of 8
arXiv:1207.0698v1 [hep-ex] 3 Jul 2012
B
A
B
AR
-PUB-12/020
SLAC-PUB-15134
Evidence of
B
+
τ
+
ν
decays with hadronic
B
tags
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
J. Garra Tico,
2
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
G. Lynch,
5
H. Koch,
6
T. Schroeder,
6
D. J. Asgeirsson,
7
C. Hearty,
7
T. S. Mattison,
7
J. A. McKenna,
7
R. Y. So,
7
A. Khan,
8
V. E. Blinov,
9
A. R. Buzykaev,
9
V. P. Druzhinin,
9
V. B. Golubev,
9
E. A. Kravchenko,
9
A. P. Onuchin,
9
S. I. Serednyakov,
9
Yu. I. Skovpen,
9
E. P. Solodov,
9
K. Yu. Todyshev,
9
A. N. Yushkov,
9
M. Bondioli,
10
D. Kirkby,
10
A. J. Lankford,
10
M. Mandelkern,
10
H. Atmacan,
11
J. W. Gary,
11
F. Liu,
11
O. Long,
11
G. M. Vitug,
11
C. Campagnari,
12
T. M. Hong,
12
D. Kovalskyi,
12
J. D. Richman,
12
C. A. West,
12
A. M. Eisner,
13
J. Kroseberg,
13
W. S. Lockman,
13
A. J. Martinez,
13
B. A. Schumm,
13
A. Seiden,
13
D. S. Chao,
14
C. H. Cheng,
14
B. Echenard,
14
K. T. Flood,
14
D. G. Hitlin,
14
P. Ongmongkolkul,
14
F. C. Porter,
14
A. Y. Rakitin,
14
R. Andreassen,
15
Z. Huard,
15
B. T. Meadows,
15
M. D. Sokoloff,
15
L. Sun,
15
P. C. Bloom,
16
W. T. Ford,
16
A. Gaz,
16
U. Nauenberg,
16
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17,
W. H. Toki,
17
B. Spaan,
18
K. R. Schubert,
19
R. Schwierz,
19
D. Bernard,
20
M. Verderi,
20
P. J. Clark,
21
S. Playfer,
21
D. Bettoni
a
,
22
C. Bozzi
a
,
22
R. Calabrese
ab
,
22
G. Cibinetto
ab
,
22
E. Fioravanti
ab
,
22
I. Garzia
ab
,
22
E. Luppi
ab
,
22
M. Munerato
ab
,
22
L. Piemontese
a
,
22
V. Santoro
a
,
22
R. Baldini-Ferroli,
23
A. Calcaterra,
23
R. de Sangro,
23
G. Finocchiaro,
23
P. Patteri,
23
I. M. Peruzzi,
23,
M. Piccolo,
23
M. Rama,
23
A. Zallo,
23
R. Contri
ab
,
24
E. Guido
ab
,
24
M. Lo Vetere
ab
,
24
M. R. Monge
ab
,
24
S. Passaggio
a
,
24
C. Patrignani
ab
,
24
E. Robutti
a
,
24
B. Bhuyan,
25
V. Prasad,
25
C. L. Lee,
26
M. Morii,
26
A. J. Edwards,
27
A. Adametz,
28
U. Uwer,
28
H. M. Lacker,
29
T. Lueck,
29
P. D. Dauncey,
30
U. Mallik,
31
C. Chen,
32
J. Cochran,
32
W. T. Meyer,
32
S. Prell,
32
A. E. Rubin,
32
A. V. Gritsan,
33
Z. J. Guo,
33
N. Arnaud,
34
M. Davier,
34
D. Derkach,
34
G. Grosdidier,
34
F. Le Diberder,
34
A. M. Lutz,
34
B. Malaescu,
34
P. Roudeau,
34
M. H. Schune,
34
A. Stocchi,
34
G. Wormser,
34
D. J. Lange,
35
D. M. Wright,
35
C. A. Chavez,
36
J. P. Coleman,
36
J. R. Fry,
36
E. Gabathuler,
36
D. E. Hutchcroft,
36
D. J. Payne,
36
C. Touramanis,
36
A. J. Bevan,
37
F. Di Lodovico,
37
R. Sacco,
37
M. Sigamani,
37
G. Cowan,
38
D. N. Brown,
39
C. L. Davis,
39
A. G. Denig,
40
M. Fritsch,
40
W. Gradl,
40
K. Griessinger,
40
A. Hafner,
40
E. Prencipe,
40
R. J. Barlow,
41,
G. Jackson,
41
G. D. Lafferty,
41
E. Behn,
42
R. Cenci,
42
B. Hamilton,
42
A. Jawahery,
42
D. A. Roberts,
42
C. Dallapiccola,
43
R. Cowan,
44
D. Dujmic,
44
G. Sciolla,
44
R. Cheaib,
45
D. Lindemann,
45
P. M. Patel,
45,
§
S. H. Robertson,
45
P. Biassoni
ab
,
46
N. Neri
a
,
46
F. Palombo
ab
,
46
S. Stracka
ab
,
46
L. Cremaldi,
47
R. Godang,
47,
R. Kroeger,
47
P. Sonnek,
47
D. J. Summers,
47
X. Nguyen,
48
M. Simard,
48
P. Taras,
48
G. De Nardo
ab
,
49
D. Monorchio
ab
,
49
G. Onorato
ab
,
49
C. Sciacca
ab
,
49
M. Martinelli,
50
G. Raven,
50
C. P. Jessop,
51
J. M. LoSecco,
51
W. F. Wang,
51
K. Honscheid,
52
R. Kass,
52
J. Brau,
53
R. Frey,
53
N. B. Sinev,
53
D. Strom,
53
E. Torrence,
53
E. Feltresi
ab
,
54
N. Gagliardi
ab
,
54
M. Margoni
ab
,
54
M. Morandin
a
,
54
M. Posocco
a
,
54
M. Rotondo
a
,
54
G. Simi
a
,
54
F. Simonetto
ab
,
54
R. Stroili
ab
,
54
S. Akar,
55
E. Ben-Haim,
55
M. Bomben,
55
G. R. Bonneaud,
55
H. Briand,
55
G. Calderini,
55
J. Chauveau,
55
O. Hamon,
55
Ph. Leruste,
55
G. Marchiori,
55
J. Ocariz,
55
S. Sitt,
55
M. Biasini
ab
,
56
E. Manoni
ab
,
56
S. Pacetti
ab
,
56
A. Rossi
ab
,
56
C. Angelini
ab
,
57
G. Batignani
ab
,
57
S. Bettarini
ab
,
57
M. Carpinelli
ab
,
57,
∗∗
G. Casarosa
ab
,
57
A. Cervelli
ab
,
57
F. Forti
ab
,
57
M. A. Giorgi
ab
,
57
A. Lusiani
ac
,
57
B. Oberhof
ab
,
57
E. Paoloni
ab
,
57
A. Perez
a
,
57
G. Rizzo
ab
,
57
J. J. Walsh
a
,
57
D. Lopes Pegna,
58
J. Olsen,
58
A. J. S. Smith,
58
A. V. Telnov,
58
F. Anulli
a
,
59
R. Faccini
ab
,
59
F. Ferrarotto
a
,
59
F. Ferroni
ab
,
59
M. Gaspero
ab
,
59
L. Li Gioi
a
,
59
M. A. Mazzoni
a
,
59
G. Piredda
a
,
59
C. B ̈unger,
60
O. Gr ̈unberg,
60
T. Hartmann,
60
T. Leddig,
60
H. Schr ̈oder,
60,
§
C. Voss,
60
R. Waldi,
60
T. Adye,
61
E. O. Olaiya,
61
F. F. Wilson,
61
S. Emery,
62
G. Hamel de Monchenault,
62
G. Vasseur,
62
Ch. Y`eche,
62
D. Aston,
63
D. J. Bard,
63
R. Bartoldus,
63
J. F. Benitez,
63
C. Cartaro,
63
M. R. Convery,
63
J. Dorfan,
63
G. P. Dubois-Felsmann,
63
W. Dunwoodie,
63
M. Ebert,
63
R. C. Field,
63
M. Franco Sevilla,
63
B. G. Fulsom,
63
A. M. Gabareen,
63
M. T. Graham,
63
P. Grenier,
63
C. Hast,
63
W. R. Innes,
63
M. H. Kelsey,
63
P. Kim,
63
M. L. Kocian,
63
D. W. G. S. Leith,
63
P. Lewis,
63
B. Lindquist,
63
S. Luitz,
63
V. Luth,
63
H. L. Lynch,
63
D. B. MacFarlane,
63
D. R. Muller,
63
H. Neal,
63
S. Nelson,
63
M. Perl,
63
T. Pulliam,
63
B. N. Ratcliff,
63
A. Roodman,
63
A. A. Salnikov,
63
R. H. Schindler,
63
A. Snyder,
63
D. Su,
63
M. K. Sullivan,
63
J. Va’vra,
63
A. P. Wagner,
63
W. J. Wisniewski,
63
M. Wittgen,
63
D. H. Wright,
63
H. W. Wulsin,
63
C. C. Young,
63
V. Ziegler,
63
W. Park,
64
M. V. Purohit,
64
R. M. White,
64
J. R. Wilson,
64
A. Randle-Conde,
65
S. J. Sekula,
65
M. Bellis,
66
P. R. Burchat,
66
T. S. Miyashita,
66
M. S. Alam,
67
J. A. Ernst,
67
R. Gorodeisky,
68
N. Guttman,
68
D. R. Peimer,
68
A. Soffer,
68
P. Lund,
69
S. M. Spanier,
69
J. L. Ritchie,
70
A. M. Ruland,
70
R. F. Schwitters,
70
B. C. Wray,
70
J. M. Izen,
71
X. C. Lou,
71
F. Bianchi
ab
,
72
D. Gamba
ab
,
72
S. Zambito
ab
,
72
L. Lanceri
ab
,
73
L. Vitale
ab
,
73
F. Martinez-Vidal,
74
A. Oyanguren,
74
2
H. Ahmed,
75
J. Albert,
75
Sw. Banerjee,
75
F. U. Bernlochner,
75
H. H. F. Choi,
75
G. J. King,
75
R. Kowalewski,
75
M. J. Lewczuk,
75
I. M. Nugent,
75
J. M. Roney,
75
R. J. Sobie,
75
N. Tasneem,
75
T. J. Gershon,
76
P. F. Harrison,
76
T. E. Latham,
76
E. M. T. Puccio,
76
H. R. Band,
77
S. Dasu,
77
Y. Pan,
77
R. Prepost,
77
and S. L. Wu
77
(The
B
A
B
AR
Collaboration)
1
Laboratoire d’Annecy-le-Vieux de Physique des Particules
(LAPP),
Universit ́e de Savoie, CNRS/IN2P3, F-74941 Annecy-Le-Vie
ux, France
2
Universitat de Barcelona, Facultat de Fisica, Departament
ECM, E-08028 Barcelona, Spain
3
INFN Sezione di Bari
a
; Dipartimento di Fisica, Universit`a 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 Ca
lifornia, Berkeley, California 94720, USA
6
Ruhr Universit ̈at Bochum, Institut f ̈ur Experimentalphys
ik 1, D-44780 Bochum, Germany
7
University of British Columbia, Vancouver, British Columb
ia, Canada V6T 1Z1
8
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kin
gdom
9
Budker Institute of Nuclear Physics, Novosibirsk 630090, R
ussia
10
University of California at Irvine, Irvine, California 926
97, USA
11
University of California at Riverside, Riverside, Califor
nia 92521, USA
12
University of California at Santa Barbara, Santa Barbara, C
alifornia 93106, USA
13
University of California at Santa Cruz, Institute for Parti
cle 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, U
SA
18
Technische Universit ̈at Dortmund, Fakult ̈at Physik, D-44
221 Dortmund, Germany
19
Technische Universit ̈at Dresden, Institut f ̈ur Kern- und T
eilchenphysik, D-01062 Dresden, Germany
20
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS
/IN2P3, F-91128 Palaiseau, France
21
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
22
INFN Sezione di Ferrara
a
; Dipartimento di Fisica, Universit`a di Ferrara
b
, I-44100 Ferrara, Italy
23
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, I
taly
24
INFN Sezione di Genova
a
; Dipartimento di Fisica, Universit`a di Genova
b
, I-16146 Genova, Italy
25
Indian Institute of Technology Guwahati, Guwahati, Assam,
781 039, India
26
Harvard University, Cambridge, Massachusetts 02138, USA
27
Harvey Mudd College, Claremont, California 91711, USA
28
Universit ̈at Heidelberg, Physikalisches Institut, Philo
sophenweg 12, D-69120 Heidelberg, Germany
29
Humboldt-Universit ̈at zu Berlin, Institut f ̈ur Physik, Ne
wtonstr. 15, D-12489 Berlin, Germany
30
Imperial College London, London, SW7 2AZ, United Kingdom
31
University of Iowa, Iowa City, Iowa 52242, USA
32
Iowa State University, Ames, Iowa 50011-3160, USA
33
Johns Hopkins University, Baltimore, Maryland 21218, USA
34
Laboratoire de l’Acc ́el ́erateur Lin ́eaire, IN2P3/CNRS et
Universit ́e Paris-Sud 11,
Centre Scientifique d’Orsay, B. P. 34, F-91898 Orsay Cedex, F
rance
35
Lawrence Livermore National Laboratory, Livermore, Calif
ornia 94550, USA
36
University of Liverpool, Liverpool L69 7ZE, United Kingdom
37
Queen Mary, University of London, London, E1 4NS, United Kin
gdom
38
University of London, Royal Holloway and Bedford New Colleg
e, Egham, Surrey TW20 0EX, United Kingdom
39
University of Louisville, Louisville, Kentucky 40292, USA
40
Johannes Gutenberg-Universit ̈at Mainz, Institut f ̈ur Ker
nphysik, D-55099 Mainz, Germany
41
University of Manchester, Manchester M13 9PL, United Kingd
om
42
University of Maryland, College Park, Maryland 20742, USA
43
University of Massachusetts, Amherst, Massachusetts 0100
3, USA
44
Massachusetts Institute of Technology, Laboratory for Nuc
lear Science, Cambridge, Massachusetts 02139, USA
45
McGill University, Montr ́eal, Qu ́ebec, Canada H3A 2T8
46
INFN Sezione di Milano
a
; Dipartimento di Fisica, Universit`a di Milano
b
, I-20133 Milano, Italy
47
University of Mississippi, University, Mississippi 38677
, USA
48
Universit ́e de Montr ́eal, Physique des Particules, Montr ́
eal, Qu ́ebec, Canada H3C 3J7
49
INFN Sezione di Napoli
a
; Dipartimento di Scienze Fisiche,
Universit`a di Napoli Federico II
b
, I-80126 Napoli, Italy
50
NIKHEF, National Institute for Nuclear Physics and High Ene
rgy Physics, NL-1009 DB Amsterdam, The Netherlands
51
University of Notre Dame, Notre Dame, Indiana 46556, USA
52
Ohio State University, Columbus, Ohio 43210, USA
53
University of Oregon, Eugene, Oregon 97403, USA
54
INFN Sezione di Padova
a
; Dipartimento di Fisica, Universit`a di Padova
b
, I-35131 Padova, Italy
3
55
Laboratoire de Physique Nucl ́eaire et de Hautes Energies,
IN2P3/CNRS, Universit ́e Pierre et Marie Curie-Paris6,
Universit ́e Denis Diderot-Paris7, F-75252 Paris, France
56
INFN Sezione di Perugia
a
; Dipartimento di Fisica, Universit`a di Perugia
b
, I-06100 Perugia, Italy
57
INFN Sezione di Pisa
a
; Dipartimento di Fisica,
Universit`a di Pisa
b
; Scuola Normale Superiore di Pisa
c
, I-56127 Pisa, Italy
58
Princeton University, Princeton, New Jersey 08544, USA
59
INFN Sezione di Roma
a
; Dipartimento di Fisica,
Universit`a di Roma La Sapienza
b
, I-00185 Roma, Italy
60
Universit ̈at Rostock, D-18051 Rostock, Germany
61
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX
11 0QX, United Kingdom
62
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, F
rance
63
SLAC National Accelerator Laboratory, Stanford, Californ
ia 94309 USA
64
University of South Carolina, Columbia, South Carolina 292
08, USA
65
Southern Methodist University, Dallas, Texas 75275, USA
66
Stanford University, Stanford, California 94305-4060, US
A
67
State University of New York, Albany, New York 12222, USA
68
Tel Aviv University, School of Physics and Astronomy, Tel Av
iv, 69978, Israel
69
University of Tennessee, Knoxville, Tennessee 37996, USA
70
University of Texas at Austin, Austin, Texas 78712, USA
71
University of Texas at Dallas, Richardson, Texas 75083, USA
72
INFN Sezione di Torino
a
; Dipartimento di Fisica Sperimentale, Universit`a di Tori
no
b
, I-10125 Torino, Italy
73
INFN Sezione di Trieste
a
; Dipartimento di Fisica, Universit`a di Trieste
b
, I-34127 Trieste, Italy
74
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spa
in
75
University of Victoria, Victoria, British Columbia, Canad
a V8W 3P6
76
Department of Physics, University of Warwick, Coventry CV4
7AL, United Kingdom
77
University of Wisconsin, Madison, Wisconsin 53706, USA
(Dated: July 3, 2012)
We present a search for the decay
B
+
τ
+
ν
using 467
.
8
×
10
6
B
B
pairs collected at the
Υ
(4
S
)
resonance with the
B
A
B
AR
detector at the SLAC PEP-II
B
-Factory. We select a sample of events
with one completely reconstructed
B
in the hadronic decay mode (
B
D
(
)0
X
and
B
J/ψX
). We examine the rest of the event to search for a
B
+
τ
+
ν
decay. We identify the
τ
+
lepton in the following modes:
τ
+
e
+
ν
ν
,
τ
+
μ
+
ν
ν
,
τ
+
π
+
ν
and
τ
+
ρ
+
ν
. We find an
excess of events with respect to the expected background, wh
ich excludes the null signal hypothesis
at the level of 3.8
σ
(including systematic uncertainties) and corresponds to a
branching fraction
central value of
B
(
B
+
τ
+
ν
) = (1
.
83
+0
.
53
0
.
49
(stat.)
±
0
.
24(syst.))
×
10
4
.
PACS numbers: 13.20.-v, 13.25.Hw
The study of the purely leptonic decay
B
+
τ
+
ν
[1] is of particular interest to test the predictions of the
Standard Model (SM) and to probe of new physics ef-
fects. It is sensitive to the product of the
B
meson de-
cay constant
f
B
, and the absolute value of the Cabibbo-
Kobayashi-Maskawa matrix element
|
V
ub
|
[2]. In the SM
the branching fraction is given by:
B
(
B
+
τ
+
ν
) =
G
2
F
m
B
m
2
τ
8
π
[
1
m
2
τ
m
2
B
]
2
f
2
B
|
V
ub
|
2
τ
B
+
,
(1)
Now at the University of Tabuk, Tabuk 71491, Saudi Arabia
Also with Universit`a di Perugia, Dipartimento di Fisica, P
erugia,
Italy
Now at the University of Huddersfield, Huddersfield HD1 3DH,
UK
§
Deceased
Now at University of South Alabama, Mobile, Alabama 36688,
USA
∗∗
Also with Universit`a di Sassari, Sassari, Italy
where
G
F
is the Fermi constant,
m
B
and
m
τ
are, the
B
+
meson and
τ
lepton masses, respectively, and
τ
B
+
is the
B
+
lifetime. Using the Lattice QCD calculation
of
f
B
= (189
±
4) MeV [3], and the
B
A
B
AR
measure-
ment of
|
V
ub
|
from charmless semileptonic B exclusive
decays [4], the predicted SM value of the brancing frac-
tion is
B
SM
(
B
+
τ
+
ν
) = (0
.
62
±
0
.
12)
×
10
4
. If
we use the
B
A
B
AR
measurement of
|
V
ub
|
from inclusive
charmless semileptonic B decays [5], the SM prediction
is
B
SM
(
B
+
τ
+
ν
) = (1
.
18
±
0
.
16)
×
10
4
.
The process is sensitive to possible extensions of
the SM. For instance, in two-Higgs doublet mod-
els (2HDM) [6] and in minimal supersymmetric exten-
sions [7] it can be mediated by a charged Higgs boson.
A branching fraction measurement can, therefore, also
be used to constrain the parameter space of new physics
models.
The data used in this analysis were collected with the
B
A
B
AR
detector at the PEP-II storage ring. The sample
corresponds to an integrated luminosity of 426 fb
1
at the
Υ
(4
S
) resonance. The sample contains (467
.
8
±
5
.
1)
×
10
6
4
TABLE I: Published results for
B
+
τ
+
ν
from
B
A
B
AR
and
Belle collaborations.
Experiment Tag
Branching Fraction (
×
10
4
)
B
A
B
AR
hadronic [8]
1
.
8
+0
.
9
0
.
8
±
0
.
4
±
0
.
2
B
A
B
AR
semileptonic [9]
1
.
7
±
0
.
8
±
0
.
2
Belle
hadronic [10]
1
.
79
+0
.
56
0
.
49
+0
.
46
0
.
51
Belle
semileptonic [11]
1
.
54
+0
.
38
0
.
37
+0
.
29
0
.
31
B
B
decays (
N
B
B
). The detector is described in detail
elsewhere [12]. Charged particle trajectories are mea-
sured in the tracking system composed of a five-layer
double-sided silicon vertex tracker and a 40-layer drift
chamber, operating in a 1.5 T solenoidal magnetic field.
A Cherenkov detector is used for charged
π
K
discrimi-
nation, a CsI calorimeter for photon and electron identifi-
cation, and the flux return of the solenoid, which consists
of layers of iron interspersed with resistive plate chambers
or limited streamer tubes, for muon and neutral hadron
identification.
We use a Monte Carlo (MC) simulation based on
GEANT4
[13] to estimate signal selection efficiencies and to
study backgrounds. In MC simulated signal events, one
B
+
meson decays as
B
+
τ
+
ν
and the other decays in
any final state. The
B
B
and continuum MC samples are
equivalent to approximately 3 times and 1.5 times the
data sample, respectively. Beam-related background and
detector noise are sampled from data and overlaid on the
simulated events.
We reconstruct an exclusive decay of one of the
B
mesons in the event (which we refer to as the tag-
B
)
and examine the rest of the event for the experimen-
tal signature of
B
+
τ
+
ν
. The tag-
B
reconstruction
can be performed by looking at both hadronic
B
decays
and semileptonic
B
decays. Published results from both
B
A
B
AR
and Belle are summarized in Table I.
We reconstruct the tag-
B
candidate in the set of
hadronic decays
B
M
0
X
, where
M
0
denotes a
D
(
)0
or a
J/ψ
, and
X
denotes a system of hadrons
with total charge
1 composed of
n
1
π
±
,
n
2
K
±
,
n
3
π
0
,
n
4
K
0
S
where
n
1
+
n
2
5,
n
2
,
n
3
and
n
4
2. We recon-
struct the
D
0
as
D
0
K
π
+
,
K
π
+
π
0
,
K
π
+
π
π
+
,
K
0
S
π
0
,
K
0
S
π
+
π
,
K
0
S
π
+
π
π
0
,
K
+
K
, or
π
+
π
. We
reconstruct the
D
0
meson as
D
0
D
0
π
0
,D
0
γ
, and
the
J/ψ
meson via their decays
J/ψ
e
+
e
+
μ
.
Two kinematic variables are used to discriminate be-
tween correctly reconstructed tag-
B
candidates and mis-
reconstructed events: the beam energy-substituted mass
m
ES
s/
4
p
2
B
, and the energy difference ∆
E
E
B
s/
2, where
s
is the total energy in the
Υ
(4
S
)
center-of-mass system (CM) and
p
B
and
E
B
respectively
denote the momentum and the energy of the tag-
B
can-
didate in the CM. The resolution on ∆
E
is measured to
be
σ
E
= 10
35 MeV, depending on the decay mode;
we require
|
E
|
<
3
σ
E
. Events with a tag-
B
can-
didate arise from two possible classes with different
m
ES
distributions. One class includes signal events with a cor-
rectly reconstructed tag-
B
, and background events from
Υ
(4
S
)
B
+
B
with a correctly reconstructed tag-
B
.
All these events are characterized by an
m
ES
distribution
peaked at the nominal
B
mass(signal and peaking back-
ground). The other classes of events consist of continuum
background,
e
+
e
q
q
(
q
=
u
, ,
.
s
, ) ̧ and
e
+
e
τ
+
τ
,
and combinatorial background,
Υ
(4
S
)
B
0
B
0
or
B
+
B
in which the tag-
B
is misreconstructed. These events are
characterized by a smooth
m
ES
distribution.
If multiple tag-
B
candidates are reconstructed in the
event, we select the one with the lowest value of
|
E
|
. Af-
ter the reconstruction of the tag-
B
, we require the pres-
ence of only one well-reconstructed track (signal track),
with charge opposite to that of the tag-
B
. The pu-
rity
P
of each reconstructed tag-
B
decay mode is esti-
mated as the ratio of the number of peaking events with
m
ES
>
5
.
27 GeV to the total number of events in the
same range. The yield in data is determined by means
of an extended unbinned maximum likelihood fit to the
m
ES
distribution, as shown in Fig. 1. We use a phe-
nomenologically motivated threshold function (ARGUS
function [14]) as probability density function (PDF) to
describe the continuum and combinatorial background
components in the fit, while for the correctly recon-
structed tag-
B
component we use a Gaussian distribu-
tion plus an exponential tail for the PDF (Crystal Ball
function) [15]. We use only events with the tag-
B
recon-
structed in decay modes with
P
>
0
.
1. Combinatorial
and continuum background distributions in any discrim-
inating variable are estimated from a sideband in
m
ES
(5
.
209 GeV
< m
ES
<
5
.
260 GeV) and are extrapolated
into the signal region (
m
ES
>
5
.
270 GeV) using the re-
sults of a fit to an ARGUS function. The peaking
B
+
B
background shape is determined from
B
+
B
MC, after
subtraction of the combinatorial component to avoid dou-
ble counting.
FIG. 1: Fit to the
m
ES
distribution in data. Dots are data,
the upper curve is the global fit result and the lower curve rep
-
resents the fitted combinatorial and continuum background.
5
The signal-side
τ
lepton is reconstructed in four de-
cay modes:
τ
+
e
+
ν
̄
ν
,
τ
+
μ
+
ν
̄
ν
,
τ
+
π
+
ν
, and
τ
+
ρ
+
ν
, totaling approximately 70% of all
τ
decays.
We separate the event sample into four categories using
particle identification criteria applied to the signal track
(
e
+
,
μ
+
, and
π
+
). The
τ
+
ρ
+
ν
sample is obtained by
associating the signal track
π
+
with a
π
0
reconstructed
from a pair of neutral clusters with an invariant mass
between 115 MeV
/c
2
and 155 MeV
/c
2
.
In order to remove the
e
+
e
τ
+
τ
background, we
impose
τ
mode dependent requirements on the ratio be-
tween the 2
nd
and the 0
th
Fox-Wolfram moments R2 [16]
calculated using all the tracks and neutral clusters of the
event. This preserves 90% of the
B
+
τ
+
ν
signal.
To reject continuum background, we use the absolute
value of cos
θ
T B
, the cosine of the angle in the CM frame
between the thrust axis [17] of the tag-
B
and the thrust
axis of the remaining charged and neutral candidates
in the event. For correctly reconstructed tag-B candi-
dates the
|
cos
θ
T B
|
distribution is expected to be uniform,
while for jet-like
e
+
e
q
q
continuum events it peaks
strongly at 1. In order to reject background from events
with a correctly reconstructed tag-
B
, we study the distri-
bution of several discriminating variables exploiting the
different kinematics between the signal and background
of the remaining reconstructed candidates. We use the
missing momentum polar angle in the laboratory frame
~p
miss
=
~p
CM
~p
tagB
~p
trk
neut
~p
i
, where
~p
CM
is
the total momentum of the beams,
~p
tagB
is the recon-
structed momentum of the tag-
B
, and
~p
trk
is the recon-
structed track momentum, and the sum is extended on
all the neutral candidates reconstructed in the calorime-
ter not assigned to the tag-
B
. For the
τ
+
π
+
ν
mode,
we combine
p
trk
(where the star denotes the CM frame)
and the cosine of the angle between
~p
miss
and the beam
axis (cos
θ
miss
) in a likelihood ratio
L
P
=
L
S
(
p
trk
,
cos
θ
miss
)
(
L
S
(
p
trk
,
cos
θ
miss
) +
L
B
(
p
trk
,
cos
θ
miss
))
,
(2)
where the signal (S) and background (B) likeli-
hoods have been obtained from the product of
the PDFs of the two discriminating variables:
L
S
(
p
trk
,
cos
θ
miss
) =
P
S
(
p
trk
)
P
S
(cos
θ
miss
) and
L
B
(
p
trk
,
cos
θ
miss
) =
P
B
(
p
trk
)
P
B
(cos
θ
miss
). Sim-
ilarly, for the
τ
+
ρ
+
ν
mode we combine four
discriminating variables in the likelihood ratio
L
P
:
cos
θ
miss
, the invariant mass of the
π
0
candidate, the
ρ
+
candidate momentum, and the invariant mass of
the
π
+
π
0
pair used to make the
ρ
+
candidate. The
PDFs used in the likelihood ratio for the signal and
background are determined from signal and
B
+
B
MC
samples, respectively.
The most powerful discriminating variable is
E
extra
,
defined as the sum of the energies of the neutral clus-
ters not associated with the tag-
B
or with the signal
π
0
from the
τ
+
ρ
+
ν
mode, and passing a minimum en-
ergy requirement (60 MeV). Signal events tend to peak
at low
E
extra
. Background events, which contain addi-
tional sources of neutral clusters, tend to be distributed
at higher values. The signal region in data is kept blind
until the end of the analysis chain when we extract the
signal yield, meaning that we do not use events in data
with
E
extra
<
400 MeV during the selection optimization
procedure and for the evaluation of background shapes.
We optimize the selection requirements, including
those on the purity
P
of the tag-
B
and the minimum
energy of the neutral clusters, minimizing the expected
uncertainty in the branching fraction fit. In order to es-
timate the uncertainty, which includes the statistical and
the dominant systematic sources, we run 1000 MC simu-
lated pseudo experiments extracted from the background
and signal expected
E
extra
distributions for a set of pos-
sible selection requirements, assuming a signal branching
fraction of 1
.
8
×
10
4
[8].
Table II summarizes the signal selection requirements
and Fig. 2 shows the
E
extra
distribution with all the selec-
tion requirements applied. The background events popu-
lating the low
E
extra
region are mostly semileptonic
B
de-
cays for the leptonic modes. For the
τ
+
π
+
ν
mode the
background is composed mostly of charmless hadronic
B
decays and semileptonic
B
decays with a muon in the
final state. For the
τ
+
ρ
+
ν
mode the backgrounds are
charmed hadronic
B
decays, semileptonic
B
decays with
a muon in the final state and a small fraction with a
τ
.
TABLE II: Optimized signal selection criteria for each
τ
mode.
Variable
e
+
μ
+
π
+
ρ
+
P
>
10%
Cluster energy ( MeV)
>
60
R
2
<
0
.
57
<
0
.
56
<
0
.
56
<
0
.
51
|
cos
θ
T B
|
<
0
.
95
<
0
.
90
<
0
.
65
<
0
.
8
L
P
>
0
.
30
>
0
.
45
We use an extended unbinned maximum likelihood fit
to the measured
E
extra
distribution to extract the
B
+
τ
+
ν
branching fraction. The likelihood function for the
N
k
candidates reconstructed in one of the four
τ
decay
modes
k
is
L
k
=
e
(
n
s,k
+
n
b,k
)
N
k
!
N
k
i
=1
{
n
s,k
P
s
k
(
E
i,k
) +
n
b,k
P
b
k
(
E
i,k
)
}
,
(3)
where
n
s,k
is the signal yield,
n
b,k
is the background yield,
E
i,k
is the
E
extra
value of the
i
th
event,
P
s
k
is the PDF of
signal events, and
P
b
k
is the PDF of background events.
The background yields in each decay mode are permitted
to float independently of each other in the fit, while the
signal yields are constrained to a single branching ratio
via the relation:
n
s,k
=
N
B
B
×
ǫ
k
× B
(4)
where
ǫ
k
is the reconstruction efficiency of a particular
τ
decay mode, and
B
is the
B
+
τ
+
ν
branching frac-
tion. The parameters
N
B
B
and
ǫ
k
are fixed in the fit
6
[GeV]
extra
E
0
0.2
0.4
0.6
0.8
/100 MeV
evt
N
0
50
100
150
200
250
300
(a)
[GeV]
extra
E
0
0.2
0.4
0.6
0.8
/100 MeV
evt
N
0
20
40
60
80
100
120
140
(b)
[GeV]
extra
E
0
0.2
0.4
0.6
0.8
/100 MeV
evt
N
0
20
40
60
80
100
120
(c)
[GeV]
extra
E
0
0.2
0.4
0.6
0.8
/100 MeV
evt
N
0
2
4
6
8
10
(d)
[GeV]
extra
E
0
0.2
0.4
0.6
0.8
/100 MeV
evt
N
0
5
10
15
20
25
(e)
FIG. 2:
E
extra
distribution in data (points with error bars)
with all selection requirements applied and fit results over
laid.
The hatched histogram is the background and the dashed
component is the best-fit signal excess distribution. Plot (
a)
shows all
τ
decay modes fitted simultaneously. Lower plots
show the projection of the simultaneous fit result on the four
analyzed
τ
decay modes: (b)
τ
+
e
+
ν
̄
ν
,
(c)
τ
+
μ
+
ν
̄
ν
,
(d)
τ
+
π
+
ν
, (e)
τ
+
ρ
+
ν
.
while
B
is allowed to vary. The reconstruction efficiencies
ǫ
k
, which include the
τ
branching fractions, are obtained
from MC-simulated signal events (see Table III). Since
the tag-
B
reconstruction efficiency is included in
ǫ
k
and
is estimated from the signal MC, we apply a correction
factor of
R
data
/
MC
= 0
.
926
±
0
.
010 to take into account
data/MC differences. This is derived from the ratio of
the peaking component of the
m
ES
distribution for the
hadronic tag-
B
in data and in MC simulated events.
The signal PDF is obtained from a high statistics sig-
nal sample of MC simulated data. We use a sample of
fully reconstructed events to correct the signal PDF for
data/MC disagreement In addition to the reconstructed
tag-
B
, a second
B
is reconstructed in the hadronic or the
semileptonic decay mode using tracks and neutral clus-
ters not assigned to the tag-
B
. In order to estimate the
correction to the signal PDF, we compare the distribu-
tion of
E
extra
in this double tagged event sample from
experimental data and MC simulations. The MC distri-
butions are normalized to the experimental data and the
comparison is shown in Fig. 3. We extract the correction
function by taking the ratio of the two distributions and
fitting it with a second order polynomial.
FIG. 3:
E
extra
distribution for double tagged events. The
“signal”
B
is reconstructed in hadronic decays (left plot) or
semileptonic decays (right plot). Points are data and boxes
are MC simulation.
We determine the PDF of the combinatorial back-
ground from the
m
ES
sideband. The normalization of
this component in the signal region is obtained by fit-
ting the
m
ES
distribution after the selection has been
applied. The shape of the peaking background is taken
from
B
+
B
MC. The two background components are
added together into a single background PDF. We esti-
mate the branching fraction by minimizing
ln
L
, where
L
= Π
4
k
=1
L
k
, and
L
k
is given in Eq. 3. The projections
of the fit results are shown in Fig. 2.
We observe an excess of events with respect to the ex-
pected background level and measure a branching frac-
tion of
B
(
B
+
τ
+
ν
) = (1
.
83
+0
.
53
0
.
49
)
×
10
4
, where the
uncertainty is statistical. Table III summarizes the re-
sults from the fit. We evaluate the significance of the
observed signal, including only statistical uncertainty, as
S
=
2 ln(
L
s
+
b
/
L
b
), where
L
s
+
b
and
L
b
denote the
obtained maximum likelihood values in the signal and
background, and the background only hypotheses, re-
spectively. We find
S
= 4
.
2
σ
.
Additive systematic uncertainties are due to the un-
certainties in the signal and background
E
extra
PDF