
:
75
< E=p <
1
:
3
, the cluster shape and
size are consistent with an electromagnetic shower, and
the energy loss in the DCH is consistent with that for an
electron. If an EMC cluster close to the electron track is
consistent with originating from a bremsstrahlung pho-
ton, it is combined with the electron candidate.
Muon candidates must satisfy requirements on the
number of interaction lengths of IFR iron penetrated
(
N
>
2
), the difference between the measured and ex-
pected interaction lengths penetrated (
j
N
N
exp
j
<
2
),
the position match between the extrapolated DCH track
and the IFR hits, and the average and spread of the
number of IFR strips hit per layer.
Pion candidates are accepted if they originate from
close to the beam spot and are not consistent with being
a kaon. The algorithm uses
dE=dx
information from the
SVT and DCH, and the Cherenkov angle and number of
photons from the DIRC.
Tracks are required to lie in polar-angle ranges where
particle identification efficiency is measured with known
control samples. The allowed ranges correspond approxi-
mately to the geometrical acceptances of the EMC for
electrons, the IFR for muons, and the DIRC for pions.
Identified electron and muon pairs are fit to a common
vertex and must lie in the
J=
invariant mass interval
2.95 (3.06) to
3
:
14 GeV
=c
2
for the
e
e
(
)
channel.
B
0
candidates are formed by combining a
J=
candi-
date with a pair of oppositely charged pion candidates
consistent with coming from a common decay point. We
also require the vertex positions of the lepton and pion
pairs to be consistent. Further selection requirements are
made using two kinematic variables: the difference,
E
,
between the energy of the candidate and the beam energy
E
cm
beam
in the center-of-mass frame and the beam-energy
substituted mass,
m
ES
E
cm
beam
2
p
cm
B
2
q
. After ap-
plying the loose requirements
5
:
2
<m
ES
<
5
:
3 GeV
=c
2
and
j
E
j
<
0
:
12 GeV
, approximately one-quarter of the
events contain more than one
B
0
candidate, from which
we keep the one with the smallest
j
E
j
. The distribution
of the candidates in
E
and
m
ES
is shown in Fig. 1.
For the final signal sample, we require
j
m
ES
5279
:
0 MeV
=c
2
j
<
9
:
9 MeV
=c
2
and
j
E
j
<
39 MeV
,
which correspond to
4
and
3
ranges in the resolutions
for
m
ES
and
E
. After all selection criteria have been
applied, 213 events remain.
An unbinned, extended maximum-likelihood [7]
fit is performed on the invariant mass distribution
of the two pions for the selected events, to determine
the various contributions to the
B
0
!
J=
events. We consider five categories: (i)
B
0
!
J=
0
events;
(ii)
B
0
!
J= K
0
S
K
0
S
!
events;
(iii)
B
0
!
J=
(non-
0
signal)
events;
-0.1
-0.05
0
0.05
0.1
∆
E (GeV)
0
10
20
30
40
5.2
5.225
5.25
5.275
5.3
Events/2.5 MeV/c
2
m
ES
(GeV/c
2
)
FIG. 1 (color online).
Signal for
B
0
!
J=
.The
upper plot shows the distribution of events in the
E
-
m
ES
plane, where the box represents the
fi
nal selection criteria.
The lower plot shows the distribution in
m
ES
of events
with
j
E
j
<
39 MeV
, where the dashed (solid) line corre-
sponds to events in the
K
0
S
(non-
K
0
S
) region in
M
(
0
:
45
–
0
:
55 GeV
=c
2
). The vertical lines represent the
fi
nal
selection.
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(iv) background from events without a real
J=
;and
(v) inclusive-
J=
background from events containing a
real
J=
. A probability density function (PDF) is con-
structed for each of these
fi
ve cases. The total PDF is then
formed from the sum of the
fi
ve PDFs and
fi
t to the data.
The
B
0
!
J= K
0
S
mode is not considered to be a signal
for the purposes of determining the branching fraction
for
B
0
!
J=
.
The PDF used to model the
0
resonance in the
B
0
!
J=
0
mode is a relativistic
P
-wave Breit-Wigner func-
tion [8]:
F
m
m
m
P
2
L
eff
1
=
m
2
m
2
2
m
2
m
2
;
where
m
0
q=q
0
3
m
=m
1
R
2
q
2
0
=
1
R
2
q
2
.
q
m
is the pion momentum in the dipion rest frame,
with
q
0
q
m
.
m
M
is the two-pion invariant
mass and
P
is the
J=
momentum in the
B
0
rest
frame.
m
770 MeV
=c
2
,
0
150 MeV
=c
2
,and
m
140 MeV
=c
2
.
L
eff
is the effective orbital angular momen-
tum between the
J=
and the
0
, which can take any
value between 0 and 2 and so is allowed to
fl
oat in the
fi
t.
R
is the Blatt-Weisskopf barrier-factor radius [9]. The
fi
tis
performed with
R
equal to two values (0.5 and 1.0 fm
[10]) and the results of the two
fi
ts are averaged.
The PDF for the
B
0
!
J= K
0
S
mode is a single
Gaussian function with the mass and width
fi
xed to values
obtained by
fi
tting a sample of simulated
J= K
0
S
events.
Allowing these parameters to vary in the
fi
nal
M
fi
t does not change the results.
The PDF used to model the
B
0
!
J=
(non-
0
signal)
contains a three-body
phase-space factor
q
m
P
m
and a factor of
P
m
2
motivated by angular
momentum conservation:
F
ph
m
q
m
P
m
3
.Ifthe
is in an
S
wave, angular momentum conservation
results in a factor of
P
m
2
, while a
D
wave yields a
second power of
P
m
or higher. We choose to use the
simple
S
wave for this PDF but take into account the
possibility of a
D
-wave contribution by allowing an
f
2
1270
resonance in the
fi
t as a systematic check.
The PDF for the
M
distribution for background
events without a real
J=
is derived from a fake-
J=
sample selected in data as described above except that at
least one of the lepton candidates must fail the appropri-
ate particle identi
fi
cation requirements. A Monte Carlo
study con
fi
rms that the
M
distribution obtained
with this procedure correctly describes the shape of the
non-
J=
background. The resulting distribution is pa-
rametrized using the sum of two Weibull functions [11]
and a Breit-Wigner. The Breit-Wigner describes the
0
component of the non-
J=
background.
The PDF for the
M
shape for background
events containing a real
J=
is obtained from a simulated
B
!
J= X
sample equivalent to a luminosity of
81 fb
1
.
Events in which the system
X
is
(nonresonant),
0
,
or
K
0
S
(
) are removed from the sample. The result-
ing shape is described by a Weibull function.
The normalization of the background components is
obtained from samples in data and simulation. The level
of non-
J=
background is obtained from sidebands of the
J=
mass distribution in data. The
m
ES
distribution for
these sideband candidates is then
fi
t to an ARGUS func-
tion [12] to determine how many events pass the
fi
nal
selection criterion. Scaling to the equivalent background
in the
J=
mass region, using an exponential to describe
the background shape in the
J=
mass distribution, the
expected non-
J=
background is found to be
35
:
7
1
:
2
events.
The level of inclusive-
J=
background is obtained
from the distribution of
m
ES
for events in the
E
signal
region in both data and simulation. In each case, the
m
ES
distribution is parametrized by a Gaussian function (to
represent signal or peaking background) and an ARGUS
function. Peaking background originates from
B
!
J= X
decays such as
B
!
J= K
,
B
!
J=
,and
B
!
J= K
1
that accumulate near
m
ES
5
:
279 GeV
=c
2
.
The nonpeaking component of the inclusive-
J=
back-
ground is determined by subtracting the non-
J=
contri-
bution, on the basis of the scaled sideband events
described above, from the total ARGUS background in
data. The peaking component is determined from the
Gaussian part of the
m
ES
distribution in
B
!
J= X
simu-
lation, where events with
X
(nonresonant),
0
,
and
K
0
S
have been removed. The sum of peaking
and nonpeaking components of the inclusive-
J=
back-
ground is found to be
61
11
events, of which the peak-
ing component comprises six events. Thus, any associated
uncertainties, such as branching fractions used in the
J= X
simulation, will not contribute signi
fi
cantly to the
fi
nal systematic uncertainty.
The branching fraction is obtained from
B
B
0
!
J=
N
J=
N
B
0
$
J=
B
J=
!
‘
‘
;
(1)
where
N
J=
is the total signal yield obtained from the
fi
t,
N
B
0
is the total number of
B
0
and
B
0
in the data sample
[6], and
$
J=
is the signal ef
fi
ciency. The
J=
branching
fraction
B
J=
!
‘
‘
is
fi
xed to 11.81% [3]. We as-
sume that the branching fraction for
4
S
!
B
0
B
0
is
one-half.
The signal ef
fi
ciencies for all requirements apart from
particle identi
fi
cation criteria are derived from simula-
tion. Lepton and pion identi
fi
cation ef
fi
ciencies are deter-
mined with samples of known muons, electrons, and
pions in the data from the following processes:
&
,
e
e
,
e
e
,
e
e
&
,
D
!
D
0
(
D
0
!
K
), and
K
0
S
!
. The ef
fi
ciencies are
determined as a function of momentum and polar
and azimuthal angle. The typical average ef
fi
ciencies
(misidenti
fi
cation rates) for these particle identi
fi
cation
algorithms are 97% (2%), 87% (7%), and 95% (5%) for
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electrons, muons, and pions, respectively. The
fi
nal signal
ef
fi
ciency of
27
:
1
0
:
2
%
is the average of the
J=
0
[
27
:
1
0
:
3
%
]and
J=
(nonresonant) [
27
:
0
0
:
3
%
]ef
fi
ciencies, where the error is from Monte Carlo
statistics.
A likelihood
fi
t is performed on the
M
distri-
bution in data with the normalization of the non-
J=
background
fi
xed to 35.7 events and the inclusive-
J=
background to 61. Thus, only the yields for
J=
0
,
J=
(non-
0
signal), and
J= K
0
S
events are al-
lowed to vary. The results of the
fi
t are overlaid on the
data points in Fig. 2. The goodness-of-
fi
t
'
2
is 33.4 for
38 data points.
The result of the
fi
tis
84
13
signal events, of which
28
10
are in the
0
component and
55
15
are in the
non-
0
signal component. The number of events in the
K
0
S
component is
28
5
. Inserting the result into Eq. (1)
yields the branching fraction
B
B
0
!
J=
4
:
6
0
:
7
10
5
, where the error is statistical.
The signal yield and statistical error can be checked by
counting the number of events passing all the selection
criteria and subtracting the estimated numbers of back-
ground and
J= K
0
S
events. This method gives
88
15
J=
events.
The systematic errors on the
fi
nal branching fraction
measurement arise from uncertainties on the signal ef
fi
-
ciency,
fi
tted yield, number of
B
B
pairs produced, and
J=
!
‘
‘
branching fraction.
N
B
B
is known to
1
:
1%
with the dominant contribution to the uncertainty coming
from the error on the
B
0
B
0
selection ef
fi
ciency.
B
J=
!
‘
‘
is known to 1.2% (fractional) [3].
The uncertainty on the pion identi
fi
cation ef
fi
ciency is
1.8% per pion. Contributions to this error come from the
limited size of the data sample used to determine the
ef
fi
ciency and the uncertainty on the kaon contamination
in the sample. Uncertainties on electron and muon par-
ticle identi
fi
cation ef
fi
ciencies come from studies using
B
!
J= X
events in data. Fits to the
M
J=
distribution
in these events, under different selection criteria, give
estimates of the electron and muon identi
fi
cation ef
fi
cien-
cies and their errors, yielding an uncertainty of 1.3%.
The tracking ef
fi
ciency uncertainty is 1.3% per track
and is summed for the four tracks from the
B
0
decay. The
ef
fi
ciency of the convergence requirement on the
ver tex
fi
t has been studied with a sample of
2
S
!
‘
‘
decays; the associated uncertainty is 1%. The un-
known
0
helicity in the
J=
0
component of the
fi
nal
sample introduces a systematic error on the ef
fi
ciency of
2.5%. The limited amount of simulated data leads to an
uncertainty in signal ef
fi
ciency of 0.7%. To determine the
effect of the signal and background shapes and the back-
ground yields on the
fi
tted yields, the
fi
xed parameters of
these PDFs are varied within their uncertainties, allow-
ing for correlations. This produces a total systematic error
due to
fi
t parameter variation of 9.7%, which is domi-
nated by the errors in the background yields. The
fi
nal
fi
t
neglects resonances such as
f
0
980
,
f
2
1270
,and
0
1450
. Allowing for the addition of such terms in the
likelihood function results in a systematic uncertainty on
the yield of 2.1%. Varying the Blatt-Weisskopf radius
between 0.5 and 1.0 fm gives no change in the total yield,
while the variation of
L
eff
leads to a systematic uncer-
tainty of 0.1%. The total fractional systematic uncertainty
from all sources is found to be 12.3%.
The analysis is repeated with variations in the selection
criteria. Taking into account statistical correlations be-
tween the results, we
fi
nd that variations are consistent
with statistical
fl
uctuations due to the addition or removal
of some of the events in the sample.
The branching fractions are measured separately for
the modes
J=
!
e
e
and
J=
!
yielding the
results
B
B
0
!
J=
ee
5
:
3
1
:
1
10
5
and
B
B
0
!
J=
4
:
0
1
:
0
10
5
, where the
errors are purely statistical.
The
M
distribution shows a clea r peak at the
0
mass. The
fi
t result of
28
10
events for the
0
signal
leads to a branching fraction of
B
B
0
!
J=
0
1
:
6
0
:
6
stat
0
:
4
syst
10
5
. The systematic error
includes a contribution from the effect of using an alter-
native PDF to describe the non-
0
signal. The shape is
from a polynomial
fi
t to data recorded in
-
scattering
experiments [13] and thus provides an empirically de-
rived shape, in contrast to the default non-
0
signal PDF,
which is based on a phase-space assumption. The assump-
tion that the non-
0
signal is predominantly
S
wave, and
therefore interference with the
0
can be neglected, has
been checked on data. A signi
fi
cant
S
-wave contribution
means that the leptons from the
J=
have a helicity
angle distribution
/
sin
2
J=
. For events in data with
M
>
1
:
1 GeV
=c
2
, we subtract the helicity cosine
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8
2
2.2 2.4
0
5
10
15
20
25
30
35
40
)
2
) (GeV/c
-
π
+
π
M(
2
Events / 0.050 GeV/c
B
A
B
AR
FIG. 2 (color online).
Distribution of the invariant mass
M
for events passing all selection criteria. The solid
line is the result of the unbinned likelihood
fi
t. The dashed line
represents the sum of background and non-
0
signal compo-
nents. The
dotted (dot-dashed) line shows the total
(inclusive-
J=
) background. The spike corresponds to
B
0
!
J= K
0
S
events.
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distribution for events with
m
ES
<
5
:
27 MeV
=c
2
from the
distribution for events in the signal
m
ES
region and
fi
nd
that the shape of the resulting distribution is consistent
with
sin
2
J=
. Interference between
S
-and
P
-wave sig-
nal components integrates out in the
M
projec-
tion, as long as the acceptance is symmetric in the cosine
of the dipion helicity angle,
. Studies using simulated
nonresonant
S
-wave events show that there is no signi
fi
-
cant odd component to the acceptance function in
cos
. Consequently, there is no such interference
contribution to the
mass distribution. The contri-
bution to the fractional systematic uncertainty on
B
B
0
!
J=
0
is 1.8% from varying the Blatt-
Weisskopf radius and 3.3% from varying
L
eff
.
In summary, the branching fraction for
B
0
meson
decay to the
fi
nal state
J=
has been measured
for the
fi
rst time. The result,
B
B
0
!
J=
4
:
6
0
:
7
stat
0
:
6
syst
10
5
, is consistent with
the standard model prediction [2]. In addition, the tech-
nique of
fi
tting the
M
distribution allows a mea-
surement of the branching fraction for the
J=
0
component. The result is
B
B
0
!
J=
0
1
:
6
0
:
6
stat
0
:
4
syst
10
5
.
We are grateful for the excellent luminosity and ma-
chine conditions provided by our PEP-II colleagues and
for the substantial dedicated effort from the computing
organizations that support
BA BAR
. The collaborating in-
stitutions thank SLAC for its support and kind hospital-
ity. This work is supported by DOE and NSF (USA),
NSERC (Canada), IHEP (China), CEA and CNRS-
IN2P3 (France), BMBF and DFG (Germany), INFN
(Italy), NFR (Norway), MIST (Russia), and PPARC
(United Kingdom). Individuals have received support
from the A. P. Sloan Foundation, Research Corporation,
and Alexander von Humboldt Foundation.
*Also with Universita
`
di Perugia, I-06100 Perugia, Italy.
[1] I. Dunietz, H. R. Quinn, A. Snyder, W. Toki, and H. J.
Lipkin, Phys. Rev. D
43
, 2193 (1991).
[2] The world average values [3] for the branching fractions
B
0
!
J= K
0
and
B
0
!
J= K
have been scaled by
0
:
5
j
V
cd
j
2
=
j
V
cs
j
2
and
j
V
cd
j
2
=
j
V
cs
j
2
, respectively. The
number quoted in the text is the sum of the two scaled
values and assumes that the tree amplitude is dominant.
[3] Particle Data Group, K. Hagiwara
et al.
, Phys. Rev. D
66
,
010001 (2002).
[4] CLEO Collaboration, M. Bishai
et al.
, Phys. Lett. B
369
,
186 (1996).
[5]
BA BAR
Collaboration, B. Aubert
et al.
, Nucl. Instrum.
Methods Phys. Res., Sect. A
479
, 1 (2002).
[6] For a description of the
B
B
event selection, see, for
example, Sec. VI of
BA BAR
Collaboration, B. Aubert
et al.
, Phys. Rev. D
65
, 032001 (2002).
[7] R. J. Barlow, Nucl. Instrum. Methods Phys. Res., Sect. A
297
, 496 (1990).
[8] J. Pisut and M. Roos, Nucl. Phys.
B6
, 325 (1968).
[9] J. M. Blatt and V. F. Weisskopf,
Theoretical Nuclear
Physics
(Wiley, New York, 1952), p. 361.
[10] Typical values of the Blatt-Weisskopf barrier-factor ra-
dius can be estimated from
q
q
meson models [see, for
example, S. Godfrey and N. Isgur, Phys. Rev. D
32
,189
(1985), especially Fig. 12] and from
fi
ts to experimental
data [see, for example, D. Aston
et al.
, Nucl. Phys.
B296
,
493 (1988)].
[11] The Weibull function can be written as
W
m
CV
m
M
on
C
1
exp
V
m
M
max
C
;
where
V
C
1
=
C
M
max
M
on
C
.
M
max
is the po-
sition of the function maximum,
M
on
is the lower kine-
matic cutoff, and
C
is a general shape parameter.
[12] ARGUS Collaboration, H. Albrecht
et al.
, Z. Phys. C
48
,
543 (1990).
[13] M. Gaspero, Nucl. Phys.
A562
, 407 (1993).
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