Search for Lepton Flavor Violation in
Υ
ð
3
S
Þ
→
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,
7b
C. Hearty,
7a,7b
T. S. Mattison,
7b
J. A. McKenna,
7b
R. Y. So,
7b
V. E. Blinov,
8a,8b,8c
A. R. Buzykaev,
8a
V. P. Druzhinin,
8a,8b
V. B. Golubev,
8a,8b
E. A. Kozyrev,
8a,8b
E. A. Kravchenko,
8a,8b
A. P. Onuchin,
8a,8b,8c
,*
S. I. Serednyakov,
8a,8b
Yu. I. Skovpen,
8a,8b
E. P. Solodov,
8a,8b
K. Yu. Todyshev,
8a,8b
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. Röhrken,
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,
16a
C. Bozzi,
16a
R. Calabrese,
16a,16b
G. Cibinetto,
16a,16b
E. Fioravanti,
16a,16b
I. Garzia,
16a,16b
E. Luppi,
16a,16b
V. Santoro,
16a
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,
35a,35b
R. M. Seddon,
35a
N. Neri,
36a
F. Palombo,
36a,36b
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,
43a
M. Margoni,
43a,43b
M. Posocco,
43a
G. Simi,
43a,43b
F. Simonetto,
43a,43b
R. Stroili,
43a,43b
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,
45a,45b
E. Manoni,
45a
A. Rossi,
45a
G. Batignani,
46a,46b
S. Bettarini,
46a,46b
M. Carpinelli,
46a,46b
,
††
G. Casarosa,
46a,46b
M. Chrzaszcz,
46a
F. Forti,
46a,46b
M. A. Giorgi,
46a,46b
A. Lusiani,
46a,46c
B. Oberhof,
46a,46b
E. Paoloni,
46a,46b
M. Rama,
46a
G. Rizzo,
46a,46b
J. J. Walsh,
46a
L. Zani,
46a,46b
A. J. S. Smith,
47
F. Anulli,
48a
R. Faccini,
48a,48b
F. Ferrarotto,
48a
F. Ferroni,
48a
,
‡‡
A. Pilloni,
48a,48b
,*
G. Piredda,
48a
C. Bünger,
49
S. Dittrich,
49
O. Grünberg,
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,
62a,62b
F. De Mori,
62a,62b
A. Filippi,
62a
D. Gamba,
62a,62b
L. Lanceri,
63
L. Vitale,
63
F. Martinez-Vidal,
64
A. Oyanguren,
64
J. Albert,
65b
A. Beaulieu,
65b
F. U. Bernlochner,
65b
G. J. King,
65b
R. Kowalewski,
65b
T. Lueck,
65b
I. M. Nugent,
65b
J. M. Roney ,
65b
R. J. Sobie,
65a,65b
N. Tasneem,
65b
T. J. Gershon,
66
P. F. Harrison,
66
T. E. Latham,
66
R. Prepost,
67
and S. L. Wu
67
(
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 Universität Bochum, Institut für Experimentalphysik 1, D-44780 Bochum, Germany
7a
Institute of Particle Physics, Vancouver, British Columbia V6T 1Z1, Canada
7b
University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
8a
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090, Russia
8b
Novosibirsk State University, Novosibirsk 630090, Russia
8c
Novosibirsk State Technical University, Novosibirsk 630092, 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
PHYSICAL REVIEW LETTERS
128,
091804 (2022)
0031-9007
=
22
=
128(9)
=
091804(7)
091804-1
Published by the American Physical Society
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
16a
INFN Sezione di Ferrara, I-44122 Ferrara, Italy
16b
Dipartimento di Fisica e Scienze della Terra, Universit`
a di Ferrara, 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-Universität zu Berlin, Institut für 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-Universität Mainz, Institut für 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
35a
Institute of Particle Physics, Montr ́
eal, Qu ́
ebec H3A 2T8, Canada
35b
McGill University, Montr ́
eal, Qu ́
ebec H3A 2T8, Canada
36a
INFN Sezione di Milano, I-20133 Milano, Italy
36b
Dipartimento di Fisica, Universit`
a di Milano, 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 H3C 3J7, Canada
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, Netherlands
41
University of Notre Dame, Notre Dame, Indiana 46556, USA
42
Ohio State University, Columbus, Ohio 43210, USA
43a
INFN Sezione di Padova, I-35131 Padova, Italy
43b
Dipartimento di Fisica, Universit`
a di Padova, 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
45a
INFN Sezione di Perugia, I-06123 Perugia, Italy
45b
Dipartimento di Fisica, Universit`
a di Perugia, I-06123 Perugia, Italy
46a
INFN Sezione di Pisa, I-56127 Pisa, Italy
46b
Dipartimento di Fisica, Universit`
a di Pisa, I-56127 Pisa, Italy
46c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
47
Princeton University, Princeton, New Jersey 08544, USA
48a
INFN Sezione di Roma, I-00185 Roma, Italy
48b
Dipartimento di Fisica, Universit`
a di Roma La Sapienza, I-00185 Roma, Italy
49
Universität 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 B2G 2W5, Canada
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
62a
INFN Sezione di Torino, I-10125 Torino, Italy
PHYSICAL REVIEW LETTERS
128,
091804 (2022)
091804-2
62a
Dipartimento di Fisica, Universit`
a di Torino, 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
65a
Institute of Particle Physics, Victoria, British Columbia V8W 3P6, Canada
65b
University of Victoria, Victoria, British Columbia V8W 3P6, Canada
66
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
67
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 10 September 2021; accepted 27 January 2022; published 3 March 2022)
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
ð
3
S
Þ
→
e
±
μ
∓
in a sample of 118 million
Υ
ð
3
S
Þ
mesons from
27
fb
−
1
of data collected with the
BABAR
detector at the SLAC PEP-II
e
þ
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
½
Υ
ð
3
S
Þ
→
e
±
μ
∓
<
3
.
6
×
10
−
7
at 90% C. L. 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 (NP).
DOI:
10.1103/PhysRevLett.128.091804
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 and are not observable in current or
planned experiments. 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.
Recently, reported tensions between the SM expectations of
the universality of leptonic couplings and measurements
involving
b
quarks
[1]
further motivate searches for
violation of lepton flavor
[2]
. 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
[3]
. This Letter
describes the first search for electron-muon LFV in the
decay of a
b
̄
b
bound state.
Indirect theoretical constraints on LFV decays of vector
(i.e., spin
¼
1
, parity
¼
−
1
)
b
̄
b
bound states [referred to as
the
Υ
ð
nS
Þ
mesons,
n
¼
1
;
2
;
3
;
4
;
...
] can be derived using
an argument based on the nonobservation of LFV decays of
the muon in conjunction with unitarity considerations
[4]
.
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
ð
3
S
Þ
(with mass
M
Υ
ð
3
S
Þ
¼
10
.
36
GeV)
obtained in this way is
B
½
Υ
ð
3
S
Þ
→
e
±
μ
∓
≤
2
.
5
×
10
−
8
,
which uses the reported limit on the branching fraction
B
ð
μ
→
3
e
Þ
<
1
.
0
×
10
−
12
[5]
. Using LFV limits from
μ
−
e
conversions, Ref.
[6]
sets an upper bound at
3
.
9
×
10
−
6
. However, it has been noted, in Ref.
[4]
, that
the size of the vector boson exchange contribution to the
μ
→
3
e
decay amplitude can be significantly reduced if
there are kinematical suppressions. Such suppressions are
possible when the effective vector boson couplings involve
derivatives (or momentum factors). This possibility means
there could be effective tensor and pseudotensor LFV
couplings in the
μ
→
3
e
decay, which would reduce
the contribution of virtual
Υ
ð
nS
Þ
bosons as they only
have vector couplings. Reference
[4]
estimates that
the contribution of the virtual
Υ
ð
3
S
Þ
→
e
±
μ
∓
to
the
μ
→
3
e
rate would be reduced by approxima-
tely
M
2
μ
=
ð
2
M
2
Υ
ð
3
S
Þ
Þ
, leading to a modified bound on
B
½
Υ
ð
3
S
Þ
→
e
±
μ
∓
≤
1
×
10
−
3
. The measurement 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 in a model-independent effective
field theory.
Our sample of
Υ
ð
3
S
Þ
meson data was collected with the
BABAR
detector at the PEP-II asymmetric-energy
e
þ
e
−
collider at the SLAC National Accelerator Laboratory. The
detector was operated from 1999 to 2008 and collected data at
the center-of-mass (c.m.) energies of the
Υ
ð
4
S
Þ
(10.58 GeV),
Υ
ð
3
S
Þ
(10.36 GeV), and
Υ
ð
2
S
Þ
(10.02 GeV) resonances, as
well as at energies in the vicinity of these resonances. In this
Published by the American Physical Society under the terms of
the
Creative Commons Attribution 4.0 International
license.
Further distribution of this work must maintain attribution to
the author(s) and the published article
’
s title, journal citation,
and DOI. Funded by SCOAP
3
.
PHYSICAL REVIEW LETTERS
128,
091804 (2022)
091804-3
Letter, we describe a direct search for LFV decays in a sample
of 122 million
Υ
ð
3
S
Þ
decays corresponding to an integrated
luminosity of
27
.
96
±0
.
17
fb
−
1
[7]
collected during 2008
(referred to as Run 7). Data collected at the
Υ
ð
4
S
Þ
in 2007
(referred to as Run 6) with an integrated luminosity of
78
.
31
±0
.
35
fb
−
1
[7]
, data taken 40 MeV below the
Υ
ð
4
S
Þ
resonance corresponding to
7
.
752
±0
.
036
fb
−
1
[7]
,
and data taken 40 MeV below the
Υ
ð
3
S
Þ
resonance corre-
sponding to
2
.
623
±0
.
017
fb
−
1
[7]
constitute control sam-
ples. These are used to evaluate nonresonant contributions to
the background and to study systematic effects in a signal-free
sample. We employ a blind analysis strategy
[8]
in which
0
.
93
fb
−
1
of the
Υ
ð
3
S
Þ
sample is used solely in the stage prior
to unblinding, during which, selection criteria are optimized
and all systematic uncertainties evaluated. The data sample
reserved for the LFV search is based on
ð
117
.
7
±1
.
2
Þ
×
10
6
Υ
ð
3
S
Þ
decays, corresponding to
27
.
02
±0
.
16
fb
−
1
,and
excludes the
0
.
93
fb
−
1
sample.
In the
BABAR
detector, which is described in detail
elsewhere
[9,10]
, the trajectories of charged particles are
measured in a five-layer silicon vertex tracker surrounded
by a 40-layer cylindrical drift chamber. This charged
particle tracking system is inside a 1.5 T solenoid with
its field running approximately parallel to the
e
þ
e
−
beams,
and together, they form a magnetic spectrometer. In order to
identify and measure the energies and directional informa-
tion of photons and electrons, an electromagnetic calorim-
eter (EMC) composed of an array of 6580 thallium doped
CsI crystals, located just inside the superconducting mag-
net, is used. Muons and neutral hadrons are identified by
arrays of resistive plate chambers or limited steamer-tube
detectors inserted into gaps in the steel of the instrumented
flux return (IFR) of the magnet. The
Υ
ð
4
S
Þ
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 was completed prior to the beginning of
Run 6.
The signature for
Υ
ð
3
S
Þ
→
e
±
μ
∓
events consists of
exactly two oppositely charged primary particles, an
electron and a muon, each with an energy close to half
the total energy of the
e
þ
e
−
collision in the c.m. frame,
E
B
. There are two main sources of background:
(i)
e
þ
e
−
→
μ
þ
μ
−
ð
γ
Þ
events in which one of the muons
is misidentified, decays in flight, or generates an electron in
a material interaction; and (ii)
e
þ
e
−
→
e
þ
e
−
ð
γ
Þ
events in
which one of the electrons is misidentified. Background
from
e
þ
e
−
→
τ
þ
τ
−
→
e
±
μ
∓
2
ν
2
̄
ν
is efficiently removed
with the kinematic requirements described below. Generic
Υ
ð
3
S
Þ
decays to two charged particles where there is
particle misidentification are also a potential background.
The
Υ
ð
4
S
Þ
Run 6 data, which is at a c.m. energy above
the
Υ
ð
3
S
Þ
mass, is used as a high statistics control sample
to estimate the continuum, i.e., non-
Υ
ð
3
S
Þ
, background in
Run 7 data. Because the large width of the
Υ
ð
4
S
Þ
→
B
̄
B
strong decays suppresses the branching fractions of lepton-
pair
Υ
ð
4
S
Þ
decays as well as any potential LFV decays,
Run 6 data provides a reliable continuum control sample.
The KK2F Monte Carlo (MC) generator
[11]
is used to
simulate
μ
-pair and
τ
-pair events. The BHWIDE generator
[12]
is used to simulate Bhabha events. Both generators
take into account initial- and final-state radiation. They are
also used to cross check the Run 6 data-driven estimates of
the continuum background. The
EVTGEN
generator
[13]
is
used to simulate hadronic continuum events and generic
Υ
ð
3
S
Þ
decays, as well as the signal
Υ
ð
3
S
Þ
→
e
±
μ
∓
decays, in which the electron and muon have a
ð
1
þ
cos
2
θ
Þ
distribution, where
θ
is the c.m. polar angle relative
to the
e
−
beam. The simulated
μ
-pair,
τ
-pair and generic
Υ
ð
3
S
Þ
samples correspond to approximately twice the
number of events expected in the
Υ
ð
3
S
Þ
data set, while
the Bhabha sample corresponds to approximately half the
number of events. The
GEANT
4
[14]
suite of programs is
used to simulate the response of the
BABAR
detector.
Event selection proceeds in two stages. In the first stage,
a dedicated
e
μ
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 passing either the drift
chamber or electromagnetic calorimeter higher level trig-
gers, are required to have exactly two tracks of opposite
charge that are separated by more than 90° in the c.m. 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 deposited in the EMC
associated with the track of momentum
p
. The preselection
has an 80% efficiency for signal events, including geo-
metrical acceptance. The first row in Table
I
documents this
preselection efficiency along with the numbers of back-
ground events expected from generic
Υ
ð
3
S
Þ
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
ð
3
S
Þ
data sample.
In the second stage of the analysis, we apply tighter and
optimized particle identification (PID) and kinematic cri-
teria. Applying PID to select events with one muon and one
electron is the most effective means of reducing the
background while maintaining an acceptable efficiency.
All components of the
BABAR
detector contribute to PID.
Different PID selectors have been developed by
BABAR
to distinguish each particle type in a set of multivariate
analyses. These are described in more detail in Ref.
[10]
.
Selectors for this analysis based on error-correcting output
codes
[15]
and decision trees are used to identify electrons,
pions, and kaons, whereas bagged decision tree selectors
[16]
are used to identify muons. The selectors can be
deployed to provide different nested levels of particle
efficiency and background, from
“
super loose
”
(most
efficient, least pure) to
“
super tight
”
(least efficient, most
pure), where candidates selected by tighter criteria of the
PHYSICAL REVIEW LETTERS
128,
091804 (2022)
091804-4
selector are a subset of events that pass looser selection
criteria. We optimize the choice of the electron and muon
selectors to maximize
ε
e
μ
=
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
þ
N
BG
p
, where
ε
e
μ
is the final
efficiency as determined from signal MC, and
N
BG
is the
number of expected background (BG) events as predicted
by data control samples in Run 6 and generic
Υ
ð
3
S
Þ
MC
events. In the optimized selection, electron candidates are
required to pass the
“
super tight
”
electron selector and
muon candidates 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
selector.
Kinematic requirements are also applied to suppress
e
þ
e
−
→
τ
þ
τ
−
→
e
±
μ
∓
2
ν
2
̄
ν
events, radiative Bhabha
and
μ
-pair events, the
e
þ
e
−
→
e
þ
e
−
e
þ
e
−
and
e
þ
e
−
→
e
þ
e
−
μ
þ
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
)isthe
ratio of the electron (muon) momentum to the beam energy in
the c.m. frame, the distribution of
e
−
μ
signal events peaks at
(1,1). Events must lie within a circle about that peak: namely,
we require
ð
p
e
=E
B
−
1
Þ
2
þð
p
μ
=E
B
−
1
Þ
2
<
0
.
01
. Figure
1
shows the distribution of
ð
p
e
=E
B
−
1
Þ
2
þð
p
μ
=E
B
−
1
Þ
2
after all other selection criteria have been applied, for the
signal,
Υ
ð
3
S
Þ
data sample, and continuum backgrounds
estimated from Run 6 data. The angle between the two
lepton tracks in the c.m. is required to be more than 179°. In
order to reduce continuum background from
μ
pairs and to
suppress 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 deposit 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
½
23
.
42
±0
.
13
ð
stat
Þ
%
. Figure
2
shows the
e
μ
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
ð
3
S
Þ
MC sample survive
the selection. We estimate an uncertainty of
±0
.
9
events in
this source of potentially misidentified generic
Υ
ð
3
S
Þ
decays by loosening the PID selectors and use the uncer-
tainty in the surviving number of events with this loosened
selection as the uncertainty in this background. The
determination of the continuum backgrounds obtained
using the Run 6 data, as described above, predicts a
background of
12
.
2
±2
.
1
events from continuum
processes. 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
þ
d
̄
d
þ
s
̄
s
, or generic
Υ
ð
3
S
Þ
MC pass the selection.
The MC predicts that
0
.
16
±0
.
05
continuum
μ
-pair events
survive the final selection. An uncertainty of
±2
.
3
events
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.
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 preselection. The last row provides the information after applying all selection
criteria. Rows 2
–
7 provide information when all requirements are applied except the criterion associated 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
þ
e
−
→
Υ
ð
3
S
Þ
MC and the data-driven continuum background events estimated from the
e
þ
e
−
→
Υ
ð
4
S
Þ
sample, respectively. The last column
represents the number of events in the
Υ
ð
3
S
Þ
data sample after unblinding.
Selection criterion
Signal efficiency (%)
Υ
ð
3
S
Þ
BG
Continuum BG
Events in data
Preselection
80
.
20
±0
.
12
75 516
± 180
725 003
± 500
945 480
Optimized PID
50
.
74
±0
.
15
5180
±50
320 910
± 330
358 322
Two tracks in final state
23
.
54
±0
.
13
0
14
.
1
±2
.
2
18
Lepton momentum
26
.
84
±0
.
12
87
±6
253
±9
302
Back-to-back
24
.
02
±0
.
13
0
.
5
±0
.
5
36
±6
39
EMC acceptance
24
.
95
±0
.
13
0
13
.
5
±2
.
2
17
Energy on EMC
24
.
52
±0
.
13
0
16
.
9
±2
.
4
19
All criteria
23
.
42
±0
.
13
0
12
.
2
±2
.
1
15
PHYSICAL REVIEW LETTERS
128,
091804 (2022)
091804-5
is assigned to the total background estimate, calculated as
the quadrature sum of the uncertainties in the
Υ
ð
3
S
Þ
and
continuum background estimates.
Table
I
summarizes the signal efficiency, estimates of the
numbers of background events, and numbers of events in
the
Υ
ð
3
S
Þ
data sample at the various stages of the selection.
We assess the systematic uncertainties in the signal
efficiency by determining the ratio of data to MC yields
for a control sample of
e
þ
e
−
→
τ
þ
τ
−
→
e
±
μ
∓
2
ν
2
̄
ν
events
in an
e
μ
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 identifica-
tion, 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
e
μ
for the data and
MC in the
τ
control sample. We evaluate the associ-
ated correction to the signal efficiency by measuring the
ratio
ð
N
Data
−
N
MC
non
−
τ
þ
τ
−
Þ
=N
MC
τ
þ
τ
−
in the sideband region
6
GeV
<M
e
μ
<
8
GeV, which is just below our signal
region, where
N
Data
is the number of events in the data,
N
MC
τ
þ
τ
−
is the number of
τ
-pair events predicted in MC, and
N
MC
non
−
τ
þ
τ
−
the number of MC-predicted events that are
not
τ
pairs. We obtain a value of
1
.
007
±0
.
010
ð
stat
Þ
for
this ratio. We take the quadratic sum of the statistical
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 uncertainties
associated with the two major kinematic requirements that
are reversed for the
τ
control sample selection by using
them to select a
μ
-pair control sample having very similar
kinematic properties to the signal. We conservatively vary
the values of the two selection criteria from the default
values to account for data-MC difference in the control
sample studies, and assign the differences in the selection
efficiencies between MC and data for the
μ
-pair control
samples as the relative efficiency uncertainties associated
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 variations. Table
II
summarizes the systematic uncertainties. The signal effi-
ciency is (
23
.
4
±0
.
8
)
%
, where the quoted uncertainty is
FIG. 2. The distribution of the
e
μ
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 normalization. The grey
histogram shows the estimate of the continuum background from
the Run 6 control sample data with the rate scaled to the amounts
expected at 10.36 GeV for a data sample of
27
.
02
fb
−
1
and the
mass scaled to
10
.
36
=
10
.
58
.
FIG. 3. The distribution of the
e
μ
invariant mass of events in a
control sample dominated by
τ
-pair events obtained by reversing
the two major kinematic requirements in the selection. The green,
light grey, and yellow colored histograms represent the MC
predictions for
τ
þ
τ
−
, generic
Υ
ð
3
S
Þ
decays, and Bhabha events,
respectively, while the histogram represented by the black line
with error bars represents data from the
Υ
ð
3
S
Þ
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 sideband region indicated by the dashed
vertical lines.
TABLE II. Summary of systematic uncertainties. The values of
the efficiency, background, and number of
Υ
ð
3
S
Þ
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
Signal efficiency:
0.2342
Lepton momentum cut: 0.0068 (2.9%)
Back-to-back cut:
0.0026 (1.1%)
All other cuts:
0.0028 (1.2%)
MC statistics:
0.0003 (0.13%)
±0
.
0078
(3.3%)
N
Υ
:
117
.
7
×
10
6
±1
.
2
×
10
6
(1.0%)
BG: 12.2
±2
.
3
(19%)
PHYSICAL REVIEW LETTERS
128,
091804 (2022)
091804-6
determined by summing in quadrature the individual
contributions.
After unblinding the data, we find
N
cand
¼
15
candidate
events and have an expected background of
12
.
2
±2
.
3
events from a sample of
ð
117
.
7
±1
.
2
Þ
×
10
6
Υ
ð
3
S
Þ
mes-
ons. Calculating the branching fraction from
ð
N
cand
−
N
BG
Þ
=
ð
ε
sig
N
Υ
ð
3
S
Þ
Þ
gives
B
½
Υ
ð
3
S
Þ
→
e
±
μ
∓
¼½
1
.
0
±1
.
4
ð
stat
Þ
±0
.
8
ð
syst
Þ
×
10
−
7
ð
1
Þ
where the statistical uncertainty is that from
N
cand
, and all
other uncertainties are included in the systematic uncer-
tainty. As this result is consistent with no signal, we set an
upper limit at 90% confidence level (C.L.) on the branching
fraction by using the
“
CLs
”
method, a modified frequentist
method described in reference
[17]
that accommodates
potential large downward fluctuations in backgrounds
B
½
Υ
ð
3
S
Þ
→
e
±
μ
∓
<
3
.
6
×
10
−
7
@ 90%
C
:
L
:
ð
2
Þ
The CLs expected 90% C.L. upper limit, given the number
of background events and assuming no signal, is
2
.
8
×
10
−
7
.
This result is the first reported experimental upper limit
on
Υ
ð
3
S
Þ
→
e
±
μ
∓
and from any
b
̄
b
bound state. It can be
interpreted as a limit on NP using the relationship
ð
g
2
NP
=
Λ
NP
Þ
2
=
ð
4
πα
3
S
Q
b
=M
Υ
ð
3
S
Þ
Þ
2
¼
B
½
Υ
ð
3
S
Þ
→
e
μ
=
B
½
Υ
ð
3
S
Þ
→
μμ
, ignoring small kinematic factors, and where
Q
b
¼
−
1
=
3
is the
b
-quark charge and
α
3
S
is the fine structure
constant at the
M
Υ
ð
3
S
Þ
energy scale. Using the world
average
B
½
Υ
ð
3
S
Þ
→
μ
þ
μ
−
¼
2
.
18
±0
.
21
[3]
gives a
90% C.L. upper limit of
Λ
NP
=g
2
NP
>
80
TeV.
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 (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 Foundation (USA).
*
Deceased.
†
Present address: Wuhan University, Wuhan 430072, China.
‡
Present address: Universit`
a di Bologna and INFN Sezione
di Bologna, I-47921 Rimini, Italy.
§
Present address: King
’
s College, London WC2R 2LS,
United Kingdom.
∥
Present address: Western Kentucky University, Bowling
Green, Kentucky 42101, USA.
¶
Present address: University of Huddersfield, Huddersfield
HD1 3DH, United Kingdom.
**
Present address: 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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