Search for Long-Lived Particles in
e
þ
e
−
Collisions
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
E. Grauges,
2
A. Palano,
3a,3b
G. Eigen,
4
B. Stugu,
4
D. N. Brown,
5
L. T. Kerth,
5
Yu. G. Kolomensky,
5
M. J. Lee,
5
G. Lynch,
5
H. Koch,
6
T. Schroeder,
6
C. Hearty,
7
T. S. Mattison,
7
J. A. McKenna,
7
R. Y. So,
7
A. Khan,
8
V. E. Blinov,
9a,9b,9c
A. R. Buzykaev,
9a
V. P. Druzhinin,
9a,9b
V. B. Golubev,
9a,9b
E. A. Kravchenko,
9a,9b
A. P. Onuchin,
9a,9b,9c
S. I. Serednyakov,
9a,9b
Yu. I. Skovpen,
9a,9b
E. P. Solodov,
9a,9b
K. Yu. Todyshev,
9a,9b
A. J. Lankford,
10
B. Dey,
11
J. W. Gary,
11
O. Long,
11
C. Campagnari,
12
M. Franco Sevilla,
12
T. M. Hong,
12
D. Kovalskyi,
12
J. D. Richman,
12
C. A. West,
12
A. M. Eisner,
13
W. S. Lockman,
13
W. Panduro Vazquez,
13
B. A. Schumm,
13
A. Seiden,
13
D. S. Chao,
14
C. H. Cheng,
14
B. Echenard,
14
K. T. Flood,
14
D. G. Hitlin,
14
T. S. Miyashita,
14
P. Ongmongkolkul,
14
F. C. Porter,
14
M. Röhrken,
14
R. Andreassen,
15
Z. Huard,
15
B. T. Meadows,
15
B. G. Pushpawela,
15
M. D. Sokoloff,
15
L. Sun,
15
P. C. Bloom,
16
W. T. Ford,
16
A. Gaz,
16
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17
,
†
W. H. Toki,
17
B. Spaan,
18
D. Bernard,
19
M. Verderi,
19
S. Playfer,
20
D. Bettoni,
21a
C. Bozzi,
21a
R. Calabrese,
21a,21b
G. Cibinetto,
21a,21b
E. Fioravanti,
21a,21b
I. Garzia,
21a,21b
E. Luppi,
21a,21b
L. Piemontese,
21a
V. Santoro,
21a
A. Calcaterra,
22
R. de Sangro,
22
G. Finocchiaro,
22
S. Martellotti,
22
P. Patteri,
22
I. M. Peruzzi,
22
,
‡
M. Piccolo,
22
M. Rama,
22
A. Zallo,
22
R. Contri,
23a,23b
M. Lo Vetere,
23a,23b
M. R. Monge,
23a,23b
S. Passaggio,
23a
C. Patrignani,
23a,23b
E. Robutti,
23a
B. Bhuyan,
24
V. Prasad,
24
A. Adametz,
25
U. Uwer,
25
H. M. Lacker,
26
U. Mallik,
27
C. Chen,
28
J. Cochran,
28
S. Prell,
28
H. Ahmed,
29
A. V. Gritsan,
30
N. Arnaud,
31
M. Davier,
31
D. Derkach,
31
G. Grosdidier,
31
F. Le Diberder,
31
A. M. Lutz,
31
B. Malaescu,
31
,§
P. Roudeau,
31
A. Stocchi,
31
G. Wormser,
31
D. J. Lange,
32
D. M. Wright,
32
J. P. Coleman,
33
J. R. Fry,
33
E. Gabathuler,
33
D. E. Hutchcroft,
33
D. J. Payne,
33
C. Touramanis,
33
A. J. Bevan,
34
F. Di Lodovico,
34
R. Sacco,
34
G. Cowan,
35
D. N. Brown,
36
C. L. Davis,
36
A. G. Denig,
37
M. Fritsch,
37
W. Gradl,
37
K. Griessinger,
37
A. Hafner,
37
K. R. Schubert,
37
R. J. Barlow,
38
,
∥
G. D. Lafferty,
38
R. Cenci,
39
B. Hamilton,
39
A. Jawahery,
39
D. A. Roberts,
39
R. Cowan,
40
G. Sciolla,
40
R. Cheaib,
41
P. M. Patel,
41
,*
S. H. Robertson,
41
N. Neri,
42a
F. Palombo,
42a,42b
L. Cremaldi,
43
R. Godang,
43
,¶
P. Sonnek,
43
D. J. Summers,
43
M. Simard,
44
P. Taras,
44
G. De Nardo,
45a,45b
G. Onorato,
45a,45b
C. Sciacca,
45a,45b
M. Martinelli,
46
G. Raven,
46
C. P. Jessop,
47
J. M. LoSecco,
47
K. Honscheid,
48
R. Kass,
48
E. Feltresi,
49a,49b
M. Margoni,
49a,49b
M. Morandin,
49a
M. Posocco,
49a
M. Rotondo,
49a
G. Simi,
49a,49b
F. Simonetto,
49a,49b
R. Stroili,
49a,49b
S. Akar,
50
E. Ben-Haim,
50
M. Bomben,
50
G. R. Bonneaud,
50
H. Briand,
50
G. Calderini,
50
J. Chauveau,
50
Ph. Leruste,
50
G. Marchiori,
50
J. Ocariz,
50
M. Biasini,
51a,51b
E. Manoni,
51a
S. Pacetti,
51a,51b
A. Rossi,
51a
C. Angelini,
52a,52b
G. Batignani,
52a,52b
S. Bettarini,
52a,52b
M. Carpinelli,
52a,52b
,**
G. Casarosa,
52a,52b
A. Cervelli,
52a,52b
M. Chrzaszcz,
52a
F. Forti,
52a,52b
M. A. Giorgi,
52a,52b
A. Lusiani,
52a,52c
B. Oberhof,
52a,52b
E. Paoloni,
52a,52b
A. Perez,
52a
G. Rizzo,
52a,52b
J. J. Walsh,
52a
D. Lopes Pegna,
53
J. Olsen,
53
A. J. S. Smith,
53
F. Anulli,
54a
R. Faccini,
54a,54b
F. Ferrarotto,
54a
F. Ferroni,
54a,54b
M. Gaspero,
54a,54b
L. Li Gioi,
54a
A. Pilloni,
54a,54b
G. Piredda,
54a
C. Bünger,
55
S. Dittrich,
55
O. Grünberg,
55
M. Hess,
55
T. Leddig,
55
C. Voß,
55
R. Waldi,
55
T. Adye,
56
E. O. Olaiya,
56
F. F. Wilson,
56
S. Emery,
57
G. Vasseur,
57
D. Aston,
58
D. J. Bard,
58
C. Cartaro,
58
M. R. Convery,
58
J. Dorfan,
58
G. P. Dubois-Felsmann,
58
W. Dunwoodie,
58
M. Ebert,
58
R. C. Field,
58
B. G. Fulsom,
58
M. T. Graham,
58
C. Hast,
58
W. R. Innes,
58
P. Kim,
58
D. W. G. S. Leith,
58
D. Lindemann,
58
S. Luitz,
58
V. Luth,
58
H. L. Lynch,
58
D. B. MacFarlane,
58
D. R. Muller,
58
H. Neal,
58
M. Perl,
58
,*
T. Pulliam,
58
B. N. Ratcliff,
58
A. Roodman,
58
A. A. Salnikov,
58
R. H. Schindler,
58
A. Snyder,
58
D. Su,
58
M. K. Sullivan,
58
J. Va
’
vra,
58
W. J. Wisniewski,
58
H. W. Wulsin,
58
M. V. Purohit,
59
R. M. White,
59
,
††
J. R. Wilson,
59
A. Randle-Conde,
60
S. J. Sekula,
60
M. Bellis,
61
P. R. Burchat,
61
E. M. T. Puccio,
61
M. S. Alam,
62
J. A. Ernst,
62
R. Gorodeisky,
63
N. Guttman,
63
D. R. Peimer,
63
A. Soffer,
63
S. M. Spanier,
64
J. L. Ritchie,
65
R. F. Schwitters,
65
B. C. Wray,
65
J. M. Izen,
66
X. C. Lou,
66
F. Bianchi,
67a,67b
F. De Mori,
67a,67b
A. Filippi,
67a
D. Gamba,
67a,67b
L. Lanceri,
68a,68b
L. Vitale,
68a,68b
F. Martinez-Vidal,
69
A. Oyanguren,
69
P. Villanueva-Perez,
69
J. Albert,
70
Sw. Banerjee,
70
A. Beaulieu,
70
F. U. Bernlochner,
70
H. H. F. Choi,
70
G. J. King,
70
R. Kowalewski,
70
M. J. Lewczuk,
70
T. Lueck,
70
I. M. Nugent,
70
J. M. Roney,
70
R. J. Sobie,
70
N. Tasneem,
70
T. J. Gershon,
71
P. F. Harrison,
71
T. E. Latham,
71
H. R. Band,
72
S. Dasu,
72
Y. Pan,
72
R. Prepost,
72
and S. L. Wu
72
(The
B
A
B
AR
Collaboration)
1
Laboratoire d
’
Annecy-le-Vieux de Physique des Particules (LAPP), Université de Savoie, CNRS/IN2P3,
F-74941 Annecy-Le-Vieux, France
2
Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain
3a
INFN Sezione di Bari, I-70126 Bari, Italy
3b
Dipartimento di Fisica, Università di Bari, I-70126 Bari, Italy
PRL
114,
171801 (2015)
PHYSICAL REVIEW LETTERS
week ending
1 MAY 2015
0031-9007
=
15
=
114(17)
=
171801(7)
171801-1
© 2015 American Physical Society
4
University of Bergen, Institute of Physics, N-5007 Bergen, Norway
5
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
6
Ruhr Universität Bochum, Institut für Experimentalphysik 1, D-44780 Bochum, Germany
7
University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
8
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
9a
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090, Russia
9b
Novosibirsk State University, Novosibirsk 630090, Russia
9c
Novosibirsk State Technical University, Novosibirsk 630092, Russia
10
University of California at Irvine, Irvine, California 92697, USA
11
University of California at Riverside, Riverside, California 92521, USA
12
University of California at Santa Barbara, Santa Barbara, California 93106, USA
13
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
14
California Institute of Technology, Pasadena, California 91125, USA
15
University of Cincinnati, Cincinnati, Ohio 45221, USA
16
University of Colorado, Boulder, Colorado 80309, USA
17
Colorado State University, Fort Collins, Colorado 80523, USA
18
Technische Universität Dortmund, Fakultät Physik, D-44221 Dortmund, Germany
19
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
20
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
21a
INFN Sezione di Ferrara, I-44122 Ferrara, Italy
21b
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, I-44122 Ferrara, Italy
22
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
23a
INFN Sezione di Genova, I-16146 Genova, Italy
23b
Dipartimento di Fisica, Università di Genova, I-16146 Genova, Italy
24
Indian Institute of Technology Guwahati, Guwahati, Assam 781 039, India
25
Universität Heidelberg, Physikalisches Institut, D-69120 Heidelberg, Germany
26
Humboldt-Universität zu Berlin, Institut für Physik, D-12489 Berlin, Germany
27
University of Iowa, Iowa City, Iowa 52242, USA
28
Iowa State University, Ames, Iowa 50011-3160, USA
29
Physics Department, Jazan University, Jazan 22822, Kingdom of Saudia Arabia
30
Johns Hopkins University, Baltimore, Maryland 21218, USA
31
Laboratoire de l
’
Accélérateur Linéaire, IN2P3/CNRS et Université Paris-Sud 11,
Centre Scientifique d
’
Orsay, F-91898 Orsay Cedex, France
32
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
33
University of Liverpool, Liverpool L69 7ZE, United Kingdom
34
Queen Mary, University of London, London E1 4NS, United Kingdom
35
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
36
University of Louisville, Louisville, Kentucky 40292, USA
37
Johannes Gutenberg-Universität Mainz, Institut für Kernphysik, D-55099 Mainz, Germany
38
University of Manchester, Manchester M13 9PL, United Kingdom
39
University of Maryland, College Park, Maryland 20742, USA
40
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
41
McGill University, Montréal, Québec H3A 2T8, Canada
42a
INFN Sezione di Milano, I-20133 Milano, Italy
42b
Dipartimento di Fisica, Università di Milano, I-20133 Milano, Italy
43
University of Mississippi, University, Mississippi 38677, USA
44
Université de Montréal, Physique des Particules, Montréal, Québec H3C 3J7, Canada
45a
INFN Sezione di Napoli, I-80126 Napoli, Italy
45b
Dipartimento di Scienze Fisiche, Università di Napoli Federico II, I-80126 Napoli, Italy
46
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, Netherlands
47
University of Notre Dame, Notre Dame, Indiana 46556, USA
48
Ohio State University, Columbus, Ohio 43210, USA
49a
INFN Sezione di Padova, I-35131 Padova, Italy
49b
Dipartimento di Fisica, Università di Padova, I-35131 Padova, Italy
50
Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS, Université Pierre et Marie Curie-Paris6,
Université Denis Diderot-Paris7, F-75252 Paris, France
51a
INFN Sezione di Perugia, I-06123 Perugia, Italy
51b
Dipartimento di Fisica, Università di Perugia, I-06123 Perugia, Italy
52a
INFN Sezione di Pisa, I-56127 Pisa, Italy
52b
Dipartimento di Fisica, Università di Pisa, I-56127 Pisa, Italy
PRL
114,
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PHYSICAL REVIEW LETTERS
week ending
1 MAY 2015
171801-2
52c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
53
Princeton University, Princeton, New Jersey 08544, USA
54a
INFN Sezione di Roma, I-00185 Roma, Italy
54b
Dipartimento di Fisica, Università di Roma La Sapienza, I-00185 Roma, Italy
55
Universität Rostock, D-18051 Rostock, Germany
56
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
57
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
58
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
59
University of South Carolina, Columbia, South Carolina 29208, USA
60
Southern Methodist University, Dallas, Texas 75275, USA
61
Stanford University, Stanford, California 94305-4060, USA
62
State University of New York, Albany, New York 12222, USA
63
School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
64
University of Tennessee, Knoxville, Tennessee 37996, USA
65
University of Texas at Austin, Austin, Texas 78712, USA
66
University of Texas at Dallas, Richardson, Texas 75083, USA
67a
INFN Sezione di Torino, I-10125 Torino, Italy
67b
Dipartimento di Fisica, Università di Torino, I-10125 Torino, Italy
68a
INFN Sezione di Trieste, I-34127 Trieste, Italy
68b
Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
69
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
70
University of Victoria, Victoria, British Columbia V8W 3P6, Canada
71
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
72
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 10 February 2015; published 29 April 2015)
We present a search for a neutral, long-lived particle
L
that is produced in
e
þ
e
−
collisions and decays at a
significant distance from the
e
þ
e
−
interaction point into various flavor combinations of two oppositely
charged tracks. The analysis uses an
e
þ
e
−
data sample with a luminosity of
489
.
1
fb
−
1
collected by the
BABAR
detector at the
Υ
ð
4
S
Þ
,
Υ
ð
3
S
Þ
, and
Υ
ð
2
S
Þ
resonances and just below the
Υ
ð
4
S
Þ
. Fitting the two-
track mass distribution in search of a signal peak, we do not observe a significant signal, and set 90%
confidence level upper limits on the product of the
L
production cross section, branching fraction, and
reconstruction efficiency for six possible two-body
L
decay modes as a function of the
L
mass. The
efficiency is given for each final state as a function of the mass, lifetime, and transverse momentum of the
candidate, allowing application of the upper limits to any production model. In addition, upper limits are
provided on the branching fraction
B
ð
B
→
X
s
L
Þ
, where
X
s
is a strange hadronic system.
DOI:
10.1103/PhysRevLett.114.171801
PACS numbers: 13.66.Hk, 14.80.Ec
Recent anomalous astrophysical observations
[1
–
3]
have
generated interest in GeV-scale hidden-sector states that
may be long-lived
[4
–
12]
. Searches for long-lived particles
have been performed both in the sub-GeV
[13
–
15]
and
multi-GeV
[16
–
21]
mass ranges. Dedicated experiments to
search for such particles have been proposed
[22]
or are
under construction
[23]
. However, the
O
ð
1
GeV
=c
2
Þ
mass
range has remained mostly unexplored, especially in a
heavy-flavor environment.
B
factories offer an ideal labo-
ratory to probe this regime. Previously, the only
B
-factory
results were from a search for a heavy neutralino by the
Belle Collaboration
[24]
.
We search, herein, for a neutral, long-lived particle
L
,
which decays into any of the final states
f
¼
e
þ
e
−
,
μ
þ
μ
−
,
e
μ
∓
,
π
þ
π
−
,
K
þ
K
−
,or
K
π
∓
. A displaced vertex and two-
body decay kinematics constitute the main means for
background suppression, and the search is performed by
fitting the distribution of the
L
-candidate mass.
The results are presented in two ways. In the
“
model-
independent
”
presentation, no assumption is made regard-
ing the production mechanism of the
L
. Rather, we present
limits on the product of the inclusive production cross
section
σ
ð
e
þ
e
−
→
LX
Þ
, branching fraction
B
ð
L
→
f
Þ
, and
efficiency
ε
ð
f
Þ
for each of the two-body final states
f
,
where
X
is any set of particles. As Supplemental Material to
this Letter
[25]
, we provide tables of the efficiency as a
function of
L
mass
m
, transverse
[26]
momentum
p
T
in the
center-of-mass (c.m.) frame, and proper decay distance
c
τ
,
assuming the
L
to be a spin-zero particle. The provided
upper limits, efficiencies, and
p
T
distributions of the
simulated events used to obtain the efficiencies facilitate
the application of the model-independent presentation of
the results to any specific model of
L
production. In the
“
model-dependent
”
presentation, we provide limits on the
branching fraction for the decay
B
→
X
s
L
, where
X
s
is a
hadronic system with strangeness
−
1
. This presentation is
PRL
114,
171801 (2015)
PHYSICAL REVIEW LETTERS
week ending
1 MAY 2015
171801-3
motivated by Higgs-portal models of dark matter and other
hidden sectors
[8
–
11]
.
The data were collected with the
BABAR
detector at the
PEP-II asymmetric-energy
e
þ
e
−
collider at SLAC National
Accelerator Laboratory. The sample consists of
404
.
0
1
.
7
fb
−
1
collected at a c.m. energy corresponding to the
Υ
ð
4
S
Þ
resonance, an
“
off-resonance
”
sample of
43
.
74
0
.
20
fb
−
1
collected about 40 MeV below the
Υ
ð
4
S
Þ
peak,
27
.
85
0
.
18
fb
−
1
collected at the
Υ
ð
3
S
Þ
, and
13
.
56
0
.
09
fb
−
1
taken at the
Υ
ð
2
S
Þ
[27]
. The
Υ
ð
4
S
Þ
sample
contains
ð
448
.
4
2
.
2
Þ
×
10
6
B
̄
B
pairs, and the
Υ
ð
3
S
Þ
and
Υ
ð
2
S
Þ
samples have
ð
121
.
3
1
.
2
Þ
×
10
6
Υ
ð
3
S
Þ
and
ð
98
.
3
0
.
9
Þ
×
10
6
Υ
ð
2
S
Þ
mesons, respectively
[28]
.An
additional
Υ
ð
4
S
Þ
sample of
20
.
37
0
.
09
fb
−
1
is used to
validate the analysis procedure and is not included in the
final analysis.
The
BABAR
detector and its operation are described in
detail in Refs.
[29]
and
[30]
. Charged-particle momenta are
measured in a tracking system consisting of a five-layer,
double-sided silicon vertex detector (SVT) and a 40-layer
drift chamber (DCH), both located in a 1.5 Taxial magnetic
field. Electron and photon energies are measured in a CsI
(Tl) electromagnetic calorimeter (EMC) inside the magnet
coil. Charged-particle identification (PID) is performed
using an internally reflecting, ring-imaging Cherenkov
detector, as well as the energy loss measured by the
SVT, DCH, and EMC. Muons are identified mainly with
the instrumented magnetic-flux return.
Using Monte Carlo (MC) simulations, we determine
both the signal mass resolution and reconstruction effi-
ciency. The events are produced with the
EVTGEN
[31]
event generator, taking the
L
spin to be zero. We generate
two types of signal MC samples. In the first type, which is
used to create the efficiency tables
[25]
for the model-
independent presentation, the
L
is produced at 11 different
masses,
m
MC
0
¼
0
.
5
,1,2,3,4,5,6,7,8,9,and
9
.
5
GeV
=c
2
.For
m
MC
0
≤
4
GeV
=c
2
, the
L
is created in
the process
e
þ
e
−
→
B
̄
B
, with one
B
meson decaying to
L
þ
N
π
(
N
¼
1
, 2, or 3) and the other
B
decaying
generically. At higher masses, the production process is
Υ
ð
4
S
Þ
→
L
þ
N
π
. In both cases, the
L
is produced uni-
formly throughout the available phase space, with an
average transverse decay distance of 20 cm. The events
are subsequently reweighted to obtain efficiencies for other
decay lengths. Note that these specific processes do not
reflect preferred hypotheses about the production mecha-
nism, nor do the results depend on these processes. Rather,
they are a convenient method to populate the kinematic
range for the efficiency tables.
The second type of signal MC sample, used for the
model-dependent presentation of the results, contains
B
→
X
s
L
decays, for the seven mass values
m
MC
0
¼
0
.
5
,
1, 2, 3, 3.5, 4, and
4
.
5
GeV
=c
2
. The
X
s
is nominally taken
to be 10%
K
, 25%
K
ð
892
Þ
, and 65%
K
ð
1680
Þ
[32]
,
with the high-mass tail of the
X
s
spectrum suppressed by
phase-space limitations, especially for heavy
L
states. This
choice of
X
s
composition results in an
L
-momentum
spectrum as a function of
m
MC
0
that reproduces the dimuon
spectrum for
B
→
X
s
μ
þ
μ
−
in events generated with
EVTGEN
using the
BTOXSLL
model
[31]
. The other
B
meson
in the event decays generically.
In addition to the signal MC samples, background MC
samples are used for optimizing the event selection criteria
and studying the signal extraction method. The background
samples are
e
þ
e
−
→
B
̄
B
(produced with
EVTGEN
[31]
),
τ
þ
τ
−
,
μ
þ
μ
−
(KK2F
[33]
),
e
þ
e
−
(
BHWIDE
[34]
), and
q
̄
q
events (
JETSET
[35]
), where
q
is a
u
,
d
,
s
,or
c
quark. The
detector response is simulated with
GEANT
4
[36]
.
The
L
candidates are reconstructed from pairs of
oppositely charged tracks, identified as either
e
þ
e
−
,
μ
þ
μ
−
,
e
μ
∓
,
π
þ
π
−
,
K
þ
K
−
,or
K
π
∓
. The PID efficiency
depends on the track momentum, and is in the range 0.96
–
0.99 for electrons, 0.60
–
0.88 for muons, and 0.90
–
0.98 for
kaons and pions. The pion misidentification probability is
less than 0.01 for the electron PID criteria, less than 0.03 for
the muon criteria, and averages at 0.06 for the kaon criteria.
A track may have different PID assignments and may
appear in multiple pairs. Each track must satisfy
d
0
=
σ
d
0
>
3
, where
d
0
is the transverse distance of closest
approach of the track to the
e
þ
e
−
interaction point (IP), and
σ
d
0
is the
d
0
uncertainty, calculated from the SVTand DCH
hit position uncertainties during the track reconstruction.
The two tracks are fit to a common vertex, and the
χ
2
value
of the fit is required to be smaller than 10 for one degree of
freedom. The two-dimensional vector
~
r
between the IP and
the vertex in the transverse plane must have length
r
≡
j
~
r
j
in the range
1
<r<
50
cm, and the uncertainty on
r
is
required to satisfy
σ
r
<
0
.
2
cm. We require the angle
α
between
~
r
and the
L
-candidate transverse-momentum
vector to satisfy
α
<
0
.
01
rad. The uncertainty
σ
m
on the
measured
L
-candidate mass
m
must be less than
0
.
2
GeV
=c
2
. The
L
candidate is discarded if either of
the tracks has SVT or DCH hits located between the IP
and the vertex, or if the vertex is within the material of
the beam pipe wall, the DCH support tube, or the DCH
inner cylinder. Candidates must satisfy the following
decay-mode-specific invariant-mass criteria:
m
e
þ
e
−
>
0
.
44
GeV
=c
2
,
m
μ
þ
μ
−
<
0
.
37
GeV
=c
2
or
m
μ
þ
μ
−
>
0
.
5
GeV
=c
2
,
m
e
μ
∓
>
0
.
48
GeV
=c
2
,
m
π
þ
π
−
>
0
.
86
GeV
=c
2
,
m
K
þ
K
−
>
1
.
35
GeV
=c
2
, and
m
K
π
∓
>
1
.
05
GeV
=c
2
.
These criteria reject background from
K
0
S
→
π
þ
π
−
and
Λ
→
p
π
−
decays. In addition, other than in the
μ
þ
μ
−
mode,
they exclude low-mass regions in which the mass distri-
butions of background MC events are not smooth and,
therefore, are incompatible with the background descrip-
tion method outlined below. We require at least one of the
tracks of
L
→
μ
þ
μ
−
candidates with
m
≥
8
GeV
=c
2
to
have an SVT hit. This rejects candidates that decay into
μ
þ
μ
−
within the material of the final-focusing magnets and,
thus, have poor mass resolution. These selection criteria are
PRL
114,
171801 (2015)
PHYSICAL REVIEW LETTERS
week ending
1 MAY 2015
171801-4