Transverse momentum dependence of eta meson suppression in Au+Au collisions at sqrt(s_NN) = 200 GeV

Abstract

New measurements by the PHENIX experiment at RHIC for eta production at
midrapidity as a function of transverse momentum (p_T) and collision centrality
in sqrt(s_NN) = 200 GeV Au+Au and p+p collisions are presented. They indicate
nuclear modification factors (R_AA) that are similar both in magnitude and
trend to those found in earlier pi^0 measurements. Linear fits to R_AA in the
5--20 GeV/c p_T region show that the slope is consistent with zero within two
standard deviations at all centralities although a slow rise cannot be
excluded. Having different statistical and systematic uncertainties the pi^0
and eta measurements are complementary at high p_T; thus, along with the
extended p_T range of these data they can provide additional constraints for
theoretical modeling and the extraction of transport properties.

Full text

Page 1

arXiv:1005.4916v1 [nucl-ex] 26 May 2010
Transverse momentum dependence of η meson suppression
in Au+Au collisions at√sNN= 200 GeV
A. Adare,12S. Afanasiev,27C. Aidala,40N.N. Ajitanand,57Y. Akiba,51,52H. Al-Bataineh,46J. Alexander,57
K. Aoki,33,51L. Aphecetche,59Y. Aramaki,11J. Asai,51E.T. Atomssa,34R. Averbeck,58T.C. Awes,47B. Azmoun,6
V. Babintsev,23M. Bai,5G. Baksay,19L. Baksay,19A. Baldisseri,15K.N. Barish,7P.D. Barnes,36B. Bassalleck,45
A.T. Basye,1S. Bathe,7S. Batsouli,47V. Baublis,50C. Baumann,41A. Bazilevsky,6S. Belikov,6, ∗R. Belmont,63
R. Bennett,58A. Berdnikov,54Y. Berdnikov,54A.A. Bickley,12J.G. Boissevain,36J.S. Bok,66H. Borel,15K. Boyle,58
M.L. Brooks,36H. Buesching,6V. Bumazhnov,23G. Bunce,6,52S. Butsyk,36C.M. Camacho,36S. Campbell,58
B.S. Chang,66W.C. Chang,2J.-L. Charvet,15C.-H. Chen,58S. Chernichenko,23C.Y. Chi,13M. Chiu,6, 24I.J. Choi,66
R.K. Choudhury,4P. Christiansen,38T. Chujo,62P. Chung,57A. Churyn,23O. Chvala,7V. Cianciolo,47Z. Citron,58
B.A. Cole,13M. Connors,58P. Constantin,36M. Csan´ ad,17T. Cs¨ org˝ o,30T. Dahms,58S. Dairaku,33,51I. Danchev,63
K. Das,20A. Datta,40G. David,6A. Denisov,23D. d’Enterria,34A. Deshpande,52,58E.J. Desmond,6O. Dietzsch,55
A. Dion,58M. Donadelli,55O. Drapier,34A. Drees,58K.A. Drees,5A.K. Dubey,65J.M. Durham,58A. Durum,23
D. Dutta,4V. Dzhordzhadze,7S. Edwards,20Y.V. Efremenko,47F. Ellinghaus,12T. Engelmore,13A. Enokizono,35
H. En’yo,51,52S. Esumi,62K.O. Eyser,7B. Fadem,42D.E. Fields,45, 52M. Finger,Jr.,8M. Finger,8F. Fleuret,34
S.L. Fokin,32Z. Fraenkel,65, ∗J.E. Frantz,58A. Franz,6A.D. Frawley,20K. Fujiwara,51Y. Fukao,33,51T. Fusayasu,44
I. Garishvili,60A. Glenn,12H. Gong,58M. Gonin,34J. Gosset,15Y. Goto,51,52R. Granier de Cassagnac,34
N. Grau,13S.V. Greene,63M. Grosse Perdekamp,24,52T. Gunji,11H.-˚ A. Gustafsson,38, ∗A. Hadj Henni,59
J.S. Haggerty,6K.I. Hahn,18H. Hamagaki,11J. Hamblen,60J. Hanks,13R. Han,49E.P. Hartouni,35K. Haruna,22
E. Haslum,38R. Hayano,11M. Heffner,35S. Hegyi,30T.K. Hemmick,58T. Hester,7X. He,21J.C. Hill,26
M. Hohlmann,19W. Holzmann,13,57K. Homma,22B. Hong,31T. Horaguchi,11,22,51,61D. Hornback,60S. Huang,63
T. Ichihara,51,52R. Ichimiya,51J. Ide,42H. Iinuma,33,51Y. Ikeda,62K. Imai,33,51J. Imrek,16M. Inaba,62
D. Isenhower,1M. Ishihara,51T. Isobe,11M. Issah,57,63A. Isupov,27D. Ivanischev,50B.V. Jacak,58, †J. Jia,6,13,57
J. Jin,13B.M. Johnson,6K.S. Joo,43D. Jouan,48D.S. Jumper,1F. Kajihara,11S. Kametani,51N. Kamihara,52
J. Kamin,58J.H. Kang,66J. Kapustinsky,36D. Kawall,40,52M. Kawashima,53,51A.V. Kazantsev,32T. Kempel,26
A. Khanzadeev,50K.M. Kijima,22J. Kikuchi,64B.I. Kim,31D.H. Kim,43D.J. Kim,28,66E.J. Kim,9E. Kim,56
S.H. Kim,66Y.J. Kim,24E. Kinney,12K. Kiriluk,12 ´A. Kiss,17E. Kistenev,6J. Klay,35C. Klein-Boesing,41
L. Kochenda,50B. Komkov,50M. Konno,62J. Koster,24D. Kotchetkov,45A. Kozlov,65A. Kr´ al,14A. Kravitz,13
G.J. Kunde,36K. Kurita,53,51M. Kurosawa,51M.J. Kweon,31Y. Kwon,60,66G.S. Kyle,46R. Lacey,57Y.S. Lai,13
J.G. Lajoie,26D. Layton,24A. Lebedev,26D.M. Lee,36J. Lee,18K.B. Lee,31K. Lee,56K.S. Lee,31T. Lee,56
M.J. Leitch,36M.A.L. Leite,55E. Leitner,63B. Lenzi,55P. Liebing,52L.A. Linden Levy,12T. Liˇ ska,14
A. Litvinenko,27H. Liu,36,46M.X. Liu,36X. Li,10B. Love,63R. Luechtenborg,41D. Lynch,6C.F. Maguire,63
Y.I. Makdisi,5A. Malakhov,27M.D. Malik,45V.I. Manko,32E. Mannel,13Y. Mao,49,51L. Maˇ sek,8,25H. Masui,62
F. Matathias,13M. McCumber,58P.L. McGaughey,36N. Means,58B. Meredith,24Y. Miake,62A.C. Mignerey,39
P. Mikeˇ s,8,25K. Miki,62A. Milov,6M. Mishra,3J.T. Mitchell,6A.K. Mohanty,4Y. Morino,11A. Morreale,7
D.P. Morrison,6T.V. Moukhanova,32D. Mukhopadhyay,63J. Murata,53,51S. Nagamiya,29J.L. Nagle,12
M. Naglis,65M.I. Nagy,17I. Nakagawa,51,52Y. Nakamiya,22T. Nakamura,22,29K. Nakano,51,61J. Newby,35
M. Nguyen,58T. Niita,62R. Nouicer,6A.S. Nyanin,32E. O’Brien,6S.X. Oda,11C.A. Ogilvie,26K. Okada,52
M. Oka,62Y. Onuki,51A. Oskarsson,38M. Ouchida,22K. Ozawa,11R. Pak,6A.P.T. Palounek,36V. Pantuev,58
V. Papavassiliou,46I.H. Park,18J. Park,56S.K. Park,31W.J. Park,31S.F. Pate,46H. Pei,26J.-C. Peng,24
H. Pereira,15V. Peresedov,27D.Yu. Peressounko,32C. Pinkenburg,6R.P. Pisani,6M. Proissl,58M.L. Purschke,6
A.K. Purwar,36H. Qu,21J. Rak,28,45A. Rakotozafindrabe,34I. Ravinovich,65K.F. Read,47,60S. Rembeczki,19
K. Reygers,41V. Riabov,50Y. Riabov,50E. Richardson,39D. Roach,63G. Roche,37S.D. Rolnick,7M. Rosati,26
C.A. Rosen,12S.S.E. Rosendahl,38P. Rosnet,37P. Rukoyatkin,27P. Ruˇ ziˇ cka,25V.L. Rykov,51B. Sahlmueller,41
N. Saito,29,33,51,52T. Sakaguchi,6S. Sakai,62K. Sakashita,51,61V. Samsonov,50S. Sano,11,64T. Sato,62
S. Sawada,29K. Sedgwick,7J. Seele,12R. Seidl,24A.Yu. Semenov,26V. Semenov,23R. Seto,7D. Sharma,65
I. Shein,23T.-A. Shibata,51,61K. Shigaki,22M. Shimomura,62K. Shoji,33,51P. Shukla,4A. Sickles,6C.L. Silva,55
D. Silvermyr,47C. Silvestre,15K.S. Sim,31B.K. Singh,3C.P. Singh,3V. Singh,3M. Sluneˇ cka,8A. Soldatov,23
R.A. Soltz,35W.E. Sondheim,36S.P. Sorensen,60I.V. Sourikova,6N.A. Sparks,1F. Staley,15P.W. Stankus,47
E. Stenlund,38M. Stepanov,46A. Ster,30S.P. Stoll,6T. Sugitate,22C. Suire,48A. Sukhanov,6J. Sziklai,30
E.M. Takagui,55A. Taketani,51,52R. Tanabe,62Y. Tanaka,44K. Tanida,33,51,52,56M.J. Tannenbaum,6S. Tarafdar,3
A. Taranenko,57P. Tarj´ an,16H. Themann,58T.L. Thomas,45M. Togawa,33,51A. Toia,58L. Tom´ aˇ sek,25Y. Tomita,62

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H. Torii,22,51R.S. Towell,1V-N. Tram,34I. Tserruya,65Y. Tsuchimoto,22C. Vale,6,26H. Valle,63H.W. van Hecke,36
E. Vazquez-Zambrano,13A. Veicht,24J. Velkovska,63R. V´ ertesi,16,30A.A. Vinogradov,32M. Virius,14V. Vrba,25
E. Vznuzdaev,50X.R. Wang,46D. Watanabe,22K. Watanabe,62Y. Watanabe,51,52F. Wei,26R. Wei,57
J. Wessels,41S.N. White,6D. Winter,13J.P. Wood,1C.L. Woody,6R.M. Wright,1M. Wysocki,12W. Xie,52
Y.L. Yamaguchi,11,64K. Yamaura,22R. Yang,24A. Yanovich,23J. Ying,21S. Yokkaichi,51,52G.R. Young,47
I. Younus,45Z. You,49I.E. Yushmanov,32W.A. Zajc,13O. Zaudtke,41C. Zhang,47S. Zhou,10and L. Zolin27
(PHENIX Collaboration)
1Abilene Christian University, Abilene, Texas 79699, USA
2Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
3Department of Physics, Banaras Hindu University, Varanasi 221005, India
4Bhabha Atomic Research Centre, Bombay 400 085, India
5Collider-Accelerator Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
6Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
7University of California - Riverside, Riverside, California 92521, USA
8Charles University, Ovocn´ y trh 5, Praha 1, 116 36, Prague, Czech Republic
9Chonbuk National University, Jeonju 561-756, Korea
10China Institute of Atomic Energy (CIAE), Beijing, People’s Republic of China
11Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
12University of Colorado, Boulder, Colorado 80309, USA
13Columbia University, New York, New York 10027 and Nevis Laboratories, Irvington, New York 10533, USA
14Czech Technical University, Zikova 4, 166 36 Prague 6, Czech Republic
15Dapnia, CEA Saclay, F-91191, Gif-sur-Yvette, France
16Debrecen University, H-4010 Debrecen, Egyetem t´ er 1, Hungary
17ELTE, E¨ otv¨ os Lor´ and University, H - 1117 Budapest, P´ azm´ any P. s. 1/A, Hungary
18Ewha Womans University, Seoul 120-750, Korea
19Florida Institute of Technology, Melbourne, Florida 32901, USA
20Florida State University, Tallahassee, Florida 32306, USA
21Georgia State University, Atlanta, Georgia 30303, USA
22Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan
23IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, 142281, Russia
24University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
25Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
26Iowa State University, Ames, Iowa 50011, USA
27Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia
28Helsinki Institute of Physics and University of Jyv¨ askyl¨ a, P.O.Box 35, FI-40014 Jyv¨ askyl¨ a, Finland
29KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
30KFKI Research Institute for Particle and Nuclear Physics of the Hungarian Academy
of Sciences (MTA KFKI RMKI), H-1525 Budapest 114, POBox 49, Budapest, Hungary
31Korea University, Seoul 136-701, Korea
32Russian Research Center “Kurchatov Institute”, Moscow, Russia
33Kyoto University, Kyoto 606-8502, Japan
34Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128, Palaiseau, France
35Lawrence Livermore National Laboratory, Livermore, California 94550, USA
36Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
37LPC, Universit´ e Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France
38Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
39University of Maryland, College Park, Maryland 20742, USA
40Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003-9337, USA
41Institut fur Kernphysik, University of Muenster, D-48149 Muenster, Germany
42Muhlenberg College, Allentown, Pennsylvania 18104-5586, USA
43Myongji University, Yongin, Kyonggido 449-728, Korea
44Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan
45University of New Mexico, Albuquerque, New Mexico 87131, USA
46New Mexico State University, Las Cruces, New Mexico 88003, USA
47Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
48IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406, Orsay, France
49Peking University, Beijing, People’s Republic of China
50PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad region, 188300, Russia
51RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, JAPAN
52RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
53Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan
54Saint Petersburg State Polytechnic University, St. Petersburg, Russia

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55Universidade de S˜ ao Paulo, Instituto de F´ ısica, Caixa Postal 66318, S˜ ao Paulo CEP05315-970, Brazil
56Seoul National University, Seoul 151-742, Korea
57Chemistry Department, Stony Brook University, Stony Brook, SUNY, New York 11794-3400, USA
58Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, New York 11794, USA
59SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universit´ e de Nantes) BP 20722 - 44307, Nantes, France
60University of Tennessee, Knoxville, Tennessee 37996, USA
61Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan
62Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
63Vanderbilt University, Nashville, Tennessee 37235, USA
64Waseda University, Advanced Research Institute for Science and
Engineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan
65Weizmann Institute, Rehovot 76100, Israel
66Yonsei University, IPAP, Seoul 120-749, Korea
(Dated: May 27, 2010)
New measurements by the PHENIX experiment at RHIC for η production at midrapidity as a
function of transverse momentum (pT) and collision centrality in√sNN = 200 GeV Au+Au and
p + p collisions are presented. They indicate nuclear modification factors (RAA) which are similar
both in magnitude and trend to those found in earlier π0measurements. Linear fits to RAA as
a function of pT in 5–20 GeV/c show that the slope is consistent with zero within two standard
deviations at all centralities although a slow rise cannot be excluded. Having different statistical and
systematic uncertainties, the π0and η measurements are complementary at high pT; thus, along
with the extended pT range of these data they can provide additional constraints for theoretical
modeling and the extraction of transport properties.
PACS numbers: 25.75.Dw, 13.85.Qk, 13.20.Fc, 13.20.He
Suppression of high pT hadron production in Au+Au
collisions at RHIC [1, 2] and its absence in d+Au col-
lisions [3] provided the first direct evidence that an ex-
tremely dense medium is formed in heavy ion collisions
at RHIC energies. This suppression relative to the yield
expected from the convolution of independent nucleon-
nucleon scatterings, measured by the nuclear modifica-
tion factor RAA, is now confirmed up to
with identified π0and attributed to the energy loss of
the hard scattered partons in the dense medium. Sev-
eral models with very different assumptions describe the
magnitude of the observed π0suppression, but predict
slightly different evolution with increasing pT. Calcu-
lations based on perturbative QCD (pQCD) and static
plasma predict that the fractional parton energy loss de-
creases with pT like log(pT)/pT leading to a slow rise of
the RAAwith pT(for a recent review see [4]). In contrast,
some AdS/CFT calculations find that the fractional en-
ergy loss is proportional to pT. Therefore, RAAdecreases
with increasing transverse momentum [5–8]. The univer-
sal upper bound model [9] predicts that RAA remains
almost independent of the energy of the original gluon
or quark. Other effects (modified nuclear parton distri-
bution functions, Cronin-effect, modified fragmentation
functions, the quark/gluon ratio) at given xT (2pT/√s)
can also dependence of RAA, and it is clear change the pT
dependence of RAA, and it is clear that a precise mea-
surement of the evolution of RAA with pT would help
20 GeV/c
∗Deceased
†PHENIX Spokesperson: jacak@skipper.physics.sunysb.edu
in confirming or rejecting classes of theories and putting
tight constraints on the free parameters of the remaining
ones. The first rigorous attempt to confront the observed
π0suppression with various pQCD-based parton energy
loss calculations and to put quantitative constraints on
the transport properties of the medium was made in [10]
using PHENIX π0data. One intriguing result was that a
linear fit with a slope consistent with zero described the
evolution of RAAwith pT slightly better than any of the
pQCD models predicting a slow rise. However, the large
statistical and systematic uncertainties of the high pT π0
points prevented a clear distinction between constant or
slowly rising RAA.
One reason the π0data [2] allow such ambiguous in-
terpretations is that the experimental uncertainties rise
rapidly as we move to higher pT (>12–14 GeV/c), due
to “shower merging,” as explained below. In the case of
the η this problem is absent for pT up to 50 GeV/c, sig-
nificantly beyond the pT range expected to be accessible
at RHIC. While the yield of the actually reconstructed η
mesons is smaller except at the highest pT, the improve-
ment in systematic uncertainties can help provide better
constraints in comparisons to theory at high pT and thus
complement the π0results. Of course some caution in in-
terpreting the results is warranted: while both π0and η
consist of light quarks, η does have a hidden strangeness
(s¯ s) content so it is not a priori obvious that the π0and
η results are interchangeable. Earlier measurements [11]
have shown that at least up to 12 GeV/c, the π0and
η nuclear modification factors in Au+Au agree within
uncertainties and the η/π0ratio is constant for pT≥4
GeV/c in p+p [11]. Using recent, more precise measure-
ments in PHENIX, we will re-examine whether π0and

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0
500
1000
1500
2000
2500
3000
<10 GeV/c
T
9<p
0.4 0.50.60.7
0
100
200
0
10
20
30
40
50
60
<18 GeV/c
T
16<p
0 0.1 0.20.3
invariant mass (GeV/c
0.40.5 0.60.7 0.8
)
2
γ γ
-1
)
2
Counts (20 MeV/c
FIG. 1: (Color online) γγ invariant mass distribution for two
different pair pT bins (minimum bias data). Top: 9<pT<10
GeV/c, the combinatorial background has been subtracted
by using mixed events. Note the large difference between π0
and η raw yields. Insert: the η region magnified. Bottom:
16<pT<18 GeV/c region, where mixed event subtraction is
no longer necessary. Also, here a cut on the γ-pair energy
asymmetry, α < 0.6 has been applied, which greatly improves
the signal/background ratio at the η peak but cuts into the
lower part of the π0peak due to cluster merging.
η production at midrapidity is indeed similar and study
the asymptotic behavior of RAA.
This analysis used 3.25B minimum bias (MB)√sNN=
200 GeV Au+Au events, corresponding to 0.511 nb−1
recorded in 2007 as well as 429M minimum bias (18.7
nb−1) and 2.06B triggered (6.90 pb−1)√s = 200 GeV
p+p events recorded in 2006 in the PHENIX experiment
at RHIC. Both the Au+Au and p+p data sets were ana-
lyzed using the same analysis chain and cuts; thus, some
of the systematic uncertainties cancel when we calculate
the nuclear modification factor RAAfor Au+Au.
Collision centrality in Au+Au has been established by
the beam-beam counters [13] (BBC, 3.0 < |η| < 3.9). A
Glauber-model Monte Carlo along with a simulation of
the BBC response was used to estimate the average num-
ber of participating nucleons (Npart) and binary nucleon-
nucleon collisions (Ncoll) for each centrality bin [12].
The η mesons were measured via their η → γγ decay
channel. The photons were reconstructed in the lead-
scintillator (PbSc) sectors of the PHENIX Electromag-
netic Calorimeter (EMCal) [14] covering 3/8 of the full
azimuth and −0.35 < η < 0.35 in pseudorapidity, and the
η yield was extracted from two-photon invariant mass dis-
tributions. This analysis is similar to the one described
in [11, 15]. There are three important differences. In
the case of π0starting around pT=12 GeV/c the min-
imum opening angle of the two decay photons is small
enough for the photon showers to merge and become in-
distinguishable. As pT increases, this effect leads to an
increasing loss of observed π0, resulting in large correc-
tions and corresponding systematic uncertainties (which
are in fact the dominant systematic uncertainties at high
pT). Since the mass of the η is about four times larger
TABLE I: Typical systematic uncertainties on η spectra and
RAA. See text for explanation of error types.
Source
raw yield
Type
B
B
B
A
B
C
C
C
C
C
Au+Au
7%
1.5%
3%
3%
8%
1.3%
5%
N/A
N/A
N/A
p+p
3%
1.5%
3%
3%
8%
N/A
5%
9.7%
3.8%
6.2%
RAA
6.3%
2.1%
3%
4.2%
11.3%
1.3%
N/A
9.7%
3.8%
6.2%
acceptance variations
photon PID
acceptance×efficiency
energy scale
conversion (HBD)
conversion (other)
BBC cross section
BBC efficiency
ERT norm.
than the π0, this is not a problem for the η measurement
up to pT∼50 GeV/c. On the other hand the observable
η rates are much lower at low and medium pT, as seen
in the invariant mass distributions in Fig. 1, because of
the smaller branching ratio into two photons (39%) and
the small η/π0≈0.5 production ratio. The raw yields be-
come comparable only around 20
the η analysis we applied an α < 0.6 photon pair energy
asymmetry cut (as opposed to α < 0.8 for π0) in order
to improve the signal/background ratio in the η region.
GeV/c. Finally, in
TABLE II: Parameters of the power-law fits A/pn
and p+p .The errors used for fit are the statistical and
pT-uncorrelated (Type A) systematic uncertainties added in
quadrature. The pT range of the fits is 5–22 GeV/c.
Tfor Au+Au
System/Cent.
Au+Au 0–5%
Au+Au 0–10%
Au+Au 10–20%
Au+Au 0–20%
Au+Au 20–40%
Au+Au 40–60%
Au+Au 20–60%
Au+Au 60–92%
Au+Au MinBias
p+p
Anχ2/NDF
3.1/7
10.6/8
10.2/9
10.5/7
17.2/8
5.5/8
11.2/9
2.98/6
9.41/9
8.33/9
27.2±11.9
17.6±5.5
19.1±5.9
18.5±4.3
17.3±4.2
9.53±2.65
14.5±2.5
1.13±0.40
10.4±1.4
8.84±0.99
7.90±0.22
7.77±0.15
7.89±0.16
7.84±0.12
8.01±0.12
8.05±0.15
8.07±0.08
7.78±0.18
8.04±0.08
8.21±0.05
The raw η yield is always counted by integrating the
histogram bin content in the η mass window (typically
±30 MeV/c2), but the way we treat the underlying com-
binatorial background varies as a function of pT.
Au+Au up to 10 GeV/c, mixed event subtraction is
used. The η region is then fitted with a polynomial and
Gaussian (see insert in Fig. 1) to estimate the residual
background. When the signal/background ratio reaches
1.0, already in the 7–10 GeV/c range, depending on
centrality, mixed event subtraction is no longer needed;
a polynomial and Gaussian fit is used on the original in-
In

Page 5

5
(GeV/c)
T
p
46810 1214 16182022
)
3
c
-2
(mb GeV
σ
3
d
) or E
3
c
-2
(GeV
N
3
d
E
3
dp
3
dp
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
1
10
2
10
3
10
4
10
7
Minimum Bias x10
5
0-5% x10
0-20% x10
20-60% x10
60-92% x10
p+p
4
3
2
= 200 GeV
NN
s spectra in p+p and Au+Au
T
p
η
PHENIX
FIG. 2: (Color online) Cross section of p + p → η + X from
the 2006 p+p data set (solid circles) and η invariant yield in
Au+Au collisions of various centralities (open symbols) and
minimum bias (solid squares) from the 2007 data set. p+p
is shown at the true pT value, all other spectra are shifted
alternately by ±0.1 GeV/c for better visibility of the error
bars and upper limits.
variant mass distribution to estimate the background. At
even higher pT (12–16 GeV/c) we estimate the residual
background under the peak simply from the average bin
content of the sidebands (the regions above and below
the peak).
Systematic uncertainties are classified into three types:
Type A is pT-uncorrelated (“point-by-point”) and for the
purposes of fitting and plotting, is added in quadrature
to the statistical errors. Type C is the overall normaliza-
tion uncertainty allowing all points to move by the same
fraction up or down. Type B is all other pT-correlated
uncertainties (including the cases where the shape of the
correlation function is not known). Table I lists typi-
cal uncertainties on the spectra and RAA. “Conversion
(HBD)” stands for loss due to photon conversion in the
Hadron Blind Detector, which was present in one of the
two central arms during the 2007 (Au+Au) data taking.
“ERT norm.” stands for the normalization uncertainty of
the EMCal-RICH Trigger, selecting high pT photons and
electrons. “Acceptance variations” are small day-by-day
changes of dead areas in the detector and thus are inde-
pendent for the p+p and Au+Au runs. The systematic
uncertainties on raw yield, photon PID and conversion
(other) are common in p+p and Au+Au, and hence were
partially cancelled out in the RAAcalculation.
0
0.2
0.4
0.6
0.8
1
1.2
1
1.4
1.6
Au+Au 0-5%
= 200 GeV
NN
s
PHENIX
0
0.2
0.4
0.6
0.8
1
1.2
1
1.4
1.6
Au+Au 0-20%
= 200 GeV
NN
s
PHENIX
0
0.2
0.4
0.6
0.8
1
1.2
1
1.4
1.6
Au+Au 20-60%
= 200 GeV
NN
s
PHENIX
0
0.2
0.4
0.6
0.8
1
1.2
1
1.4
1.6
Au+Au 60-92%
= 200 GeV
NN
s
PHENIX
468 10 12
p
1416182022
(GeV/c)
T
0.2
0.4
0.6
0.8
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1.2
1.4
1.6
AA
R
η
FIG. 3:
at various centralities, calculated using the measured p+p
points. Dark (green) band around 1 indicates the absolute
normalization error from p+p, light (grey) band is the (cen-
trality dependent) absolute normalization error from Au+Au.
Error bars include statistical and pT-uncorrelated systematic
errors. Also shown: linear fits to the data with 1σ error bands.
(Color online) Nuclear modification factor for η
Cross sections for p+p → η + X and invariant yield
of inclusive η production in Au+Au collisions for dif-
ferent centralities are shown in Fig. 2. They cover the
5 < pT < 22 GeV/c range and five orders of magnitude
in cross section (invariant yield). The overall normal-
ization uncertainties (Type C) are 13% for p+p and 5%
for Au+Au. Parameters of simple power-law fits (A/pn
to various, partially overlapping centrality selections, in-
cluding ones not shown in Fig. 2, are given in Table II.
Fits include all available points in the 5 <pT< 22 GeV/c
range but exclude upper limits. Only statistical and pT-
uncorrelated uncertainties were used in the fits. Note
that for π0in Au+Au collisions the power n was con-
sistent within uncertainties at all centralities [2] ranging
from 8.00±0.12 in 0–5% to 8.06±0.08 in 80–92%, and
for π0in p+p the power n was 8.22±0.09. In this mea-
surement we find that for η production p + p → η + X
the power n is the same as it was for π0. The powers
obtained for η in Au+Au are also consistent with those
from π0within two standard deviations.
T)

Page 6

6
(GeV/c)
T
p
02468 101214 1618 2022

AA
R
η
or
AA
R
0
π
0
0.2
0.4
0.6
0.8
1
1.2
, PRL 101, 232301(2008)
0
π
η
= 200 GeV
NN
s Au+Au Minimum Bias
PHENIX
FIG. 4: (Color online) Nuclear modification factor RAA for
π0(open squares, points shifted for clarity, data from [2])
and η (solid circles, this analysis) in MB Au+Au collisions.
Error bars include statistical and pT-uncorrelated systematic
errors, bands show pT-correlated systematic errors. The pair
of bands at RAA=1 are the absolute normalization error for
p+p (larger, dark) and Au+Au (lighter) for π0(left) and η
(right).
The nuclear modification factor RAAis defined as
RAA=
1/NevtdN/dydpT
?TAB?dσpp/dydpT
where σpp is the production cross section of the parti-
cle in p+p collisions, and ?TAB? is the nuclear thick-
ness function averaged over a range of impact parame-
ters for the given centrality, calculated within a Glauber
model [16]. When calculating RAA, the measured p+p
points are used. RAAfor η production is shown in Fig. 3
for four centralities, along with linear fits to RAA. Fit
parameters are listed in Table III. In the measured pT
range we observe strong suppression in all but the most
peripheral collisions. As shown in Fig. 4, for the mini-
mum bias case the suppression is quite comparable to the
one observed for π0, and above 13 GeV/c the (relative)
systematic errors are smaller.
TABLE III: Parameters from linear function fit to η RAA.
CentralityNpart
Slopeχ2/NDF
0–5%
0–10%
10–20%
0–20%
20–40%
40–60%
20–60%
60–92%
MinBias
351
326
236
280
142
61.6
102
11.8
109
0.008±0.008
0.011±0.007
0.010+0.009
−0.008
0.010+0.007
−0.006
0.004±0.010
0.010+0.018
−0.017
0.005±0.011
0.056+0.043
−0.038
0.006±0.007
2.77/7
9.79/7
11.7/8
10.8/7
15.7/8
4.64/7
11.7/8
1.52/6
10.1/8
Based upon the most central (0–5%) collisions in [10]

-1
Slope (GeV/c)
-0.01
0
0.01
0.02
0.03
0.04
0
π
=351)
p
(N
)
AA
Intercept (=R
0.2
0.4
0.6
0.8
1
1.2
At 20GeV/c
At 5GeV/c
0 50100 150 200250300350
part
N
FIG. 5: (Color online) Top: slopes of the linear fits (like the
ones shown in Fig. 3) along with the fitting errors. Centrality
is shown in terms of participating nucleons Npart. Open sym-
bols are overlapping, solid symbols are non-overlapping cen-
trality bins (0–10%, 10–20%, 20–40%, 40–60% and 60–92%).
Also shown: slope of the linear fit to 0–5% π0data [10], shifted
for better visibility. Bottom: value of RAA calculated from
the fit at 5 GeV/c (blue) and 20 GeV/c (red).
we found that the π0RAA is consistent with a com-
pletely flat pT dependence when fitted in the 5 < pT <
18 GeV/c region, namely the slope of a linear fit was
m = 0.0017+0.0035
−0.0039c/GeV. Fitting the current η RAA
data with straight lines gives the slopes and uncertain-
ties listed in Table III and shown in Fig. 5 where central-
ity is expressed in terms of participating nucleons Npart.
All slopes are consistent with zero; the largest deviation

-1
Slope (GeV/c)
-0.010 0.010.02 0.03
Intercept at 20 GeV/c
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
)
σ
Contour (Lines show 1 and 2
2
χ
0-5%
0-20%
20-60%
FIG. 6: (Color online) One and two standard deviation χ2
contours of the linear fits to RAA in Au+Au collisions for
0–5%, 0–20% and 20–60% centralities.

Page 7

7
is less than 2σ (for the 0–20% centrality bin). One and
two standard deviation χ2contours for selected centrality
bins are shown in Fig. 6. For 0–5% centrality we repeated
the linear fits using only the first 3,4,...,(n − 1) points
and found that the slope already stabilizes around its fi-
nal value with the first few points; data above 10 GeV/c
improve the significance but barely change the central
value itself. The same is true for other centralities.
While the above result indicates that RAAfor η is con-
sistent with a pT-independent, constant value, and disfa-
vors a decreasing RAA, a slow rise (∼0.01c/GeV) of RAA
with increasing pTcannot be excluded. In fact, a detailed
statistical analysis, comparing to various theories like the
study done for π0in [10] is necessary once theoretical
calculations of η production are available. However, as-
suming the linear dependence we can calculate the RAA
values at 5 GeV/c (where the suppression is already at
its maximum) and 20 GeV/c; the results are shown in
the bottom panel of Fig. 5.
In summary, we measured invariant yields of η in
√sNN= 200 GeV Au+Au collisions at various centrali-
ties, as well as the η production cross section in√s = 200
GeV p+p collisions in the 5 <pT< 22
verse momentum range using the PbSc calorimeter of the
PHENIX experiment at RHIC. The nuclear modification
factor for η in minimum bias collisions is consistent with
earlier π0results. In conclusion, linear fits to RAAas a
function of pT indicate that RAAis consistent with con-
stant at all centralities, although a slow rise cannot be
excluded.
We thank the staff of the Collider-Accelerator and
Physics Departments at BNL for their vital contribu-
tions. We acknowledge support from the Office of Nu-
clear Physics in DOE Office of Science, NSF, and a
sponsored research grant from Renaissance Technolo-
gies (USA), MEXT and JSPS (Japan), CNPq and
FAPESP (Brazil), NSFC (China), MSMT (Czech Repub-
lic), IN2P3/CNRS and CEA (France), BMBF, DAAD,
and AvH (Germany), OTKA (Hungary), DAE and DST
(India), ISF (Israel), NRF (Korea), MES, RAS, and
FAAE (Russia), VR and KAW (Sweden), U.S. CRDF
for the FSU, US-Hungary Fulbright, and US-Israel BSF.
GeV/c trans-
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