Search for the highly suppressed decays
B
!
K
þ
and
B
!
K
K
þ
B. Aubert,
1
M. Bona,
1
Y. Karyotakis,
1
J. P. Lees,
1
V. Poireau,
1
E. Prencipe,
1
X. Prudent,
1
V. Tisserand,
1
J. Garra Tico,
2
E. Grauges,
2
L. Lopez,
3a,3b
A. Palano,
3a,3b
M. Pappagallo,
3a,3b
G. Eigen,
4
B. Stugu,
4
L. Sun,
4
G. S. Abrams,
5
M. Battaglia,
5
D. N. Brown,
5
R. N. Cahn,
5
R. G. Jacobsen,
5
L. T. Kerth,
5
Yu. G. Kolomensky,
5
G. Lynch,
5
I. L. Osipenkov,
5
M. T. Ronan,
5,
*
K. Tackmann,
5
T. Tanabe,
5
C. M. Hawkes,
6
N. Soni,
6
A. T. Watson,
6
H. Koch,
7
T. Schroeder,
7
D. Walker,
8
D. J. Asgeirsson,
9
B. G. Fulsom,
9
C. Hearty,
9
T. S. Mattison,
9
J. A. McKenna,
9
M. Barrett,
10
A. Khan,
10
V. E. Blinov,
11
A. D. Bukin,
11
A. R. Buzykaev,
11
V. P. Druzhinin,
11
V. B. Golubev,
11
A. P. Onuchin,
11
S. I. Serednyakov,
11
Yu. I. Skovpen,
11
E. P. Solodov,
11
K. Yu. Todyshev,
11
M. Bondioli,
12
S. Curry,
12
I. Eschrich,
12
D. Kirkby,
12
A. J. Lankford,
12
P. Lund,
12
M. Mandelkern,
12
E. C. Martin,
12
D. P. Stoker,
12
S. Abachi,
13
C. Buchanan,
13
J. W. Gary,
14
F. Liu,
14
O. Long,
14
B. C. Shen,
14,
*
G. M. Vitug,
14
Z. Yasin,
14
L. Zhang,
14
V. Sharma,
15
C. Campagnari,
16
T. M. Hong,
16
D. Kovalskyi,
16
M. A. Mazur,
16
J. D. Richman,
16
T. W. Beck,
17
A. M. Eisner,
17
C. J. Flacco,
17
C. A. Heusch,
17
J. Kroseberg,
17
W. S. Lockman,
17
A. J. Martinez,
17
T. Schalk,
17
B. A. Schumm,
17
A. Seiden,
17
M. G. Wilson,
17
L. O. Winstrom,
17
C. H. Cheng,
18
D. A. Doll,
18
B. Echenard,
18
F. Fang,
18
D. G. Hitlin,
18
I. Narsky,
18
T. Piatenko,
18
F. C. Porter,
18
R. Andreassen,
19
G. Mancinelli,
19
B. T. Meadows,
19
K. Mishra,
19
M. D. Sokoloff,
19
P. C. Bloom,
20
W. T. Ford,
20
A. Gaz,
20
J. F. Hirschauer,
20
M. Nagel,
20
U. Nauenberg,
20
J. G. Smith,
20
K. A. Ulmer,
20
S. R. Wagner,
20
R. Ayad,
21,
+
A. Soffer,
21,
‡
W. H. Toki,
21
R. J. Wilson,
21
D. D. Altenburg,
22
E. Feltresi,
22
A. Hauke,
22
H. Jasper,
22
M. Karbach,
22
J. Merkel,
22
A. Petzold,
22
B. Spaan,
22
K. Wacker,
22
M. J. Kobel,
23
W. F. Mader,
23
R. Nogowski,
23
K. R. Schubert,
23
R. Schwierz,
23
A. Volk,
23
D. Bernard,
24
G. R. Bonneaud,
24
E. Latour,
24
M. Verderi,
24
P. J. Clark,
25
S. Playfer,
25
J. E. Watson,
25
M. Andreotti,
26a,26b
D. Bettoni,
26a
C. Bozzi,
26a
R. Calabrese,
26a,26b
A. Cecchi,
26a,26b
G. Cibinetto,
26a,26b
P. Franchini,
26a,26b
E. Luppi,
26a,26b
M. Negrini,
26a,26b
A. Petrella,
26a,26b
L. Piemontese,
26a
V. Santoro,
26a,26b
R. Baldini-Ferroli,
27
A. Calcaterra,
27
R. de Sangro,
27
G. Finocchiaro,
27
S. Pacetti,
27
P. Patteri,
27
I. M. Peruzzi,
27,
x
M. Piccolo,
27
M. Rama,
27
A. Zallo,
27
A. Buzzo,
28a
R. Contri,
28a,28b
M. Lo Vetere,
28a,28b
M. M. Macri,
28a
M. R. Monge,
28a,28b
S. Passaggio,
28a
C. Patrignani,
28a,28b
E. Robutti,
28a
A. Santroni,
28a,28b
S. Tosi,
28a,28b
K. S. Chaisanguanthum,
29
M. Morii,
29
A. Adametz,
30
J. Marks,
30
S. Schenk,
30
U. Uwer,
30
V. Klose,
31
H. M. Lacker,
31
D. J. Bard,
32
P. D. Dauncey,
32
J. A. Nash,
32
M. Tibbetts,
32
P. K. Behera,
33
X. Chai,
33
M. J. Charles,
33
U. Mallik,
33
J. Cochran,
34
H. B. Crawley,
34
L. Dong,
34
W. T. Meyer,
34
S. Prell,
34
E. I. Rosenberg,
34
A. E. Rubin,
34
Y. Y. Gao,
34
A. V. Gritsan,
35
Z. J. Guo,
35
C. K. Lae,
35
N. Arnaud,
36
J. Be
́
quilleux,
36
A. D’Orazio,
36
M. Davier,
36
J. Firmino da Costa,
36
G. Grosdidier,
36
A. Ho
̈
cker,
36
V. Lepeltier,
36
F. Le Diberder,
36
A. M. Lutz,
36
S. Pruvot,
36
P. Roudeau,
36
M. H. Schune,
36
J. Serrano,
36
V. Sordini,
36,
k
A. Stocchi,
36
G. Wormser,
36
D. J. Lange,
37
D. M. Wright,
37
I. Bingham,
38
J. P. Burke,
38
C. A. Chavez,
38
J. R. Fry,
38
E. Gabathuler,
38
R. Gamet,
38
D. E. Hutchcroft,
38
D. J. Payne,
38
C. Touramanis,
38
A. J. Bevan,
39
C. K. Clarke,
39
K. A. George,
39
F. Di Lodovico,
39
R. Sacco,
39
M. Sigamani,
39
G. Cowan,
40
H. U. Flaecher,
40
D. A. Hopkins,
40
S. Paramesvaran,
40
F. Salvatore,
40
A. C. Wren,
40
D. N. Brown,
41
C. L. Davis,
41
A. G. Denig,
41
M. Fritsch,
42
W. Gradl,
42
G. Schott,
42
K. E. Alwyn,
43
D. Bailey,
43
R. J. Barlow,
43
Y. M. Chia,
43
C. L. Edgar,
43
G. Jackson,
43
G. D. Lafferty,
43
T. J. West,
43
J. I. Yi,
43
J. Anderson,
44
C. Chen,
44
A. Jawahery,
44
D. A. Roberts,
44
G. Simi,
44
J. M. Tuggle,
44
C. Dallapiccola,
45
X. Li,
45
E. Salvati,
45
S. Saremi,
45
R. Cowan,
46
D. Dujmic,
46
P. H. Fisher,
46
G. Sciolla,
46
M. Spitznagel,
46
F. Taylor,
46
R. K. Yamamoto,
46
M. Zhao,
46
P. M. Patel,
47
S. H. Robertson,
47
A. Lazzaro,
48a,48b
V. Lombardo,
48a
F. Palombo,
48a,48b
J. M. Bauer,
49
L. Cremaldi,
49
R. Godang,
49,
{
R. Kroeger,
49
D. A. Sanders,
49
D. J. Summers,
49
H. W. Zhao,
49
M. Simard,
50
P. Taras,
50
F. B. Viaud,
50
H. Nicholson,
51
G. De Nardo,
52a,52b
L. Lista,
52a
D. Monorchio,
52a,52b
G. Onorato,
52a,52b
C. Sciacca,
52a,52b
G. Raven,
53
H. L. Snoek,
53
C. P. Jessop,
54
K. J. Knoepfel,
54
J. M. LoSecco,
54
W. F. Wang,
54
G. Benelli,
55
L. A. Corwin,
55
K. Honscheid,
55
H. Kagan,
55
R. Kass,
55
J. P. Morris,
55
A. M. Rahimi,
55
J. J. Regensburger,
55
S. J. Sekula,
55
Q. K. Wong,
55
N. L. Blount,
56
J. Brau,
56
R. Frey,
56
O. Igonkina,
56
J. A. Kolb,
56
M. Lu,
56
R. Rahmat,
56
N. B. Sinev,
56
D. Strom,
56
J. Strube,
56
E. Torrence,
56
G. Castelli,
57a,57b
N. Gagliardi,
57a,57b
M. Margoni,
57a,57b
M. Morandin,
57a
M. Posocco,
57a
M. Rotondo,
57a
F. Simonetto,
57a,57b
R. Stroili,
57a,57b
C. Voci,
57a,57b
P. del Amo Sanchez,
58
E. Ben-Haim,
58
H. Briand,
58
G. Calderini,
58
J. Chauveau,
58
P. David,
58
L. Del Buono,
58
O. Hamon,
58
Ph. Leruste,
58
J. Ocariz,
58
A. Perez,
58
J. Prendki,
58
S. Sitt,
58
L. Gladney,
59
M. Biasini,
60a,60b
R. Covarelli,
60a,60b
E. Manoni,
60a,60b
C. Angelini,
61a,61b
G. Batignani,
61a,61b
S. Bettarini,
61a,61b
M. Carpinelli,
61a,61b,
**
A. Cervelli,
61a,61b
F. Forti,
61a,61b
M. A. Giorgi,
61a,61b
A. Lusiani,
61a,61c
G. Marchiori,
61a,61b
M. Morganti,
61a,61b
N. Neri,
61a,61b
E. Paoloni,
61a,61b
G. Rizzo,
61a,61b
J. J. Walsh,
61a
D. Lopes Pegna,
62
C. Lu,
62
J. Olsen,
62
A. J. S. Smith,
62
A. V. Telnov,
62
F. Anulli,
63a
E. Baracchini,
63a,63b
G. Cavoto,
63a
D. del Re,
63a,63b
E. Di Marco,
63a,63b
R. Faccini,
63a,63b
F. Ferrarotto,
63a
F. Ferroni,
63a,63b
M. Gaspero,
63a,63b
P. D. Jackson,
63a
L. Li Gioi,
63a
M. A. Mazzoni,
63a
S. Morganti,
63a
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
1550-7998
=
2008
=
78(9)
=
091102(9)
091102-1
Ó
2008 The American Physical Society
G. Piredda,
63a
F. Polci,
63a,63b
F. Renga,
63a,63b
C. Voena,
63a
M. Ebert,
64
T. Hartmann,
64
H. Schro
̈
der,
64
R. Waldi,
64
T. Adye,
65
B. Franek,
65
E. O. Olaiya,
65
F. F. Wilson,
65
S. Emery,
66
M. Escalier,
66
L. Esteve,
66
S. F. Ganzhur,
66
G. Hamel de Monchenault,
66
W. Kozanecki,
66
G. Vasseur,
66
Ch. Ye
`
che,
66
M. Zito,
66
X. R. Chen,
67
H. Liu,
67
W. Park,
67
M. V. Purohit,
67
R. M. White,
67
J. R. Wilson,
67
M. T. Allen,
68
D. Aston,
68
R. Bartoldus,
68
P. Bechtle,
68
J. F. Benitez,
68
R. Cenci,
68
J. P. Coleman,
68
M. R. Convery,
68
J. C. Dingfelder,
68
J. Dorfan,
68
G. P. Dubois-Felsmann,
68
W. Dunwoodie,
68
R. C. Field,
68
A. M. Gabareen,
68
S. J. Gowdy,
68
M. T. Graham,
68
P. Grenier,
68
C. Hast,
68
W. R. Innes,
68
J. Kaminski,
68
M. H. Kelsey,
68
H. Kim,
68
P. Kim,
68
M. L. Kocian,
68
D. W. G. S. Leith,
68
S. Li,
68
B. Lindquist,
68
S. Luitz,
68
V. Luth,
68
H. L. Lynch,
68
D. B. MacFarlane,
68
H. Marsiske,
68
R. Messner,
68
D. R. Muller,
68
H. Neal,
68
S. Nelson,
68
C. P. O’Grady,
68
I. Ofte,
68
A. Perazzo,
68
M. Perl,
68
B. N. Ratcliff,
68
A. Roodman,
68
A. A. Salnikov,
68
R. H. Schindler,
68
J. Schwiening,
68
A. Snyder,
68
D. Su,
68
M. K. Sullivan,
68
K. Suzuki,
68
S. K. Swain,
68
J. M. Thompson,
68
J. Va’vra,
68
A. P. Wagner,
68
M. Weaver,
68
C. A. West,
68
W. J. Wisniewski,
68
M. Wittgen,
68
D. H. Wright,
68
H. W. Wulsin,
68
A. K. Yarritu,
68
K. Yi,
68
C. C. Young,
68
V. Ziegler,
68
P. R. Burchat,
69
A. J. Edwards,
69
S. A. Majewski,
69
T. S. Miyashita,
69
B. A. Petersen,
69
L. Wilden,
69
S. Ahmed,
70
M. S. Alam,
70
J. A. Ernst,
70
B. Pan,
70
M. A. Saeed,
70
S. B. Zain,
70
S. M. Spanier,
71
B. J. Wogsland,
71
R. Eckmann,
72
J. L. Ritchie,
72
A. M. Ruland,
72
C. J. Schilling,
72
R. F. Schwitters,
72
B. W. Drummond,
73
J. M. Izen,
73
X. C. Lou,
73
F. Bianchi,
74a,74b
D. Gamba,
74a,74b
M. Pelliccioni,
74a,74b
M. Bomben,
75a,75b
L. Bosisio,
75a,75b
C. Cartaro,
75a,75b
G. Della Ricca,
75a,75b
L. Lanceri,
75a,75b
L. Vitale,
75a,75b
V. Azzolini,
76
N. Lopez-March,
76
F. Martinez-Vidal,
76
D. A. Milanes,
76
A. Oyanguren,
76
J. Albert,
77
Sw. Banerjee,
77
B. Bhuyan,
77
H. H. F. Choi,
77
K. Hamano,
77
R. Kowalewski,
77
M. J. Lewczuk,
77
I. M. Nugent,
77
J. M. Roney,
77
R. J. Sobie,
77
T. J. Gershon,
78
P. F. Harrison,
78
J. Ilic,
78
T. E. Latham,
78
G. B. Mohanty,
78
H. R. Band,
79
X. Chen,
79
S. Dasu,
79
K. T. Flood,
79
Y. Pan,
79
M. Pierini,
79
R. Prepost,
79
C. O. Vuosalo,
79
and S. L. Wu
79
(
B
A
B
AR
Collaboration)
1
Laboratoire de Physique des Particules, IN2P3/CNRS et Universite
́
de Savoie, 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
Dipartmento di Fisica, Universita
`
di Bari, I-70126 Bari, Italy
4
University of Bergen, Institute of Physics, N-5007 Bergen, Norway
5
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
6
University of Birmingham, Birmingham, B15 2TT, United Kingdom
7
Ruhr Universita
̈
t Bochum, Institut fu
̈
r Experimentalphysik 1, D-44780 Bochum, Germany
8
University of 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, USA
13
University of California at Los Angeles, Los Angeles, California 90024, USA
14
University of California at Riverside, Riverside, California 92521, USA
15
University of California at San Diego, La Jolla, California 92093, USA
16
University of California at Santa Barbara, Santa Barbara, California 93106, USA
17
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
18
California Institute of Technology, Pasadena, California 91125, USA
19
University of Cincinnati, Cincinnati, Ohio 45221, USA
20
University of Colorado, Boulder, Colorado 80309, USA
21
Colorado State University, Fort Collins, Colorado 80523, USA
22
Technische Universita
̈
t Dortmund, Fakulta
̈
t Physik, D-44221 Dortmund, Germany
23
Technische Universita
̈
t Dresden, Institut fu
̈
r Kern- und Teilchenphysik, D-01062 Dresden, Germany
24
Laboratoire Leprince-Ringuet, CNRS/IN2P3, Ecole Polytechnique, F-91128 Palaiseau, France
25
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
26a
INFN Sezione di Ferrara, I-44100 Ferrara, Italy;
26b
Dipartimento di Fisica, Universita
`
di Ferrara, I-44100 Ferrara, Italy
27
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
28a
INFN Sezione di Genova, I-16146 Genova, Italy;
28b
Dipartimento di Fisica, Universita
`
di Genova, I-16146 Genova, Italy
29
Harvard University, Cambridge, Massachusetts 02138, USA
30
Universita
̈
t Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany
B. AUBERT
et al.
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-2
31
Humboldt-Universita
̈
t zu Berlin, Institut fu
̈
r Physik, Newtonstr. 15, D-12489 Berlin, Germany
32
Imperial College London, London, SW7 2AZ, United Kingdom
33
University of Iowa, Iowa City, Iowa 52242, USA
34
Iowa State University, Ames, Iowa 50011-3160, USA
35
Johns Hopkins University, Baltimore, Maryland 21218, USA
36
Laboratoire de l’Acce
́
le
́
rateur Line
́
aire, IN2P3/CNRS et Universite
́
Paris-Sud 11,
Centre Scientifique d’Orsay, BP 34, F-91898 Orsay Cedex, France
37
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
38
University of Liverpool, Liverpool L69 7ZE, United Kingdom
39
Queen Mary, University of London, London, E1 4NS, United Kingdom
40
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
41
University of Louisville, Louisville, Kentucky 40292, USA
42
Johannes Gutenberg-Universita
̈
t Mainz, Institut fu
̈
r Kernphysik, D-55099 Mainz, Germany
43
University of Manchester, Manchester M13 9PL, United Kingdom
44
University of Maryland, College Park, Maryland 20742, USA
45
University of Massachusetts, Amherst, Massachusetts 01003, USA
46
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
47
McGill University, Montre
́
al, Que
́
bec, Canada H3A 2T8
48a
INFN Sezione di Milano, I-20133 Milano, Italy;
48b
Dipartimento di Fisica, Universita
`
di Milano, I-20133 Milano, Italy
49
University of Mississippi, University, Mississippi 38677, USA
50
Universite
́
de Montre
́
al, Physique des Particules, Montre
́
al, Que
́
bec, Canada H3C 3J7
51
Mount Holyoke College, South Hadley, Massachusetts 01075, USA
52a
INFN Sezione di Napoli, I-80126 Napoli, Italy;
52b
Dipartimento di Scienze Fisiche, Universita
`
di Napoli Federico II, I-80126 Napoli, Italy
53
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
54
University of Notre Dame, Notre Dame, Indiana 46556, USA
55
Ohio State University, Columbus, Ohio 43210, USA
56
University of Oregon, Eugene, Oregon 97403, USA
57a
INFN Sezione di Padova, I-35131 Padova, Italy;
57b
Dipartimento di Fisica, Universita
`
di Padova, I-35131 Padova, Italy
58
Laboratoire de Physique Nucle
́
aire et de Hautes Energies, IN2P3/CNRS,
Universite
́
Pierre et Marie Curie-Paris6, Universite
́
Denis Diderot-Paris7, F-75252 Paris, France
59
University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
60a
INFN Sezione di Perugia, I-06100 Perugia, Italy;
60b
Dipartimento di Fisica, Universita
`
di Perugia, I-06100 Perugia, Italy
61a
INFN Sezione di Pisa, I-56127 Pisa, Italy;
61b
Dipartimento di Fisica, Universita
`
di Pisa, I-56127 Pisa, Italy;
61c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
62
Princeton University, Princeton, New Jersey 08544, USA
63a
INFN Sezione di Roma, I-00185 Roma, Italy;
63b
Dipartimento di Fisica, Universita
`
di Roma La Sapienza, I-00185 Roma, Italy
64
Universita
̈
t Rostock, D-18051 Rostock, Germany
65
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
66
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
67
University of South Carolina, Columbia, South Carolina 29208, USA
68
Stanford Linear Accelerator Center, Stanford, California 94309, USA
69
Stanford University, Stanford, California 94305-4060, USA
70
State University of New York, Albany, New York 12222, USA
71
University of Tennessee, Knoxville, Tennessee 37996, USA
72
University of Texas at Austin, Austin, Texas 78712, USA
73
University of Texas at Dallas, Richardson, Texas 75083, USA
{
Now at University of South Alabama, Mobile, AL 36688, USA.
k
Also with Universita
`
di Roma La Sapienza, I-00185 Roma, Italy.
x
Also with Universita
`
di Perugia, Dipartimento di Fisica, Perugia, Italy.
‡
Now at Tel Aviv University, Tel Aviv, 69978, Israel.
+
Now at Temple University, Philadelphia, PA 19122, USA.
**
Also with Universita
`
di Sassari, Sassari, Italy.
*
Deceased.
SEARCH FOR THE HIGHLY SUPPRESSED DECAYS
...
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-3
74a
INFN Sezione di Torino, I-10125 Torino, Italy;
74b
Dipartimento di Fisica Sperimentale, Universita
`
di Torino, I-10125 Torino, Italy
75a
INFN Sezione di Trieste, I-34127 Trieste, Italy;
75b
Dipartimento di Fisica, Universita
`
di Trieste, I-34127 Trieste, Italy
76
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
77
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
78
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
79
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 7 August 2008; published 13 November 2008)
We report a search for the decays
B
!
K
þ
and
B
!
K
K
þ
, which are highly suppressed
in the standard model. Using a sample of
ð
467
5
Þ
10
6
B
B
pairs collected with the
BABAR
detector, we
do not see any evidence of these decays and determine 90% confidence level upper limits of
B
ð
B
!
K
þ
Þ
<
9
:
5
10
7
and
B
ð
B
!
K
K
þ
Þ
<
1
:
6
10
7
on the corresponding branching frac-
tions, including systematic uncertainties.
DOI:
10.1103/PhysRevD.78.091102
PACS numbers: 13.25.Hw, 12.60.
i
The decays
B
!
K
þ
and
B
!
K
K
þ
pro-
ceed via
b
!
dd
s
and
b
!
ss
d
quark transitions, respec-
tively. These are highly suppressed in the standard model
(SM). Compared with the penguin (loop) transitions
b
!
q
qd
, and
b
!
q
qs
they are additionally suppressed by the
small Cabibbo-Kobayashi-Maskawa matrix [
1
,
2
] element
factor
j
V
td
V
ts
j’
3
10
4
, leading to predicted branching
fractions of only
O
ð
10
14
Þ
and
O
ð
10
11
Þ
, respectively
[
3
,
4
]. Example SM decay diagrams can be seen in Fig.
1
.
These branching fractions can be significantly enhanced
in SM extensions such as the minimal supersymmetric
standard model with or without conserved
R
parity, or in
models containing extra
U
ð
1
Þ
gauge bosons. For example,
the branching fraction for the
b
!
dd
s
transition
B
!
K
0
can be enhanced from about
10
16
in the SM to
about
10
6
in the presence of an extra
Z
0
boson [
4
]. The
branching fraction for the
b
!
ss
d
decay
B
!
K
K
0
can be enhanced from about
7
10
14
in the SM to about
6
10
9
in the minimal supersymmetric standard model
[
5
].
Observations of the decays
B
!
K
þ
and
B
!
K
K
þ
would be clear experimental signals for the
b
!
dd
s
and
b
!
ss
d
quark transitions [
6
,
7
]. These decay
modes have been previously searched for [
8
–
11
], and the
most restrictive 90% confidence level experimental upper
limits
B
ð
B
!
K
þ
Þ
<
1
:
8
10
6
and
B
ð
B
!
K
K
þ
Þ
<
1
:
3
10
6
[
10
] were obtained from an
analysis of
81
:
8fb
1
of
BABAR
data. Upper limits on
b
!
ss
d
transitions have also been set using the decays
B
!
K
K
þ
[
12
],
B
0
!
K
0
K
þ
[
13
], and
B
0
!
K
0
K
0
[
14
].
We report the results of a search for the decays
B
!
K
þ
and
B
!
K
K
þ
. Inclusion of the charge
conjugate modes is implied throughout this paper. The data
used in this analysis, collected at the PEP-II asymmetric
energy
e
þ
e
collider [
15
], consist of an integrated lumi-
nosity of
426 fb
1
recorded at the
ð
4
S
Þ
resonance. In
addition,
44 fb
1
of data were collected 40 MeV below the
resonance and are used for background characterization.
These samples are referred to as on-resonance and off-
resonance data, respectively. The on-resonance data sam-
ple contains
ð
467
5
Þ
10
6
B
B
pairs. Beyond the larger
sample size, we utilize improved analysis techniques for
background rejection and signal identification compared
with our previous study [
10
].
The
BABAR
detector is described in detail elsewhere
[
16
]. Charged particles are detected and their momenta
measured with a five-layer silicon vertex tracker and a
40-layer drift chamber (DCH) inside a 1.5 T solenoidal
magnet. Surrounding the DCH is a detector of internally
reflected Cherenkov radiation. Energy deposited by elec-
trons and photons is measured by a CsI(Tl) crystal elec-
tromagnetic calorimeter.
FIG. 1. Example standard model decay diagrams for the de-
cays
B
!
K
þ
and
B
!
K
K
þ
, respectively.
B. AUBERT
et al.
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-4
We select
B
!
K
þ
candidates by combining a
charged kaon candidate with two charged pion candidates,
each of which has charge opposite to the kaon. Similarly,
B
!
K
K
þ
candidates are selected by combining
two charged kaon candidates with a charged pion candi-
date. Each track is required to have a minimum transverse
momentum of
50 MeV
=c
, and to be consistent with having
originated from the interaction region. Identification of
charged pions and kaons is accomplished using energy
loss (
d
E=
d
x
) information from the silicon vertex tracker
and DCH, and the Cherenkov angle and number of photons
measured in the detector of internally reflected Cherenkov
radiation. The efficiency for kaon selection is approxi-
mately 80% including geometrical acceptance, while the
probability of misidentification of pions as kaons is below
5%. The corresponding efficiency and kaon misindentifi-
cation rate for the pion selection criteria are 95% and less
than 10%, respectively. We require all charged particle
candidates to be inconsistent with the electron hypothesis,
based on a cut-based selection algorithm that uses infor-
mation from
d
E=
d
x
, shower shapes in the electromagnetic
calorimeter, and the ratio of the shower energy and track
momentum.
To avoid a potentially large source of background aris-
ing from decays mediated by the favored
b
!
c
transition,
we veto
B
candidates for which pairs of daughter tracks
have invariant mass combinations in the ranges
1
:
76
<
m
K
<
1
:
94 GeV
=c
2
,
2
:
85
<m
K
<
3
:
25 GeV
=c
2
, and
3
:
65
<m
K
<
3
:
75 GeV
=c
2
. These remove events con-
taining the decays
D
0
!
K
þ
,
J=
c
!
‘
þ
‘
, and
c
ð
2
S
Þ!
‘
þ
‘
, respectively, where the leptons in the
J=
c
and
c
ð
2
S
Þ
decays are misidentified as pions or kaons.
Continuum
e
þ
e
!
q
q
(
q
¼
u
,
d
,
s
,
c
) events are the
dominant background. To discriminate this type of event
from signal, we use a neural network that combines five
variables. The first of these is the ratio of
L
2
to
L
0
, with
L
j
¼
P
i
p
?
i
j
cos
?
i
j
j
, where
p
?
i
is the particle momentum,
?
i
is the angle between the particle and the thrust axis
determined from the
B
candidate decay products, the sum
is over all tracks and neutral clusters not associated with
the
B
candidate, and all quantities are calculated in the
e
þ
e
center-of-mass (CM) frame. The other four variables
are the absolute value of the cosine of the angle between
the
B
direction and the beam axis; the magnitude of the
cosine of the angle between the
B
thrust axis and the beam
axis; the product of the
B
candidate’s charge and the output
of a multivariate algorithm that identifies the flavor of the
recoiling
B
meson [
17
]; and the proper time difference
between the decays of the two
B
mesons divided by its
uncertainty. The angles with respect to the beam axis are
calculated in the CM frame. The neural network output
NN
out
is distributed such that it peaks around 0 for con-
tinuum background and around 1 for signal. We require
NN
out
>
0
:
5
(
NN
out
>
0
:
4
) for
B
!
K
þ
(
B
!
K
K
þ
) candidates. These requirements retain approxi-
mately 90% of the signal, while rejecting approximately
80% of the continuum background.
In addition to the neural network output, we distinguish
signal from background events using two kinematic vari-
ables: the difference
E
between the CM energy of the
B
candidate and
ffiffiffi
s
p
=
2
, and the beam-energy substituted mass
m
ES
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s=
4
p
?
2
B
q
, where
ffiffiffi
s
p
is the total CM energy and
p
?
B
is the momentum of the candidate
B
meson in the CM
frame. The
E
distribution peaks near zero with a resolu-
tion of around 19 MeV, while the
m
ES
distribution for
signal events peaks near the
B
mass with a resolution of
around
2
:
4 MeV
=c
2
. We select signal candidates that sat-
isfy
5
:
260
<m
ES
<
5
:
286 GeV
=c
2
and
j
E
j
<
0
:
100 GeV
. This region includes a sufficiently large range
of
m
ES
below the signal peak to determine properties of the
continuum distribution.
The efficiency for signal events to pass the selection
criteria is 21.6% (17.8%) for
B
!
K
þ
(
B
!
K
K
þ
), determined with a Monte Carlo (MC) simula-
tion in which the decays are generated uniformly in three-
body phase space. The
BABAR
detector Monte Carlo simu-
lation is based on
GEANT4
[
18
] and
EvtGen
[
19
]. We find
that 8.2% (5.1%) of
B
!
K
þ
(
B
!
K
K
þ
)
selected events contain more than one candidate, in which
case we choose the one with the highest probability that the
three tracks originate from a common vertex.
We study possible residual backgrounds from
B
B
events
using MC event samples. Backgrounds arise from decays
with topologies similar to the signal but with some mis-
reconstruction. Such effects include kaon/pion misidenti-
fication, the loss of a soft neutral particle, and the
association of a particle from the decay of the other
B
in
the event with the signal candidate or vice versa. We find
that the backgrounds can be conveniently divided into five
categories for both the
K
þ
and
K
K
þ
channels,
each of which is dominated by one or two particular decays
but also includes other decay modes that result in similar
m
ES
and
E
shapes. Table
I
provides details of the com-
position of the background categories.
In order to obtain the
B
!
K
þ
and
B
!
K
K
þ
signal yields, we perform unbinned extended
maximum likelihood fits to the candidate events using
three variables:
m
ES
,
E
, and
NN
out
. For each event hy-
pothesis
j
(signal, continuum background, or one of the
five
B
B
background categories), we define a probability
density function (PDF)
P
i
j
P
j
ð
m
ES
i
;
E
i
Þ
P
j
ð
NN
i
out
Þ
;
(1)
where
i
denotes the event index. For the signal, continuum
background, and the
B
B
background categories with small
correlations between
m
ES
and
E
, the PDF is further
factorized
P
j
ð
m
ES
i
;
E
i
Þ¼
P
j
ð
m
ES
i
Þ
P
j
ð
E
i
Þ
:
(2)
SEARCH FOR THE HIGHLY SUPPRESSED DECAYS
...
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-5
The extended likelihood function is
L
¼
exp
X
k
n
k
Y
i
X
j
n
j
P
i
j
;
(3)
where
n
j
ð
n
k
Þ
is the yield belonging to the event hypothesis
j
ð
k
Þ
.
The signal
m
ES
and
E
shapes are parametrized with the
sum of a Gaussian and a Crystal Ball function [
20
–
22
] and
the sum of two Gaussian functions, respectively. We de-
termine the shape parameters by taking the values obtained
from signal MC and correcting for differences between
data and MC seen in a control sample of
B
!
D
0
with
D
0
!
K
þ
decays. The continuum background
m
ES
shape is described by the function
x
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
x
2
p
exp
½
ð
1
x
2
Þ
, with
x
2
m
ES
=
ffiffiffi
s
p
and
a free parameter [
23
], while
the continuum
E
shape is modeled with a linear function.
We describe the
m
ES
and
E
shapes of each
B
B
back-
ground category using either independent 1D histograms
or a 2D histogram determined from MC samples. The
decision to use 1D or 2D histograms is made based on
the magnitude of the correlations between these variables
for each category and the effect on the signal yield of
neglecting such correlations, discussed below. The PDFs
for categories 1, 2, and 3, for both
B
!
K
þ
and
B
!
K
K
þ
, are modeled using 2D histograms. We
use 1D histograms to describe all
NN
out
distributions.
These histograms are obtained from MC samples for the
signal and
B
B
background categories, and from a combi-
nation of on-resonance data, in a continuum-dominated
sideband of
m
ES
and
E
, and off-resonance data for the
continuum background.
The nine free parameters in our fits are the yields of the
signal, continuum and all five
B
B
background categories,
the
parameter of the continuum
m
ES
shape, and the slope
of the continuum
E
shape.
We test the fitting procedure by applying it to ensembles
of simulated experiments where events are generated from
the PDF shapes described above for all seven categories of
events. We repeat the exercise with
q
q
events generated
from the PDF while signal events are randomly extracted
from the MC samples. The
B
B
background events are
either generated from PDF shapes or drawn from MC
samples. In all cases, these tests confirm that our fit per-
forms as expected, with very small biases on the fitted
signal yields, for which we correct the measured yields
and include systematic uncertainties.
We apply the fit described above to the 26 478
B
!
K
þ
and 7822
B
!
K
K
þ
candidate events
selected from the data recorded at the
ð
4
S
Þ
resonance.
We find
22
43
and
26
19
signal events, respectively,
(statistical uncertainties only). The yields of continuum
and all
B
B
background categories (shown in Table
I
) are
generally consistent with expectations. The yields of
B
B
background categories 3 and 5 from the fit to
B
!
K
þ
candidates do not show perfect agreement; how-
ever, the sum of their yields is consistent with the expec-
tation and, owing to the strong negative correlation
between the yields of these categories, the discrepancy
with the expectation is not significant. Such behavior was
seen in the fit validations and has been shown not to effect
the signal yield. The results of the fits are shown in Fig.
2
.
We determine the branching fractions for
B
!
K
þ
and
B
!
K
K
þ
by applying corrections
TABLE I. Summary of the
B
background categories, giving the dominant decay mode, numbers of expected and observed events and
the character of the
m
ES
and
E
distributions. ‘‘Peaking’’ indicates that the shape is similar to that of the signal. ‘‘Broad peak,’’ ‘‘left
peak,’’ and ‘‘right peak’’ differ from the signal in being wider or shifted to lower or higher values, respectively. The number of expected
and observed events are also given for the continuum background.
B
!
K
þ
Category
1
2
3
4
5
Continuum
Dominant mode(s)
B
!
D
0
;
D
0
!
K
K
þ
B
!
þ
B
!
K
þ
&
B
0
!
K
þ
0
B
0
!
K
þ
Generic
B
B
Number of expected events
80
357
4
472
24
43
1
917
19 25552
495
Number of observed events
61
70
153
94
1116
347
26
152
197
273 25261
198
m
ES
Structure
Peaking
Peaking
Broad peak
Broad peak Continuum-like
E
Structure
Left peak
Right peak
Broad peak
Right peak Continuum-like
B
!
K
K
þ
Category
1
2
3
4
5
Continuum
Dominant mode(s)
B
!
K
K
þ
K
B
!
K
þ
B
!
D
0
;
D
0
!
K
þ
0
Generic
B
þ
B
Generic
B
0
B
0
Number of expected events
190
9
198
961
4
312
11
173
8
6088
241
Number of observed events
213
41
240
37
34
55
380
117
95
107
6953
100
m
ES
Structure
Peaking
Peaking
Broad peak
Broad peak Continuum-like
E
Structure
Left peak
Right peak
Left peak
Continuum-like Continuum-like
B. AUBERT
et al.
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-6
for the small biases evaluated in the MC studies (
3
:
6
2
:
4
and
0
:
5
1
:
0
events, respectively) and then dividing by
the selection efficiencies and the total number of
B
B
pairs
in the data sample. We assume equal decay rates of
ð
4
S
Þ!
B
þ
B
and
B
0
B
0
. Systematic uncertainties on
the fitted yields arise from uncertainties in the PDF shapes
(10.0 and 3.5 events, respectively) including possible data/
MC differences in the signal PDF shapes studied using the
B
!
D
0
control samples discussed above. We esti-
mate the uncertainty on the fit bias (3.1 and 1.0 events,
respectively) to be half the value of the correction com-
bined in quadrature with the precision with which the bias
is known. Uncertainties on the efficiency arise from pos-
sible data/MC differences for tracking (1.2%) and particle
identification (4.2%). We consider two sources of uncer-
tainty related to the Dalitz plot distributions of the signal
decays. The first is related to the variation of the efficiency
over the parts of the Dalitz plots that are included in the
analysis: from MC studies, the uncertainties are found to
be 13.0% for
B
!
K
þ
and 13.5% for
B
!
K
K
þ
. The second is due to the correction for the
vetoed parts of the Dalitz plots, which we estimate for
various signal decay distributions. In addition to the nomi-
nal phase-space distribution, we consider decays domi-
nated by the intermediate states
K
0
ð
892
Þ
or
K
0
0
ð
1430
Þ
(modeled using the LASS [
24
] shape, as implemented in
our Dalitz plot analysis of
B
þ
!
K
þ
þ
[
25
]). We
mimic a possible enhancement at low
or
K
K
invariant mass by employing an
ad hoc
doubly charged
scalar resonance with mass
1500 MeV
=c
2
and width
300 MeV
=c
2
. The efficiency of the veto requirement is
larger than that for the phase-space MC in all alternative
models, so we assign asymmetric systematic errors of
þ
0%
18%
for
B
!
K
þ
and
þ
25%
0%
for
B
!
K
K
þ
. The
uncertainty on the number of
B
B
pairs is 1.1%. Including
all systematic uncertainties, we obtain the following results
for the branching fractions:
B
ð
B
!
K
þ
Þ¼ð
1
:
8
4
:
3
0
:
9
Þ
10
7
and
B
ð
B
!
K
K
þ
Þ¼
ð
3
:
2
2
:
3
þ
1
:
0
0
:
6
Þ
10
7
, where the first uncertainties are
statistical and the second are systematic.
We have also calculated the branching fractions using
event-by-event efficiencies applied to signal weights ob-
tained from the fit result [
26
,
27
]. We obtain results con-
sistent with our main results within the efficiency variation
systematic uncertainty. We have also checked that remov-
ing each of the discriminating variables from the fit, in
turn, gives consistent results.
To obtain 90% confidence level upper limits on the
branching fractions, we use the frequentist approach of
Feldman and Cousins [
28
]. We determine 90% confidence
region bands that relate the true values of the branching
fractions to the measured numbers of signal events. These
bands are constructed using the results of MC studies that
account for relevant biases in the fit procedure and include
systematic uncertainties. The construction of the confi-
dence region bands is shown in Fig.
3
. The 90% confidence
level upper limits are found to be
B
ð
B
!
K
þ
Þ
<
9
:
5
10
7
and
B
ð
B
!
K
K
þ
Þ
<
1
:
6
10
7
. To aid
)
2
(GeV/c
ES
m
5.26
5.27
5.28
)
2
Events / (2.6 MeV/c
0
50
100
150
E (GeV)
∆
-0.1
-0.05
0
0.05
0.1
Events / (0.02 GeV)
0
50
100
150
out
NN
0.6
0.8
1
Events / (0.065 Units)
0
100
200
300
)
2
(GeV/c
ES
m
5.26
5.27
5.28
)
2
Events / (2.6 MeV/c
0
20
40
60
E (GeV)
∆
-0.1
-0.05
0
0.05
0.1
Events / (0.02 GeV)
0
20
40
60
out
NN
0.4
0.6
0.8
1
Events / (0.075 Units)
0
50
FIG. 2 (color online). Projections of the selected events with the fit results overlaid. The top (bottom) set of plots are for
B
!
K
þ
(
B
!
K
K
þ
). From left to right the plots show the projections onto
m
ES
,
E
, and the output of the neural network.
The black points are the data, the solid blue curve is the total fit, the dotted red curve is the continuum background, the dashed green
curve is the total background, and the dash-dotted black curve at the bottom is the signal. The continuum component has been
suppressed in these plots by applying an additional requirement on the ratio of the signal likelihood to the sum of the signal and
continuum likelihoods, calculated without use of the plotted variable. The value of the requirement for each plot is chosen to reject
about 95% of the continuum background while retaining about 55% of the signal.
SEARCH FOR THE HIGHLY SUPPRESSED DECAYS
...
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-7
comparison with other experiments, we also extract the
sensitivities
B
0
defined as the 90% confidence level upper
limits that would be obtained in the case of zero fitted
signal yield. The sensitivities are
B
0
ð
B
!
K
þ
Þ
<
7
:
4
10
7
and
B
0
ð
B
!
K
K
þ
Þ
<
4
:
2
10
7
.
In conclusion, we present searches for the standard
model suppressed
B
meson decays
B
!
K
þ
and
B
!
K
K
þ
. We do not see any evidence of these
decays and obtain improved 90% confidence level upper
limits on the branching fractions. These results supersede
those of our previous publication [
10
] and can be used to
constrain models of new physics.
We are grateful for the excellent luminosity and machine
conditions provided by our PEP-II colleagues, and for the
substantial dedicated effort from the computing organiza-
tions that support
BABAR
. The collaborating institutions
wish to thank SLAC for its support and kind hospitality.
This work is supported by DOE and NSF (USA) , NSERC
(Canada), CEA and CNRS-IN2P3 (France) , BMBF and
DFG (Germany) , INFN (Italy), FOM (The Netherlands),
NFR (Norway), MES (Russia), MEC (Spain), and STFC
(United Kingdom). Individuals have received support from
the Marie Curie EIF (European Union) and the A. P. Sloan
Foundation.
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fit
N
-50
0
50
100
150
200
250
)
-
π
-
π
+
K
→
-
BF(B
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
-6
10
×
fit
N
-50
0
50
100
150
200
250
)
+
π
-
K
-
K
→
-
BF(B
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
-6
10
×
FIG. 3 (color online). Construction of the confidence region bands. The left (right) plot is for
B
!
K
þ
(
B
!
K
K
þ
).
In each figure the blue dotted line shows the expected central value of
N
fit
as a function of the true branching fraction, the green solid
(red dashed) lines show the 90% confidence level upper and lower limits including statistical and systematic errors (statistical errors
only), the black dashed horizontal line marks the position of the previous upper limit [
10
], and the black dash-dotted lines indicate the
results of this study.
B. AUBERT
et al.
PHYSICAL REVIEW D
78,
091102(R) (2008)
RAPID COMMUNICATIONS
091102-8