B
A
B
AR
-PUB-21/003
SLAC-PUB-17617
Search for Lepton Flavor Violation in
Υ
(3
S
)
→
e
±
μ
∓
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
E. Grauges,
2
A. Palano,
3
G. Eigen,
4
D. N. Brown,
5
Yu. G. Kolomensky,
5
M. Fritsch,
6
H. Koch,
6
T. Schroeder,
6
R. Cheaib
b
,
7
C. Hearty
ab
,
7
T. S. Mattison
b
,
7
J. A. McKenna
b
,
7
R. Y. So
b
,
7
V. E. Blinov
abc
,
8
A. R. Buzykaev
a
,
8
V. P. Druzhinin
ab
,
8
V. B. Golubev
ab
,
8
E. A. Kozyrev
ab
,
8
E. A. Kravchenko
ab
,
8
A. P. Onuchin
abc
,
8,
∗
S. I. Serednyakov
ab
,
8
Yu. I. Skovpen
ab
,
8
E. P. Solodov
ab
,
8
K. Yu. Todyshev
ab
,
8
A. J. Lankford,
9
B. Dey,
10
J. W. Gary,
10
O. Long,
10
A. M. Eisner,
11
W. S. Lockman,
11
W. Panduro Vazquez,
11
D. S. Chao,
12
C. H. Cheng,
12
B. Echenard,
12
K. T. Flood,
12
D. G. Hitlin,
12
J. Kim,
12
Y. Li,
12
D. X. Lin,
12
S. Middleton,
12
T. S. Miyashita,
12
P. Ongmongkolkul,
12
J. Oyang,
12
F. C. Porter,
12
M. R ̈ohrken,
12
Z. Huard,
13
B. T. Meadows,
13
B. G. Pushpawela,
13
M. D. Sokoloff,
13
L. Sun,
13,
†
J. G. Smith,
14
S. R. Wagner,
14
D. Bernard,
15
M. Verderi,
15
D. Bettoni
a
,
16
C. Bozzi
a
,
16
R. Calabrese
ab
,
16
G. Cibinetto
ab
,
16
E. Fioravanti
ab
,
16
I. Garzia
ab
,
16
E. Luppi
ab
,
16
V. Santoro
a
,
16
A. Calcaterra,
17
R. de Sangro,
17
G. Finocchiaro,
17
S. Martellotti,
17
P. Patteri,
17
I. M. Peruzzi,
17
M. Piccolo,
17
M. Rotondo,
17
A. Zallo,
17
S. Passaggio,
18
C. Patrignani,
18,
‡
B. J. Shuve,
19
H. M. Lacker,
20
B. Bhuyan,
21
U. Mallik,
22
C. Chen,
23
J. Cochran,
23
S. Prell,
23
A. V. Gritsan,
24
N. Arnaud,
25
M. Davier,
25
F. Le Diberder,
25
A. M. Lutz,
25
G. Wormser,
25
D. J. Lange,
26
D. M. Wright,
26
J. P. Coleman,
27
E. Gabathuler,
27,
∗
D. E. Hutchcroft,
27
D. J. Payne,
27
C. Touramanis,
27
A. J. Bevan,
28
F. Di Lodovico,
28,
§
R. Sacco,
28
G. Cowan,
29
Sw. Banerjee,
30
D. N. Brown,
30,
¶
C. L. Davis,
30
A. G. Denig,
31
W. Gradl,
31
K. Griessinger,
31
A. Hafner,
31
K. R. Schubert,
31
R. J. Barlow,
32,
∗∗
G. D. Lafferty,
32
R. Cenci,
33
A. Jawahery,
33
D. A. Roberts,
33
R. Cowan,
34
S. H. Robertson
ab
,
35
R. M. Seddon
b
,
35
N. Neri
a
,
36
F. Palombo
ab
,
36
L. Cremaldi,
37
R. Godang,
37,
††
D. J. Summers,
37,
∗
P. Taras,
38
G. De Nardo,
39
C. Sciacca,
39
G. Raven,
40
C. P. Jessop,
41
J. M. LoSecco,
41
K. Honscheid,
42
R. Kass,
42
A. Gaz
a
,
43
M. Margoni
ab
,
43
M. Posocco
a
,
43
G. Simi
ab
,
43
F. Simonetto
ab
,
43
R. Stroili
ab
,
43
S. Akar,
44
E. Ben-Haim,
44
M. Bomben,
44
G. R. Bonneaud,
44
G. Calderini,
44
J. Chauveau,
44
G. Marchiori,
44
J. Ocariz,
44
M. Biasini
ab
,
45
E. Manoni
a
,
45
A. Rossi
a
,
45
G. Batignani
ab
,
46
S. Bettarini
ab
,
46
M. Carpinelli
ab
,
46,
‡‡
G. Casarosa
ab
,
46
M. Chrzaszcz
a
,
46
F. Forti
ab
,
46
M. A. Giorgi
ab
,
46
A. Lusiani
ac
,
46
B. Oberhof
ab
,
46
E. Paoloni
ab
,
46
M. Rama
a
,
46
G. Rizzo
ab
,
46
J. J. Walsh
a
,
46
L. Zani
ab
,
46
A. J. S. Smith,
47
F. Anulli
a
,
48
R. Faccini
ab
,
48
F. Ferrarotto
a
,
48
F. Ferroni
a
,
48,
§§
A. Pilloni
ab
,
48
G. Piredda
a
,
48,
∗
C. B ̈unger,
49
S. Dittrich,
49
O. Gr ̈unberg,
49
M. Heß,
49
T. Leddig,
49
C. Voß,
49
R. Waldi,
49
T. Adye,
50
F. F. Wilson,
50
S. Emery,
51
G. Vasseur,
51
D. Aston,
52
C. Cartaro,
52
M. R. Convery,
52
J. Dorfan,
52
W. Dunwoodie,
52
M. Ebert,
52
R. C. Field,
52
B. G. Fulsom,
52
M. T. Graham,
52
C. Hast,
52
W. R. Innes,
52,
∗
P. Kim,
52
D. W. G. S. Leith,
52,
∗
S. Luitz,
52
D. B. MacFarlane,
52
D. R. Muller,
52
H. Neal,
52
B. N. Ratcliff,
52
A. Roodman,
52
M. K. Sullivan,
52
J. Va’vra,
52
W. J. Wisniewski,
52
M. V. Purohit,
53
J. R. Wilson,
53
A. Randle-Conde,
54
S. J. Sekula,
54
H. Ahmed,
55
M. Bellis,
56
P. R. Burchat,
56
E. M. T. Puccio,
56
M. S. Alam,
57
J. A. Ernst,
57
R. Gorodeisky,
58
N. Guttman,
58
D. R. Peimer,
58
A. Soffer,
58
S. M. Spanier,
59
J. L. Ritchie,
60
R. F. Schwitters,
60
J. M. Izen,
61
X. C. Lou,
61
F. Bianchi
ab
,
62
F. De Mori
ab
,
62
A. Filippi
a
,
62
D. Gamba
ab
,
62
L. Lanceri,
63
L. Vitale,
63
F. Martinez-Vidal,
64
A. Oyanguren,
64
J. Albert
b
,
65
A. Beaulieu
b
,
65
F. U. Bernlochner
b
,
65
G. J. King
b
,
65
R. Kowalewski
b
,
65
T. Lueck
b
,
65
I. M. Nugent
b
,
65
J. M. Roney
b
,
65
R. J. Sobie
ab
,
65
N. Tasneem
b
,
65
T. J. Gershon,
66
P. F. Harrison,
66
T. E. Latham,
66
R. Prepost,
67
and S. L. Wu
67
(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-Vieux, France
2
Universitat de Barcelona, Facultat de Fisica, Departament ECM, E-08028 Barcelona, Spain
3
INFN Sezione 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
Ruhr Universit ̈at Bochum, Institut f ̈ur Experimentalphysik 1, D-44780 Bochum, Germany
7
Institute of Particle Physics
a
; University of British Columbia
b
, Vancouver, British Columbia, Canada V6T 1Z1
8
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090
a
,
Novosibirsk State University, Novosibirsk 630090
b
,
Novosibirsk State Technical University, Novosibirsk 630092
c
, Russia
9
University of California at Irvine, Irvine, California 92697, USA
10
University of California at Riverside, Riverside, California 92521, USA
11
University of California at Santa Cruz, Institute for Particle Physics, Santa Cruz, California 95064, USA
arXiv:2109.03364v1 [hep-ex] 7 Sep 2021
12
California Institute of Technology, Pasadena, California 91125, USA
13
University of Cincinnati, Cincinnati, Ohio 45221, USA
14
University of Colorado, Boulder, Colorado 80309, USA
15
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
16
INFN Sezione di Ferrara
a
; Dipartimento di Fisica e Scienze della Terra, Universit`a di Ferrara
b
, I-44122 Ferrara, Italy
17
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
18
INFN Sezione di Genova, I-16146 Genova, Italy
19
Harvey Mudd College, Claremont, California 91711, USA
20
Humboldt-Universit ̈at zu Berlin, Institut f ̈ur Physik, D-12489 Berlin, Germany
21
Indian Institute of Technology Guwahati, Guwahati, Assam, 781 039, India
22
University of Iowa, Iowa City, Iowa 52242, USA
23
Iowa State University, Ames, Iowa 50011, USA
24
Johns Hopkins University, Baltimore, Maryland 21218, USA
25
Universit ́e Paris-Saclay, CNRS/IN2P3, IJCLab, F-91405 Orsay, France
26
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
27
University of Liverpool, Liverpool L69 7ZE, United Kingdom
28
Queen Mary, University of London, London, E1 4NS, United Kingdom
29
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
30
University of Louisville, Louisville, Kentucky 40292, USA
31
Johannes Gutenberg-Universit ̈at Mainz, Institut f ̈ur Kernphysik, D-55099 Mainz, Germany
32
University of Manchester, Manchester M13 9PL, United Kingdom
33
University of Maryland, College Park, Maryland 20742, USA
34
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
35
Institute of Particle Physics
a
; McGill University
b
, Montr ́eal, Qu ́ebec, Canada H3A 2T8
36
INFN Sezione di Milano
a
; Dipartimento di Fisica, Universit`a di Milano
b
, I-20133 Milano, Italy
37
University of Mississippi, University, Mississippi 38677, USA
38
Universit ́e de Montr ́eal, Physique des Particules, Montr ́eal, Qu ́ebec, Canada H3C 3J7
39
INFN Sezione di Napoli and Dipartimento di Scienze Fisiche,
Universit`a di Napoli Federico II, I-80126 Napoli, Italy
40
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
41
University of Notre Dame, Notre Dame, Indiana 46556, USA
42
Ohio State University, Columbus, Ohio 43210, USA
43
INFN Sezione di Padova
a
; Dipartimento di Fisica, Universit`a di Padova
b
, I-35131 Padova, Italy
44
Laboratoire de Physique Nucl ́eaire et de Hautes Energies, Sorbonne Universit ́e,
Paris Diderot Sorbonne Paris Cit ́e, CNRS/IN2P3, F-75252 Paris, France
45
INFN Sezione di Perugia
a
; Dipartimento di Fisica, Universit`a di Perugia
b
, I-06123 Perugia, Italy
46
INFN Sezione di Pisa
a
; Dipartimento di Fisica,
Universit`a di Pisa
b
; Scuola Normale Superiore di Pisa
c
, I-56127 Pisa, Italy
47
Princeton University, Princeton, New Jersey 08544, USA
48
INFN Sezione di Roma
a
; Dipartimento di Fisica,
Universit`a di Roma La Sapienza
b
, I-00185 Roma, Italy
49
Universit ̈at Rostock, D-18051 Rostock, Germany
50
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom
51
IRFU, CEA, Universit ́e Paris-Saclay, F-91191 Gif-sur-Yvette, France
52
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
53
University of South Carolina, Columbia, South Carolina 29208, USA
54
Southern Methodist University, Dallas, Texas 75275, USA
55
St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5
56
Stanford University, Stanford, California 94305, USA
57
State University of New York, Albany, New York 12222, USA
58
Tel Aviv University, School of Physics and Astronomy, Tel Aviv, 69978, Israel
59
University of Tennessee, Knoxville, Tennessee 37996, USA
60
University of Texas at Austin, Austin, Texas 78712, USA
61
University of Texas at Dallas, Richardson, Texas 75083, USA
62
INFN Sezione di Torino
a
; Dipartimento di Fisica, Universit`a di Torino
b
, I-10125 Torino, Italy
63
INFN Sezione di Trieste and Dipartimento di Fisica, Universit`a di Trieste, I-34127 Trieste, Italy
64
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
65
Institute of Particle Physics
a
; University of Victoria
b
, Victoria, British Columbia, Canada V8W 3P6
66
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
67
University of Wisconsin, Madison, Wisconsin 53706, USA
We report on the first search for electron-muon lepton flavor violation (LFV) in the decay of a
b
quark and
b
antiquark bound state. We look for the LFV decay
Υ
(3
S
)
→
e
±
μ
∓
in a sample of
118 million
Υ
(3
S
) mesons from 27 fb
−
1
of data collected with the
B
A
B
AR
detector at the SLAC
2
PEP-II
e
+
e
−
collider operating with a 10.36 GeV center-of-mass energy. No evidence for a signal
is found and we set a limit on the branching fraction
B
(
Υ
(3
S
)
→
e
±
μ
∓
)
<
3
.
6
×
10
−
7
at 90% CL.
This result can be interpreted as a limit
Λ
NP
/g
2
NP
>
80 TeV on the energy scale
Λ
NP
divided by
the coupling-squared
g
2
NP
of relevant new physics.
PACS numbers: 11.30.j, 11.30.Fs, 13.20.Gd,13.20.v, 14.40.Gx, 14.40.n, 14.60.-z, 14.60.Cd, 14.60.Ef, 14.65.Fy
In the standard model (SM), the three lepton flavors
(electron, muon, tau) are carried by the charged leptons
(
e
−
,
μ
−
, and
τ
−
) and their associated neutrinos (
ν
e
,
ν
μ
,
ν
τ
). Were it not for the fact that neutrinos oscillate from
one flavor to another, lepton flavor would be strictly
conserved in all reactions in the SM. Although mixing
between the neutrino flavor eigenstates permits charged
lepton flavor violating (LFV) processes at higher-order,
these are extremely suppressed in the SM by powers of
the small neutrino masses. Therefore, observation of
charged LFV would be a clear signature of new physics
(NP), and placing experimentally stringent limits on the
branching fractions of such processes tightly constrains
NP models. Searches for electron-tau and muon-tau LFV
in decays of bound states of a
b
quark and
b
antiquark
(
b
̄
b
) have yielded no evidence of a signal and upper limits
ranging from 3.1
×
10
−
6
to 6.0
×
10
−
6
on their branching
fractions have been set [1]. This paper describes the first
search for electron-muon LFV in the decay of a
b
̄
b
bound-
state.
Indirect theoretical constraints on LFV decays of vec-
tor (i.e., spin = 1
,
parity =
−
1)
b
̄
b
bound states (referred
to as the
Υ
(
nS
) mesons,
n
= 1
,
2
,
3
,
4
...
) can be derived
using an argument based on the non-observation of LFV
decays of the muon in conjunction with unitarity con-
siderations [2]. In these calculations, it is assumed that
a virtual
Υ
meson could potentially contribute to the
muon LFV decay. The most stringent indirect bound
on electron-muon LFV decays of the
Υ
(3
S
) (with mass
M
Υ
(3
S
)
= 10
.
36 GeV) obtained in this way is
B
(
Υ
(3
S
)
→
e
±
μ
∓
)
≤
2
.
5
×
10
−
8
, which uses the reported limit on the
branching fraction
B
(
μ
→
3
e
)
<
1
.
0
×
10
−
12
[3]. Using
LFV limits from
μ
-
e
conversions, Ref. [4] sets an up-
per bound at 3
.
9
×
10
−
6
. However, it has been noted
in Ref. [2] that the size of the vector boson exchange
contribution to the
μ
→
3
e
decay amplitude can be sig-
nificantly reduced if there are kinematical suppressions.
Such suppressions are possible when the effective vec-
tor boson couplings involve derivatives (or momentum
factors). This possibility means there could be effective
tensor and pseudo-tensor LFV couplings in the
μ
→
3
e
decay, which would reduce the contribution of virtual
Υ
(nS) bosons as they only have vector couplings. Ref-
erence [2] estimates that the contribution of the virtual
Υ
(3
S
)
→
e
±
μ
∓
to the
μ
→
3
e
rate would be reduced by
approximately
M
2
μ
/
(2
M
2
Υ
(3
S
)
), leading to a re-calculated
bound on
B
(
Υ
(3
S
)
→
e
±
μ
∓
)
≤
1
×
10
−
3
. The mea-
surement we report here is several orders of magnitude
more sensitive than this indirect limit. We use our result
to place constraints on
Λ
NP
/
g
2
NP
of NP processes that
include LFV, where
g
NP
is the coupling of the NP and
Λ
NP
is the energy scale of the NP.
Our sample of
Υ
(3
S
) meson data was collected with
the
B
A
B
AR
detector at the PEP-II asymmetric-energy
e
+
e
−
collider at the SLAC National Accelerator Labo-
ratory. The detector was operated from 1999 to 2008
and collected data at the center-of-mass (CM) energies
of the
Υ
(4
S
) (10.58 GeV),
Υ
(3
S
) (10.36 GeV), and
Υ
(2
S
)
(10.02 GeV) resonances, as well as at energies in the
vicinity of these resonances. In this paper we describe
a direct search for LFV decays in a sample of 122 mil-
lion
Υ
(3S) decays corresponding to an integrated lumi-
nosity of 27.96
±
0.17 fb
−
1
[5] collected during 2008 (re-
ferred to as Run 7). Data collected at the
Υ
(4
S
) in 2007
(referred to as Run 6) with an integrated luminosity of
78.31
±
0.35 fb
−
1
[5], data taken 40 MeV below the
Υ
(4
S
)
resonance corresponding to 7.752
±
0.036 fb
−
1
[5], and
data taken 40 MeV below the
Υ
(3
S
) resonance corre-
sponding to 2.623
±
0.017 fb
−
1
[5] constitute control sam-
ples. These are used to evaluate non-resonant contribu-
tions to the background and to study systematic effects
in a signal-free sample. We employ a blind analysis strat-
egy [6] in which 0.93 fb
−
1
of the
Υ
(3
S
) sample is used
solely in the stage prior to unblinding, during which selec-
tion criteria are optimized and all systematic uncertain-
ties evaluated. The data sample reserved for the LFV
search is based on (117.7
±
1.2)
×
10
6
Υ
(3
S
) decays, corre-
sponding to 27.02
±
0.16 fb
−
1
, and excludes the 0.93 fb
−
1
sample.
In the
B
A
B
AR
detector, which is described in detail
elsewhere [7, 8], the trajectories of charged particles are
measured in a 5-layer silicon vertex tracker (SVT) sur-
rounded by a 40-layer cylindrical drift chamber (DCH).
This charged particle tracking system is inside a 1.5 T
solenoid with its field running approximately parallel to
the
e
+
e
−
beams and together they form a magnetic spec-
trometer. In order to identify and measure the energies
and directional information of photons and electrons, an
electromagnetic calorimeter (EMC) composed of an array
of 6580 thallium doped CsI crystals, located just inside
the superconducting magnet, is used. Muons and neutral
hadrons are identified by arrays of resistive plate cham-
bers or limited steamer-tube detectors inserted into gaps
in the steel of the Instrumented Flux Return (IFR) of the
magnet. The
Υ
(4
S
) control sample data for this analysis
are restricted to Run 6 to ensure that the control (Run 6)
and signal (Run 7) data sets have the same IFR detector
configurations following an IFR upgrade program that
3
was completed prior to the beginning of Run 6.
The signature for
Υ
(3
S
)
→
e
±
μ
∓
events consists of ex-
actly two oppositely charged primary particles, an elec-
tron and a muon, each with an energy close to half
the total energy of the
e
+
e
−
collision in the CM frame,
E
B
. There are two main sources of background: (i)
e
+
e
−
→
μ
+
μ
−
(
γ
) events in which one of the muons is
misidentified, decays in flight, or generates an electron in
a material interaction; and (ii)
e
+
e
−
→
e
+
e
−
(
γ
) events
in which one of the electrons is misidentified. Back-
ground from
e
+
e
−
→
τ
+
τ
−
→
e
±
μ
∓
2
ν
2 ̄
ν
is efficiently
removed with the kinematic requirements described be-
low.
Generic
Υ
(3
S
) decays to two charged particles
where there is particle misidentification are also a po-
tential background.
The
Υ
(4
S
) Run 6 data, which is at a CM energy
above the
Υ
(3
S
) mass, is used as a high statistics con-
trol sample to estimate the continuum, i.e., non-
Υ
(3
S
),
background in Run 7 data. Although collected at the
Υ
(4
S
) resonance, because the large width of the
Υ
(4
S
)
→
B
̄
B
strong decays suppresses the branching fractions of
lepton-pair
Υ
(4
S
) decays as well as any potential LFV
decays, Run 6 data provides a reliable continuum con-
trol sample. The KK2F Monte Carlo (MC) generator [9]
is used to simulate
μ
-pair and
τ
-pair events. The BH-
WIDE generator [10] is used to simulate Bhabha events.
Both generators take into account initial- and final-state
radiation. They also are used to cross-check the Run 6
data-driven estimates of the continuum background. The
EvtGen generator [11] is used to simulate hadronic con-
tinuum events and generic
Υ
(3
S
) decays, as well as the
signal
Υ
(3
S
)
→
e
±
μ
∓
decays, in which the electron and
muon have a (1 + cos
2
θ
) distribution, where
θ
is the
CM polar angle relative to the
e
−
beam. The simu-
lated
μ
-pair,
τ
-pair and generic
Υ
(3
S
) samples corre-
spond to approximately twice the number of events ex-
pected in the
Υ
(3
S
) data set, while the Bhabha sample
corresponds to approximately half the number of events.
The GEANT4 [12] suite of programs is used to simulate
the response of the
B
A
B
AR
detector.
Event selection proceeds in two stages. In the first
stage, a dedicated
eμ
filter is used to preselect events
with only an electron candidate and a muon candidate in
the detector. In this filter all events, in addition to pass-
ing either the drift chamber or electromagnetic calorime-
ter higher level triggers, are required to have exactly two
tracks of opposite charge that are separated by more than
90
◦
in the CM. One of the tracks must pass a very loose
electron selection (
E/p >
0
.
8) and the other a very loose
muon requirement (
E/p <
0
.
8), where
E
is the energy de-
posited in the EMC associated with the track of momen-
tum
p
. The preselection has an 80% efficiency for signal
events, including geometrical acceptance. The first row
in Table I documents this preselection efficiency along
with the numbers of background events expected from
generic
Υ
(3
S
) decays, as predicted by EvtGen, and the
amount of background from the continuum as determined
by the Run 6 data control sample. It also includes the
number of events preselected from the unblinded
Υ
(3S)
data sample.
TABLE I: Impact of each component of the selection on the
signal efficiency, number of background events, and number of
events in the data. The first row provides information on the
pre-selection. The last row provides the information after ap-
plying all selection criteria. Rows 2 to 7 provide information
when all requirements are applied except the criterion asso-
ciated with the particular row. The luminosity-normalized
expected number of events in the third and fourth columns
are the background events from the generic
e
+
e
−
→
Υ
(3
S
)
MC and the data-driven continuum background events es-
timated from the
e
+
e
−
→
Υ
(4
S
) sample, respectively. The
last column represents the number of events in the
Υ
(3
S
) data
sample after unblinding.
Selection
Signal
Υ
(3
S
)
Continuum
Events
Criterion
Efficiency
(%)
BG
BG
in Data
Pre-Selec.
80.20
75516
725003
945480
±
0.12
±
180
±
500
Optimized
50.74
5180
320910
358322
PID
±
0.15
±
50
±
330
2 tracks
23.54
0
14.1
18
in final
±
0.13
±
2.2
state
Lep. Mom.
26.84
87
253
302
±
0.12
±
6
±
9
Back-to-
24.02
0.5
36
39
back
±
0.13
±
0.5
±
6
EMC
24.95
0
13.5
17
Accept.
±
0.13
±
2.2
Energy on
24.52
0
16.9
19
EMC
±
0.13
±
2.4
All Criteria
23.42
0
12.2
15
±
0.13
±
2.1
In the second stage of the analysis, we apply tighter
and optimized particle identification (PID) and kine-
matic criteria. Applying PID to select events with one
muon and one electron is the most effective means of
reducing the background while maintaining an accept-
able efficiency. All components of the
B
A
B
AR
detector
contribute to PID. Different PID selectors have been de-
veloped by
B
A
B
AR
to distinguish each particle type in
a set of multivariate analyses. These are described in
more detail in Ref. [8]. Selectors for this analysis based
on error-correcting output codes [13] and decision trees
are used to identify electrons, pions, and kaons, whereas
bagged decision tree selectors [14] are used to identify
muons. The selectors can be deployed to provide differ-
ent nested levels of particle efficiency and background,
from “Super Loose” (most efficient, least pure) to “Su-
per Tight” (least efficient, most pure), where candidates
selected by tighter criteria of the selector are a subset of
4
events that pass looser selection criteria. We optimize
the choice of the electron and muon selectors to maxi-
mize
ε
eμ
/
√
1 +
N
BG
, where
ε
eμ
is the final efficiency as
determined from signal MC, and
N
BG
is the number of
expected background events as predicted by data control
samples in Run 6 and generic
Υ
(3
S
) MC events. In the
optimized selection, electron candidates are required to
pass the “Super Tight” electron selector and muon can-
didates to pass the “Tight” muon selector. In addition,
electron candidates are required to fail the “Tight” muon
selector and muons are required to fail the “Super Tight”
electron selector. Each track is also required to fail the
“Loose” pion selector as well as the “Loose” kaon selec-
tor.
Kinematic requirements are also applied to sup-
press
e
+
e
−
→
τ
+
τ
−
→
e
±
μ
∓
2
ν
2 ̄
ν
events, radiative
Bhabha and
μ
-pair events, the
e
+
e
−
→
e
+
e
−
e
+
e
−
and
e
+
e
−
→
e
+
e
−
μ
+
μ
−
two-photon processes, and beam-
gas interactions. In the
p
e
/E
B
vs
p
μ
/E
B
plane, where
p
e
/E
B
(
p
μ
/E
B
) is the ratio of the electron (muon) mo-
mentum to the beam energy in the CM frame, the distri-
bution of
e
-
μ
signal events peaks at (1,1). Events must
lie within a circle about that peak: namely we require
(
p
e
/E
B
−
1)
2
+ (
p
μ
/E
B
−
1)
2
<
0
.
01. Figure 1 shows the
distribution of (
p
e
/E
B
−
1)
2
+(
p
μ
/E
B
−
1)
2
after all other
selection criteria have been applied, for the signal,
Υ
(3
S
)
data sample, and continuum backgrounds estimated from
Run 6 data. The angle between the two lepton tracks in
the CM is required to be more than 179
◦
. In order to
reduce continuum background from
μ
-pairs and to sup-
press Bhabha events in which an electron is misidentified
as a muon because it passes through the space between
crystals, the primary muon candidate is required to de-
posit at least 50 MeV in the EMC. We require that the
lepton tracks fall within the angular acceptance (24
◦
<
θ
Lab
<
130
◦
) of the EMC, where
θ
Lab
is the polar angle
of lepton tracks in the lab frame.
The signal efficiency, as determined from signal MC,
is (23.42
±
0.13 (stat))%. Figure 2 shows the
eμ
invariant
mass distribution of the data candidates and background
events, as well as the potential signal, after all selection
requirements have been applied.
No events from the generic
Υ
(3
S
) MC sample survive
the selection. We estimate an uncertainty of
±
0
.
9 events
in this source of potentially misidentified generic
Υ
(3
S
)
decays by loosening the PID selectors and use the un-
certainty in the surviving number of events with this
loosened selection as the uncertainty in this background.
The determination of the continuum backgrounds ob-
tained using the Run 6 data, as described above, predicts
a background of 12.2
±
2.1 events from continuum pro-
cesses. The MC samples of the continuum are only used
as a cross check on backgrounds at various stages of the
analysis. No events from Bhabha,
τ
-pairs,
c
̄
c
,
u
̄
u
+
d
̄
d
+
s
̄
s
,
or generic
Υ
(3
S
) MC pass the selection. The MC predicts
that 0.16
±
0.05 continuum
μ
-pair events survive the final
FIG. 1: Distribution of the squared distance from the point
(1,1) in the plane of the scaled electron
vs
muon momenta for
events satisfying all other selection criteria in the data (points
with error bars), continuum background (gray histogram,
from the Run 6 control sample normalized to 27.02 fb
−
1
),
and simulated signal (red histogram, arbitrarily normalized).
The dashed line indicates the position of the 0.01 requirement.
selection. An uncertainty of
±
2.3 events is assigned to
the total background estimate, calculated as the quadra-
ture sum of the uncertainties in the
Υ
(3
S
) and continuum
background estimates.
Table I summarizes the signal efficiency, estimates of
the numbers of background events, and numbers of events
in the
Υ
(3
S
) data sample at the various stages of the
selection.
FIG. 2: The distribution of the
eμ
invariant mass of events
surviving all selection criteria. The data sample is presented
as the histogram in black with error bars and the open red
histogram represents the signal MC with arbitrary normaliza-
tion. The grey histogram shows the estimate of the contiuum
background from the Run 6 control sample data with the rate
scaled to the amounts expected at 10.36 GeV for a data sam-
ple of 27.02 fb
−
1
and the mass scaled to 10.36/10.58.
We assess the systematic uncertainties in the signal ef-
ficiency by determining the ratio of data to MC yields for
5
a control sample of
e
+
e
−
→
τ
+
τ
−
→
e
±
μ
∓
2
ν
2 ̄
ν
events in
an
eμ
mass sideband. For this study, we reverse the two
major kinematic requirements, the
E
B
-normalized lepton
momentum cut and the requirement on the angle between
the two tracks, in order to obtain a large control sample
of
τ
-pair events. This
τ
control sample study measures
the systematic uncertainty associated with particle iden-
tification, tracking, kinematics, trigger selection criteria,
and all other effects except those associated with the two
major kinematic requirements used to select the control
sample. Figure 3 shows the distribution of
M
eμ
for the
data and MC in the
τ
control sample. We evaluate the as-
sociated correction to the signal efficiency by measuring
the ratio (
N
Data
−
N
MC
non
−
τ
+
τ
−
)
/N
MC
τ
+
τ
−
in the sideband re-
gion 6 GeV
<M
eμ
<
8 GeV, which is just below our signal
region, where
N
Data
is the number of events in the data,
N
MC
τ
+
τ
−
is the number of
τ
-pair events predicted in MC,
and
N
MC
non
−
τ
+
τ
−
the number of MC-predicted events that
are not
τ
-pairs. We obtain a value of 1.007
±
0.010(stat)
for this ratio. We take the quadratic sum of the statisti-
cal uncertainty in this ratio and difference from unity as
this part of the relative systematic uncertainty, 1.2%, in
the signal efficiency. We evaluate the systematic uncer-
tainties associated with the two major kinematic require-
ments that are reversed for the
τ
control sample selection
by using them to select a
μ
-pair control sample having
very similar kinematic properties as the signal. We con-
servatively vary the values of the two selection criteria
from the default values and assign the differences in the
selection efficiencies between MC and data for the
μ
-pair
control samples as the relative efficiency uncertainties as-
sociated with these requirements. The
E
B
-normalized
momentum requirement is varied by
±
0.0015 and the
back-to-back angle requirement by
±
0.1
◦
. The number
of signal events remains unchanged under these varia-
tions. Table II summarizes the systematic uncertainties.
The signal efficiency is (23.4
±
0.8)%, where the quoted
uncertainty is determined by summing in quadrature the
individual contributions.
TABLE II: Summary of systematic uncertainties. The values
of the efficiency, background, and number of
Υ
(3
S
) decays are
presented in the first column and their uncertainties in the
second column. The different contributions to the efficiency
systematic uncertainties are also presented.
Component Value
Uncertainties by Source
Lep. Mom. cut:
0.0068 (2.9 %)
Back-to-back cut:
0.0026 (1.1 %)
All other cuts:
0.0028 (1.2 %)
Signal
MC statistics:
0.0003 (0.13 %)
Efficiency: 0.2342
±
0
.
0078 (3.3 %)
N
Υ
: 117.7
×
10
6
±
1
.
2
×
10
6
(1.0 %)
BG: 12.2
±
2
.
3 (19 %)
FIG. 3: The distribution of the
eμ
invariant mass of events
in a control sample dominated by
τ
-pair events obtained by
reversing the two major kinematic requirements in the se-
lection. The green, light grey, and yellow colored histograms
represent the MC predictions for
τ
+
τ
−
, generic
Υ
(3
S
) decays,
and Bhabha events, respectively, while the histogram repre-
sented by the black line with error bars represents data from
the
Υ
(3
S
) data sample. The systematic uncertainty in the
signal efficiency associated with all requirements but those on
the two kinematic requirements used to define this sample is
obtained by comparing the MC expectations with the data in
the side-band region indicated by the dashed vertical lines.
After unblinding the data, we find
N
cand
=15 can-
didate events and have an expected background of
12.2
±
2.3 events from a sample of (117.7
±
1.2)
×
10
6
Υ
(3
S
) mesons. Calculating the branching fraction from
(
N
cand
−
N
BG
)
/
(
ε
sig
N
Υ
(3
S
)
) gives:
B
(
Υ
(3
S
)
→
e
±
μ
∓
) = (1
.
0
±
1
.
4 (stat)
±
0
.
8 (syst))
×
10
−
7
(1)
where the statistical uncertainty is that from
N
cand
and
all other uncertainties are included in the systematic un-
certainty. As this result is consistent with no signal, we
set an upper limit at 90% confidence level (CL) on the
branching fraction by using the CLs method [15], a modi-
fied frequentist method that accomodates potential large
downward fluctuations in backgrounds:
B
(
Υ
(3
S
)
→
e
±
μ
∓
)
<
3
.
6
×
10
−
7
@ 90% CL
.
(2)
The CLs expected 90% CL upper limit, given the num-
ber of background events and assuming no signal, is
2.8
×
10
−
7
.
This result is the first reported experimental upper
limit on
Υ
(3
S
)
→
e
±
μ
∓
and from any
b
̄
b
bound state.
It can be interpreted as a limit on NP using the relation-
ship
(
g
2
NP
/Λ
NP
)
2
/
(
4
πα
3
S
Q
b
/M
Υ
(3
S
)
)
2
=
B
(
Υ
(3
S
)
→
eμ
)
/
B
(
Υ
(3
S
)
→
μμ
), ignoring small kinematic factors,
and where
Q
b
=
−
1
/
3 is the
b
-quark charge and
α
3
S
is
the fine structure constant at the
M
Υ
(3
S
)
energy scale.
Using the world average
B
(
Υ
(3
S
)
→
μ
+
μ
−
) = 2
.
18
±
0
.
21 [1] gives a 90% CL upper limit of
Λ
NP
/
g
2
NP
>
80 TeV.
6
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 comput-
ing organizations that support
B
A
B
AR
. The collaborat-
ing 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),
MINECO (Spain), STFC (United Kingdom), BSF (USA-
Israel). Individuals have received support from the Marie
Curie EIF (European Union) and the A. P. Sloan Foun-
dation (USA).
∗
Deceased
†
Now at: Wuhan University, Wuhan 430072, China
‡
Now at: Universit`a di Bologna and INFN Sezione di
Bologna, I-47921 Rimini, Italy
§
Now at: King’s College, London, WC2R 2LS, UK
¶
Now at: Western Kentucky University, Bowling Green,
Kentucky 42101, USA
∗∗
Now at: University of Huddersfield, Huddersfield HD1
3DH, UK
††
Now at: University of South Alabama, Mobile, Alabama
36688, USA
‡‡
Also at: Universit`a di Sassari, I-07100 Sassari, Italy
§§
Also at: Gran Sasso Science Institute, I-67100 LAquila,
Italy
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7