Transverse momentum and centrality dependence of dihadron correlations in Au+Au collisions at √sNN=200 GeV: Jet quenching and the response of partonic matter

Abstract

Azimuthal angle (Δϕ) correlations are presented for charged hadrons from dijets for 0.4<pT<10 GeV/c in Au+Au collisions at √sNN=200 GeV. With increasing pT, the away-side distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side can be divided into a partially suppressed “head” region centered at Δϕ~π and an enhanced “shoulder” region centered at Δϕ~π±1.1. The pT spectrum for the head region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the shoulder region is independent of centrality and trigger pT, which offers constraints on energy transport mechanisms and suggests that it contains the medium response to energetic jets.

Full text

arXiv:0705.3238v1 [nucl-ex] 22 May 2007
Transverse momentum and centrality dependence of dihadron correlations in Au+Au
collisions at √sNN = 200 GeV: Jet-quenching and the response of partonic matter
A. Adare,8S. Afanasiev,22 C. Aidala,9N.N. Ajitanand,49 Y. Akiba,43, 44 H. Al-Bataineh,38 J. Alexander,49
A. Al-Jamel,38 K. Aoki,28, 43 L. Aphecetche,51 R. Armendariz,38 S.H. Aronson,3J. Asai,44 E.T. Atomssa,29
R. Averbeck,50 T.C. Awes,39 B. Azmoun,3V. Babintsev,18 G. Baksay,14 L. Baksay,14 A. Baldisseri,11 K.N. Barish,4
P.D. Barnes,31 B. Bassalleck,37 S. Bathe,4S. Batsouli,9, 39 V. Baublis,42 F. Bauer,4A. Bazilevsky,3S. Belikov,3, 21
R. Bennett,50 Y. Berdnikov,46 A.A. Bickley,8M.T. Bjorndal,9J.G. Boissevain,31 H. Borel,11 K. Boyle,50
M.L. Brooks,31 D.S. Brown,38 D. Bucher,34 H. Buesching,3V. Bumazhnov,18 G. Bunce,3, 44 J.M. Burward-Hoy,31
S. Butsyk,31, 50 S. Campbell,50 J.-S. Chai,23 B.S. Chang,58 J.-L. Charvet,11 S. Chernichenko,18 J. Chiba,24
C.Y. Chi,9M. Chiu,9, 19 I.J. Choi,58 T. Chujo,55 P. Chung,49 A. Churyn,18 V. Cianciolo,39 C.R. Cleven,16
Y. Cobigo,11 B.A. Cole,9M.P. Comets,40 P. Constantin,21,31 M. Csan´ad,13 T. Cs¨org˝o,25 T. Dahms,50 K. Das,15
G. David,3M.B. Deaton,1K. Dehmelt,14 H. Delagrange,51 A. Denisov,18 D. d’Enterria,9A. Deshpande,44,50
E.J. Desmond,3O. Dietzsch,47 A. Dion,50 M. Donadelli,47 J.L. Drachenberg,1O. Drapier,29 A. Drees,50
A.K. Dubey,57 A. Durum,18 V. Dzhordzhadze,4, 52 Y.V. Efremenko,39 J. Egdemir,50 F. Ellinghaus,8W.S. Emam,4
A. Enokizono,17, 30 H. En’yo,43, 44 B. Espagnon,40 S. Esumi,54 K.O. Eyser,4D.E. Fields,37, 44 M. Finger,5, 22
F. Fleuret,29 S.L. Fokin,27 B. Forestier,32 Z. Fraenkel,57 J.E. Frantz,9, 50 A. Franz,3A.D. Frawley,15 K. Fujiwara,43
Y. Fukao,28, 43 S.-Y. Fung,4T. Fusayasu,36 S. Gadrat,32 I. Garishvili,52 F. Gastineau,51 M. Germain,51 A. Glenn,8, 52
H. Gong,50 M. Gonin,29 J. Gosset,11 Y. Goto,43, 44 R. Granier de Cassagnac,29 N. Grau,21 S.V. Greene,55
M. Grosse Perdekamp,19, 44 T. Gunji,7H.-˚
A. Gustafsson,33 T. Hachiya,17, 43 A. Hadj Henni,51 C. Haegemann,37
J.S. Haggerty,3M.N. Hagiwara,1H. Hamagaki,7R. Han,41 H. Harada,17 E.P. Hartouni,30 K. Haruna,17 M. Harvey,3
E. Haslum,33 K. Hasuko,43 R. Hayano,7M. Heffner,30 T.K. Hemmick,50 T. Hester,4J.M. Heuser,43 X. He,16
H. Hiejima,19 J.C. Hill,21 R. Hobbs,37 M. Hohlmann,14 M. Holmes,55 W. Holzmann,49 K. Homma,17 B. Hong,26
T. Horaguchi,43, 53 D. Hornback,52 M.G. Hur,23 T. Ichihara,43,44 K. Imai,28, 43 M. Inaba,54 Y. Inoue,45, 43
D. Isenhower,1L. Isenhower,1M. Ishihara,43 T. Isobe,7M. Issah,49 A. Isupov,22 B.V. Jacak,50, ∗J. Jia,9J. Jin,9
O. Jinnouchi,44 B.M. Johnson,3K.S. Joo,35 D. Jouan,40 F. Kajihara,7, 43 S. Kametani,7, 56 N. Kamihara,43, 53
J. Kamin,50 M. Kaneta,44 J.H. Kang,58 H. Kanou,43, 53 T. Kawagishi,54 D. Kawall,44 A.V. Kazantsev,27
S. Kelly,8A. Khanzadeev,42 J. Kikuchi,56 D.H. Kim,35 D.J. Kim,58 E. Kim,48 Y.-S. Kim,23 E. Kinney,8
A. Kiss,13 E. Kistenev,3A. Kiyomichi,43 J. Klay,30 C. Klein-Boesing,34 L. Kochenda,42 V. Kochetkov,18
B. Komkov,42 M. Konno,54 D. Kotchetkov,4A. Kozlov,57 A. Kr´al,10 A. Kravitz,9P.J. Kroon,3J. Kubart,5, 20
G.J. Kunde,31 N. Kurihara,7K. Kurita,45, 43 M.J. Kweon,26 Y. Kwon,52, 58 G.S. Kyle,38 R. Lacey,49 Y.-S. Lai,9
J.G. Lajoie,21 A. Lebedev,21 Y. Le Bornec,40 S. Leckey,50 D.M. Lee,31 M.K. Lee,58 T. Lee,48 M.J. Leitch,31
M.A.L. Leite,47 B. Lenzi,47 H. Lim,48 T. Liˇska,10 A. Litvinenko,22 M.X. Liu,31 X. Li,6X.H. Li,4B. Love,55
D. Lynch,3C.F. Maguire,55 Y.I. Makdisi,3A. Malakhov,22 M.D. Malik,37 V.I. Manko,27 Y. Mao,41, 43 L. Maˇsek,5, 20
H. Masui,54 F. Matathias,9, 50 M.C. McCain,19 M. McCumber,50 P.L. McGaughey,31 Y. Miake,54 P. Mikeˇs,5, 20
K. Miki,54 T.E. Miller,55 A. Milov,50 S. Mioduszewski,3G.C. Mishra,16 M. Mishra,2J.T. Mitchell,3M. Mitrovski,49
A. Morreale,4D.P. Morrison,3J.M. Moss,31 T.V. Moukhanova,27 D. Mukhopadhyay,55 J. Murata,45, 43
S. Nagamiya,24 Y. Nagata,54 J.L. Nagle,8M. Naglis,57 I. Nakagawa,43,44 Y. Nakamiya,17 T. Nakamura,17
K. Nakano,43, 53 J. Newby,30 M. Nguyen,50 B.E. Norman,31 A.S. Nyanin,27 J. Nystrand,33 E. O’Brien,3S.X. Oda,7
C.A. Ogilvie,21 H. Ohnishi,43 I.D. Ojha,55 H. Okada,28,43 K. Okada,44 M. Oka,54 O.O. Omiwade,1A. Oskarsson,33
I. Otterlund,33 M. Ouchida,17 K. Ozawa,7R. Pak,3D. Pal,55 A.P.T. Palounek,31 V. Pantuev,50 V. Papavassiliou,38
J. Park,48 W.J. Park,26 S.F. Pate,38 H. Pei,21 J.-C. Peng,19 H. Pereira,11 V. Peresedov,22 D.Yu. Peressounko,27
C. Pinkenburg,3R.P. Pisani,3M.L. Purschke,3A.K. Purwar,31,50 H. Qu,16 J. Rak,21, 37 A. Rakotozafindrabe,29
I. Ravinovich,57 K.F. Read,39,52 S. Rembeczki,14 M. Reuter,50 K. Reygers,34 V. Riabov,42 Y. Riabov,42
G. Roche,32 A. Romana,29, †M. Rosati,21 S.S.E. Rosendahl,33 P. Rosnet,32 P. Rukoyatkin,22 V.L. Rykov,43
S.S. Ryu,58 B. Sahlmueller,34 N. Saito,28, 43, 44 T. Sakaguchi,3, 7, 56 S. Sakai,54 H. Sakata,17 V. Samsonov,42
H.D. Sato,28, 43 S. Sato,3, 24, 54 S. Sawada,24 J. Seele,8R. Seidl,19 V. Semenov,18 R. Seto,4D. Sharma,57 T.K. Shea,3
I. Shein,18 A. Shevel,42, 49 T.-A. Shibata,43, 53 K. Shigaki,17 M. Shimomura,54 T. Shohjoh,54 K. Shoji,28, 43
A. Sickles,50 C.L. Silva,47 D. Silvermyr,39 C. Silvestre,11 K.S. Sim,26 C.P. Singh,2V. Singh,2S. Skutnik,21
M. Sluneˇcka,5, 22 W.C. Smith,1A. Soldatov,18 R.A. Soltz,30 W.E. Sondheim,31 S.P. Sorensen,52 I.V. Sourikova,3
F. Staley,11 P.W. Stankus,39 E. Stenlund,33 M. Stepanov,38 A. Ster,25 S.P. Stoll,3T. Sugitate,17 C. Suire,40

2
J.P. Sullivan,31 J. Sziklai,25 T. Tabaru,44 S. Takagi,54 E.M. Takagui,47 A. Taketani,43, 44 K.H. Tanaka,24
Y. Tanaka,36 K. Tanida,43, 44 M.J. Tannenbaum,3A. Taranenko,49 P. Tarj´an,12 T.L. Thomas,37 M. Togawa,28, 43
A. Toia,50 J. Tojo,43 L. Tom´aˇsek,20 H. Torii,43 R.S. Towell,1V-N. Tram,29 I. Tserruya,57 Y. Tsuchimoto,17,43
S.K. Tuli,2H. Tydesj¨o,33 N. Tyurin,18 C. Vale,21 H. Valle,55 H.W. van Hecke,31 J. Velkovska,55 R. Vertesi,12
A.A. Vinogradov,27 M. Virius,10 V. Vrba,20 E. Vznuzdaev,42 M. Wagner,28, 43 D. Walker,50 X.R. Wang,38
Y. Watanabe,43, 44 J. Wessels,34 S.N. White,3N. Willis,40 D. Winter,9C.L. Woody,3M. Wysocki,8W. Xie,4, 44
Y. Yamaguchi,56 A. Yanovich,18 Z. Yasin,4J. Ying,16 S. Yokkaichi,43, 44 G.R. Young,39 I. Younus,37
I.E. Yushmanov,27 W.A. Zajc,9O. Zaudtke,34 C. Zhang,9, 39 S. Zhou,6J. Zim´anyi,25, †and L. Zolin22
(PHENIX Collaboration)
1Abilene Christian University, Abilene, TX 79699, U.S.
2Department of Physics, Banaras Hindu University, Varanasi 221005, India
3Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.
4University of California - Riverside, Riverside, CA 92521, U.S.
5Charles University, Ovocn´y trh 5, Praha 1, 116 36, Prague, Czech Republic
6China Institute of Atomic Energy (CIAE), Beijing, People’s Republic of China
7Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
8University of Colorado, Boulder, CO 80309, U.S.
9Columbia University, New York, NY 10027 and Nevis Laboratories, Irvington, NY 10533, U.S.
10Czech Technical University, Zikova 4, 166 36 Prague 6, Czech Republic
11Dapnia, CEA Saclay, F-91191, Gif-sur-Yvette, France
12Debrecen University, H-4010 Debrecen, Egyetem t´er 1, Hungary
13ELTE, E¨otv¨os Lor´and University, H - 1117 Budapest, P´azm´any P. s. 1/A, Hungary
14Florida Institute of Technology, Melbourne, FL 32901, U.S.
15Florida State University, Tallahassee, FL 32306, U.S.
16Georgia State University, Atlanta, GA 30303, U.S.
17Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan
18IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, 142281, Russia
19University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.
20Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
21Iowa State University, Ames, IA 50011, U.S.
22Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia
23KAERI, Cyclotron Application Laboratory, Seoul, South Korea
24KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
25KFKI Research Institute for Particle and Nuclear Physics of the Hungarian Academy
of Sciences (MTA KFKI RMKI), H-1525 Budapest 114, POBox 49, Budapest, Hungary
26Korea University, Seoul, 136-701, Korea
27Russian Research Center “Kurchatov Institute”, Moscow, Russia
28Kyoto University, Kyoto 606-8502, Japan
29Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128, Palaiseau, France
30Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.
31Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.
32LPC, Universit´e Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France
33Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
34Institut f¨ur Kernphysik, University of Muenster, D-48149 Muenster, Germany
35Myongji University, Yongin, Kyonggido 449-728, Korea
36Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan
37University of New Mexico, Albuquerque, NM 87131, U.S.
38New Mexico State University, Las Cruces, NM 88003, U.S.
39Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.
40IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406, Orsay, France
41Peking University, Beijing, People’s Republic of China
42PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad region, 188300, Russia
43RIKEN, The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan
44RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.
45Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan
46Saint Petersburg State Polytechnic University, St. Petersburg, Russia
47Universidade de S˜ao Paulo, Instituto de F´ısica, Caixa Postal 66318, S˜ao Paulo CEP05315-970, Brazil
48System Electronics Laboratory, Seoul National University, Seoul, South Korea
49Chemistry Department, Stony Brook University, Stony Brook, SUNY, NY 11794-3400, U.S.
50Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, NY 11794, U.S.
51SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universit´e de Nantes) BP 20722 - 44307, Nantes, France

3
52University of Tennessee, Knoxville, TN 37996, U.S.
53Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan
54Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Vanderbilt University, Nashville, TN 37235, U.S.
56Waseda University, Advanced Research Institute for Science and
Engineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan
57Weizmann Institute, Rehovot 76100, Israel
58Yonsei University, IPAP, Seoul 120-749, Korea
(Dated: February 1, 2008)
Azimuthal angle (∆φ) correlations are presented for charged hadrons from dijets for 0.4< pT<
10 GeV/cin Au+Au collisions at √sNN = 200 GeV. With increasing pT, the away-side distribution
evolves from a broad to a concave shape, then to a convex shape. Comparisons to p+pdata suggest
that the away-side can be divided into a partially suppressed “head” region centered at ∆φ∼π, and
an enhanced “shoulder” region centered at ∆φ∼π±1.1. The pTspectrum for the “head” region
softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for
the “shoulder” region is independent of centrality and trigger pT, which offers constraints on energy
transport mechanisms and suggests that the “shoulder” region contains the medium response to
energetic jets.
PACS numbers: 25.75.Dw
High transverse momentum (pT) partons are valuable
probes of the high energy density matter created at the
Relativistic Heavy-Ion Collider (RHIC). These partons
lose a large fraction of their energy in the matter prior to
forming hadrons, a phenomenon known as jet-quenching.
Such energy loss is predicted to lead to strong suppres-
sion of both single- and correlated away-side dihadron
yields at high pT[1], consistent with experimental find-
ings [2, 3]. The exact mechanism for energy loss is not
yet understood. Recent results of dihadron azimuthal
angle (∆φ) correlations have indicated strong modifica-
tion of the away-side jet [3, 4, 5, 6]. For high pThadron
pairs, such modification is manifested by a partially sup-
pressed away-side peak at ∆φ∼π[3]. This has been
interpreted as evidence for the fragmentation of jets that
survive their passage through the medium.
For intermediate pTcharged hadron pairs, the away-
side jet was observed to peak at ∆φ∼π±1.1 [4, 5],
suggesting that the energy lost by high pTpartons is
transported to lower pThadrons at angles away from
∆φ∼π. The proposed mechanisms for such energy
transport include medium deflection of hard [7] or shower
partons [8],large-angle gluon radiation [9, 10], Cherenkov
gluon radiation [11], and “Mach Shock” medium excita-
tions [12].
In this letter we present a detailed “mapping” of the
pTand centrality dependence of away-side jet shapes and
yields. These measurements (1) allow a detailed inves-
tigation of the jet distributions centered around ∆φ∼
π±1.1 and ∆φ∼π, (2) provide new insight on the in-
terplay between jet quenching and the response of the
medium to the lost energy, and (3) provide new con-
straints for distinguishing the competing mechanisms for
energy transport.
The results presented here are based on minimum-
bias (MB) Au+Au and p+p datasets as well as a “pho-
ton” level-1 triggered (PT) p+p dataset [13] collected
with the PHENIX detector [14] at √sNN=200 GeV, dur-
ing the 2004-2005 RHIC running periods. The collision
vertex zwas required to be within |z|<30cm of the
nominal crossing point. The event centrality was deter-
mined via the method in Ref. [14]. A total of 840 million
Au+Au events were analyzed. Charged particles were
reconstructed in the two central arms of PHENIX, each
covering -0.35 to 0.35 in pseudo-rapidity and 90◦in az-
imuth. The tracking system consists of the drift cham-
bers and two layers of multi-wire proportional chambers
with pad readout (PC1 and PC3), achieving a momen-
tum resolution of 0.7% L1.1% p(GeV/c) [2].
Dihadron azimuthal angle correlations are obtained by
correlating “trigger” (type A) hadrons with “partner”
(type B) hadrons. The MB and PT p+p datasets are
used for trigger pT<5 GeV/cand pT>5 GeV/c,
respectively. To reduce background from decays and
conversions, tracks are required to have a matching hit
within a ±2.3σwindow in PC3. For pT>4 GeV/c, ad-
ditional matching hit at the electromagnetic calorimeter
(EMC) was required to suppress background tracks that
randomly associate with the PC3 [2]. For triggers with
pT>5 GeV/c, a pTdependent energy cut in the EMC
and a tight ±1.5σmatching cut at the PC3 were applied
to reduce the background to <10% [15]. This energy
cut greatly reduces PT trigger bias effects. The PT p+p
results are consistent with the MB p+p data for trigger
pT>5 GeV/c.
The jet associated partner yield per trigger, Yjet (∆φ),
is obtained from the ∆φcorrelations as [4, 15]:
Yjet =»Ns(∆φ)
Nm(∆φ)−b0“1 + 2vA
2vB
2cos 2∆φ”–Rd∆φNm(∆φ)
2πNAεB
(1)
where NAis the number of triggers, εBis the single
particle efficiency for partners in the full azimuth and
|η|<0.35; Ns(∆φ) and Nm(∆φ) are pair distributions

4
from the same- and mixed-events, respectively. Mixed-
event pairs are obtained by selecting partners from dif-
ferent events with similar centrality and vertex. The
εBvalues include detector acceptance and reconstruc-
tion efficiency, with an uncertainty of ∼10% [2, 16]. The
harmonic term, 2vA
2vB
2cos 2∆φ, reflects the elliptic flow
modulation of the combinatoric pairs in Au+Au colli-
sions [4]. Values for vA
2and vB
2for each centrality class
are measured via the reaction plane (RP) method [17]
using the Beam-Beam Counters at 3 <|η|<4. The sys-
tematic errors on v2are dominated by the RP resolution,
and are estimated to be ∼6% for central and mid-central
collisions, and ∼10% for the peripheral collisions [4].
To fix the value of b0, we followed the subtraction pro-
cedure of Refs. [4, 18] and assumed that Yjet has zero
yield at its minimum ∆φmin (ZYAM). To estimate the
possible over-subtraction at ∆φmin, we calculate b0val-
ues independently by fitting Yjet(∆φ) to a function con-
sisting of one near-side and two symmetric away-side
Gaussians. The fitting procedure is similar to that used
in [5], except that a region around π(|∆φ−π|<1) is
excluded to avoid “punch-through” jets around π(see
Fig.1). This fit accounts for the overlap of the near- and
away-side Gaussians at ∆φmin, and thus gives system-
atically lower b0values than that for ZYAM. We assign
the differences as one-sided systematic errors on b0. This
over-subtraction error is only significant in central colli-
sions and at pTA,B <3 GeV/c.
The per-trigger yield distributions for p+pand 0-
20% central Au+Au collisions are compared in Fig. 1
for various combinations of trigger and partner pTranges
(pTA⊗pTB) as indicated. The p+pdata show essentially
Gaussian away-side peaks centered at ∆φ∼πfor all pTA
and pTB. In contrast, the Au+Au data show substantial
shape modifications dependent on pTAand pTB. For a
fixed value of pTA, Figs. 1(a)-(d) reveal a striking evolu-
tion from a broad, roughly flat peak to a local minimum
at ∆φ∼πwith side-peaks at ∆φ∼π±1.1. Interestingly,
the location of the side-peaks in ∆φis roughly constant
with increasing pTB(see also [5]). Such pTindependence
is compatible with the away-side jet modification ex-
pected from a medium-induced “Mach Shock” [12] but
disfavors models which incorporate large angle gluon ra-
diation [9, 10], Cherenkov gluon radiation [11] or de-
flected jets [7, 8].
For relatively high values of pTA⊗pTB, Figs. 1(e)-(h)
show that the away-side jet shape for Au+Au gradually
becomes peaked as for p+p, albeit suppressed. This “re-
appearance” of the away-side peak seems due to a reduc-
tion of the yield centered at ∆φ∼π±1.1 relative to that
at ∆φ∼π, rather than a merging of the peaks centered
at ∆φ∼π±1.1. This is consistent with the dominance
of dijet fragmentation at large pTA⊗pTB, possibly due
to jets that “punch-through” the medium [19] or those
emitted tangentially to the medium’s surface [20].
The evolution of the away-side jet shape with pT(cf.
0
0.2
0.4 0.4-1 GeV/c⊗3-4 1-2 GeV/c⊗3-4 1.5×
0
0.02
0.04
SR HR SR
2-3 GeV/c⊗3-4 3-4 GeV/c⊗3-4
3.5×
0
0.05
0.1
0.15 2-3 GeV/c⊗5-10 4-5 GeV/c⊗4-5
10×
0 2 4
0
0.02
0.04
0.06 3-5 GeV/c⊗5-10
0 2 4
5-10 GeV/c⊗5-10
2.5×
(rad)φ∆
=
jet
Yφ∆/d
AB
dN
A
1/N
(a) (b)
(c) (d)
(e) (f)
(g) (h)
Au + Au 0-20%
p + p
FIG. 1: Per-trigger yield versus ∆φfor various trigger and
partner pT(pT
A⊗pT
B), arranged by increasing pair mo-
mentum (sum of pT
Aand pB
T), in p+pand 0-20% Au+Au
collisions. The Data in some panels are scaled as indicated.
Solid lines (shaded bands) indicate elliptic flow (ZYAM) un-
certainties. Arrows in (c) indicate “head” (HR) and “shoul-
der” (SR) regions.
Fig. 1) suggests separate contributions from a medium-
induced component centered at ∆φ∼π±1.1 and a frag-
mentation component centered at ∆φ∼π. A model
independent study of these contributions can be made
by dividing the away-side jet function into equal-sized
“head” (|∆φ−π|< π/6, HR) and “shoulder” (π/6<
|∆φ−π|< π/2, SR) regions, as indicated in Fig. 1(c).
We characterize the relative amplitude of these two re-
gions with the ratio, RHS ,
RHS =R∆φ∈HR d∆φYjet(∆φ)
∆φHR R∆φ∈SR d∆φYjet(∆φ)
∆φSR
(2)
Since NAin Eq.1 cancels in the ratio, RHS is a
pure pair variable and is symmetric w.r.t pA
Tand pB
T:
RHS(pA
T, pB
T) = RHS(pB
T, pA
T). For concave and convex
shapes, one expects RHS <1 and RHS >1, respectively.
Figure 2 summarizes the pB
Tdependence of RHS for
both p+pand central Au+Au collisions in four pA
Tbins.
The ratios for p+pare always above one and increase
with pB
T. This reflects the narrowing of a peaked jet
shape with increasing pTB[15]. In contrast, the ra-
tios for Au+Au show a non-monotonic dependence on
pTA,B. They evolve from RHS ∼1 for pA,B
T.1 GeV/c
through RHS <1 for 1 .pA,B
T.4 GeV/cfollowed by
RHS >1 for pA,B
T&5 GeV/c. These trends reflect the
competition between medium-induced modification and
jet fragmentation, and suggest that the latter dominates
at pA,B
T&5 GeV/c. The results shown in Fig. 1 indicate
that, relative to p+p, the Au+Au yield is suppressed in

5
1
10
<3.0 GeV/c
A
T
2.0<p <4.0 GeV/c
A
T
3.0<p
0 2 4 6
HS
R
1
10
<5.0 GeV/c
A
T
4.0<p
2 4 6
<10.0 GeV/c
A
T
5.0<p
(GeV/c)
B
T
p
p + p
Au + Au 0-20%
FIG. 2: RHS versus pT
Bfor p+p(open) and Au+Au
(filled) collisions for four trigger selections. Since RHS is
purely hadron pair variable, the result is unchanged by swap-
ping pT
Aand pT
B. Shaded bars (brackets) represent pT-
correlated uncertainties due to elliptic flow (ZYAM proce-
dure).
the HR but is enhanced in the SR. We quantify this sup-
pression/enhancement via IAA, the ratio of jet yield Yjet
between Au+Au and p+pcollisions over a ∆φregion,
W, IW
AA =R∆φ∈Wd∆φY Au+Au
jet .R∆φ∈Wd∆φY p+p
jet .
Figure 3 shows IAA as a function of pTBfor the HR
and the HR+SR, respectively, in four pTAbins. For trig-
gers of 2 < pTA<3 GeV/c,IAA for HR+SR exceeds one
at low pTB, but falls and crosses one at ∼3.5 GeV/c. A
similar trend is observed for the higher pTtriggers, but
the enhancement (at low pTB) is smaller and the sup-
pression (at high pTB) is stronger. The IAA values in HR
are lower relative to HR+SR for all pTA,B. For the low
pTtriggers, the suppression sets in around 1 .pTB.3
GeV/c, followed by a fall-off for pTB&4 GeV/c. For
higher pTtriggers, a constant level of ∼0.2−0.3 is ob-
served above ∼2 GeV/csimilar to the suppression level
of inclusive hadrons [2]. These results provide clear ev-
idence for significant yield enhancement in the SR and
suppression in the HR. The data suggest that the SR
reflects the dissipative processes that redistribute the en-
ergy lost in the medium; The suppression for the HR is
consistent with jet quenching. However, we note that the
IAA values for the HR are upper limit estimates for the
jet fragmentation component. This is because the HR
yield includes possible contributions from the tails of the
SR, as well as from bremsstrahlung gluon radiations [9].
To further explore the interplay between the HR and
the SR, we focus on the intermediate pTregion, 1 <
pTB<5 GeV/c, where the medium-induced component
dominates the away-side yield. We characterize the in-
verse local slope of the partner yield in this pTrange via
a truncated mean pT,hpT′i ≡ hpTBi|1<pTB<5GeV/c- 1
GeV/c.hpT′iis calculated from the jet yields used to
0
1
2
3
4<3.0 GeV/c
A
T
2.0<p
0-20%
<4.0 GeV/c
A
T
3.0<p
0 1 2 3 4 5
0
1
2
3
4<5.0 GeV/c
A
T
4.0<p
1 2 3 4 5
<10.0 GeV/c
A
T
5.0<p
AA
I
(GeV/c)
B
T
p
Head + Shoulder
Head region only
FIG. 3: IAA versus pT
Bfor four trigger pTbins in HR+SR
(|∆φ−π|< π/2) and HR (|∆φ−π|< π/6). The total
systematic errors for the two regions, represented by shaded
bars and brackets respectively, are strongly correlated. Grey
bands around IAA = 1 represent 14% combined uncertainty
on the single particle efficiency in Au+Au and p+p.
make IAA in Fig. 3. Fig. 4 shows the hpT′ivalues for
the HR, SR and a near-side region (|∆φ|< π/3, NR),
as a function of the number of participating nucleons,
Npart. The hpT′ivalues for NR have a weak central-
ity dependence. Their overall levels for Npart >100 are
0.533 ±0.024, 0.605 ±0.032 and 0.698 ±0.040 GeV/cfor
the pTAranges 2-3, 3-4 and 4-5 GeV/c, respectively [21].
This finding is consistent with the dominance of jet frag-
mentation on the near-side, i.e. a harder spectrum for
partner hadrons is expected for higher pTtrigger hadrons.
A very weak centrality dependence is observed for
the SR for Npart &100. In this case, the values for
hpT′iare lower (≈0.45 GeV/c) and do not depend on
pTA. They are, however, larger than the values mea-
sured for inclusive charged hadrons (0.38 GeV/cshown
by solid lines) [2]. The relatively sharp increase in hpT′i
for Npart .100 may reflect a significant jet fragmenta-
tion contribution in peripheral collisions. In contrast,
the hpT′ivalues for the HR show a gradual decrease
with Npart, starting close to that for the near-side jet,
and approaches the value for the inclusive spectrum for
Npart &150.
The different patterns observed for the yields in the HR
and SR suggest a different origin for these yields. The
suppression of the HR yield and the softening of its spec-
trum are consistent with a depletion of yield due to jet
quenching. The observed HR yield could be comprised
of contributions from “punch-through” jets, radiated glu-
ons and feed-in from the SR. By contrast, the enhance-
ment of the SR yield for pTA,B <4 GeV/csuggests a
remnant of the lost energy from quenched jets. How-
ever, the very weak dependence on pTand centrality (for
Npart &100) for its peak location and mean pTmay re-

6
0.4
0.6
0.8
0.4
0.6
0.8
0 100 200 300
0.4
0.6
0.8
0 100 200 300
2.0<p <3.0 GeV/c
A
T
3.0<p <4.0 GeV/c
A
T
4.0<p <5.0 GeV/c
A
T
Nearside
Awayside Shoulder Awayside Head
Inclusive
part
N
< 5.0 GeV/c]
B
T
[1.0 < p
> (GeV/c)
’
T
<p
FIG. 4: Truncated mean hpT′iin 1 < pT
B<5 GeV/cver-
sus Npart for the near-side (diamonds), away-side shoulder
(circles) and head (squares) regions for Au+Au (filled) and
p+p (open) for three trigger pTbins. Solid lines represent
measured values for inclusive charged hadrons [2]. Error bars
represent the statistical errors. Shaded bars represent the sum
of Npart-correlated elliptic flow and ZYAM error.
flect an intrinsic property of the response of the medium
to the energetic jets. These observations are inconsistent
with simple deflected jet [7, 8] and Cherenkov gluon ra-
diation [11] models, since both the deflection/radiation
angle and jet spectra slope would depend on the pTAor
pTB. However, these results are consistent with expecta-
tions for “Mach Shock” in a near-ideal hydrodynamical
medium [12, 22], and thus they can be used to constrain
medium transport properties such as speed of sound and
viscosity to entropy ratio.
In conclusion, we have observed strong medium mod-
ification of away-side shapes and yields for jet-induced
pairs in Au+Au collisions at √sNN=200 GeV. The de-
tailed dependence of these results on pTand centrality
gives strong evidence for two distinct contributions from
the regions of ∆φ∼πand ∆φ∼π±1.1. The former is
consistent with jet quenching. The latter exhibits pTand
centrality independent shape and mean pT, possibly re-
flecting an intrinsic property of the medium response to
energetic jets. These results provide strong constraints
on competing mechanisms for the energy transport.
We thank the staff of the Collider-Accelerator and
Physics Departments at BNL for their vital contribu-
tions. We acknowledge support from the Department of
Energy and NSF (U.S.A.), MEXT and JSPS (Japan),
CNPq and FAPESP (Brazil), NSFC (China), MSMT
(Czech Republic), IN2P3/CNRS and CEA (France),
BMBF, DAAD, and AvH (Germany), OTKA (Hun-
gary), DAE (India), ISF (Israel), KRF and KOSEF
(Korea), MES, RAS, and FAAE (Russia), VR and
KAW (Sweden), U.S. CRDF for the FSU, US-Hungarian
NSFOTKA- MTA, and US-Israel BSF.
∗PHENIX Spokesperson: jacak@skipper.physics.sunysb.edu
†Deceased
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