High-p
T
charged hadron suppression in Au+Au collisions at
Î
s
NN
=200 GeV
S. S. Adler,
5
S. Afanasiev,
17
C. Aidala,
5
N. N. Ajitanand,
43
Y. Akiba,
20,38
J. Alexander,
43
R. Amirikas,
12
L. Aphecetche,
45
S. H. Aronson,
5
R. Averbeck,
44
T. C. Awes,
35
R. Azmoun,
44
V. Babintsev,
15
A. Baldisseri,
10
K. N. Barish,
6
P. D. Barnes,
27
B. Bassalleck,
33
S. Bathe,
30
S. Batsouli,
9
V. Baublis,
37
A. Bazilevsky,
39,15
S. Belikov,
16,15
Y. Berdnikov,
40
S. Bhagavatula,
16
J. G. Boissevain,
27
H. Borel,
10
S. Borenstein,
25
M. L. Brooks,
27
D. S. Brown,
34
N. Bruner,
33
D. Bucher,
30
H. Buesching,
30
V. Bumazhnov,
15
G. Bunce,
5,39
J. M. Burward-Hoy,
26,44
S. Butsyk,
44
X. Camard,
45
J.-S. Chai,
18
P. Chand,
4
W. C. Chang,
2
S. Chernichenko,
15
C. Y. Chi,
9
J. Chiba,
20
M. Chiu,
9
I. J. Choi,
52
J. Choi,
19
R. K. Choudhury,
4
T. Chujo,
5
V. Cianciolo,
35
Y. Cobigo,
10
B. A. Cole,
9
P. Constantin,
16
D. G. d’Enterria,
45
G. David,
5
H. Delagrange,
45
A. Denisov,
15
A. Deshpande,
39
E. J. Desmond,
5
O. Dietzsch,
41
O. Drapier,
25
A. Drees,
44
R. du Rietz,
29
A. Durum,
15
D. Dutta,
4
Y. V. Efremenko,
35
K. El Chenawi,
49
A. Enokizono,
14
H. En’yo,
38,39
S. Esumi,
48
L. Ewell,
5
D. E. Fields,
33,39
F. Fleuret,
25
S. L. Fokin,
30
B. D. Fox,
39
Z. Fraenkel,
51
J. E. Frantz,
9
A. Franz,
5
A. D. Frawley,
12
S.-Y. Fung,
6
S. Garpman,
29,
*
T. K. Ghosh,
49
A. Glenn,
46
G. Gogiberidze,
46
M. Gonin,
25
J. Gosset,
10
Y. Goto,
39
R. Granier de Cassagnac,
25
N. Grau,
16
S. V. Greene,
49
M. Grosse Perdekamp,
39
W. Guryn,
5
H.-Å. Gustafsson,
29
T. Hachiya,
14
J. S. Haggerty,
5
H. Hamagaki,
8
A. G. Hansen,
27
E. P. Hartouni,
26
M. Harvey,
5
R. Hayano,
8
X. He,
13
M. Heffner,
26
T. K. Hemmick,
44
J. M. Heuser,
44
M. Hibino,
50
J. C. Hill,
16
W. Holzmann,
43
K. Homma,
14
B. Hong,
22
A. Hoover,
34
T. Ichihara,
38,39
V. V. Ikonnikov,
23
K. Imai,
24,38
D. Isenhower,
1
M. Ishihara,
38
M. Issah,
43
A. Isupov,
17
B. V. Jacak,
44
W. Y. Jang,
22
Y. Jeong,
19
J. Jia,
44
O. Jinnouchi,
38
B. M. Johnson,
5
S. C. Johnson,
26
K. S. Joo,
31
D. Jouan,
36
S. Kametani,
8,50
N. Kamihara,
47,38
J. H. Kang,
52
S. S. Kapoor,
4
K. Katou,
50
S. Kelly,
9
B. Khachaturov,
51
A. Khanzadeev,
37
J. Kikuchi,
50
D. H. Kim,
31
D. J. Kim,
52
D. W. Kim,
19
E. Kim,
42
G.-B. Kim,
25
H. J. Kim,
52
E. Kistenev,
5
A. Kiyomichi,
48
K. Kiyoyama,
32
C. Klein-Boesing,
30
H. Kobayashi,
38,39
L. Kochenda,
37
V. Kochetkov,
15
D. Koehler,
33
T. Kohama,
14
M. Kopytine,
44
D. Kotchetkov,
6
A. Kozlov,
51
P. J. Kroon,
5
C. H. Kuberg,
1,27
K. Kurita,
39
Y. Kuroki,
48
M. J. Kweon,
22
Y. Kwon,
52
G. S. Kyle,
34
R. Lacey,
43
V. Ladygin,
17
J. G. Lajoie,
16
A. Lebedev,
16,23
S. Leckey,
44
D. M. Lee,
27
S. Lee,
19
M. J. Leitch,
27
X. H. Li,
6
H. Lim,
42
A. Litvinenko,
17
M. X. Liu,
27
Y. Liu,
36
C. F. Maguire,
49
Y. I. Makdisi,
5
A. Malakhov,
17
V. I. Manko,
23
Y. Mao,
7,38
G. Martinez,
45
M. D. Marx,
44
H. Masui,
48
F. Matathias,
44
T. Matsumoto,
8,50
P. L. McGaughey,
27
E. Melnikov,
15
F. Messer,
44
Y. Miake,
48
J. Milan,
43
T. E. Miller,
49
A. Milov,
44,51
S. Mioduszewski,
5
R. E. Mischke,
27
G. C. Mishra,
13
J. T. Mitchell,
5
A. K. Mohanty,
4
D. P. Morrison,
5
J. M. Moss,
27
F. Mühlbacher,
44
D. Mukhopadhyay,
51
M. Muniruzzaman,
6
J. Murata,
38,39
S. Nagamiya,
20
J. L. Nagle,
9
T. Nakamura,
14
B. K. Nandi,
6
M. Nara,
48
J. Newby,
46
P. Nilsson,
29
A. S. Nyanin,
23
J. Nystrand,
29
E. O’Brien,
5
C. A. Ogilvie,
16
H. Ohnishi,
5,38
I. D. Ojha,
49,3
K. Okada,
38
M. Ono,
48
V. Onuchin,
15
A. Oskarsson,
29
I. Otterlund,
29
K. Oyama,
8
K. Ozawa,
8
D. Pal,
51
A. P. T. Palounek,
27
V. S. Pantuev,
44
V. Papavassiliou,
34
J. Park,
42
A. Parmar,
33
S. F. Pate,
34
T. Peitzmann,
30
J.-C. Peng,
27
V. Peresedov,
17
C. Pinkenburg,
5
R. P. Pisani,
5
F. Plasil,
35
M. L. Purschke,
5
A. K. Purwar,
44
J. Rak,
16
I. Ravinovich,
51
K. F. Read,
35,46
M. Reuter,
44
K. Reygers,
30
V. Riabov,
37,40
Y. Riabov,
37
G. Roche,
28
A. Romana,
25
M. Rosati,
16
P. Rosnet,
28
S. S. Ryu,
52
M. E. Sadler,
1
N. Saito,
38,39
T. Sakaguchi,
8,50
M. Sakai,
32
S. Sakai,
48
V. Samsonov,
37
L. Sanfratello,
33
R. Santo,
30
H. D. Sato,
24,38
S. Sato,
5,48
S. Sawada,
20
Y. Schutz,
45
V. Semenov,
15
R. Seto,
6
M. R. Shaw,
1,27
T. K. Shea,
5
T.-A. Shibata,
47,38
K. Shigaki,
14,20
T. Shiina,
27
C. L. Silva,
41
D. Silvermyr,
27,29
K. S. Sim,
22
C. P. Singh,
3
V. Singh,
3
M. Sivertz,
5
A. Soldatov,
15
R. A. Soltz,
26
W. E. Sondheim,
27
S. P. Sorensen,
46
I. V. Sourikova,
5
F. Staley,
10
P. W. Stankus,
35
E. Stenlund,
29
M. Stepanov,
34
A. Ster,
21
S. P. Stoll,
5
T. Sugitate,
14
J. P. Sullivan,
27
E. M. Takagui,
41
A. Taketani,
38,39
M. Tamai,
50
K. H. Tanaka,
20
Y. Tanaka,
32
K. Tanida,
38
M. J. Tannenbaum,
5
P. Tarján,
11
J. D. Tepe,
1,27
T. L. Thomas,
33
J. Tojo,
24,38
H. Torii,
24,38
R. S. Towell,
1
I. Tserruya,
51
H. Tsuruoka,
48
S. K. Tuli,
3
H. Tydesjö,
29
N. Tyurin,
15
H. W. van Hecke,
27
J. Velkovska,
5,44
M. Velkovsky,
44
L. Villatte,
46
A. A. Vinogradov,
23
M. A. Volkov,
23
E. Vznuzdaev,
37
X. R. Wang,
13
Y. Watanabe,
38,39
S. N. White,
5
F. K. Wohn,
16
C. L. Woody,
5
W. Xie,
6
Y. Yang,
7
A. Yanovich,
15
S. Yokkaichi,
38,39
G. R. Young,
35
I. E. Yushmanov,
23
W. A. Zajc,
9,†
C. Zhang,
9
S. Zhou,
7,51
and
L. Zolin
17
(PHENIX Collaboration)
1
Abilene Christian University, Abilene, Texas 79699, USA
2
Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
3
Department of Physics, Banaras Hindu University, Varanasi 221005, India
4
Bhabha Atomic Research Centre, Bombay 400 085, India
5
Brookhaven National Laboratory, Upton, New York 11973-5000, USA
6
University of California, Riverside, Riverside, California 92521, USA
7
China Institute of Atomic Energy (CIAE), Beijing, People’s Republic of China
8
Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
9
Columbia University, New York, New York 10027 and Nevis Laboratories, Irvington, New York 10533, USA
10
Dapnia, CEA Saclay, F-91191 Gif-sur-Yvette, France
11
Debrecen University, H-4010 Debrecen, Egyetem tér 1, Hungary
12
Florida State University, Tallahassee, Florida 32306, USA
13
Georgia State University, Atlanta, Georgia 30303, USA
14
Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan
15
Institute for High Energy Physics (IHEP), Protvino, Russia
PHYSICAL REVIEW C 69, 034910 (2004)
0556-2813/2004/69(3)/034910(20)/$22.50 ©2004 The American Physical Society69 034910-1
16
Iowa State University, Ames, Iowa 50011, USA
17
Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia
18
KAERI, Cyclotron Application Laboratory, Seoul, South Korea
19
Kangnung National University, Kangnung 210-702, South Korea
20
KEK, High Energy Accelerator Research Organization, Tsukuba-shi, Ibaraki-ken 305-0801, Japan
21
KFKI Research Institute for Particle and Nuclear Physics (RMKI), P. O. Box 49, H-1525 Budapest 114, Hungary
22
Korea University, Seoul 136-701, Korea
23
Russian Research Center “Kurchatov Institute,” Moscow, Russia
24
Kyoto University, Kyoto 606, Japan
25
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128 Palaiseau, France
26
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
27
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
28
LPC, Université Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France
29
Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
30
Institut fuer Kernphysik, University of Muenster, D-48149 Muenster, Germany
31
Myongji University, Yongin, Kyonggido 449-728, Korea
32
Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan
33
University of New Mexico, Albuquerque, New Mexico 87131, USA
34
New Mexico State University, Las Cruces, New Mexico 88003, USA
35
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
36
IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, Boîte Postale 1, F-91406 Orsay, France
37
PNPI, Petersburg Nuclear Physics Institute, Gatchina, Russia
38
RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan
39
RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
40
St. Petersburg State Technical University, St. Petersburg, Russia
41
Universidade de São Paulo, Instituto de Física, Caixa Postal 66318, São Paulo CEP05315-970, Brazil
42
System Electronics Laboratory, Seoul National University, Seoul, South Korea
43
Chemistry Department, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA
44
Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, New York 11794, USA
45
SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Université de Nantes) Boîte Postale 20722-44307, Nantes, France
46
University of Tennessee, Knoxville, Tennessee 37996, USA
47
Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
48
Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
49
Vanderbilt University, Nashville, Tennessee 37235, USA
50
Waseda University, Advanced Research Institute for Science and Engineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan
51
Weizmann Institute, Rehovot 76100, Israel
52
Yonsei University, IPAP, Seoul 120-749, Korea
(Received 8 August 2003; published 30 March 2004)
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured charged hadron yields at
midrapidity over a wide range of transverse momenta s0.5, p
T
,10 GeV/cd in Au+Au collisions at
Î
s
NN
=200 GeV. The data are compared to
p
0
measurements from the same experiment. For both charged hadrons
and neutral pions, the yields per nucleon-nucleon collision are significantly suppressed in central compared to
peripheral and nucleon-nucleon collisions. The suppression sets in gradually and increases with increasing
centrality of the collisions. Above 4–5 GeV/c in p
T
, a constant and almost identical suppression of charged
hadrons and
p
0
’s is observed. The p
T
spectra are compared to published spectra from Au+Au at
Î
s
NN
=130 in
terms of x
T
scaling. Central and peripheral
p
0
as well as peripheral charged spectra exhibit the same x
T
scaling
as observed in p +p data.
DOI: 10.1103/PhysRevC.69.034910 PACS number(s): 25.75.Dw
*
Deceased.
†
Email address: zajc@nevis.columbia.edu
S. S. ADLER et al. PHYSICAL REVIEW C 69, 034910 (2004)
034910-2
I. INTRODUCTION
Lattice quantum chromodynamic (QCD) calculations pre-
dict a new state of matter of deconfined quarks and gluons at
an energy density exceeding ,1 GeV/ fm
3
[1]. It has long
been suggested that such a “quark gluon plasma” may be
produced in collisions between ultrarelativistic heavy nuclei
[2]. Indeed, measurements of transverse energy produced in
high-energy Pb+Pb and Au+Au collisions suggest that en-
ergy densities above 3 GeV/fm
3
at the CERN SPS [3] and
5 GeV/fm
3
at the Relativistic Heavy Ion Collider (RHIC)
[4,5] have been reached. However, this conclusion relies on
model assumptions [6–9] to relate the properties of the had-
ronic final state to the initial state dynamics.
The spectra of high transverse momentum sp
T
d hadrons
resulting from the fragmentation of hard-scattered partons
potentially provide a direct probe of the properties of the
initial state. Theoretical calculations show that the outgoing
high-p
T
partons radiate substantially more energy when
propagating through dense matter than when propagating in
the vacuum, resulting in a softening of the hadron p
T
spec-
trum [10], with the energy loss of the partons depending on
the gluon density of the matter [11,12]. Formation time con-
siderations suggest that hard scattered partons are “pro-
duced” at the earliest stage of the collision, thus directly
probe the dense matter from the time of their creation. There-
fore, a detailed analysis of high-p
T
hadron production may
reveal information on the properties of the dense medium
created early in the collisions [12–14].
At the energies reached at RHIC, high-p
T
hadrons are
copiously produced. In nucleon-nucleon collisions, it has
been well established that hadrons with p
T
ù2 GeV/c result
primarily from the fragmentation of hard-scattered partons,
and that the p
T
spectra of these hadrons can be calculated
using perturbative QCD (pQCD)[15,16]. Initial measure-
ments of hadron p
T
spectra in Au+Au collisions at
Î
s
NN
=130 GeV led to the discovery of a substantial suppression
of hadron yields per nucleon-nucleon collision relative to pp
data [17–19]. Data from
Î
s
NN
=200 GeV confirm these re-
sults [20–23]. The suppression is observed in central but not
in peripheral collisions. These observations are consistent
with pQCD-inspired modeling of parton energy loss in dense
matter [24,25]. However, alternative interpretations that do
not assume the formation of a deconfined phase have been
proposed based on the modifications of the parton distribu-
tion functions in the initial state [26] or final-state hadronic
interactions [27].
In addition to hadron suppression, an unexpectedly large
fraction of baryons has been observed in central Au+Au
collisions for p
T
up to 4–5 GeV/c [28–30], which compli-
cates the interpretation of the high-p
T
results. The observed
baryon to meson ratio from PHENIX [29] is inconsistent
with jet fragmentation in p + p [31] and e
+
e
−
collisions [32].
While the origin of this effect is unclear, it could point to-
wards bulk particle production (“soft physics”) contributing
to the p
T
spectra out to 4–5 GeV/c. It has been suggested
that coalescence of thermalized quarks combining with en-
ergy loss of hard-scattered partons can account for the un-
usual particle composition, which shifts the region domi-
nated by hard scattering to higher p
T
[33].
Systematic measurements of the p
T
, centrality, particle
species, and
Î
s
NN
dependence of the suppression can con-
strain competing descriptions of high-p
T
hadron production.
In this paper, we present new data on inclusive charged had-
ron production for 0.5, p
T
,10 GeV/c, measured over a
broad range of centrality in Au+Au collisions at
Î
s
NN
=200 GeV by the PHENIX Collaboration at RHIC. These
data are compared to data on neutral pion production [21]
and to data from Au+Au collisions at
Î
s
NN
=130 GeV
[17,19], all measured within the same experiment.
The remainder of the paper is organized as follows. Sec-
tion II gives a detailed account of the charged particle analy-
sis. Centrality and p
T
dependence of the charged hadron p
T
spectra are discussed in Sec. III A. Section III B studies the
charged hadron suppression and compares the results to
p
0
data. In Sec. III C, we discuss the
Î
s
NN
dependence of both
charged hadron and neutral pion production and test possible
x
T
scaling. A summary is given in Sec. IV.
II. DATA ANALYSIS
A. PHENIX detector
The PHENIX experiment consists of four spectrometer
arms—two around midrapidity (the central arms) and two at
forward rapidity (the muon arms)—and a set of global detec-
tors. The central arm and south muon arm detectors were
completed in 2001 and took data during Au+Au operation of
RHIC the same year (RUN-2). The layout of the PHENIX
experiment during RUN-2 is shown in Fig. 1. Each central
arm covers u
h
u,0.35° in pseudorapidity and 90° in azi-
muthal angle
f
. In each of the central arms, charged particles
are tracked by a drift chamber (DC) positioned from 2.0 to
2.4 m radially outward from the beam axis and two or three
layers of pixel pad chambers [PC1, (PC2), PC3 located at
2.4 m, s4.2 md, 5 m in radial direction, respectively]. Par-
ticle identification is provided by ring imaging Cerenkov
counters (RICH), a time of flight scintillator wall (TOF), and
two types of electromagnetic calorimeters (lead scintillator
and lead glass ). The magnetic field for the central spectrom-
eter is axially symmetric around the beam axis. Its compo-
nent parallel to the beam axis has an approximately Gaussian
dependence on the radial distance from the beam axis, drop-
ping from 0.48 T at the center to 0.096 T (0.048 T) at the
inner (outer) radius of the DC. A pair of zero-degree calo-
rimeters (ZDC) and a pair of beam-beam counters (BBC)
were used for global event characterization. Further details
about the design and performance of PHENIX can be found
in Ref. [34].
B. Event selection
During RUN-2, PHENIX sampled an integrated luminos-
ity of 24
m
b
−1
for Au+Au collisions at
Î
s
NN
=200 GeV.
Minimum bias events were selected by a coincidence be-
tween the ZDCs and the BBCs. This selection corresponds to
92.2
−3.0
+2.5
% of the 6.9 b Au+Au inelastic cross section. The
event centrality is determined by correlating the charge de-
tected in the BBCs with the energy measured in the ZDCs.
Two sets of centrality definitions are used in this analysis: a
HIGH-p
T
CHARGED HADRON SUPPRESSION IN Au+Au… PHYSICAL REVIEW C 69, 034910 (2004)
034910-3
“Fine” set of centralities, which corresponds to 0–5%, ...,
15–20%, 20–30%, ..., 80–92%, and a “Coarse” set of cen-
tralities, which corresponds to 0–10%, 10–20%, 20–30%,
..., 80–92%. A Glauber model Monte Carlo simulation
[35–38] that includes the responses of BBC and ZDC gives
an estimate of the average number of binary collisions
kN
coll
l, participating nucleons kN
part
l, and nuclear overlap
function kT
AuAu
l for each centrality class. The calculated val-
ues of kN
coll
l, kN
part
l, and kT
AuAu
l for each centrality class
are listed in Table I.
In addition to the event selection, the BBCs also allow us
to reconstruct the collision vertex in the beam direction szd
with a resolution of 0.5 cm. An offline z-vertex cut,
uz
v
tx
u,30 cm, was applied to the minimum bias events. After
this selection, a total of 273 10
6
minimum bias Au+Au
events were analyzed to obtain the charged hadron spectra
presented in this paper.
C. Charged particle tracking and momentum measurement
Charged hadron tracks are measured using information
from the DC, PC1, PC2, and PC3 detectors of the west cen-
tral arm and the BBC. The projections of the charged particle
trajectories into a plane perpendicular to the beam axis are
detected typically in 12 wire planes in the DC. The wire
planes are spaced at 0.6 cm intervals along the radial direc-
tion from the beam axis. Each wire provides a projective
measurement, with better than 150
m
m spacial resolution in
the azimuthal s
f
d direction. Eight additional wire planes in
the DC provide stereoscopic projections, which together with
the space point measured at the PC1 and the vertex position
measured by the BBC determine the polar angle of the track.
Trajectories are confirmed by requiring matching hits at both
PC2 and PC3 to reduce the secondary background.
Tracks are then projected back to the collision vertex
through the magnetic field to determine the momentum p
W
.
The transverse momentum p
T
is related to the deflection
angle
a
measured at the DC with respect to an infinite mo-
mentum trajectory. For tracks emitted perpendicular to the
beam axis, this relation can be approximated by
a
.
K
p
T
, s1d
where K =87 mrad GeV/c is the effective field integral.
The momentum scale is verified by comparing the known
proton mass to the value measured for charged particles
identified as protons from their time of flight. The flight time
is measured in the TOF detector, which cover
p
/4 of the
FIG. 1. (Color online) PHENIX experimental
layout for the Au+Au run in 2001. The top panel
shows the PHENIX central arm spectrometers
viewed along the beam axis. The bottom panel
shows a side view of the PHENIX muon arm
spectrometers.
S. S. ADLER et al. PHYSICAL REVIEW C 69, 034910 (2004)
034910-4
azimuthal acceptance in the east arm. The absolute value of
the momentum scale is known to be correct to better than
0.7%.
The momentum resolution is directly related to the
a
resolution,
d
p/p =
d
a
/
a
=
1
K
Î
S
s
ms
b
D
2
+ s
s
a
pd
2
, s2d
where
d
a
is the measured angular spread, which can be de-
composed into the contribution from multiple scattering
s
ms
and the contribution from the intrinsic pointing resolution
s
a
of the DC. At high p
T
,
s
a
is the dominating contribution, i.e.,
d
a
.
s
a
. We measure
s
a
<0.84±0.05 mrad/sGeV/cd using
zero field data, where we select high-momentum tracks by
requiring energetic hadronic showers in the electromag-
netic calorimeters. The width of the proton mass as func-
tion of p
T
independently confirms the momentum resolu-
tion. In summary, the momentum resolution is determined
to be
d
p/p .0.7% % 1.0%p sGeV/cd. Further details on
track reconstruction and momentum determination can be
found in Ref. f39g.
D. Background rejection and subtraction
Approximately 95% of the tracks reconstructed by the DC
originate from the event vertex. The remainder have to be
investigated as potential background to the charged particle
measurement. The main background sources include second-
ary particles from decays and e
+
e
−
pairs from the conversion
of photons in materials between the vertex and the DC. De-
pending on how close the conversion or decay point is to the
DC, or depending on the Q value of the decay, these tracks
may have a small deflection angle
a
at the DC. Thus, accord-
ing to Eq. (1), they are incorrectly assigned a large momen-
tum. In this analysis, the p
T
range over which charged par-
ticle production is accessible in PHENIX is limited by this
background. We exploit the track match to PC2 and PC3 to
reject as much of the background as possible, then employ a
statistical method to measure and subtract the irreducible
background.
For primary tracks, the distance in both the r −
f
and the z
direction between the track projection point and the mea-
sured PC hit position is approximately Gaussian with a mean
of 0 and a width given by
s
match
=
Î
s
det
match
2
+
S
s
ms
match
p
b
D
2
, s3d
where
s
det
match
is the finite detector resolution fwhich in-
cludes DC pointing sor
a
d resolution and the PC2, PC3
spacial resolutiong, and
s
ms
match
is the multiple scattering
contribution.
Despite being incorrectly reconstructed with large p
T
, the
majority of the background particles have low momenta.
While traveling from the DC to the PC2 and PC3, they mul-
tiple scatter and receive an additional deflection from the
fringe field. This causes a correlated deflection between the
measured positions at PC2, PC3, and the projections calcu-
lated from tracks measured by the DC and PC1. The dis-
placements in r −
f
and z directions are represented by D
f
and D
z
. Since the residual bend depends on the z component
of the fringe field, which decreases rapidly at large u
h
u,a
fiducial cut of u
h
u,0.18 was applied to ensure that the re-
sidual bend due to the fringe field is almost independent of z.
FIG. 2. D
f
pc2
(the difference between projection and hit location
in r-
f
direction at PC2) vs D
f
pc3
in centimeters for tracks with
reconstructed p
T
.4 GeV/c. PC2, PC3 matching differences are
correlated, with signal tracks peaked around 0 and background
tracks extend along the D
f
+
direction. The double-peak structure
along D
f
−
is related to the finite granularity of PC2 and PC3 pads.
The positive directions of D
f
+
and D
f
−
are indicated by the arrow.
A±2
s
cut on these variables is illustrated by the box region inside
the dashed lines.
TABLE I. Centrality classes, average number of N +N colli-
sions, average number of participant nucleons, and average nuclear
overlap function obtained from a Glauber Monte Carlo simulation
of the BBC and ZDC responses for Au+Au at
Î
s
NN
=200 GeV.
Each centrality class is expressed as a percentage of
s
AuAu
=6.9 b.
Two sets of centrality definitions are used in this analysis: a “Fine”
set of centralities, which corresponds to 0–5%, . . ., 15–20%,
20–30%,..., 80–92%, and a “Coarse” set of centralities, which
corresponds to 0–10%, 10–20%, 20–30%, ..., 80–92%.
Centrality (%) kN
coll
lkN
part
lkT
AuAu
lsmb
−1
d
0–5 1065±105.5 351.4±2.9 25.37±1.77
5–10 854.4±82.1 299±3.8 20.13±1.36
10–15 672.4±66.8 253.9±4.3 16.01±1.15
15–20 532.7±52.1 215.3±5.3 12.68±0.86
0–10 955.4±93.6 325.2±3.3 22.75±1.56
10–20 602.6±59.3 234.6±4.7 14.35±1.00
20–30 373.8±39.6 166.6±5.4 8.90±0.72
30–40 219.8±22.6 114.2±4.4 5.23±0.44
40–50 120.3±13.7 74.4±3.8 2.86±0.28
50–60 61.0±9.9 45.5±3.3 1.45±0.23
60–70 28.5±7.6 25.7±3.8 0.68±0.18
70–80 12.4±4.2 13.4±3.0 0.30±0.10
80–92 4.9±1.2 6.3±1.2 0.12±0.03
60–92 14.5±4 14.5±2.5 0.35±0.10
Minimum bias 257.8±25.4 109.1±4.1 6.14±0.45
HIGH-p
T
CHARGED HADRON SUPPRESSION IN Au+Au… PHYSICAL REVIEW C 69, 034910 (2004)
034910-5
