of 8
Search for di-muon decays of a low-mass Higgs boson in radiative decays of the

ð
1
S
Þ
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
J. Garra Tico,
2
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
G. Lynch,
5
H. Koch,
6
T. Schroeder,
6
D. J. Asgeirsson,
7
C. Hearty,
7
T. S. Mattison,
7
J. A. McKenna,
7
R. Y. So,
7
A. Khan,
8
V. E. Blinov,
9
A. R. Buzykaev,
9
V. P. Druzhinin,
9
V. B. Golubev,
9
E. A. Kravchenko,
9
A. P. Onuchin,
9
S. I. Serednyakov,
9
Yu. I. Skovpen,
9
E. P. Solodov,
9
K. Yu. Todyshev,
9
A. N. Yushkov,
9
M. Bondioli,
10
D. Kirkby,
10
A. J. Lankford,
10
M. Mandelkern,
10
H. Atmacan,
11
J. W. Gary,
11
F. Liu,
11
O. Long,
11
G. M. Vitug,
11
C. Campagnari,
12
T. M. Hong,
12
D. Kovalskyi,
12
J. D. Richman,
12
C. A. West,
12
A. M. Eisner,
13
J. Kroseberg,
13
W. S. Lockman,
13
A. J. Martinez,
13
B. A. Schumm,
13
A. Seiden,
13
D. S. Chao,
14
C. H. Cheng,
14
B. Echenard,
14
K. T. Flood,
14
D. G. Hitlin,
14
P. Ongmongkolkul,
14
F. C. Porter,
14
A. Y. Rakitin,
14
R. Andreassen,
15
Z. Huard,
15
B. T. Meadows,
15
M. D. Sokoloff,
15
L. Sun,
15
P. C. Bloom,
16
W. T. Ford,
16
A. Gaz,
16
U. Nauenberg,
16
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17,
*
W. H. Toki,
17
B. Spaan,
18
K. R. Schubert,
19
R. Schwierz,
19
D. Bernard,
20
M. Verderi,
20
P. J. Clark,
21
S. Playfer,
21
D. Bettoni,
22a
C. Bozzi,
22a
R. Calabrese,
22a,22b
G. Cibinetto,
22a,22b
E. Fioravanti,
22a,22b
I. Garzia,
22a,22b
E. Luppi,
22a,22b
L. Piemontese,
22a
V. Santoro,
22a
R. Baldini-Ferroli,
23
A. Calcaterra,
23
R. de Sangro,
23
G. Finocchiaro,
23
P. Patteri,
23
I. M. Peruzzi,
23,
M. Piccolo,
23
M. Rama,
23
A. Zallo,
23
R. Contri,
24a,24b
E. Guido,
24a,24b
M. Lo Vetere,
24a,24b
M. R. Monge,
24a,24b
S. Passaggio,
24a
C. Patrignani,
24a,24b
E. Robutti,
24a
B. Bhuyan,
25
V. Prasad,
25
C. L. Lee,
26
M. Morii,
26
A. J. Edwards,
27
A. Adametz,
28
U. Uwer,
28
H. M. Lacker,
29
T. Lueck,
29
P. D. Dauncey,
30
U. Mallik,
31
C. Chen,
32
J. Cochran,
32
W. T. Meyer,
32
S. Prell,
32
A. E. Rubin,
32
A. V. Gritsan,
33
Z. J. Guo,
33
N. Arnaud,
34
M. Davier,
34
D. Derkach,
34
G. Grosdidier,
34
F. Le Diberder,
34
A. M. Lutz,
34
B. Malaescu,
34
P. Roudeau,
34
M. H. Schune,
34
A. Stocchi,
34
G. Wormser,
34
D. J. Lange,
35
D. M. Wright,
35
C. A. Chavez,
36
J. P. Coleman,
36
J. R. Fry,
36
E. Gabathuler,
36
D. E. Hutchcroft,
36
D. J. Payne,
36
C. Touramanis,
36
A. J. Bevan,
37
F. Di Lodovico,
37
R. Sacco,
37
M. Sigamani,
37
G. Cowan,
38
D. N. Brown,
39
C. L. Davis,
39
A. G. Denig,
40
M. Fritsch,
40
W. Gradl,
40
K. Griessinger,
40
A. Hafner,
40
E. Prencipe,
40
R. J. Barlow,
41,
G. Jackson,
41
G. D. Lafferty,
41
E. Behn,
42
R. Cenci,
42
B. Hamilton,
42
A. Jawahery,
42
D. A. Roberts,
42
C. Dallapiccola,
43
R. Cowan,
44
D. Dujmic,
44
G. Sciolla,
44
R. Cheaib,
45
D. Lindemann,
45
P. M. Patel,
45,
§
S. H. Robertson,
45
P. Biassoni,
46a,46b
N. Neri,
46a
F. Palombo,
46a,46b
S. Stracka,
46a,46b
L. Cremaldi,
47
R. Godang,
47,
k
R. Kroeger,
47
P. Sonnek,
47
D. J. Summers,
47
X. Nguyen,
48
M. Simard,
48
P. Taras,
48
G. De Nardo,
49a,49b
D. Monorchio,
49a,49b
G. Onorato,
49a,49b
C. Sciacca,
49a,49b
M. Martinelli,
50
G. Raven,
50
C. P. Jessop,
51
J. M. LoSecco,
51
W. F. Wang,
51
K. Honscheid,
52
R. Kass,
52
J. Brau,
53
R. Frey,
53
N. B. Sinev,
53
D. Strom,
53
E. Torrence,
53
E. Feltresi,
54a,54b
N. Gagliardi,
54a,54b
M. Margoni,
54a,54b
M. Morandin,
54a
M. Posocco,
54a
M. Rotondo,
54a
G. Simi,
54a
F. Simonetto,
54a,54b
R. Stroili,
54a,54b
S. Akar,
55
E. Ben-Haim,
55
M. Bomben,
55
G. R. Bonneaud,
55
H. Briand,
55
G. Calderini,
55
J. Chauveau,
55
O. Hamon,
55
Ph. Leruste,
55
G. Marchiori,
55
J. Ocariz,
55
S. Sitt,
55
M. Biasini,
56a,56b
E. Manoni,
56a,56b
S. Pacetti,
56a,56b
A. Rossi,
56a,56b
C. Angelini,
57a,57b
G. Batignani,
57a,57b
S. Bettarini,
57a,57b
M. Carpinelli,
57a,57b,
{
G. Casarosa,
57a,57b
A. Cervelli,
57a,57b
F. Forti,
57a,57b
M. A. Giorgi,
57a,57b
A. Lusiani,
57a,57c
B. Oberhof,
57a,57b
E. Paoloni,
57a,57b
A. Perez,
57a
G. Rizzo,
57a,57b
J. J. Walsh,
57a
D. Lopes Pegna,
58
J. Olsen,
58
A. J. S. Smith,
58
F. Anulli,
59a
R. Faccini,
59a,59b
F. Ferrarotto,
59a
F. Ferroni,
59a,59b
M. Gaspero,
59a,59b
L. Li Gioi,
59a
M. A. Mazzoni,
59a
G. Piredda,
59a
C. Bu
̈
nger,
60
O. Gru
̈
nberg,
60
T. Hartmann,
60
T. Leddig,
60
C. Voß,
60
R. Waldi,
60
T. Adye,
61
E. O. Olaiya,
61
F. F. Wilson,
61
S. Emery,
62
G. Hamel de Monchenault,
62
G. Vasseur,
62
Ch. Ye
`
che,
62
D. Aston,
63
D. J. Bard,
63
R. Bartoldus,
63
J. F. Benitez,
63
C. Cartaro,
63
M. R. Convery,
63
J. Dorfan,
63
G. P. Dubois-Felsmann,
63
W. Dunwoodie,
63
M. Ebert,
63
R. C. Field,
63
M. Franco Sevilla,
63
B. G. Fulsom,
63
A. M. Gabareen,
63
M. T. Graham,
63
P. Grenier,
63
C. Hast,
63
W. R. Innes,
63
M. H. Kelsey,
63
P. Kim,
63
M. L. Kocian,
63
D. W. G. S. Leith,
63
P. Lewis,
63
B. Lindquist,
63
S. Luitz,
63
V. Luth,
63
H. L. Lynch,
63
D. B. MacFarlane,
63
D. R. Muller,
63
H. Neal,
63
S. Nelson,
63
M. Perl,
63
T. Pulliam,
63
B. N. Ratcliff,
63
A. Roodman,
63
A. A. Salnikov,
63
R. H. Schindler,
63
A. Snyder,
63
D. Su,
63
M. K. Sullivan,
63
J. Va’vra,
63
A. P. Wagner,
63
W. J. Wisniewski,
63
M. Wittgen,
63
D. H. Wright,
63
H. W. Wulsin,
63
C. C. Young,
63
V. Ziegler,
63
W. Park,
64
M. V. Purohit,
64
R. M. White,
64
J. R. Wilson,
64
A. Randle-Conde,
65
S. J. Sekula,
65
M. Bellis,
66
P. R. Burchat,
66
T. S. Miyashita,
66
E. M. T. Puccio,
66
M. S. Alam,
67
J. A. Ernst,
67
R. Gorodeisky,
68
N. Guttman,
68
D. R. Peimer,
68
A. Soffer,
68
S. M. Spanier,
69
J. L. Ritchie,
70
A. M. Ruland,
70
R. F. Schwitters,
70
B. C. Wray,
70
J. M. Izen,
71
X. C. Lou,
71
F. Bianchi,
72a,72b
D. Gamba,
72a,72b
S. Zambito,
72a,72b
L. Lanceri,
73a,73b
L. Vitale,
73a,73b
F. Martinez-Vidal,
74
A. Oyanguren,
74
P. Villanueva-Perez,
74
H. Ahmed,
75
J. Albert,
75
Sw. Banerjee,
75
F. U. Bernlochner,
75
H. H. F. Choi,
75
G. J. King,
75
R. Kowalewski,
75
M. J. Lewczuk,
75
I. M. Nugent,
75
J. M. Roney,
75
R. J. Sobie,
75
N. Tasneem,
75
T. J. Gershon,
76
P. F. Harrison,
76
T. E. Latham,
76
H. R. Band,
77
S. Dasu,
77
Y. Pan,
77
R. Prepost,
77
and S. L. Wu
77
PHYSICAL REVIEW D
87,
031102(R) (2013)
RAPID COMMUNICATIONS
1550-7998
=
2013
=
87(3)
=
031102(8)
031102-1
Ó
2013 American Physical Society
(
B
A
B
AR
Collaboration)
1
Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP), Universite
́
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, Universita
`
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 Universita
̈
t Bochum, Institut fu
̈
r Experimentalphysik 1, D-44780 Bochum, Germany
7
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
8
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
9
Budker Institute of Nuclear Physics, Novosibirsk 630090, 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 Universita
̈
t Dortmund, Fakulta
̈
t Physik, D-44221 Dortmund, Germany
19
Technische Universita
̈
t Dresden, Institut fu
̈
r Kern- und Teilchenphysik, D-01062 Dresden, Germany
20
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
21
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
22a
INFN Sezione di Ferrara, I-44100 Ferrara, Italy
22b
Dipartimento di Fisica, Universita
`
di Ferrara, I-44100 Ferrara, Italy
23
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
24a
INFN Sezione di Genova, I-16146 Genova, Italy
24b
Dipartimento di Fisica, Universita
`
di Genova, I-16146 Genova, Italy
25
Indian Institute of Technology Guwahati, Guwahati, Assam, 781 039, India
26
Harvard University, Cambridge, Massachusetts 02138, USA
27
Harvey Mudd College, Claremont, California 91711, USA
28
Universita
̈
t Heidelberg, Physikalisches Institut, Philosophenweg 12, D-69120 Heidelberg, Germany
29
Humboldt-Universita
̈
t zu Berlin, Institut fu
̈
r Physik, Newtonstraße 15, D-12489 Berlin, Germany
30
Imperial College London, London SW7 2AZ, United Kingdom
31
University of Iowa, Iowa City, Iowa 52242, USA
32
Iowa State University, Ames, Iowa 50011-3160, USA
33
Johns Hopkins University, Baltimore, Maryland 21218, USA
34
Laboratoire de l’Acce
́
le
́
rateur Line
́
aire, IN2P3/CNRS et Universite
́
Paris-Sud 11, Centre Scientifique d’Orsay,
B. P. 34, F-91898 Orsay Cedex, France
35
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
36
University of Liverpool, Liverpool L69 7ZE, United Kingdom
37
Queen Mary, University of London, London, E1 4NS, United Kingdom
38
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
39
University of Louisville, Louisville, Kentucky 40292, USA
40
Johannes Gutenberg-Universita
̈
t Mainz, Institut fu
̈
r Kernphysik, D-55099 Mainz, Germany
41
University of Manchester, Manchester M13 9PL, United Kingdom
42
University of Maryland, College Park, Maryland 20742, USA
43
University of Massachusetts, Amherst, Massachusetts 01003, USA
44
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge, Massachusetts 02139, USA
45
McGill University, Montre
́
al, Que
́
bec, Canada H3A 2T8
46a
INFN Sezione di Milano, I-20133 Milano, Italy
46b
Dipartimento di Fisica, Universita
`
di Milano, I-20133 Milano, Italy
47
University of Mississippi, University, Mississippi 38677, USA
48
Universite
́
de Montre
́
al, Physique des Particules, Montre
́
al, Que
́
bec, Canada H3C 3J7
49a
INFN Sezione di Napoli, I-80126 Napoli, Italy
49b
Dipartimento di Scienze Fisiche, Universita
`
di Napoli Federico II, I-80126 Napoli, Italy
50
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, The Netherlands
51
University of Notre Dame, Notre Dame, Indiana 46556, USA
J. P. LEES
et al.
PHYSICAL REVIEW D
87,
031102(R) (2013)
RAPID COMMUNICATIONS
031102-2
52
Ohio State University, Columbus, Ohio 43210, USA
53
University of Oregon, Eugene, Oregon 97403, USA
54a
INFN Sezione di Padova, I-35131 Padova, Italy
54b
Dipartimento di Fisica, Universita
`
di Padova, I-35131 Padova, Italy
55
Laboratoire de Physique Nucle
́
aire et de Hautes Energies, IN2P3/CNRS, Universite
́
Pierre et Marie Curie-Paris6,
Universite
́
Denis Diderot-Paris7, F-75252 Paris, France
56a
INFN Sezione di Perugia, I-06100 Perugia, Italy
56b
Dipartimento di Fisica, Universita
`
di Perugia, I-06100 Perugia, Italy
57a
INFN Sezione di Pisa, I-56127 Pisa, Italy
57b
Dipartimento di Fisica, Universita
`
di Pisa, I-56127 Pisa, Italy
57c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
58
Princeton University, Princeton, New Jersey 08544, USA
59a
INFN Sezione di Roma, I-00185 Roma, Italy
59b
Dipartimento di Fisica, Universita
`
di Roma La Sapienza, I-00185 Roma, Italy
60
Universita
̈
t Rostock, D-18051 Rostock, Germany
61
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
62
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
63
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
64
University of South Carolina, Columbia, South Carolina 29208, USA
65
Southern Methodist University, Dallas, Texas 75275, USA
66
Stanford University, Stanford, California 94305-4060, USA
67
State University of New York, Albany, New York 12222, USA
68
School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
69
University of Tennessee, Knoxville, Tennessee 37996, USA
70
University of Texas at Austin, Austin, Texas 78712, USA
71
University of Texas at Dallas, Richardson, Texas 75083, USA
72a
INFN Sezione di Torino, I-10125 Torino, Italy
72b
Dipartimento di Fisica Sperimentale, Universita
`
di Torino, I-10125 Torino, Italy
73a
INFN Sezione di Trieste, I-34127 Trieste, Italy
73b
Dipartimento di Fisica, Universita
`
di Trieste, I-34127 Trieste, Italy
74
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
75
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
76
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
77
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 2 October 2012; published 15 February 2013; corrected 21 February 2013)
We search for di-muon decays of a low-mass Higgs boson (
A
0
) produced in radiative

ð
1
S
Þ
decays. The

ð
1
S
Þ
sample is selected by tagging the pion pair in the

ð
2
S;
3
S
Þ!

þ



ð
1
S
Þ
transitions, using a
data sample of
92
:
8

10
6

ð
2
S
Þ
and
116
:
8

10
6

ð
3
S
Þ
events collected by the
BABAR
detector. We find
no evidence for
A
0
production and set 90% confidence level upper limits on the product branching fraction
B
ð

ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!

þ


Þ
in the range of
ð
0
:
28

9
:
7
Þ
10

6
for
0
:
212

m
A
0

9
:
20 GeV
=c
2
.
The results are combined with our previous measurements of

ð
2
S;
3
S
Þ!
A
0
,
A
0
!

þ


to set limits
on the effective coupling of the
b
quark to the
A
0
.
DOI:
10.1103/PhysRevD.87.031102
PACS numbers: 12.60.Fr, 12.60.Jv, 13.20.Gd, 13.35.Bv
Many extensions of the Standard Model (SM), such
as the next-to-minimal supersymmetric Standard Model
(NMSSM), include a light Higgs boson [
1
,
2
]. The minimal
supersymmetric Standard Model (MSSM) [
3
] solves the
hierarchy problem of the SM, whose superpotential
contains a supersymmetric Higgs mass parameter,

, that
contributes to the masses of the Higgs bosons. The MSSM
fails to explain why the value of the

parameter is of the
order of the electroweak scale, which is many orders of
magnitude below the next natural scale, the Planck scale.
The NMSSM solves this so-called ‘‘

problem’’ [
4
]by
adding a singlet chiral superfield to the MSSM, generating
an effective

term. As a result, the NMSSM Higgs sector
contains a total of three neutral
CP
-even, two neutral
CP
-odd, and two charged Higgs bosons. The lightest
CP
-odd Higgs boson (
A
0
) could have a mass smaller
than twice the mass of the
b
quark [
1
], making it detectable
*
Present address: University of Tabuk, Tabuk71491, Saudi Arabia.
Also at Universita
`
di Perugia, Dipartimento di Fisica,
Perugia, Italy.
Present address: University of Huddersfield, Huddersfield
HD1 3DH, United Kingdom.
§
Deceased.
k
Present address: University of South Alabama, Mobile,
Alabama 36688, USA.
{
Also at Universita
`
di Sassari, Sassari, Italy.
SEARCH FOR DI-MUON DECAYS OF A LOW-MASS HIGGS
...
PHYSICAL REVIEW D
87,
031102(R) (2013)
RAPID COMMUNICATIONS
031102-3
via radiative

ð
nS
Þ!
A
0
(
n
¼
1
, 2, 3) decays [
5
]. The
coupling of the
A
0
field to up-type (down-type) fermion
pairs is proportional to
cos

A
cot

(
cos

A
tan

), where

A
is the mixing angle between the singlet component and the
MSSM component of the
A
0
, and
tan

is the ratio of the
vacuum expectation values of the up- and down-type Higgs
doublets. The branching fraction of

ð
1
S
Þ!
A
0
could be
as large as
10

4
depending on the values of the
A
0
mass,
tan

and
cos

A
[
2
]. Constraints on the low-mass NMSSM
Higgs sector are also important for interpreting the SM
Higgs sector [
6
].
BABAR
has previously searched for
A
0
production
in several final states [
7
10
], including

ð
2
S;
3
S
Þ!
A
0
;A
0
!

þ


[
7
]. Similar searches have been per-
formed by CLEO in the di-muon and di-tau final states in
radiative

ð
1
S
Þ
decays [
11
], and more recently by BESIII
in
J=
c
!
A
0
,
A
0
!

þ


[
12
], and by the CMS
experiment in
pp
!
A
0
,
A
0
!

þ


[
13
]. These results
have ruled out a substantial fraction of the NMSSM
parameter space [
14
].
We report herein a search for a di-muon resonance
in the fully reconstructed decay chain of

ð
2
S;
3
S
Þ!

þ



ð
1
S
Þ
,

ð
1
S
Þ!
A
0
,
A
0
!

þ


. This search
is based on a sample of
ð
92
:
8

0
:
8
Þ
10
6

ð
2
S
Þ
and
ð
116
:
8

1
:
0
Þ
10
6

ð
3
S
Þ
mesons collected by the
BABAR
detector at the PEP-II asymmetric-energy
e
þ
e

collider located at the SLAC National Accelerator
Laboratory. A sample of

ð
1
S
Þ
mesons is selected by
tagging the di-pion transition, which results in a substantial
background reduction compared to direct searches of
A
0
in

ð
2
S;
3
S
Þ!
A
0
decays. We assume that the light Higgs
boson that we search for is a scalar or pseudoscalar particle
with a negligible decay width compared to the experimen-
tal resolution [
15
].
The
BABAR
detector is described in detail elsewhere
[
16
,
17
]. Charged particle momenta are measured in a five-
layer double-sided silicon vertex tracker and a 40-layer
drift chamber, both operating in a 1.5 T solenoidal
magnetic field. Charged particle identification is performed
using a ring-imaging Cherenkov detector and the energy
loss (
d
E=
d
x
) in the tracking system. Photon and electron
energies are measured in a CsI(Tl) electromagnetic calo-
rimeter, while muons are identified in the instrumented
magnetic flux return of the magnet.
Monte Carlo (MC) simulated events are used to study
the detector acceptance and to optimize the event selection
procedure. The EvtGen package [
18
] is used to simulate
the
e
þ
e

!
q

q
ð
q
¼
u;d;s;c
Þ
and generic

ð
2
S;
3
S
Þ
production,
BHWIDE
[
19
] to simulate the Bhabha scatter-
ing, and
KK2F
[
20
] to simulate the processes
e
þ
e

!
ð

Þ

þ


and
e
þ
e


Þ

þ


. Dedicated MC samples
of

ð
2
S;
3
S
Þ
generic decays to

þ



ð
1
S
Þ
with

ð
1
S
Þ!

þ


decays, hereafter referred to as the ‘‘nonresonant
di-muon decays’’ are also generated. Signal events are
generated using a phase-space (
P
-wave) model for the
A
0
!

þ


(

ð
1
S
Þ!
A
0
) decay, and the hadronic
matrix elements measured by the CLEO experiment [
21
]
are used for the

ð
2
S;
3
S
Þ!

þ



ð
1
S
Þ
modeling. The
detector response is simulated by
GEANT4
[
22
] and the time-
dependent detector effects, such as the variation of the
detector performance over the data-taking period and the
beam related backgrounds, are included in the simulation. A
sample corresponding to about 5% of the data set is used to
validate the selection and fitting procedure. To avoid bias,
this sample is discarded from the final data set. We perform
a blind analysis, where the rest of the

ð
2
S;
3
S
Þ
data sets are
blinded until the analysis procedure is frozen.
We select events containing exactly four charged tracks
and a single energetic photon with a center-of-mass (CM)
energy larger than 200 MeV. The tracks are required to
have a distance of closest approach to the interaction point
of less than 1.5 cm in the plane transverse to the beam axis
and less than 2.5 cm along the beam axis. At least one of
the tracks must be identified as a muon by particle ID
algorithms; the probability for misidentifying a charged
pion as a muon is 3%. Additional photons with CM ener-
gies below the threshold of 200 MeVare also allowed to be
present in the events. The two highest momentum tracks in
the CM frame with opposite charge are assumed to be
muon candidates and are required to originate from a
common vertex to form the
A
0
candidates.
The

ð
1
S
Þ
candidate is reconstructed by combining
the
A
0
candidate with the energetic photon candidate and
by requiring the invariant mass to be between 9.0 and
9
:
8 GeV
=c
2
. The

ð
2
S;
3
S
Þ
candidates are formed by
combining the

ð
1
S
Þ
candidate with the two remaining
tracks, assumed to be pions. The di-pion invariant mass
must be in the range [
2
m

;
ð
m

ð
2
S;
3
S
Þ

m

ð
1
S
Þ
Þ
], compat-
ible with the kinematic boundaries of the

ð
2
S;
3
S
Þ!

þ



ð
1
S
Þ
decay. The entire decay chain is then fit by
imposing the decay vertex of the

ð
2
S;
3
S
Þ
candidate to be
constrained to the beam interaction region, and a mass
constraint on the

ð
1
S
Þ
and

ð
2
S;
3
S
Þ
candidates, as
well as requiring the energy of the

ð
2
S;
3
S
Þ
candidate
to be consistent with the
e
þ
e

CM energy. These con-
straints improve the resolution of the di-muon invariant
mass to be less than
10 MeV
=c
2
.
To improve the purity of the

ð
1
S
Þ
sample, we train a
random forest (RF) classifier [
23
] on simulated signal and
background events, using variables that distinguish signal
from background in the

ð
2
S;
3
S
Þ!

þ



ð
1
S
Þ
transi-
tions. The following quantities are used as inputs to the
classifier: the cosine of the angle between the two pion
candidates in the laboratory frame; the transverse momen-
tum of the di-pion system in the laboratory frame; the
azimuthal angle and transverse momentum of each pion;
the di-pion invariant mass (
m

); the pion helicity
angle; the transverse position of the di-pion vertex and
the mass recoiling against the di-pion system, defined
as
m
recoil
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s
þ
m
2


2
ffiffiffi
s
p
E
CM

q
, where
ffiffiffi
s
p
is the
e
þ
e

J. P. LEES
et al.
PHYSICAL REVIEW D
87,
031102(R) (2013)
RAPID COMMUNICATIONS
031102-4
CM energy and
E
CM

is the CM energy of the di-pion
system. For signal-like events, the
m
recoil
distribution peaks
at the mass of the

ð
1
S
Þ
, with a mass resolution of about
3 MeV
=c
2
. The RF output peaks at 1 for signal-like
candidates and peaks at 0 for the backgroundlike candi-
dates. The optimum value of the RF selection is chosen to
maximize Punzi’s figure of merit,
=
ð
0
:
5
N

þ
ffiffiffiffi
B
p
Þ
[
24
],
where
N

¼
3
is the number of standard deviations
desired from the result, and

and
B
are the average
efficiency and background yield over a broad
m
A
0
range
(
0
:
212
9
:
20 GeV
=c
2
), respectively.
A total of 11,136

ð
2
S
Þ
and 3,857

ð
3
S
Þ
candidates are
selected by these criteria. Figures
1
and
2
show the
distributions of the
m
recoil
and di-muon reduced mass,
m
red
¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
m
2

þ



4
m
2

q
, together with the background
prediction estimated from the MC samples, which are
dominated by the nonresonant di-muon decays. The
reduced mass is equal to twice the momentum of the di-
muon system in the rest frame of the
A
0
, and has a smooth
distribution in the region of the kinematic threshold
m

þ



2
m

(
m
red

0
). After unblinding the data,
two peaking components corresponding to
0
and
J=
c
mesons are observed in the

ð
3
S
Þ
data set. The
0
mesons
are mainly produced in initial state radiation (ISR) events,
along with two or more pions. This peak disappears if we
require both candidates to be identified as muons in the
A
0
reconstruction. The
J=
c
mesons arise from
e
þ
e

!

ISR
c
ð
2
S
Þ
,
c
ð
2
S
Þ!

þ


J=
c
,
J=
c
!

þ


decays.
The
J=
c
and
0
events in the

ð
2
S
Þ
data set are sup-
pressed since the di-pion mass distribution in these events
is above the kinematic edge of the di-pion mass distribu-
tion of

ð
2
S
Þ
decays, but well within the range of values
allowed for the

ð
3
S
Þ
decays.
We extract the signal yield as a function of
m
A
0
in the
region
0
:
212

m
A
0

9
:
20 GeV
=c
2
by performing a
series of one-dimensional unbinned extended maximum
likelihood (ML) fits to the
m
red
distribution. We fit
over fixed intervals in the low mass region:
0
:
002

m
red

1
:
85 GeV
=c
2
for
0
:
212

m
A
0

1
:
50 GeV
=c
2
,
1
:
4

m
red

5
:
6 GeV
=c
2
for
1
:
50
<m
A
0
<
5
:
36 GeV
=c
2
and
5
:
25

m
red

7
:
3 GeV
=c
2
for
5
:
36

m
A
0

7
:
10 GeV
=c
2
. Above this range, we use sliding intervals


0
:
2 GeV
=c
2
<m
red
<
þ
0
:
15 GeV
=c
2
, where

is
the mean of the reduced mass distribution.
The probability density function (PDF) of the signal
is described by a sum of two Crystal Ball functions [
25
].
The signal PDF is determined as a function of
m
A
0
using signal MC samples generated at 26 different masses,
and by interpolating the PDF parameters between each
mass point. The resolution of the
m
red
distribution for
signal MC increases monotonically with
m
A
0
from 2 to
9 MeV
=c
2
, while the signal efficiency decreases from
38.3% (40.4%) to 31.7% (31.6%) for

ð
2
S
Þ
[

ð
3
S
Þ
]
transitions. The background for
m
A
0

1
:
5 GeV
=c
2
is
described by a threshold function
f
ð
m
red
Þ/½
Erf
ð
s
ð
m
red

m
0
ÞÞþ
1
exp
ð
c
0
þ
c
1
m
red
Þ
(1)
MC
Data
(a)
)
2
Events/(0.001 GeV/c
1000
2000
)
2
(GeV/c
recoil
m
9.44
9.46
9.48
0
200
400
MC
Data
(b)
FIG. 1 (color online). The distribution of
m
recoil
for (a) the

ð
2
S
Þ
and (b) the

ð
3
S
Þ
data sets, together with the background
predictions from the MC samples, which are dominated by the
nonresonant di-muon decays. The MC samples are normalized to
the data luminosity.
)
2
Events/(0.20 GeV/c
1
3
10
MC
Data
(a)
)
2
(GeV/c
red
m
0123456789
1
3
10
MC
Data
ψ
J/
0
ρ
(b)
FIG. 2 (color online). The distribution of
m
red
for (a) the

ð
2
S
Þ
and (b) the

ð
3
S
Þ
data sets, together with the background
predictions from the various MC samples. The MC samples
are normalized to the data luminosity. Two peaking components
corresponding to the
0
and
J=
c
mesons are observed in the

ð
3
S
Þ
data set. The contribution of
J=
c
mesons is not included
in the MC predictions, whereas the
0
meson is poorly modeled
in the MC.
SEARCH FOR DI-MUON DECAYS OF A LOW-MASS HIGGS
...
PHYSICAL REVIEW D
87,
031102(R) (2013)
RAPID COMMUNICATIONS
031102-5