Search for a light Higgs boson decaying to two gluons or
s
s
in the radiative decays of
ð
1
S
Þ
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
E. Grauges,
2
A. Palano,
3a,3b
G. Eigen,
4
B. Stugu,
4
D. N. Brown,
5
L. T. Kerth,
5
Yu. G. Kolomensky,
5
M. J. Lee,
5
G. Lynch,
5
H. Koch,
6
T. Schroeder,
6
C. Hearty,
7
T. S. Mattison,
7
J. A. McKenna,
7
R. Y. So,
7
A. Khan,
8
V. E. Blinov,
9a,9c
A. R. Buzykaev,
9a
V. P. Druzhinin,
9a,9b
V. B. Golubev,
9a,9b
E. A. Kravchenko,
9a,9b
A. P. Onuchin,
9a,9c
S. I. Serednyakov,
9a,9b
Yu. I. Skovpen,
9a,9b
E. P. Solodov,
9a,9b
K. Yu. Todyshev,
9a,9b
A. N. Yushkov,
9a
D. Kirkby,
10
A. J. Lankford,
10
M. Mandelkern,
10
B. Dey,
11
J. W. Gary,
11
O. Long,
11
G. M. Vitug,
11
C. Campagnari,
12
M. Franco Sevilla,
12
T. M. Hong,
12
D. Kovalskyi,
12
J. D. Richman,
12
C. A. West,
12
A. M. Eisner,
13
W. S. Lockman,
13
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
R. Andreassen,
15
Z. Huard,
15
B. T. Meadows,
15
B. G. Pushpawela,
15
M. D. Sokoloff,
15
L. Sun,
15
P. C. Bloom,
16
W. T. Ford,
16
A. Gaz,
16
U. Nauenberg,
16
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17,
*
W. H. Toki,
17
B. Spaan,
18
R. Schwierz,
19
D. Bernard,
20
M. Verderi,
20
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
S. Martellotti,
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
M. Morii,
26
A. Adametz,
27
U. Uwer,
27
H. M. Lacker,
28
P. D. Dauncey,
29
U. Mallik,
30
C. Chen,
31
J. Cochran,
31
W. T. Meyer,
31
S. Prell,
31
A. V. Gritsan,
32
N. Arnaud,
33
M. Davier,
33
D. Derkach,
33
G. Grosdidier,
33
F. Le Diberder,
33
A. M. Lutz,
33
B. Malaescu,
33,
‡
P. Roudeau,
33
A. Stocchi,
33
G. Wormser,
33
D. J. Lange,
34
D. M. Wright,
34
J. P. Coleman,
35
J. R. Fry,
35
E. Gabathuler,
35
D. E. Hutchcroft,
35
D. J. Payne,
35
C. Touramanis,
35
A. J. Bevan,
36
F. Di Lodovico,
36
R. Sacco,
36
G. Cowan,
37
J. Bougher,
38
D. N. Brown,
38
C. L. Davis,
38
A. G. Denig,
39
M. Fritsch,
39
W. Gradl,
39
K. Griessinger,
39
A. Hafner,
39
E. Prencipe,
39
K. Schubert,
39
R. J. Barlow,
40,
§
G. D. Lafferty,
40
E. Behn,
41
R. Cenci,
41
B. Hamilton,
41
A. Jawahery,
41
D. A. Roberts,
41
R. Cowan,
42
D. Dujmic,
42
G. Sciolla,
42
R. Cheaib,
43
P. M. Patel,
43,
∥
S. H. Robertson,
43
P. Biassoni,
44a,44b
N. Neri,
44a
F. Palombo,
44a,44b
L. Cremaldi,
45
R. Godang,
45,
¶
P. Sonnek,
45
D. J. Summers,
45
M. Simard,
46
P. Taras,
46
G. De Nardo,
47a,47b
D. Monorchio,
47a,47b
G. Onorato,
47a,47b
C. Sciacca,
47a,47b
M. Martinelli,
48
G. Raven,
48
C. P. Jessop,
49
J. M. LoSecco,
49
K. Honscheid,
50
R. Kass,
50
J. Brau,
51
R. Frey,
51
N. B. Sinev,
51
D. Strom,
51
E. Torrence,
51
E. Feltresi,
52a,52b
M. Margoni,
52a,52b
M. Morandin,
52a
M. Posocco,
52a
M. Rotondo,
52a
G. Simi,
52a
F. Simonetto,
52a,52b
R. Stroili,
52a,52b
S. Akar,
53
E. Ben-Haim,
53
M. Bomben,
53
G. R. Bonneaud,
53
H. Briand,
53
G. Calderini,
53
J. Chauveau,
53
Ph. Leruste,
53
G. Marchiori,
53
J. Ocariz,
53
S. Sitt,
53
M. Biasini,
54a,54b
E. Manoni,
54a
S. Pacetti,
54a,54b
A. Rossi,
54a
C. Angelini,
55a,55b
G. Batignani,
55a,55b
S. Bettarini,
55a,55b
M. Carpinelli,
55a,55b,
**
G. Casarosa,
55a,55b
A. Cervelli,
55a,55b
F. Forti,
55a,55b
M. A. Giorgi,
55a,55b
A. Lusiani,
55a,55c
B. Oberhof,
55a,55b
E. Paoloni,
55a,55b
A. Perez,
55a
G. Rizzo,
55a,55b
J. J. Walsh,
55a
D. Lopes Pegna,
56
J. Olsen,
56
A. J. S. Smith,
56
R. Faccini,
57a,57b
F. Ferrarotto,
57a
F. Ferroni,
57a,57b
M. Gaspero,
57a,57b
L. Li Gioi,
57a
G. Piredda,
57a
C. Bu
̈
nger,
58
O. Gru
̈
nberg,
58
T. Hartmann,
58
T. Leddig,
58
C. Voß,
58
R. Waldi,
58
T. Adye,
59
E. O. Olaiya,
59
F. F. Wilson,
59
S. Emery,
60
G. Hamel de Monchenault,
60
G. Vasseur,
60
Ch. Ye
`
che,
60
F. Anulli,
61,
††
D. Aston,
61
D. J. Bard,
61
J. F. Benitez,
61
C. Cartaro,
61
M. R. Convery,
61
J. Dorfan,
61
G. P. Dubois-Felsmann,
61
W. Dunwoodie,
61
M. Ebert,
61
R. C. Field,
61
B. G. Fulsom,
61
A. M. Gabareen,
61
M. T. Graham,
61
C. Hast,
61
W. R. Innes,
61
P. Kim,
61
M. L. Kocian,
61
D. W. G. S. Leith,
61
P. Lewis,
61
D. Lindemann,
61
B. Lindquist,
61
S. Luitz,
61
V. Luth,
61
H. L. Lynch,
61
D. B. MacFarlane,
61
D. R. Muller,
61
H. Neal,
61
S. Nelson,
61
M. Perl,
61
T. Pulliam,
61
B. N. Ratcliff,
61
A. Roodman,
61
A. A. Salnikov,
61
R. H. Schindler,
61
A. Snyder,
61
D. Su,
61
M. K. Sullivan,
61
J. Va’vra,
61
A. P. Wagner,
61
W. F. Wang,
61
W. J. Wisniewski,
61
M. Wittgen,
61
D. H. Wright,
61
H. W. Wulsin,
61
V. Ziegler,
61
W. Park,
62
M. V. Purohit,
62
R. M. White,
62,
‡‡
J. R. Wilson,
62
A. Randle-Conde,
63
S. J. Sekula,
63
M. Bellis,
64
P. R. Burchat,
64
T. S. Miyashita,
64
E. M. T. Puccio,
64
M. S. Alam,
65
J. A. Ernst,
65
R. Gorodeisky,
66
N. Guttman,
66
D. R. Peimer,
66
A. Soffer,
66
S. M. Spanier,
67
J. L. Ritchie,
68
A. M. Ruland,
68
R. F. Schwitters,
68
B. C. Wray,
68
J. M. Izen,
69
X. C. Lou,
69
F. Bianchi,
70a,70b
F. De Mori,
70a,70b
A. Filippi,
70a
D. Gamba,
70a,70b
S. Zambito,
70a,70b
L. Lanceri,
71a,71b
L. Vitale,
71a,71b
F. Martinez-Vidal,
72
A. Oyanguren,
72
P. Villanueva-Perez,
72
H. Ahmed,
73
J. Albert,
73
Sw. Banerjee,
73
F. U. Bernlochner,
73
H. H. F. Choi,
73
G. J. King,
73
R. Kowalewski,
73
M. J. Lewczuk,
73
T. Lueck,
73
I. M. Nugent,
73
J. M. Roney,
73
R. J. Sobie,
73
N. Tasneem,
73
T. J. Gershon,
74
P. F. Harrison,
74
T. E. Latham,
74
H. R. Band,
75
S. Dasu,
75
Y. Pan,
75
R. Prepost,
75
and S. L. Wu
75
PHYSICAL REVIEW D
88,
031701(R) (2013)
RAPID COMMUNICATIONS
1550-7998
=
2013
=
88(3)
=
031701(7)
031701-1
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
Departament ECM, Facultat de Fisica, Universitat de Barcelona, 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
Institute of Physics, University of Bergen, N-5007 Bergen, Norway
5
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
6
Institut fu
̈
r Experimentalphysik 1, Ruhr Universita
̈
t Bochum, D-44780 Bochum, Germany
7
University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
8
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
9a
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090, Russia
9b
Novosibirsk State University, Novosibirsk 630090, Russia
9c
Novosibirsk State Technical University, Novosibirsk 630092, Russia
10
University of California at Irvine, Irvine, California 92697, USA
11
University of California at Riverside, Riverside, California 92521, USA
12
University of California at Santa Barbara, Santa Barbara, California 93106, USA
13
Institute for Particle Physics, University of California at Santa Cruz, 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
Fakulta
̈
t Physik, Technische Universita
̈
t Dortmund, D-44221 Dortmund, Germany
19
Institut fu
̈
r Kern- und Teilchenphysik, Technische Universita
̈
t Dresden, 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-44122 Ferrara, Italy
22b
Dipartimento di Fisica e Scienze della Terra, Universita
`
di Ferrara, I-44122 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
Physikalisches Institut, Universita
̈
t Heidelberg, D-69120 Heidelberg, Germany
28
Institut fu
̈
r Physik, Humboldt-Universita
̈
t zu Berlin, D-12489 Berlin, Germany
29
Imperial College London, London SW7 2AZ, United Kingdom
30
University of Iowa, Iowa City, Iowa 52242, USA
31
Iowa State University, Ames, Iowa 50011-3160, USA
32
Johns Hopkins University, Baltimore, Maryland 21218, USA
33
Laboratoire de l’Acce
́
le
́
rateur Line
́
aire, Centre Scientifique d’Orsay,
IN2P3/CNRS et Universite
́
Paris-Sud 11, F-91898 Orsay Cedex, France
34
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
35
University of Liverpool, Liverpool L69 7ZE, United Kingdom
36
Queen Mary, University of London, London E1 4NS, United Kingdom
37
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, United Kingdom
38
University of Louisville, Louisville, Kentucky 40292, USA
39
Institut fu
̈
r Kernphysik, Johannes Gutenberg-Universita
̈
t Mainz, D-55099 Mainz, Germany
40
University of Manchester, Manchester M13 9PL, United Kingdom
41
University of Maryland, College Park, Maryland 20742, USA
42
Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
43
McGill University, Montre
́
al, Que
́
bec, Canada H3A 2T8
44a
INFN Sezione di Milano, I-20133 Milano, Italy
44b
Dipartimento di Fisica, Universita
`
di Milano, I-20133 Milano, Italy
45
University of Mississippi, University, Mississippi 38677, USA
46
Universite
́
de Montre
́
al, Physique des Particules, Montre
́
al, Que
́
bec, Canada H3C 3J7
47a
INFN Sezione di Napoli, I-80126 Napoli, Italy
47b
Dipartimento di Scienze Fisiche, Universita
`
di Napoli Federico II, I-80126 Napoli, Italy
48
NIKHEF, National Institute for Nuclear Physics and High Energy Physics, NL-1009 DB Amsterdam, Netherlands
J. P. LEES
et al.
PHYSICAL REVIEW D
88,
031701(R) (2013)
RAPID COMMUNICATIONS
031701-2
49
University of Notre Dame, Notre Dame, Indiana 46556, USA
50
Ohio State University, Columbus, Ohio 43210, USA
51
University of Oregon, Eugene, Oregon 97403, USA
52a
INFN Sezione di Padova, I-35131 Padova, Italy
52b
Dipartimento di Fisica, Universita
`
di Padova, I-35131 Padova, Italy
53
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
54a
INFN Sezione di Perugia, I-06123 Perugia, Italy
54b
Dipartimento di Fisica, Universita
`
di Perugia, I-06123 Perugia, Italy
55a
INFN Sezione di Pisa, I-56127 Pisa, Italy
55b
Dipartimento di Fisica, Universita
`
di Pisa, I-56127 Pisa, Italy
55c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
56
Princeton University, Princeton, New Jersey 08544, USA
57a
INFN Sezione di Roma, I-00185 Roma, Italy
57b
Dipartimento di Fisica, Universita
`
di Roma La Sapienza, I-00185 Roma, Italy
58
Universita
̈
t Rostock, D-18051 Rostock, Germany
59
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
60
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
61
SLAC National Accelerator Laboratory, Stanford, California 94309, USA
62
University of South Carolina, Columbia, South Carolina 29208, USA
63
Southern Methodist University, Dallas, Texas 75275, USA
64
Stanford University, Stanford, California 94305-4060, USA
65
State University of New York, Albany, New York 12222, USA
66
School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
67
University of Tennessee, Knoxville, Tennessee 37996, USA
68
University of Texas at Austin, Austin, Texas 78712, USA
69
University of Texas at Dallas, Richardson, Texas 75083, USA
70a
INFN Sezione di Torino, I-10125 Torino, Italy
70b
Dipartimento di Fisica, Universita
`
di Torino, I-10125 Torino, Italy
71a
INFN Sezione di Trieste, I-34127 Trieste, Italy
71b
Dipartimento di Fisica, Universita
`
di Trieste, I-34127 Trieste, Italy
72
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
73
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
74
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
75
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 21 July 2013; published 6 August 2013)
We search for the decay
ð
1
S
Þ!
A
0
,
A
0
!
gg
or
s
s
, where
A
0
is the pseudoscalar light Higgs
boson predicted by the next-to-minimal supersymmetric Standard Model. We use a sample of
ð
17
:
6
0
:
3
Þ
10
6
ð
1
S
Þ
mesons produced in the
BABAR
experiment via
e
þ
e
!
ð
2
S
Þ!
þ
ð
1
S
Þ
. We see no significant signal 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
!
gg
or
s
s
Þ
ranging from
10
6
to
10
2
for
A
0
masses in the
range
0
:
5
–
9
:
0 GeV
=c
2
.
DOI:
10.1103/PhysRevD.88.031701
PACS numbers: 14.80.Da, 12.60.Fr, 13.20.Gd, 14.40.Pq
The next-to-minimal supersymmetric standard model
(NMSSM), one of several extensions to the Standard
Model [
1
], predicts that there are two charged, three neutral
CP
-even, and two neutral
CP
-odd Higgs bosons. One of
the
CP
-odd Higgs bosons,
A
0
, can be lighter than two
bottom quarks [
2
]. If so, a
CP
-odd Higgs boson that
couples to bottom quarks could be produced in the radia-
tive decays of an
meson.
The
A
0
is a superposition of a singlet and a nonsinglet
state. The branching fraction
B
ð
!
A
0
Þ
depends on the
NMSSM parameter
cos
A
, which is the nonsinglet frac-
tion. The final state to which the
A
0
decays depends on
various parameters such as
tan
and the
A
0
mass [
3
].
*
Present address: University of Tabuk, Tabuk 71491, Saudi
Arabia.
†
Also at Dipartimento di Fisica, Universita
`
di Perugia,
Perugia, Italy.
‡
Also at Laboratoire de Physique Nucla
́
ire et de Hautes
Energies, IN2P3/CNRS, Paris, France.
§
Present address: University of Huddersfield, Huddersfield
HD1 3DH, United Kingdom.
∥
Deceased.
¶
Present address: University of South Alabama, Mobile, AL
36688, USA.
**
Also at Universita
`
di Sassari, Sassari, Italy.
††
Also at INFN Sezione di Roma, Roma, Italy.
‡‡
Present address: Universidad Te
́
cnica Federico Santa Maria,
Valparaiso, Chile 2390123.
SEARCH FOR A LIGHT HIGGS BOSON DECAYING TO
...
PHYSICAL REVIEW D
88,
031701(R) (2013)
RAPID COMMUNICATIONS
031701-3
BABAR
has searched for an
A
0
decaying into
þ
[
4
,
5
],
þ
[
6
,
7
], invisible states [
8
], and hadronic final states
[
9
] and has not seen a significant signal. The CMS
Collaboration has also not observed a significant signal
in the search for
A
0
decaying into
þ
[
10
]. In this
paper, we report on the first search for the decay
ð
1
S
Þ!
A
0
,
A
0
!
gg
or
s
s
. We search for the
A
0
in the mass range
0
:
5
<m
A
0
<
9
:
0 GeV
=c
2
. By tagging the dipion in the
ð
2
S
Þ!
þ
ð
1
S
Þ
transition, this analysis greatly
reduces
e
þ
e
!
q
q
background, where
q
is a
u
,
d
,or
s
quark, which is a dominant background contribution in
BABAR
’s previous
A
0
!
hadrons analysis [
9
]. Although
this analysis has been motivated by NMSSM, these results
are generally applicable to any
CP
-odd hadronic reso-
nances produced in the radiative decays of
ð
1
S
Þ
because
we search for the
A
0
excluding two-body final states. For
an
A
0
mass less than
2
m
, the
A
0
is predicted to decay
predominantly into two gluons if
tan
is of order 1, and
into
s
s
if
tan
is of order 10.
This paper uses data recorded with the
BABAR
detec-
tor at the PEP-II asymmetric-energy
e
þ
e
collider at the
SLAC National Accelerator Laboratory. The
BABAR
detector is described in detail elsewhere [
11
,
12
]. For
this analysis, we use
13
:
6fb
1
of data [
13
] taken at
the
ð
2
S
Þ
resonance (‘‘on resonance’’). An estimated
number of
ð
98
:
3
0
:
9
Þ
10
6
ð
2
S
Þ
mesons were pro-
duced. The branching fraction
B
ð
ð
2
S
Þ!
þ
ð
1
S
ÞÞ
is
ð
17
:
92
0
:
26
Þ
%
[
14
]. Therefore,
ð
17
:
6
0
:
3
Þ
10
6
ð
1
S
Þ
mesons were produced via the dipion transition.
We also use
1
:
4fb
1
of data [
13
] taken 30 MeV below
the
ð
2
S
Þ
resonance (‘‘off resonance’’) as a background
sample.
Simulated signal events with various
A
0
masses ranging
from 0.5 to
9
:
0 GeV
=c
2
are used in this analysis. The
EVTGEN
event generator [
15
] is used to simulate particle
decays. The
A
0
is simulated as a spin-0 particle decaying to
either
gg
or
s
s
. Since the width of the
A
0
is expected
to be much less than the invariant-mass resolution of
100 MeV
=c
2
, we simulate the
A
0
with a
1 MeV
=c
2
decay width.
JETSET
[
16
] is used to hadronize partons,
and
GEANT4
[
17
] is used to simulate the detector response.
We select events with two charged tracks as the dipion
system candidate, a radiative photon, and a hadronic sys-
tem, as described later in this paper. We select
ð
2
S
Þ!
þ
ð
1
S
Þ
candidates based on the invariant mass
m
R
of
the system recoiling against the dipion system:
m
2
R
¼
M
2
ð
2
S
Þ
þ
m
2
2
M
ð
2
S
Þ
E
CM
;
(1)
where
M
ð
2
S
Þ
is the world average
ð
2
S
Þ
mass [
14
],
m
is the measured dipion invariant mass, and
E
CM
is
the dipion energy in the
e
þ
e
center-of-mass (CM)
frame. The recoil mass distribution from an
ð
2
S
Þ!
þ
ð
1
S
Þ
transition has a peak near the
ð
1
S
Þ
mass
of
9
:
460 30
0
:
000 26 GeV
=c
2
[
14
]. The background
recoil mass distribution is uniform. We select events
with a recoil mass in the range
9
:
45
–
9
:
47 GeV
=c
2
.We
further suppress the background with a multilayer percep-
tron (MLP) neural network [
18
]. Using simulated
ð
2
S
Þ!
þ
ð
1
S
Þ
,
ð
1
S
Þ!
A
0
decays of various
A
0
masses,
ð
2
S
Þ
decays without dipions in the final state,
and
e
þ
e
!
q
q
events, we train an MLP using nine
dipion kinematic variables [
8
]. The variables are: opening
angle between the pions; absolute value of the cosine of
the angle formed between the
and the direction of the
ð
2
S
Þ
in the dipion frame; dipion momentum perpendicu-
lar to the beam axis; dipion invariant mass; distance from
the beam spot; the larger momentum of the two pions;
cosine of the dipion polar angle;
2
probability of the fit
of the two pion tracks to a common vertex; and cosine of
the polar angle of the more energetic pion. These quan-
tities are calculated in the
e
þ
e
CM frame unless other-
wise specified. Applying all other selection criteria, 99%
of the remaining signal
events and
80% of continuum
events pass our MLP selection. The distribution of the
recoil mass against the dipion system in data after
applying all selection criteria is shown in Fig.
1
.
We reconstruct
A
0
!
gg
using 26 channels as listed in
Table
I
. We do not use two-body decay channels because
a
CP
-odd Higgs boson cannot decay into two pseudosca-
lar mesons. Charged kaons, pions, and protons are
required to be positively identified. To reduce the number
of misreconstructed candidates in an event, we require the
number of reconstructed charged tracks in an event to
match the number of charged tracks in the corresponding
decay mode (including the
þ
). For example, we
reconstruct ten-track events only as
K
þ
K
3
þ
3
,
K
K
0
S
2
þ
2
0
(two tracks from a
K
0
S
), or
4
þ
4
.The
0
and
candidates are reconstructed
from two photon candidates. The
K
0
S
candidates are
reconstructed using two charged pions of opposite charge.
)
2
(GeV/c
R
m
9.45
9.455
9.46
9.465
9.47
bin
2
Events per 0.4 MeV/c
0
100
200
300
400
500
600
Data
data
Scaled continuum
FIG. 1. Distribution of the recoil mass against the dipion
system in on-resonance data (points with error bars) after apply-
ing all selection criteria. The histogram is the continuum
background recoil mass distribution from off-resonance data
normalized to the on-resonance integrated luminosity.
J. P. LEES
et al.
PHYSICAL REVIEW D
88,
031701(R) (2013)
RAPID COMMUNICATIONS
031701-4
We define our
A
0
!
s
s
sample as the subset of the 26
A
0
!
gg
decay channels that include two or four kaons
(channels 11–24 in Table
I
). In simulated
A
0
!
s
s
events,
there is a negligible contribution from channels that do
not include at least two kaons. We form an
A
0
candidate
by adding the four-momenta of the hadrons. Similarly, we
form an
ð
1
S
Þ
candidate by using the
A
0
candidate and a
photon with energy more than 200 MeV in the
e
þ
e
CM
frame. To improve the
A
0
mass resolution, we constrain
the photon and the
A
0
candidates to have an invariant
mass equal to the
ð
1
S
Þ
mass and a decay vertex at the
beam spot. The
2
probability of the constrained fit is
required to be greater than
10
3
. This rejects 77% of the
misreconstructed
A
0
candidates, which includes candi-
dates with misidentified charged kaons, pions, and pro-
tons. We reject
ð
1
S
Þ
candidates if the radiative photon,
when combined with anothe
r photon in the
event that is
not used in the reconstruction of a
0
or
candidate, has
an invariant mass within
50 MeV
=c
2
of the
0
mass. This
removes backgrounds where a photon from a
0
decay is
misidentified as the radiative photon. We also reject
ð
1
S
Þ
candidates if the Zernike moment
A
42
[
19
]ofthe
radiative photon is greater t
han 0.1. This r
emoves back-
grounds where showers from both photons from a
0
decay overlap a
nd are mistaken as the radiative photon.
If there is more than one
ð
2
S
Þ!
þ
ð
1
S
Þ
,
ð
1
S
Þ!
A
0
candidate that passes all the selection criteria in an
event, the ca
ndidate with the highest product of MLP
output and
2
probability is kept. Of the events with at
least one
A
0
candidate, 16% have more than one candi-
date. Figure
2
shows the
A
0
candidate invariant mass
spectra for the
A
0
!
gg
and
A
0
!
s
s
channels separately
after applying all selection criteria and selecting one
candidate per event.
We use our off-resonance sample to estimate the con-
tinuum contribution in the on-resonance sample. Fifteen
percent of the candidates in the on-resonance sample are
determined to come from non-
ð
2
S
Þ
decays.
We use simulated
ð
2
S
Þ
events to study the remaining
backgrounds, which originate mainly from
ð
1
S
Þ!
ggg
and
ð
1
S
Þ!
gg
, where the gluons hadronize to more
than one daughter. In
ð
1
S
Þ!
ggg
decays, a
0
from the
gluon hadronization is mistaken as the radiative photon.
This decay mode contributes most of the background
candidates with
A
0
masses between 7 and
9 GeV
=c
2
. The
candidates with
A
0
masses between 2 and
4 GeV
=c
2
are
mostly
ð
1
S
Þ!
gg
. CLEO measured the
ð
1
S
Þ!
f
2
ð
1270
Þ
[
20
] and
ð
1
S
Þ!
f
0
2
ð
1525
Þ
[
21
] branching
fractions. We do not expect these decays to be a back-
ground to the search for a narrow
A
0
because they mainly
decay to two-body final states and have decay widths of
100 MeV
=c
2
.
To determine the number of signal events, we define a
mass window, centered on the hypothesis
A
0
mass, that
contains 80% of simulated signal events at that mass. For
example, in simulated
3 GeV
=c
2
A
0
!
s
s
events, 80% of
the events that pass the selection criteria have a recon-
structed invariant mass for the
A
0
within
170 MeV
=c
2
of
3 GeV
=c
2
. The mass windows are estimated for several
A
0
masses for both
gg
and
s
s
and interpolated for all other
masses. A sideband region is defined as half of the mass
window size adjacent to both sides of the mass window.
Again, for example, the lower sideband for a
3 GeV
=c
2
A
0
!
s
s
would be from 2.66 to
2
:
83 GeV
=c
2
, and the
upper sideband would be from 3.17 to
3
:
34 GeV
=c
2
.
)
2
candidate mass (GeV/c
0
A
024 6810
bin
2
Events per 0.2 GeV/c
0
100
200
300
400
500
600
gg data
data
s
s
continuum data
gg scaled
continuum data
scaled
s
s
FIG. 2 (color online).
A
0
candidate mass spectra after applying
all selection criteria. We reconstruct
A
0
!
gg
using the 26
channels listed in Table
I
and
A
0
!
s
s
using the subset of the
same 26 channels that includes two or four kaons. The
A
0
candidate mass is the invariant mass of the reconstructed hadrons
in each channel. The black points with error bars are on-
resonance data for
A
0
!
gg
. The red squares with error bars
are on-resonance data for
A
0
!
s
s
. The thick blue histogram is
A
0
!
gg
in off-resonance data normalized to the on-resonance
integrated luminosity. The thin magenta histogram is
A
0
!
s
s
in
off-resonance data normalized to the on-resonance integrated
luminosity.
TABLE I. Decay modes for candidate
A
0
!
gg
and
s
s
decays,
sorted by the total mass of the decay products.
Number
Channel
Number
Channel
1
þ
0
14
K
þ
K
þ
2
þ
2
0
15
K
þ
K
þ
0
3
2
þ
2
16
K
K
0
S
þ
4
2
þ
2
0
17
K
þ
K
5
þ
18
K
þ
K
2
þ
2
6
2
þ
2
2
0
19
K
K
0
S
þ
2
0
7
3
þ
3
20
K
þ
K
2
þ
2
0
8
2
þ
2
21
K
þ
K
2
þ
2
2
0
9
3
þ
3
2
0
22
K
K
0
S
2
þ
2
0
10
4
þ
4
23
K
þ
K
3
þ
3
11
K
þ
K
0
24
2
K
þ
2
K
12
K
K
0
S
25
p
p
0
13
K
þ
K
2
0
26
p
p
þ
SEARCH FOR A LIGHT HIGGS BOSON DECAYING TO
...
PHYSICAL REVIEW D
88,
031701(R) (2013)
RAPID COMMUNICATIONS
031701-5
Using simulated events, we estimate efficiencies of
reconstructing the whole decay chain by taking the number
of events in a signal mass window, subtracting the number
of events in the sidebands, and dividing the difference
by the number of simulated events. We interpolate the
efficiencies for all hypothesis
A
0
masses.
Our efficiency measurements of
gg
and
s
s
into the 26
channels are dependent on the hadronization modeling by
JETSET
. The accuracies of the simulated branching frac-
tions of
gg
and
s
s
to different final states are difficult to
determine. We correct for this by comparing simulations
with data in
ð
1
S
Þ!
gg
decays. We count the number of
events in the 26 channels where the reconstructed
gg
mass
is between 2 and
4 GeV
=c
2
in data and compare that to
simulated
ð
2
S
Þ!
þ
ð
1
S
Þ
,
ð
1
S
Þ!
gg
events in
the same mass range. The background in this mass region
is almost entirely from
ð
1
S
Þ!
gg
decays. The number
of
ð
1
S
Þ!
gg
events is too few at masses above
4 GeV
=c
2
to allow any meaningful study. For each of the
26 channels listed in Table
I
, we calculate a weight that is
the ratio of the event yields in data and simulation. We
apply these weights to our efficiency calculations to
determine how much the signal efficiency changes. The
efficiencies change by a factor of 0.66 on average for
A
0
!
gg
and 1.09 for
A
0
!
s
s
. We correct the efficiencies by
multiplying our measured efficiencies by these factors and
assign an uncertainty due to hadronization modeling of
ð
1
0
:
66
Þ
=
0
:
66
¼
50%
to all
A
0
!
gg
and
A
0
!
s
s
effi-
ciencies since the correction is based on simulated
ð
1
S
Þ!
gg
decays but not
ð
1
S
Þ!
s
s
decays. We
do not correct for, or assign hadronization modeling
uncertainty to,
A
0
!
gg
of invariant mass from 0.5 to
0
:
6 GeV
=c
2
because a
CP
-odd
A
0
can decay to only
þ
0
in that mass region. Signal efficiencies range
from 0.07 to
4
10
4
for
gg
and 0.04 to
1
10
3
for
s
s
. The efficiencies are lower for higher
A
0
masses because
a more massive
A
0
decays to more hadrons, which
increases the probability of misreconstruction.
An
A
0
signal would appear as a narrow peak in the
candidate mass spectrum. To look for a signal, we scan
the mass spectrum in
10 MeV
=c
2
steps from 0.5 GeV to
9
:
0 GeV
=c
2
. Our null hypothesis is that the signal rate is 0
in the signal mass window. We use sidebands to estimate
the number of background events in the signal region.
Using Cousins’ method [
22
], we calculate a probability
(
p
value) of seeing the observed result or greater in the
signal mass region given the null hypothesis. We do
this separately for
A
0
!
gg
and
A
0
!
s
s
. Figure
3
is the
2
0
2468
gg p value
-2
10
-1
10
1
)
2
hypothesis mass (GeV/c
0
A
2468
p value
s
s
-2
10
-1
10
1
FIG. 3. The probability of observing at least the number of
signal events, assuming a null hypothesis for the existence of the
decay
ð
1
S
Þ!
A
0
,
A
0
!
gg
(top) and
ð
1
S
Þ!
A
0
,
A
0
!
s
s
(bottom).
2
0
2468
90% C.L. upper limits
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
gg)
→
0
) x B(A
0
A
γ
→
(1S)
Υ
B(
2
0
2468
)
2
hypothesis mass (GeV/c
0
A
2468
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
)
s
s
→
0
) x B(A
0
A
γ
→
(1S)
Υ
B(
Expected average
Observed limits
Expected (68%)
statistical errors only
Observed limits from
Expected (95%)
FIG. 4 (color online). The 90%-confidence-level upper limits
(thin solid line) on the product branching fractions
B
ð
ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!
gg
Þ
(top) and
B
ð
ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!
s
s
Þ
(bottom). We overlay limits calculated using statistical uncer-
tainties only (thin dashed line). The inner band is the expected
region of upper limits in 68% of simulated experiments. The
inner band plus the outer band is the expected region of upper
limits in 95% of simulated experiments. The bands are calcu-
lated using all uncertainties. The thick line in the center of the
inner band is the expected upper limits calculated using
simulated experiments.
J. P. LEES
et al.
PHYSICAL REVIEW D
88,
031701(R) (2013)
RAPID COMMUNICATIONS
031701-6
resulting
p
-value plot for all hypothesis masses. The mini-
mum
p
value for
A
0
!
gg
is 0.003 and occurs at an
A
0
mass of
8
:
13 GeV
=c
2
. The minimum
p
value for
A
0
!
s
s
is 0.002 and occurs at an
A
0
mass of
8
:
63 GeV
=c
2
. These
results are equivalent to Gaussian standard deviations of
2.7 and 2.9, respectively. We use
10
4
simulated experi-
ments to calculate how often such a statistical fluctuation
might occur. For
A
0
!
gg
, 86% of the simulated experi-
ments have a minimum
p
value less than 0.003. For
A
0
!
s
s
, 59% of the simulated experiments have a mini-
mum
p
value less than 0.002. Therefore, we conclude that
there is no evidence for the light
CP
-odd Higgs boson.
The dominant systematic uncertainty on the product
branching fraction upper limit is related to the efficiency,
which was described earlier in the text. Other systematic
uncertainties, which are small compared to the 50%
uncertainty due to hadronization modeling, include
Monte Carlo statistical uncertainties (1%–7%), efficiency
variations in estimating the size of the mass windows (5%),
dipion branching fraction (2%),
ð
2
S
Þ
counting (1%), and
dipion selection efficiency (1%). The systematic uncertain-
ties are summed in quadrature and total 51%.
We calculate 90%-confidence-level (C.L.) upper limits
(Fig.
4
) on the product branching fractions
B
ð
ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!
gg
Þ
and
B
ð
ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!
s
s
Þ
using a profile likelihood approach [
23
]. We do this by
calculating an upper limit of the mean number of signal
events in the signal region given the number of events
observed in the sidebands, and dividing by the efficiency,
dipion branching fraction, and the number of
ð
2
S
Þ
mesons produced. The number of background events is
assumed to be Poissonian distributed and the efficiency
distribution is assumed to be Gaussian with width equal to
the total systematic uncertainty.
In summary, we select dipions in
ð
2
S
Þ
decays to obtain
a sample of
ð
1
S
Þ
mesons. We reconstruct the
ð
1
S
Þ
decay
using a photon and a hadronic system. We observe no
signals in the hadronic invariant mass spectra and set upper
limits at 90% C.L. on the product branching fractions
B
ð
ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!
gg
Þ
from
10
6
to
10
2
and
B
ð
ð
1
S
Þ!
A
0
Þ
B
ð
A
0
!
s
s
Þ
from
10
5
to
10
3
.We
do not observe a NMSSM
A
0
or any narrow hadronic
resonance.
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), IHEP (China), CEA and CNRS-IN2P3
(France), BMBF and DFG (Germany), INFN (Italy), FOM
(Netherlands), NFR (Norway), MIST (Russia), and PPARC
(United Kingdom). Individuals have received support from
CONACyT (Mexico), A. P. Sloan Foundation, Research
Corporation, and Alexander von Humboldt Foundation.
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