of 8
Search for a Dark Leptophilic Scalar in
e
+
e
Collisions
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
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,
35b
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,
48a48b
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 and Dipartimento di Fisica, Universit`
a 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
125,
181801 (2020)
0031-9007
=
20
=
125(18)
=
181801(8)
181801-1
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
PHYSICAL REVIEW LETTERS
125,
181801 (2020)
181801-2
62a
INFN Sezione di Torino, I-10125 Torino, Italy
62b
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, Canada V8W 3P6
65b
University of Victoria, 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
(Received 6 May 2020; revised 17 September 2020; accepted 18 September 2020; published 28 October 2020)
Many scenarios of physics beyond the standard model predict the existence of new gauge singlets, which
might be substantially lighter than the weak scale. The experimental constraints on additional scalars with
masses in the MeV to GeV range could be significantly weakened if they interact predominantly with
leptons rather than quarks. At an
e
þ
e
collider, such a leptophilic scalar (
φ
L
) would be produced
predominantly through radiation from a
τ
lepton. We report herein a search for
e
þ
e
τ
þ
τ
φ
L
,
φ
L
l
þ
l
(
l
¼
e
,
μ
) using data collected by the
BABAR
experiment at SLAC. No significant signal is
observed, and we set limits on the
φ
L
coupling to leptons in the range
0
.
04
<m
φ
L
<
7
.
0
GeV. These
bounds significantly improve upon the current constraints, excluding almost entirely the parameter space
favored by the observed discrepancy in the muon anomalous magnetic moment below 4 GeV at
90% confidence level.
DOI:
10.1103/PhysRevLett.125.181801
Many theories beyond the standard model (SM) predict
the existence of additional scalars, and discovering or
constraining their existence might shed light on the physics
of electroweak symmetry breaking and the Higgs sector
(e.g., see Ref.
[1]
). Some of these particles may be
substantially lighter than the weak scale, notably in the
next-to-minimal supersymmetric standard model
[2]
,but
also in more generic singlet-extended sectors
[3,4]
. In the
MeV
GeV range, new scalars could mediate interactions
between the SM and dark matter, as well as account for the
discrepancy in the observed value of the muon anomalous
magnetic dipole moment
[5
7]
.
The possible coupling of a new scalar
φ
L
to SM
particles is constrained by SM gauge invariance. In the
simplest case, the mixing between the scalar and the SM
Higgs boson gives rise to couplings proportional to SM
fermion masses. Because the new scalar couples predomi-
nantly to heavy-flavor quarks, this minimal scenario is
strongly constrained by searches for rare flavor-changing
neutral current decays of mesons, such as
B
K
φ
and
K
πφ
[8]
. However, these bounds are evaded if the
coupling of the scalar to quarks is suppressed and the scalar
interacts preferentially with heavy-flavor leptons
[3,4,9,10]
.
We refer to such a particle as a leptophilic scalar,
φ
L
. Its
interaction Lagrangian with leptons can be described by
L
¼
ξ
X
l
¼
e;
μ
;
τ
m
l
v
̄
l
φ
L
l
;
where
ξ
denotes the flavor-independent coupling strength to
leptons and
v
¼
246
GeV is the SM Higgs vacuum expect-
ation value
[11]
. Model independent constraints relying
exclusively on the coupling to leptons are derived from a
BABAR
search for a muonic dark force
[12]
and beam dump
experiments
[13,14]
. A large fraction of the parameter space,
includingtheregionfavoredbythemeasurementofthemuon
anomalous magnetic moment, is still unexplored
[3,9,10]
.
Examples of model dependent bounds from
B
and
h
decays
for a specific UV completion of the theory can be found
in Ref.
[3]
.
The large sample of
τ
þ
τ
pairs collected by
BABAR
offers a clean environment to study model independent
φ
L
production via final-state radiation in
e
þ
e
τ
þ
τ
φ
L
. The
mass proportionality of the coupling, in particular the
feeble interaction with electrons, dictates the experimental
signature. For
2
m
e
<m
φ
L
<
2
m
μ
, the scalar decays pre-
dominantly into electrons, leading to displaced vertices for
sufficiently small values of the coupling. Prompt decays
into a pair of muons (taus) dominate when
2
m
μ
m
φ
L
<
2
m
τ
(
2
m
τ
<m
φ
L
).
We report herein the first search for a leptophilic scalar in
the reaction
e
þ
e
τ
þ
τ
φ
L
,
φ
L
l
þ
l
(
l
¼
e
,
μ
) for
0
.
04
<m
φ
L
<
7
.
0
GeV. The cross section for
m
φ
L
<
2
m
μ
is measured separately for
φ
L
lifetimes corresponding to
c
τ
φ
L
values of 0, 1, 10, and 100 mm. Above the dimuon
threshold, we determine the cross section for prompt
φ
L
μ
þ
μ
decays. In all cases, the
φ
L
width is much
smaller than the detector resolution, and the signal can be
identified as a narrow peak in the dilepton invariant mass.
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
125,
181801 (2020)
181801-3
The search is based on
514
fb
1
of data collected at the
Υ
ð
2
S
Þ
,
Υ
ð
3
S
Þ
,
Υ
ð
4
S
Þ
resonances and their vicinities
[15]
by the
BABAR
experiment at the SLAC PEP-II
e
þ
e
collider. The
BABAR
detector is described in detail else-
where
[16,17]
. A sample corresponding to about 5% of the
data, called the optimization sample, is used to optimize the
search strategy and is subsequently discarded. The remain-
ing data are examined only once the analysis procedure has
been finalized.
Signal Monte Carlo (MC) samples with prompt
decays are simulated for 36 different
φ
L
mass hypotheses
by the
MADGRAPH
event generator
[18]
and showered
using
PYTHIA
8
[19]
, including final-state radiation. For
m
φ
L
<
0
.
3
GeV, events with
c
τ
φ
L
values up to 300 mm are
also generated. We simulate the following reactions to
study the background:
e
þ
e
e
þ
e
ð
γ
Þ
(
BHWIDE
[20]
),
e
þ
e
μ
þ
μ
ð
γ
Þ
and
e
þ
e
τ
þ
τ
ð
γ
Þ
(
KK
with the
TAUOLA
library
[21,22]
),
e
þ
e
q
̄
q
with
q
¼
u
,
d
,
s
,
c
(
J
etset
[23]
), and
e
þ
e
B
̄
B
and generic
e
þ
e
Υ
ð
2
S;
3
S
Þ
decays (
E
vt
G
en
[24]
). The resonance production
e
þ
e
γψ
ð
2
S
Þ
,
ψ
ð
2
S
Þ
π
þ
π
J=
ψ
,
J=
ψ
μ
þ
μ
is
simulated with
E
vt
G
en
using a structure function technique
[25,26]
. The detector acceptance and reconstruction
efficiencies are estimated with a simulation based on
GEANT
4
[27]
.
We select events containing exactly four charged tracks
with zero net charge, focusing on
τ
lepton decays to single
tracks and any number of neutral particles. The
φ
L
l
þ
l
candidates are formed by combining two opposite-
sign tracks identified as an electron or muon pair by particle
identification (PID) algorithms
[12,16]
. We do not attempt
to select a single
φ
L
candidate per event, but simply
consider all possible combinations. Radiative Bhabha
and dimuon events in which the photon converts to an
e
þ
e
pair are suppressed by rejecting events with a total
visible mass greater than 9 GeV. We further veto
e
þ
e
e
þ
e
e
þ
e
events by requiring the cosine of the angle
between the momentum of the
φ
L
candidate and that of the
nearest track to be less than 0.98, the missing momentum
against all tracks and neutral particles to be greater than
300 MeV, and that there be three or less tracks identified as
electrons. We perform a kinematic fit to the selected
φ
L
candidates, constraining the two tracks to originate from
the same point in space. The dimuon production vertex is
required to be compatible with the beam interaction region,
while we only constrain the momentum vector of the
e
þ
e
pair to point back to the beam interaction region since the
dielectron vertex can be substantially displaced. We select
dielectron (dimuon) combinations with a value of the
χ
2
per
degree of freedom of the fit,
χ
2
=
n
:
d
:
f., less than 3 (12).
A multivariate selection based on boosted decision trees
(BDT) further improves the signal purity
[28]
. The BDTs
include variables capturing the typical
τ
and
φ
L
decay
characteristics: a well-reconstructed
l
þ
l
vertex, either
prompt or displaced; missing energy and momentum due to
neutrino emission; relatively large track momenta; low
neutral particle multiplicity; and two or more tracks
identified as electrons or muons. A few variables are also
targeted at specific backgrounds, such as
ψ
ð
2
S
Þ
π
þ
π
J=
ψ
;J=
ψ
μ
þ
μ
production in initial-state radia-
tion (ISR) events. The
φ
L
mass is specifically excluded to
limit potential bias in the classifier. A full description of
these variables can be found in the Supplemental Material
[29]
. We train a separate BDT for each of the different final
states and
c
τ
φ
L
values with signal events modeled using a
flat
m
φ
L
distribution and background events modeled using
the optimization sample data.
The final selection of
φ
L
candidates for each lifetime
selection and decay channel is made by applying a mass-
dependent criterion on the corresponding BDT score that
maximizes signal sensitivity. The distributions of the
resulting dielectron and dimuon masses for prompt decays
are shown in Fig.
1
, and spectra for other lifetimes for
φ
L
e
þ
e
decays are shown in Fig.
2
, together with the
dominant background components among the set of simu-
lated MC samples. The differences between the data and
summed-MC distributions are mainly due to processes that
(GeV)
ee
m
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24
1
10
2
10
3
10
Data
q
q
-
e
+
e
B
B
-
e
+
e
-
τ
+
τ
-
e
+
e
(GeV)
μ
μ
m
012345678
10
2
10
3
10
4
10
Data
q
q
-
e
+
e
B
B
-
e
+
e
-
τ
+
τ
-
e
+
e
ψ
J/
(2S)
ψ
Entries / 0.004 (GeV)
Entries / 0.1 (GeV)
FIG. 1. The distribution of (top) the dielectron invariant mass and
(bottom) the dimuon invariant mass for prompt decays, together
with simulated predictions for the indicated processes normalized
to the integrated luminosity of the data (stacked histograms).
PHYSICAL REVIEW LETTERS
125,
181801 (2020)
181801-4