Search for a light Higgs resonance in radiative decays
of the
Υ
ð
1
S
Þ
with a charm tag
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,9b,9c
A. R. Buzykaev,
9a
V. P. Druzhinin,
9a,9b
V. B. Golubev,
9a,9b
E. A. Kravchenko,
9a,9b
A. P. Onuchin,
9a,9b,9c
S. I. Serednyakov,
9a,9b
Yu. I. Skovpen,
9a,9b
E. P. Solodov,
9a,9b
K. Yu. Todyshev,
9a,9b
A. J. Lankford,
10
B. Dey,
11
J. W. Gary,
11
O. Long,
11
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
W. Panduro Vazquez,
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
T. S. Miyashita,
14
P. Ongmongkolkul,
14
F. C. Porter,
14
M. Röhrken,
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
J. G. Smith,
16
S. R. Wagner,
16
R. Ayad,
17
,
†
W. H. Toki,
17
B. Spaan,
18
D. Bernard,
19
M. Verderi,
19
S. Playfer,
20
D. Bettoni,
21a
C. Bozzi,
21a
R. Calabrese,
21a,21b
G. Cibinetto,
21a,21b
E. Fioravanti,
21a,21b
I. Garzia,
21a,21b
E. Luppi,
21a,21b
L. Piemontese,
21a
V. Santoro,
21a
A. Calcaterra,
22
R. de Sangro,
22
G. Finocchiaro,
22
S. Martellotti,
22
P. Patteri,
22
I. M. Peruzzi,
22
,
‡
M. Piccolo,
22
M. Rama,
22
A. Zallo,
22
R. Contri,
23a,23b
M. R. Monge,
23a,23b
S. Passaggio,
23a
C. Patrignani,
23a,23b
B. Bhuyan,
24
V. Prasad,
24
A. Adametz,
25
U. Uwer,
25
H. M. Lacker,
26
U. Mallik,
27
C. Chen,
28
J. Cochran,
28
S. Prell,
28
H. Ahmed,
29
A. V. Gritsan,
30
N. Arnaud,
31
M. Davier,
31
D. Derkach,
31
G. Grosdidier,
31
F. Le Diberder,
31
A. M. Lutz,
31
B. Malaescu,
31
,§
P. Roudeau,
31
A. Stocchi,
31
G. Wormser,
31
D. J. Lange,
32
D. M. Wright,
32
J. P. Coleman,
33
J. R. Fry,
33
E. Gabathuler,
33
D. E. Hutchcroft,
33
D. J. Payne,
33
C. Touramanis,
33
A. J. Bevan,
34
F. Di Lodovico,
34
R. Sacco,
34
G. Cowan,
35
D. N. Brown,
36
C. L. Davis,
36
A. G. Denig,
37
M. Fritsch,
37
W. Gradl,
37
K. Griessinger,
37
A. Hafner,
37
K. R. Schubert,
37
R. J. Barlow,
38
,¶
G. D. Lafferty,
38
R. Cenci,
39
B. Hamilton,
39
A. Jawahery,
39
D. A. Roberts,
39
R. Cowan,
40
R. Cheaib,
41
P. M. Patel,
41
,*
S. H. Robertson,
41
N. Neri,
42a
F. Palombo,
42a,42b
L. Cremaldi,
43
R. Godang,
43
,**
D. J. Summers,
43
M. Simard,
44
P. Taras,
44
G. De Nardo,
45a,45b
G. Onorato,
45a,45b
C. Sciacca,
45a,45b
G. Raven,
46
C. P. Jessop,
47
J. M. LoSecco,
47
K. Honscheid,
48
R. Kass,
48
M. Margoni,
49a,49b
M. Morandin,
49a
M. Posocco,
49a
M. Rotondo,
49a
G. Simi,
49a,49b
F. Simonetto,
49a,49b
R. Stroili,
49a,49b
S. Akar,
50
E. Ben-Haim,
50
M. Bomben,
50
G.
R. Bonneaud,
50
H. Briand,
50
G. Calderini,
50
J. Chauveau,
50
Ph. Leruste,
50
G. Marchiori,
50
J. Ocariz,
50
M. Biasini,
51a,51b
E. Manoni,
51a
A. Rossi,
51a
C. Angelini,
52a,52b
G. Batignani,
52a,52b
S. Bettarini,
52a,52b
M. Carpinelli,
52a,52b
,
††
G. Casarosa,
52a,52b
M. Chrzaszcz,
52a
F. Forti,
52a,52b
M. A. Giorgi,
52a,52b
A. Lusiani,
52a,52c
B. Oberhof,
52a,52b
E. Paoloni,
52a,52b
G. Rizzo,
52a,52b
J. J. Walsh,
52a
D. Lopes Pegna,
53
J. Olsen,
53
A. J. S. Smith,
53
F. Anulli,
54a
R. Faccini,
54a,54b
F. Ferrarotto,
54a
F. Ferroni,
54a,54b
M. Gaspero,
54a,54b
A. Pilloni,
54a,54b
G. Piredda,
54a
C. Bünger,
55
S. Dittrich,
55
O. Grünberg,
55
M. Hess,
55
T. Leddig,
55
C. Voß,
55
R. Waldi,
55
T. Adye,
56
E. O. Olaiya,
56
F. F. Wilson,
56
S. Emery,
57
G. Vasseur,
57
D. Aston,
58
D. J. Bard,
58
C. Cartaro,
58
M. R. Convery,
58
J. Dorfan,
58
G. P. Dubois-Felsmann,
58
W. Dunwoodie,
58
M. Ebert,
58
R. C. Field,
58
B. G. Fulsom,
58
M. T. Graham,
58
C. Hast,
58
W. R. Innes,
58
P. Kim,
58
D. W. G. S. Leith,
58
D. Lindemann,
58
S. Luitz,
58
V. Luth,
58
H. L. Lynch,
58
D. B. MacFarlane,
58
D. R. Muller,
58
H. Neal,
58
M. Perl,
58
,*
T. Pulliam,
58
B. N. Ratcliff,
58
A. Roodman,
58
R. H. Schindler,
58
A. Snyder,
58
D. Su,
58
M. K. Sullivan,
58
J. Va
’
vra,
58
W. J. Wisniewski,
58
H. W. Wulsin,
58
M. V. Purohit,
59
J. R. Wilson,
59
A. Randle-Conde,
60
S. J. Sekula,
60
M. Bellis,
61
P. R. Burchat,
61
E. M. T. Puccio,
61
M. S. Alam,
62
J. A. Ernst,
62
R. Gorodeisky,
63
N. Guttman,
63
D. R. Peimer,
63
A. Soffer,
63
S. M. Spanier,
64
J. L. Ritchie,
65
R. F. Schwitters,
65
J. M. Izen,
66
X. C. Lou,
66
F. Bianchi,
67a,67b
F. De Mori,
67a,67b
A. Filippi,
67a
D. Gamba,
67a,67b
L. Lanceri,
68a,68b
L. Vitale,
68a,68b
F. Martinez-Vidal,
69
A. Oyanguren,
69
P. Villanueva-Perez,
69
J. Albert,
70
Sw. Banerjee,
70
A. Beaulieu,
70
F. U. Bernlochner,
70
H. H. F. Choi,
70
G. J. King,
70
R. Kowalewski,
70
M. J. Lewczuk,
70
T. Lueck,
70
I. M. Nugent,
70
J. M. Roney,
70
R. J. Sobie,
70
N. Tasneem,
70
T. J. Gershon,
71
P. F. Harrison,
71
T. E. Latham,
71
H. R. Band,
72
S. Dasu,
72
Y. Pan,
72
R. Prepost,
72
and S. L. Wu
72
(
B
A
B
AR
Collaboration)
1
Laboratoire d
’
Annecy-le-Vieux de Physique des Particules (LAPP), Université 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, Università 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
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
PHYSICAL REVIEW D
91,
071102(R) (2015)
1550-7998
=
2015
=
91(7)
=
071102(9)
071102-1
© 2015 American Physical Society
RAPID COMMUNICATIONS
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
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 Universität Dortmund, Fakultät Physik, D-44221 Dortmund, Germany
19
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
20
University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom
21a
INFN Sezione di Ferrara, I-44122 Ferrara, Italy
21b
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, I-44122 Ferrara, Italy
22
INFN Laboratori Nazionali di Frascati, I-00044 Frascati, Italy
23a
INFN Sezione di Genova, I-16146 Genova, Italy
23b
Dipartimento di Fisica, Università di Genova, I-16146 Genova, Italy
24
Indian Institute of Technology Guwahati, Guwahati, Assam 781 039, India
25
Universität Heidelberg, Physikalisches Institut, D-69120 Heidelberg, Germany
26
Humboldt-Universität zu Berlin, Institut für Physik, D-12489 Berlin, Germany
27
University of Iowa, Iowa City, Iowa 52242, USA
28
Iowa State University, Ames, Iowa 50011-3160, USA
29
Physics Department, Jazan University, Jazan 22822, Kingdom of Saudi Arabia
30
Johns Hopkins University, Baltimore, Maryland 21218, USA
31
Laboratoire de l
’
Accélérateur Linéaire, IN2P3/CNRS et Université Paris-Sud 11,
Centre Scientifique d
’
Orsay, F-91898 Orsay Cedex, France
32
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
33
University of Liverpool, Liverpool L69 7ZE, United Kingdom
34
Queen Mary, University of London, London E1 4NS, United Kingdom
35
University of London, Royal Holloway and Bedford New College, Egham,
Surrey TW20 0EX, United Kingdom
36
University of Louisville, Louisville, Kentucky 40292, USA
37
Johannes Gutenberg-Universität Mainz, Institut für Kernphysik, D-55099 Mainz, Germany
38
University of Manchester, Manchester M13 9PL, United Kingdom
39
University of Maryland, College Park, Maryland 20742, USA
40
Massachusetts Institute of Technology, Laboratory for Nuclear Science, Cambridge,
Massachusetts 02139, USA
41
McGill University, Montréal, Québec, Canada H3A 2T8
42a
INFN Sezione di Milano, I-20133 Milano, Italy
42b
Dipartimento di Fisica, Università di Milano, I-20133 Milano, Italy
43
University of Mississippi, University, Mississippi 38677, USA
44
Université de Montréal, Physique des Particules, Montréal, Québec, Canada H3C 3J7
45a
INFN Sezione di Napoli, I-80126 Napoli, Italy
45b
Dipartimento di Scienze Fisiche, Università di Napoli Federico II, I-80126 Napoli, Italy
46
NIKHEF, National Institute for Nuclear Physics and High Energy Physics,
NL-1009 DB Amsterdam, The Netherlands
47
University of Notre Dame, Notre Dame, Indiana 46556, USA
48
Ohio State University, Columbus, Ohio 43210, USA
49a
INFN Sezione di Padova, I-35131 Padova, Italy
49b
Dipartimento di Fisica, Università di Padova, I-35131 Padova, Italy
50
Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS,
Université Pierre et Marie Curie-Paris6, Université Denis Diderot-Paris7,
F-75252 Paris, France
51a
INFN Sezione di Perugia, I-06123 Perugia, Italy
51b
Dipartimento di Fisica, Università di Perugia, I-06123 Perugia, Italy
52a
INFN Sezione di Pisa, I-56127 Pisa, Italy
52b
Dipartimento di Fisica, Università di Pisa, I-56127 Pisa, Italy
52c
Scuola Normale Superiore di Pisa, I-56127 Pisa, Italy
53
Princeton University, Princeton, New Jersey 08544, USA
54a
INFN Sezione di Roma, I-00185 Roma, Italy
J. P. LEES
et al.
PHYSICAL REVIEW D
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071102-2
RAPID COMMUNICATIONS
54b
Dipartimento di Fisica, Università di Roma La Sapienza, I-00185 Roma, Italy
55
Universität Rostock, D-18051 Rostock, Germany
56
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
57
CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France
58
SLAC National Accelerator Laboratory, Stanford, California 94309 USA
59
University of South Carolina, Columbia, South Carolina 29208, USA
60
Southern Methodist University, Dallas, Texas 75275, USA
61
Stanford University, Stanford, California 94305-4060, USA
62
State University of New York, Albany, New York 12222, USA
63
Tel Aviv University, School of Physics and Astronomy, Tel Aviv, 69978, Israel
64
University of Tennessee, Knoxville, Tennessee 37996, USA
65
University of Texas at Austin, Austin, Texas 78712, USA
66
University of Texas at Dallas, Richardson, Texas 75083, USA
67a
INFN Sezione di Torino, I-10125 Torino, Italy
67b
Dipartimento di Fisica, Università di Torino, I-10125 Torino, Italy
68a
INFN Sezione di Trieste, I-34127 Trieste, Italy
68b
Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
69
IFIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain
70
University of Victoria, Victoria, British Columbia, Canada V8W 3P6
71
Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
72
University of Wisconsin, Madison, Wisconsin 53706, USA
(Received 23 February 2015; published 10 April 2015)
A search is presented for the decay
Υ
ð
1
S
Þ
→
γ
A
0
,
A
0
→
c
̄
c
, where
A
0
is a candidate for the
CP
-odd
Higgs boson of the next-to-minimal supersymmetric standard model. The search is based on data collected
with the
BABAR
detector at the
Υ
ð
2
S
Þ
resonance. A sample of
Υ
ð
1
S
Þ
mesons is selected via the decay
Υ
ð
2
S
Þ
→
π
þ
π
−
Υ
ð
1
S
Þ
. The
A
0
→
c
̄
c
decay is identified through the reconstruction of hadronic
D
0
,
D
þ
,
and
D
ð
2010
Þ
þ
meson decays. No significant signal is observed. The measured 90% confidence-level
upper limits on the product branching fraction
B
ð
Υ
ð
1
S
Þ
→
γ
A
0
Þ
×
B
ð
A
0
→
c
̄
c
Þ
range from
7
.
4
×
10
−
5
to
2
.
4
×
10
−
3
for
A
0
masses from 4.00 to
8
.
95
GeV
=c
2
and 9.10 to
9
.
25
GeV
=c
2
, where the region between
8.95 and
9
.
10
GeV
=c
2
is excluded because of background from
Υ
ð
2
S
Þ
→
γχ
bJ
ð
1
P
Þ
,
χ
bJ
ð
1
P
Þ
→
γ
Υ
ð
1
S
Þ
decays.
DOI:
10.1103/PhysRevD.91.071102
PACS numbers: 12.15.Ji, 12.60.Fr, 13.20.Gd, 14.80.Da
The next-to-minimal supersymmetric standard model
(NMSSM) is an appealing extension of the standard model
(SM). It solves the
μ
problem of the minimal supersym-
metric standard model and the hierarchy problem of the SM
[1,2]
. The NMSSM has a rich Higgs sector of two charged,
three neutral
CP
-even, and two neutral
CP
-odd bosons.
Although the Higgs boson discovered at the CERN LHC
[3,4]
is consistent with the SM Higgs boson, it can also be
interpreted as one of the heavier Higgs bosons of the
NMSSM
[5]
. The least heavy of the NMSSM Higgs
bosons, denoted
A
0
, could be light enough to be produced
in the decay of an
Υ
meson
[1,6]
.
In the context of type I or type II two-Higgs-doublet
models, the branching fractions of the
A
0
depend on the
A
0
mass and the NMSSM parameter tan
β
[7]
. Below the
charm mass threshold, the
A
0
preferentially decays into two
gluons if tan
β
is of order 1, and to
s
̄
s
or to
μ
þ
μ
−
if tan
β
is
of order 10. Above the charm mass threshold, the
A
0
decays
mainly to
c
̄
c
for tan
β
of order 1 and to
τ
þ
τ
−
for tan
β
of
order 10.
BABAR
has already ruled out much of the
NMSSM parameter space for
A
0
masses below the charm
mass threshold through searches for
A
0
→
μ
þ
μ
−
[8,9]
and
for
A
0
→
gg
or
s
̄
s
[10]
. Above the charm mass threshold,
BABAR
has ruled out some of the parameter space for high
tan
β
with the
A
0
→
τ
þ
τ
−
searches
[11,12]
. None of the
searches from
BABAR
have observed a significant signal,
nor have the searches in leptonic channels from the CMS
and CLEO
[13
–
15]
Collaborations. The
A
0
→
c
̄
c
channel
is one of the last channels that has not yet been explored.
We report a search for the decay
Υ
ð
1
S
Þ
→
γ
A
0
,
A
0
→
c
̄
c
with
A
0
masses ranging between 4.00 and
9
.
25
GeV
=c
2
.
An
Υ
ð
1
S
Þ
decay is tagged by the presence of a pion pair
from
Υ
ð
2
S
Þ
→
π
þ
π
−
Υ
ð
1
S
Þ
.An
A
0
→
c
̄
c
decay is tagged
by the presence of at least one charmed meson such as
a
D
0
,a
D
þ
,ora
D
ð
2010
Þ
þ
. Therefore, candidates are
*
Deceased.
†
Now at University of Tabuk, Tabuk 71491, Saudi Arabia.
‡
Also at Università di Perugia, Dipartimento di Fisica, I-06123
Perugia, Italy.
§
Now at Laboratoire de Physique Nucléaire et de Hautes
Energies, IN2P3/CNRS, F-75252 Paris, France.
∥
Now at University of Huddersfield, Huddersfield HD1 3DH,
United Kingdom.
¶
Now at University of South Alabama, Mobile, Alabama
36688, USA.
**
Also at Università di Sassari, I-07100 Sassari, Italy.
SEARCH FOR A LIGHT HIGGS RESONANCE IN
...
PHYSICAL REVIEW D
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071102(R) (2015)
071102-3
RAPID COMMUNICATIONS
constructed from the combination of a photon, a
D
meson,
and a dipion candidate. An exclusive reconstruction of the
A
0
is not attempted. Instead, a search is performed in the
spectrum of the invariant mass of the system that recoils
against the dipion-photon system. The analysis is there-
fore sensitive to the production of any charm resonance
produced in the radiative decays of the
Υ
ð
1
S
Þ
meson.
The data were recorded with the
BABAR
detector at the
PEP-II asymmetric-energy
e
þ
e
−
collider at the SLAC
National Accelerator Laboratory. The
BABAR
detector is
described in detail elsewhere
[16,17]
. We use
13
.
6
fb
−
1
of
“
on-resonance
”
data collected at the
Υ
ð
2
S
Þ
resonance,
corresponding to
ð
98
.
3
0
.
9
Þ
×
10
6
Υ
ð
2
S
Þ
mesons
[18]
,
which includes an estimated
ð
17
.
5
0
.
3
Þ
×
10
6
Υ
ð
2
S
Þ
→
π
þ
π
−
Υ
ð
1
S
Þ
decays
[19]
. The non-
Υ
ð
2
S
Þ
backgrounds are
studied using
1
.
4
fb
−
1
of
“
off-resonance
”
data collected
30 MeV below the
Υ
ð
2
S
Þ
resonance.
The E
VT
G
EN
event generator
[20]
is used to simulate the
signal event decay chain,
e
þ
e
−
→
Υ
ð
2
S
Þ
→
π
þ
π
−
Υ
ð
1
S
Þ
,
Υ
ð
1
S
Þ
→
γ
A
0
,
A
0
→
c
̄
c
, for
A
0
masses between 4.0 and
9
.
0
GeV
=c
2
in
0
.
5
GeV
=c
2
steps and for
A
0
masses of 9.2,
9.3, and
9
.
4
GeV
=c
2
. The
A
0
decay width is assumed to be
1 MeV. The hadronization of the
c
̄
c
system is simulated
using the J
ETSET
[21]
program. The detector response is
simulated with the GEANT4
[22]
suite of programs.
Photon candidates are required to have an energy greater
than 30 MeV and a Zernike moment
A
42
[23]
less than 0.1.
The
A
42
selection reduces contributions from hadronic
showers identified as photons. Events are required to
contain at least one photon candidate. Each photon can-
didate is taken in turn to represent the radiative photon in
the
Υ
ð
1
S
Þ
→
γ
A
0
decays. We do not select a best signal
candidate, neither for the radiative photon nor for the
D
meson and dipion candidates discussed below, but rather
allow multiple candidates in an event.
Events must contain at least one
D
meson candidate,
which is reconstructed in five channels:
D
0
→
K
−
π
þ
,
D
þ
→
K
−
π
þ
π
þ
,
D
0
→
K
−
π
þ
π
þ
π
−
,
D
0
→
K
0
S
π
þ
π
−
, and
D
ð
2010
Þ
þ
→
π
þ
D
0
with
D
0
→
K
−
π
þ
π
0
. The
D
0
→
K
−
π
þ
π
0
decays are reconstructed in the
D
ð
2010
Þ
þ
pro-
duction channel to reduce a large background that would
otherwise be present. The inclusion of charge conjugate
processes is implied. The
π
0
candidates are reconstructed
from two photon candidates by requiring the invariant mass
of the reconstructed
π
0
to lie between 100 and
160
MeV
=c
2
.
The
π
0
candidates do not make use of the radiative photon
candidate. The
K
0
S
candidates are reconstructed from two
oppositely charged pion candidates. Each
K
0
S
candidate must
have a reconstructed mass within
25
MeV
=c
2
of the nominal
K
0
S
mass
[19]
and satisfy
d=
σ
d
>
3
, where
d
is the distance
between the reconstructed
e
þ
e
−
collision point and the
K
0
S
vertex, with
σ
d
the uncertainty of
d
.
The
D
0
and
D
þ
candidates are required to have
masses within
20
MeV
=c
2
of their nominal masses
[19]
,
corresponding to three to four standard deviations (
σ
) in their
mass resolution. When reconstructing
D
ð
2010
Þ
þ
candi-
dates, we constrain the
D
0
→
K
−
π
þ
π
0
candidate mass to its
nominal value
[19]
.The
D
ð
2010
Þ
þ
candidate mass distri-
bution has longer tails. The
D
ð
2010
Þ
þ
candidates are
required to lie within
5
MeV
=c
2
of its nominal mass
[19]
,
corresponding to
10
σ
in the mass resolution.
Events are required to have at least one dipion candidate,
constructed from two oppositely charged tracks. The invari-
ant mass,
m
R
, of thesystem recoiling against the dipion in the
Υ
ð
2
S
Þ
→
π
þ
π
−
Υ
ð
1
S
Þ
transition is calculated by
m
2
R
¼
M
2
Υ
ð
2
S
Þ
þ
m
2
ππ
−
2
M
Υ
ð
2
S
Þ
E
ππ
;
ð
1
Þ
where
m
ππ
is the measured dipion mass,
M
Υ
ð
2
S
Þ
is the
nominal
Υ
ð
2
S
Þ
mass
[19]
,and
E
ππ
is the dipion energy in
the
e
þ
e
−
center-of-mass (CM) frame. The two pions in the
dipion system are required to arise from a common vertex.
Signal candidates must satisfy
9
.
45
<m
R
<
9
.
47
GeV
=c
2
.
Figure
1
presents the distribution of
m
R
after application of
these criteria. A clear peak is seen at the
Υ
ð
1
S
Þ
mass.
All charged tracks and calorimeter clusters other than
those used to define the radiative photon, the
D
meson
candidate, and the dipion candidate are referred to as the
“
rest of the event.
”
The mass of the
A
0
candidate,
m
X
, is determined from
the mass of the system recoiling against the dipion and
photon through
m
2
X
¼ð
P
e
þ
e
−
−
P
π
þ
π
−
−
P
γ
Þ
2
;
ð
2
Þ
where
P
denotes four-momentum measured in the CM
frame. The four-momentum of the
e
þ
e
−
system is given
by
P
e
þ
e
−
¼ð
M
Υ
ð
2
S
Þ
;
0
;
0
;
0
Þ
.
Backgrounds are evaluated using simulated
Υ
ð
2
S
Þ
and
e
þ
e
−
→
q
̄
q
events, where
q
is a
u;d;s
,or
c
quark. Events
with low-energy photons contribute a large background for
m
X
greater than
7
.
50
GeV
=c
2
. Therefore, the analysis is
)
2
(GeV/c
R
m
9.45
9.455
9.46
9.465
9.47
2
Candidates / MeV/c
0
500
1000
1500
3
10
×
on-resonance data
on-resonance luminosity
off-resonance data normalized to
FIG. 1. The
m
R
distribution of events with a dipion, charm, and
photon tag before application of selection criteria based on the
BDT output (see text). The solid circles indicate the on-resonance
data. The open squares indicate the off-resonance data normal-
ized to the on-resonance luminosity.
J. P. LEES
et al.
PHYSICAL REVIEW D
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071102(R) (2015)
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RAPID COMMUNICATIONS
divided into a low
A
0
mass region (4.00 to
8
.
00
GeV
=c
2
)
and a high
A
0
mass region (7.50 to
9
.
25
GeV
=c
2
). The
definitions of the regions, which overlap, are motivated by
the need to have sufficient statistical precision for the
background determination in each region.
We train ten boosted decision tree (BDT) classifiers
[24]
to separate background from signal candidates (two mass
regions × five
D
channels). The BDTs are trained using
samples of simulated signal events, simulated generic
Υ
ð
2
S
Þ
events, and the off-resonance data. The BDT inputs
consist of 24 variables:
(1
–
2) Event variables:
(a) number of charged tracks in the event,
(b) number of calorimeter clusters in the event.
(3
–
12) Kinematic variables:
(a)
m
R
,
(b) dipion likelihood (defined later),
(c)
D
candidate mass,
(d)
D
candidate momentum,
(e) photon
π
0
score (defined later),
(f) energy of the most energetic charged track in the
rest of the event, calculated using a charged pion
mass hypothesis,
(g) energy of the most energetic calorimeter cluster
in the rest of the event,
(h) invariant mass of the rest of the event,
(i) CM frame momentum of the rest of the event,
(j) CM frame energy of the rest of the event.
(13
–
15) Vertex variables:
(a) transverse coordinate of a vertex formed using
all charged tracks,
(b) longitudinal coordinate of a vertex formed using
all charged tracks,
(c) the
χ
2
probability of a vertex fit using all charged
tracks.
(16
–
18) Event shape variables:
(a) the ratio of the second to zeroth Fox-Wolfram
moment
[25]
, calculated using all charged tracks
and calorimeter clusters,
(b) sphericity
[26]
of the event,
(c) magnitude of the thrust
[27]
.
(19
–
24) Opening angles in the CM frame between the
(a) dipion and photon candidate,
(b) dipion and
D
candidate,
(c) dipion and thrust axis,
(d) photon and
D
candidate,
(e) photon and thrust axis,
(f)
D
candidate and thrust axis.
The kinematic variables provide the most separation
power for all ten BDTs. The separation power of the other
variables depends on the mass region and channel. The
vertex variables suppress background without a
D
meson in
the event. The event shape variables suppress
e
þ
e
−
→
q
̄
q
backgrounds.
The dipion likelihood
[24]
is defined using the opening
angle between the two charged pions in the CM frame, the
transverse momentum of the pair, the invariant mass of the
pair, the larger of the two momenta of the pair, and the
χ
2
probability of the pair
’
s vertex fit.
To reject photon candidates from
π
0
→
γγ
decays, a
likelihood
[24]
is defined using the invariant mass of the
radiative photon candidate and a second photon (if present),
and the second photon
’
s CM energy. The lower the like-
lihood, the more
π
0
-like the photon pair. The photon
π
0
score is the minimum likelihood formed from the radiative
photon and any other photon in the event excluding photon
candidates used to reconstruct the
π
0
candidate in the
D
0
→
K
−
π
þ
π
0
decay.
For each channel and mass range, each BDT output
variable is required to exceed a value determined by
maximizing the quantity
S=
ð
0
.
5
N
σ
þ
ffiffiffiffi
B
p
Þ
[28]
, where
S
and
B
are the expected numbers of signal and background
events, respectively, based on simulation, and
N
σ
¼
3
is the
number of standard deviations desired from the result.
In the case of events with multiple signal candidates that
satisfy the selection criteria, there may be multiple values of
m
X
. Signal candidates that have the same dipion and
radiative photon candidate have the same value of
m
X
,
irrespective of which
D
candidate is used. We reject a
signal candidate if its value of
m
X
has already been used.
In total,
9
.
8
×
10
3
and
7
.
4
×
10
6
candidates satisfy the
selection criteria in the low- and high-mass regions, respec-
tively. The corresponding distributions of
m
X
are shown in
Fig.
2
. The backgrounds in the low-mass region consist of
Υ
ð
1
S
Þ
→
γ
gg
(35%), other
Υ
ð
1
S
Þ
decays, denoted
Υ
ð
1
S
Þ
→
X
(34%),
Υ
ð
2
S
Þ
decays without a dipion transi-
tion, denoted
Υ
ð
2
S
Þ
→
X
(15%), and
e
þ
e
−
→
q
̄
q
events
(16%). The corresponding background contributions in the
high-mass region are 1%, 66%, 18%, and 15%. Background
contributions from
Υ
ð
1
S
Þ
→
γ
gg
decays reach a maximum
near
5
.
5
GeV
=c
2
and decrease above
7
GeV
=c
2
.
We search for the
A
0
resonance as a peak in the
m
X
distribution. The reconstructed width of the
A
0
is expected
to strongly depend on its mass due to better photon energy
resolution at lower photon energies. Therefore, an extended
maximum likelihood fit in a local mass range is performed
as a function of test-mass values, denoted
m
A
0
. For these
fits, the parameters of the probability density function
(PDF) used to model the shape of the signal distribution are
fixed. The parameters of the background PDF, the number
of signal events
N
sig
, and the number of background events
are determined in the fit.
The signal
m
X
PDF is modeled with a Crystal Ball
function
[29]
, which consists of a Gaussian and a power-
law component. Thevalues of the signal PDFat a givenvalue
of
m
A
0
are determined through interpolation from fits of
simulated signal events at neighboring masses. The back-
ground
m
X
PDF is modeled with a second-order polynomial.
The fits are performed to the
m
X
spectrum, for various
choices of
m
A
0
, in steps of 10 and
2
MeV
=c
2
for the low- and
high-mass regions, respectively. The step sizes are at least
SEARCH FOR A LIGHT HIGGS RESONANCE IN
...
PHYSICAL REVIEW D
91,
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RAPID COMMUNICATIONS
3 times smaller than thewidth of the signal
m
X
PDF. We usea
local fitting range of
10
σ
CB
around
m
A
0
, where
σ
CB
denotes
the width of the Gaussian component of the Crystal Ball
function. The
σ
CB
parameter varies between 120 and
8
MeV
=c
2
for values of
m
A
0
between 4.00 and
9
.
25
GeV
=c
2
, as shown in Fig.
3
. We do not perform a fit
for
8
.
95
<m
A
0
<
9
.
10
GeV
=c
2
because of a large back-
ground from
Υ
ð
2
S
Þ
→
γχ
bJ
ð
1
P
Þ
,
χ
bJ
ð
1
P
Þ
→
γ
Υ
ð
1
S
Þ
decays.
The fitting procedure is validated using background-only
pseudoexperiments. The
m
X
PDF used to generate pseu-
doexperiments for the low-mass region is obtained from a
fit of a fifth-order polynomial to the low-mass region data.
The
m
X
PDF used for the high-mass region is obtained
from a fit of the sum of four exponential functions plus
six Crystal Ball functions to the high-mass region data,
with shape parameters fixed according to expectations
from simulation and with the relative normalizations
determined in the fit. The Crystal Ball functions describe
the
Υ
ð
2
S
Þ
→
γχ
bJ
ð
1
P
Þ
and
χ
bJ
ð
1
P
Þ
→
γ
Υ
ð
1
S
Þ
transitions
while the exponential terms describe the nonresonant
background. Four exponential terms are used because
the nonresonant background increases rapidly for higher
m
X
. The background fits are overlaid in Fig.
2
. The fitting
procedure returns a null signal for most
m
A
0
values but is
found to require a correction to
N
sig
for values of
m
A
0
near
4.00 or
9
.
25
GeV
=c
2
. The corrections are determined from
the average number of signal events found in the fits to the
background-only pseudoexperiments. The corrections are
applied as a function of
m
A
0
and reach a maximum of 15
and 50 candidates in the low- and high-mass regions,
respectively. The uncertainty of the correction is assumed
to be half its value.
The reconstruction efficiency takes into account the
hadronization of the
c
̄
c
system into
D
mesons, the
branching fraction of
D
mesons to the five decay channels,
detector acceptance, and the BDT selection. The efficien-
cies range from 4.0% to 2.6% for simulated
A
0
masses
between 4.00 and
9
.
25
GeV
=c
2
.
Potential bias introduced by the fitting procedure is
evaluated using pseudoexperiments with different values
of the product branching fraction
B
ð
Υ
ð
1
S
Þ
→
γ
A
0
Þ
×
B
ð
A
0
→
c
̄
c
). For various choices of
m
A
0
, the extracted
product branching fraction is found to be
ð
4
3
Þ
% higher
than the value used to generate the events. This result is used
to define a correction and its uncertainty.
Table
I
summarizes all correction factors and associated
systematic uncertainties. The fit correction systematic
uncertainty is added in quadrature with the statistical
uncertainty of
N
sig
. All other correction factors are added
in quadrature and applied to the reconstruction efficiency.
A correction of 1.00 means we do not apply any correction
but propagate the multiplicative uncertainty.
The systematic uncertainties associated with the
reconstruction efficiencies are dominated by the differences
between data and simulation, including the BDT output
modeling,
c
̄
c
hadronization,
D
-candidate mass resolu-
tion, dipion recoil mass and likelihood modeling, and
photon reconstruction. Other systematic uncertainties are
)
2
mass (GeV/c
0
Simulated A
456789
)
2
(GeV/c
CB
σ
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
FIG. 3. The
σ
CB
parameter for
A
0
decays of various simulated
masses.
)
2
(GeV/c
X
m
345678
2
Candidates / 100 MeV/c
0
200
400
600
800
1000
(a)
data
fit to data
gg
γ
→
(1S)
Υ
X
→
(1S)
Υ
X
→
(2S)
Υ
q
q
→
-
e
+
e
)
2
(GeV/c
X
m
7.5
8
8.5
9
2
Candidates / 10 MeV/c
10
2
10
3
10
4
10
(b)
FIG. 2 (color online). The
m
X
distributions of signal candidates
in the low- (a) and high- (b) mass regions after applying all
selection criteria. The points indicate the data. The solid curve
shows the result of a fit to the data under a background-only
hypothesis. The colored histograms show the cumulative back-
ground contributions from
e
þ
e
−
→
q
̄
q
(magenta dense-dot
filled),
Υ
ð
2
S
Þ
→
X
(green sparse-dot filled),
Υ
ð
1
S
Þ
→
X
(blue
dotted), and
Υ
ð
1
S
Þ
→
γ
gg
(red dashed) events.
J. P. LEES
et al.
PHYSICAL REVIEW D
91,
071102(R) (2015)
071102-6
RAPID COMMUNICATIONS
associated with the fit bias (discussed above), the dipion
branching fraction
[19]
, the finite size of the simulated
signal sample, and the
Υ
ð
2
S
Þ
counting
[18]
.
The BDT output distributions in off-resonance data and
e
þ
e
−
→
q
̄
q
simulation, shown in Fig.
4
, have consistent
shapes but are slightly shifted from one another. The
associated systematic uncertainty is estimated by shifting
the simulated distributions so that the mean values agree
with the data, and then recalculating the efficiencies. The
reconstruction efficiencies decrease by 7% and 2% in the
low- and high-mass regions, respectively.
The uncertainty associated with
c
̄
c
hadronization is
evaluated by comparing
D
meson production in off-
resonance data and
e
þ
e
−
→
c
̄
c
simulation normalized to
the same luminosity. The difference in the yield varies from
1% to 9% for the five
D
decay channels. We conservatively
assign a global multiplicative uncertainty of 9% that
includes effects due to the hadronization modeling, particle
identification, tracking,
π
0
reconstruction, and luminosity
determination of the off-resonance data.
The uncertainty due to the discrepancy between the
reconstructed
D
mass resolution in data and simulation is
estimated by Gaussian smearing of the
D
mass input in
simulation to match the data and measuring the difference
in the reconstruction efficiency.
Further corrections to account for data and simulation
differences in reconstruction efficiencies are estimated with
similar methods. Corrections are applied to account for the
dipion recoil mass reconstruction, the dipion likelihood
modeling, and the photon reconstruction
[30]
.
The highest observed local significance in the low-mass
region is 2.3 standard deviations, including statistical
uncertainties only, at
4
.
145
GeV
=c
2
. The corresponding
result for the high-mass region is 2.0 standard deviations
at
8
.
411
GeV
=c
2
. The fits are shown in Fig.
5
. Such
)
2
(GeV/c
X
m
33.544.55
2
Candidates / 0.05 GeV/c
0
20
40
60
80
100
2
Candidates / 0.05 GeV/c
(a)
2
5
5
8
4
6
4
4
4
2
4
4
8
3
6
3
4
3
2
3
3
Data
-2
0
2
2
5
5
8
4
6
4
4
4
2
4
4
8
3
6
3
4
3
2
3
3
Data-Fit
)
2
(GeV/c
X
m
8.2
8.3
8.4
8.5
8.6
2
Candidates / 0.01 GeV/c
0
200
400
600
800
1000
1200
2
Candidates / 0.01 GeV/c
(b)
Data
0
2
Data-Fit
-2
FIG. 5. The fits with the highest local significance in the low-
(a) and high- (b) mass regions. The solid line is the fit that
includes a signal. The dotted line is the background component of
the solid line.
TABLE I. Summary of corrections and their associated sys-
tematic uncertainties. All corrections are multiplicative except for
the fit correction.
Source
Low region
High region
Fit correction (candidates) Up to
15
8
Up to
50
25
BDT output modeling
0
.
93
0
.
04
0
.
98
0
.
01
Source
Both regions
c
̄
c
hadronization
1
.
00
0
.
09
Fit bias
1
.
04
0
.
03
Dipion branching fraction
1
.
00
0
.
02
Photon reconstruction
0
.
967
0
.
017
D
mass resolution
0
.
98
0
.
01
Finite simulation statistics
1
.
00
0
.
01
Υ
ð
2
S
Þ
counting
1
.
00
0
.
01
Dipion likelihood
1
.
02
0
.
01
Dipion recoil mass
0
.
991
0
.
005
100
200
(a)
π
K
10000
20000
(b)
π
K
200
400
(c)
K
ππ
20000
40000
60000
(d)
K
ππ
500
1000
(e)
K
πππ
1
00000
2
00000
(f)
K
πππ
100
200
(g)
ππ
S
K
10000
20000
(h)
ππ
S
K
-1
-0.5
0
0.5
0
50
100
(i)
0
ππ
K
-1
-0.5
0
0.5
0
10000
20000
(j)
0
ππ
K
BDT output
Candidates / 0.1
FIG. 4. The BDT distributions in off-resonance data (points) and
simulated
e
þ
e
−
→
q
̄
q
events (histograms) for the five
D
meson
decay modes. The results on the left (a, c, e, g, i) and right (b, d, f,
h, j) correspond to the low- and high-mass regions, respectively.
SEARCH FOR A LIGHT HIGGS RESONANCE IN
...
PHYSICAL REVIEW D
91,
071102(R) (2015)
071102-7
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