arXiv:1307.5306v1 [hep-ex] 19 Jul 2013
B
A
B
AR
-PUB-13/013
SLAC-PUB-15687
Search for a light Higgs boson decaying to two gluons or s
s in the radiative decays of
Υ
(
1S
)
J. P. Lees,
1
V. Poireau,
1
V. Tisserand,
1
E. Grauges,
2
A. Palano
ab
,
3
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
ac
,
9
A. R. Buzykaev
a
,
9
V. P. Druzhinin
ab
,
9
V. B. Golubev
ab
,
9
E. A. Kravchenko
ab
,
9
A. P. Onuchin
ac
,
9
S. I. Serednyakov
ab
,
9
Yu. I. Skovpen
ab
,
9
E. P. Solodov
ab
,
9
K. Yu. Todyshev
ab
,
9
A. N. Yushkov
a
,
9
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
a
,
22
C. Bozzi
a
,
22
R. Calabrese
ab
,
22
G. Cibinetto
ab
,
22
E. Fioravanti
ab
,
22
I. Garzia
ab
,
22
E. Luppi
ab
,
22
L. Piemontese
a
,
22
V. Santoro
a
,
22
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
ab
,
24
E. Guido
ab
,
24
M. Lo Vetere
ab
,
24
M. R. Monge
ab
,
24
S. Passaggio
a
,
24
C. Patrignani
ab
,
24
E. Robutti
a
,
24
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
ab
,
44
N. Neri
a
,
44
F. Palombo
ab
,
44
L. Cremaldi,
45
R. Godang,
45,
∗∗
P. Sonnek,
45
D. J. Summers,
45
M. Simard,
46
P. Taras,
46
G. De Nardo
ab
,
47
D. Monorchio
ab
,
47
G. Onorato
ab
,
47
C. Sciacca
ab
,
47
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
ab
,
52
M. Margoni
ab
,
52
M. Morandin
a
,
52
M. Posocco
a
,
52
M. Rotondo
a
,
52
G. Simi
a
,
52
F. Simonetto
ab
,
52
R. Stroili
ab
,
52
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
ab
,
54
E. Manoni
a
,
54
S. Pacetti
ab
,
54
A. Rossi
a
,
54
C. Angelini
ab
,
55
G. Batignani
ab
,
55
S. Bettarini
ab
,
55
M. Carpinelli
ab
,
55,
††
G. Casarosa
ab
,
55
A. Cervelli
ab
,
55
F. Forti
ab
,
55
M. A. Giorgi
ab
,
55
A. Lusiani
ac
,
55
B. Oberhof
ab
,
55
E. Paoloni
ab
,
55
A. Perez
a
,
55
G. Rizzo
ab
,
55
J. J. Walsh
a
,
55
D. Lopes Pegna,
56
J. Olsen,
56
A. J. S. Smith,
56
R. Faccini
ab
,
57
F. Ferrarotto
a
,
57
F. Ferroni
ab
,
57
M. Gaspero
ab
,
57
L. Li Gioi
a
,
57
G. Piredda
a
,
57
C. B ̈unger,
58
O. Gr ̈unberg,
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. Y`eche,
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
ab
,
70
F. De Mori
ab
,
70
A. Filippi
a
,
70
D. Gamba
ab
,
70
S. Zambito
ab
,
70
L. Lanceri
ab
,
71
L. Vitale
ab
,
71
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
2
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
(The
B
A
B
AR
Collaboration)
1
Laboratoire d’Annecy-le-Vieux de Physique des Particules
(LAPP),
Universit ́e de Savoie, CNRS/IN2P3, F-74941 Annecy-Le-Vie
ux, France
2
Universitat de Barcelona, Facultat de Fisica, Departament
ECM, E-08028 Barcelona, Spain
3
INFN Sezione di Bari
a
; Dipartimento di Fisica, Universit`a di Bari
b
, I-70126 Bari, Italy
4
University of Bergen, Institute of Physics, N-5007 Bergen,
Norway
5
Lawrence Berkeley National Laboratory and University of Ca
lifornia, Berkeley, California 94720, USA
6
Ruhr Universit ̈at Bochum, Institut f ̈ur Experimentalphys
ik 1, D-44780 Bochum, Germany
7
University of British Columbia, Vancouver, British Columb
ia, Canada V6T 1Z1
8
Brunel University, Uxbridge, Middlesex UB8 3PH, United Kin
gdom
9
Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630
090
a
,
Novosibirsk State University, Novosibirsk 630090
b
,
Novosibirsk State Technical University, Novosibirsk 6300
92
c
, Russia
10
University of California at Irvine, Irvine, California 926
97, USA
11
University of California at Riverside, Riverside, Califor
nia 92521, USA
12
University of California at Santa Barbara, Santa Barbara, C
alifornia 93106, USA
13
University of California at Santa Cruz, Institute for Parti
cle 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, U
SA
18
Technische Universit ̈at Dortmund, Fakult ̈at Physik, D-44
221 Dortmund, Germany
19
Technische Universit ̈at Dresden, Institut f ̈ur Kern- und T
eilchenphysik, 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
22
INFN Sezione di Ferrara
a
; Dipartimento di Fisica e Scienze della Terra, Universit`a
di Ferrara
b
, I-44122 Ferrara, Italy
23
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, I
taly
24
INFN Sezione di Genova
a
; Dipartimento di Fisica, Universit`a di Genova
b
, I-16146 Genova, Italy
25
Indian Institute of Technology Guwahati, Guwahati, Assam,
781 039, India
26
Harvard University, Cambridge, Massachusetts 02138, USA
27
Universit ̈at Heidelberg, Physikalisches Institut, D-691
20 Heidelberg, Germany
28
Humboldt-Universit ̈at zu Berlin, Institut f ̈ur Physik, 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’Acc ́el ́erateur Lin ́eaire, IN2P3/CNRS et
Universit ́e Paris-Sud 11,
Centre Scientifique d’Orsay, F-91898 Orsay Cedex, France
34
Lawrence Livermore National Laboratory, Livermore, Calif
ornia 94550, USA
35
University of Liverpool, Liverpool L69 7ZE, United Kingdom
36
Queen Mary, University of London, London, E1 4NS, United Kin
gdom
37
University of London, Royal Holloway and Bedford New Colleg
e, Egham, Surrey TW20 0EX, United Kingdom
38
University of Louisville, Louisville, Kentucky 40292, USA
39
Johannes Gutenberg-Universit ̈at Mainz, Institut f ̈ur Ker
nphysik, D-55099 Mainz, Germany
40
University of Manchester, Manchester M13 9PL, United Kingd
om
41
University of Maryland, College Park, Maryland 20742, USA
42
Massachusetts Institute of Technology, Laboratory for Nuc
lear Science, Cambridge, Massachusetts 02139, USA
43
McGill University, Montr ́eal, Qu ́ebec, Canada H3A 2T8
44
INFN Sezione di Milano
a
; Dipartimento di Fisica, Universit`a di Milano
b
, I-20133 Milano, Italy
45
University of Mississippi, University, Mississippi 38677
, USA
46
Universit ́e de Montr ́eal, Physique des Particules, Montr ́
eal, Qu ́ebec, Canada H3C 3J7
47
INFN Sezione di Napoli
a
; Dipartimento di Scienze Fisiche,
Universit`a di Napoli Federico II
b
, I-80126 Napoli, Italy
48
NIKHEF, National Institute for Nuclear Physics and High Ene
rgy Physics, NL-1009 DB Amsterdam, The Netherlands
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
52
INFN Sezione di Padova
a
; Dipartimento di Fisica, Universit`a di Padova
b
, I-35131 Padova, Italy
53
Laboratoire de Physique Nucl ́eaire et de Hautes Energies,
IN2P3/CNRS, Universit ́e Pierre et Marie Curie-Paris6,
Universit ́e Denis Diderot-Paris7, F-75252 Paris, France
3
54
INFN Sezione di Perugia
a
; Dipartimento di Fisica, Universit`a di Perugia
b
, I-06123 Perugia, Italy
55
INFN Sezione di Pisa
a
; Dipartimento di Fisica,
Universit`a di Pisa
b
; Scuola Normale Superiore di Pisa
c
, I-56127 Pisa, Italy
56
Princeton University, Princeton, New Jersey 08544, USA
57
INFN Sezione di Roma
a
; Dipartimento di Fisica,
Universit`a di Roma La Sapienza
b
, I-00185 Roma, Italy
58
Universit ̈at Rostock, D-18051 Rostock, Germany
59
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX
11 0QX, United Kingdom
60
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, F
rance
61
SLAC National Accelerator Laboratory, Stanford, Californ
ia 94309 USA
62
University of South Carolina, Columbia, South Carolina 292
08, USA
63
Southern Methodist University, Dallas, Texas 75275, USA
64
Stanford University, Stanford, California 94305-4060, US
A
65
State University of New York, Albany, New York 12222, USA
66
Tel Aviv University, School of Physics and Astronomy, Tel Av
iv, 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
70
INFN Sezione di Torino
a
; Dipartimento di Fisica, Universit`a di Torino
b
, I-10125 Torino, Italy
71
INFN Sezione di Trieste
a
; Dipartimento di Fisica, Universit`a di Trieste
b
, I-34127 Trieste, Italy
72
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spa
in
73
University of Victoria, Victoria, British Columbia, Canad
a V8W 3P6
74
Department of Physics, University of Warwick, Coventry CV4
7AL, United Kingdom
75
University of Wisconsin, Madison, Wisconsin 53706, USA
We search for the decay
Υ
(1
S
)
→
γA
0
, A
0
→
gg
or
s
s
, where
A
0
is the pseudoscalar light
Higgs boson predicted by the next-to-minimal supersymmetr
ic standard model. We use a sample
of (17
.
6
±
0
.
3)
×
10
6
Υ
(1
S
) mesons produced in the
B
A
B
AR
experiment via
e
+
e
−
→
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
). We see no significant signal and set 90%-confidence-level u
pper limits on the product
branching fraction
B
(
Υ
(1
S
)
→
γA
0
)
· B
(
A
0
→
gg
or
s
s
) ranging from 10
−
6
to 10
−
2
for
A
0
masses
in the range 0.5 to 9.0 GeV
/c
2
.
PACS numbers: 14.80.Da, 14.40.Pq, 13.20.Gd, 12.60.Fr, 12.
15.Ji
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 ra-
diative decays of an
Υ
meson.
The
A
0
is a superposition of a singlet and a non-singlet
state. The branching fraction
B
(
Υ
→
γA
0
) depends on
the NMSSM parameter cos
θ
A
, which is the non-singlet
fraction. The final state to which the
A
0
decays de-
∗
Now at the University of Tabuk, Tabuk 71491, Saudi Arabia
†
Also with Universit`a di Perugia, Dipartimento di Fisica, P
erugia,
Italy
‡
Also with Laboratoire de Physique Nucl ́aire et de Hautes Ene
rgies,
IN2P3/CNRS, Paris, France
§
Now at the University of Huddersfield, Huddersfield HD1 3DH,
UK
¶
Deceased
∗∗
Now at University of South Alabama, Mobile, Alabama 36688,
USA
††
Also with Universit`a di Sassari, Sassari, Italy
‡‡
Also with INFN Sezione di Roma, Roma, Italy
§§
Now at Universidad T ́ecnica Federico Santa Maria, Valparai
so,
Chile 2390123
pends on various parameters such as tan
β
and the
A
0
mass [3].
B
A
B
AR
has searched for an
A
0
decaying into
μ
+
μ
−
[4, 5],
τ
+
τ
−
[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
μ
+
μ
−
[10]. In
this article, we report on the first search for the decay
Υ
(1
S
)
→
γA
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
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
) transition, this
analysis greatly reduces
e
+
e
−
→
q
q
background, where
q
is a
u, d,
or
s
quark, which is a dominant background
contribution in
B
A
B
AR
’s previous
A
0
→
hadrons analy-
sis [9]. Although this analysis has been motivated by
NMSSM, these results are generally applicable to any
CP
-odd hadronic resonances produced in the radiative
decays of
Υ
(1
S
) because we search for the
A
0
excluding
two-body final states. For an
A
0
mass less than 2
m
τ
, the
A
0
is predicted to decay predominantly into two gluons
if tan
β
is of order 1, and into
s
s
if tan
β
is of order 10.
This article uses data recorded with the
B
A
B
AR
detec-
tor at the PEP-II asymmetric-energy
e
+
e
−
collider at
the SLAC National Accelerator Laboratory. The
B
A
B
AR
detector is described in detail elsewhere [11, 12]. For
this analysis, we use 13.6 fb
−
1
of data [13] taken at
the
Υ
(2
S
) resonance (“on-resonance”). An estimated
number of (98
.
3
±
0
.
9)
×
10
6
Υ
(2
S
) mesons were pro-
4
duced. The branching fraction
B
(
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
))
is (17
.
92
±
0
.
26)% [14]. Therefore, (17
.
6
±
0
.
3)
×
10
6
Υ
(1
S
)
mesons were produced via the dipion transition. We also
use 1.4 fb
−
1
of data [13] taken 30 MeV below the
Υ
(2
S
)
resonance (“off-resonance”) as a background sample.
Simulated signal events with various
A
0
masses rang-
ing 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 decay-
ing to either
gg
or
s
s
. Since the width of the
A
0
is ex-
pected 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
de-
cay 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
system, as described later in this article. We select
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
) candidates based on the invari-
ant mass
m
R
of the system recoiling against the dipion
system:
m
2
R
=
M
2
Υ
(2
S
)
+
m
2
ππ
−
2
M
Υ
(2
S
)
E
CM
ππ
,
(1)
where
M
Υ
(2
S
)
is the world average
Υ
(2
S
) mass [14],
m
ππ
is the measured dipion invariant mass, and
E
CM
ππ
is the dipion energy in the
e
+
e
−
center-of-mass (CM)
frame. The recoil mass distribution from an
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
) transition has a peak near the
Υ
(1
S
) mass
of 9
.
46030
±
0
.
00026 GeV
/c
2
[14]. The background recoil
mass distribution is uniform. We select events with a
recoil mass in the range 9.45 to 9.47 GeV
/c
2
. We fur-
ther suppress the background with a multi-layer per-
ceptron (MLP) neural network [18]. Using simulated
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
),
Υ
(1
S
)
→
γA
0
decays of various
A
0
masses,
Υ
(2
S
) decays without dipions in the final
state, and
e
+
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
π
−
and the direc-
tion of the
Υ
(2
S
) in the dipion frame; dipion momentum
perpendicular 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;
χ
2
proba-
bility of the fit of the two pion tracks to a common vertex;
and cosine of the polar angle of the more energetic pion.
These quantities are calculated in the
e
+
e
−
CM frame
unless otherwise specified. Applying all other selection
criteria, 99% of the remaining signal events and 80% of
continuum events pass our MLP selection. The distribu-
tion 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
pseudoscalar mesons. Charged kaons, pions, and pro-
tons 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
)
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 a
p-
plying all selection criteria. The histogram is the continu
um
background recoil mass distribution from off-resonance dat
a
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.
# Channel
# Channel
1
π
+
π
−
π
0
14
K
+
K
−
π
+
π
−
2
π
+
π
−
2
π
0
15
K
+
K
−
π
+
π
−
π
0
3 2
π
+
2
π
−
16
K
±
K
0
S
π
∓
π
+
π
−
4 2
π
+
2
π
−
π
0
17
K
+
K
−
η
5
π
+
π
−
η
18
K
+
K
−
2
π
+
2
π
−
6 2
π
+
2
π
−
2
π
0
19
K
±
K
0
S
π
∓
π
+
π
−
2
π
0
7 3
π
+
3
π
−
20
K
+
K
−
2
π
+
2
π
−
π
0
8 2
π
+
2
π
−
η
21
K
+
K
−
2
π
+
2
π
−
2
π
0
9 3
π
+
3
π
−
2
π
0
22
K
±
K
0
S
π
∓
2
π
+
2
π
−
π
0
10 4
π
+
4
π
−
23
K
+
K
−
3
π
+
3
π
−
11
K
+
K
−
π
0
24 2
K
+
2
K
−
12
K
±
K
0
S
π
∓
25
p
̄
pπ
0
13
K
+
K
−
2
π
0
26
p
̄
pπ
+
π
−
the corresponding decay mode (including the
π
+
π
−
).
For example, we reconstruct ten-track events only as
K
+
K
−
3
π
+
3
π
−
,
K
±
K
0
S
π
∓
2
π
+
2
π
−
π
0
(two tracks from a
K
0
S
), or 4
π
+
4
π
−
. The
π
0
and
η
candidates are recon-
structed from two photon candidates. The
K
0
S
candi-
dates are reconstructed using two charged pions of op-
posite charge. 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 simu-
lated
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
Υ
(1
S
) candidate by
using the
A
0
candidate and a photon with energy more
than 200 MeV in the
e
+
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
Υ
(1
S
)
mass and a decay vertex at the beam spot. The
χ
2
prob-
5
)
2
candidate mass (GeV/c
0
A
0
2
4
6
8
10
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:
A
0
candidate mass spectra after applying all selec-
tion 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
candi-
date 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 his-
togram is
A
0
→
gg
in off-resonance data normalized to the
on-resonance integrated luminosity. The thin magenta his-
togram is
A
0
→
s
s
in off-resonance data normalized to the
on-resonance integrated luminosity.
ability of the constrained fit is required to be greater
than 10
−
3
. This rejects 77% of the misreconstructed
A
0
candidates, which includes candidates with misidentified
charged kaons, pions, and protons. We reject
Υ
(1
S
) can-
didates if the radiative photon, when combined with an-
other photon in the event that is not used in the recon-
struction of a
π
0
or
η
candidate, has an invariant mass
within 50 MeV
/c
2
of the
π
0
mass. This removes back-
grounds where a photon from a
π
0
decay is misidentified
as the radiative photon. We also reject
Υ
(1
S
) candidates
if the Zernike moment
A
42
[19] of the radiative photon is
greater than 0.1. This removes backgrounds where show-
ers from both photons from a
π
0
decay overlap and are
mistaken as the radiative photon. If there is more than
one
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
)
, Υ
(1
S
)
→
γA
0
candidate that
passes all the selection criteria in an event, the candidate
with the highest product of MLP output and
χ
2
proba-
bility is kept. Figure 2 shows the
A
0
candidate invariant
mass spectra for the
A
0
→
gg
and
A
0
→
s
s
channels sep-
arately 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-
Υ
(2
S
) decays.
We use simulated
Υ
(2
S
) events to study the remaining
backgrounds, which originate mainly from
Υ
(1
S
)
→
ggg
and
Υ
(1
S
)
→
γgg
, where the gluons hadronize to more
than one daughter. In
Υ
(1
S
)
→
ggg
decays, a
π
0
from the gluon hadronization is mistaken as the radia-
tive 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
Υ
(1
S
)
→
γgg
. CLEO measured the
Υ
(1
S
)
→
γf
2
(1270) [20] and
Υ
(1
S
)
→
γf
′
2
(1525) [21]
branching fractions. We do not expect these decays to
be a background to the search for a narrow
A
0
because
they mainly decay to two-body final states and have de-
cay 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
.
Using simulated events, we estimate efficiencies of re-
constructing the whole decay chain by taking the number
of events in a signal mass window, subtracting the num-
ber 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 modelling
by
Jetset
. The accuracies of the simulated branch-
ing fractions of
gg
and
s
s
to different final states are
difficult to determine. We correct for this by com-
paring simulations with data in
Υ
(1
S
)
→
γgg
de-
cays. We count the number of events in the 26 chan-
nels where the reconstructed
gg
mass is between 2
and 4 GeV
/c
2
in data, and compare that to simulated
Υ
(2
S
)
→
π
+
π
−
Υ
(1
S
)
, Υ
(1
S
)
→
γgg
events in the same
mass range. The background in this mass region is al-
most entirely from
Υ
(1
S
)
→
γgg
decays. The number of
Υ
(1
S
)
→
γgg
events is too few at masses above 4 GeV
/c
2
to allow any meaningful study. For each of the 26 chan-
nels 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 mul-
tiplying our measured efficiencies by these factors and
assign an uncertainty due to hadronization modelling of
(1
−
0
.
66)/0.66 = 50% to all
A
0
→
gg
and
A
0
→
s
s
efficiencies since the correction is based on simulated
Υ
(1
S
)
→
γgg
decays but not
Υ
(1
S
)
→
γs
s
decays. We
do not correct for, or assign hadronization modelling un-
certainty 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
π
+
π
−
π
0
in that mass region. Signal efficiencies range from 0.07
6
2
4
6
8
gg p-value
-2
10
-1
10
1
)
2
hypothesis mass (GeV/c
0
A
2
4
6
8
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 existen
ce
of the decay
Υ
(1
S
)
→
γA
0
, A
0
→
gg
(top), and
Υ
(1
S
)
→
γA
0
, A
0
→
s
s
(bottom).
to 4
×
10
−
4
for
gg
and 0.04 to 1
×
10
−
3
for
s
s
. The ef-
ficiencies 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 proba-
bility (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
resulting p-value plot for all hypothesis masses. The min-
imum 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 ex-
periments have a minimum p-value less than 0.003. For
A
0
→
s
s
, 59% of the simulated experiments have a min-
imum p-value less than 0.002. Therefore, we conclude
that there is no evidence for the light
CP
-odd Higgs bo-
son.
2
0
2
4
6
8
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
2
4
6
8
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
)
2
hypothesis mass (GeV/c
0
A
2
4
6
8
-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(
)
2
hypothesis mass (GeV/c
0
A
2
4
6
8
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
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
(
Υ
(1
S
)
→
γA
0
)
· B
(
A
0
→
gg
) (top) and
B
(
Υ
(1
S
)
→
γA
0
)
· B
(
A
0
→
s
s
) (bottom). We overlay limits calculated
using statistical uncertainties only (thin dashed line). T
he
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 ex-
periments. The bands are calculated using all uncertaintie
s.
The thick line in the center of the inner band is the expected
upper limits calculated using simulated experiments.
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% un-
certainty due to hadronization modelling, include Monte
Carlo statistical uncertainties (1–7%), efficiency varia-
tions in estimating the size of the mass windows (5%),
dipion branching fraction (2%),
Υ
(2
S
) counting (1%),
and dipion selection efficiency (1%). The systematic un-
certainties are summed in quadrature and total 51%.
We calculate 90%-confidence-level (CL) upper limits
(Fig. 4) on the product branching fractions
B
(
Υ
(1
S
)
→
γA
0
)
· B
(
A
0
→
gg
) and
B
(
Υ
(1
S
)
→
γA
0
)
· B
(
A
0
→
s
s
)
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 ob-
served in the sidebands, and dividing by the efficiency, di-
pion branching fraction, and the number of
Υ
(2
S
) mesons
produced. The number of background events is assumed
to be Poissonian distributed and the efficiency distribu-