arX

iv:1

010.

1521

v1 [

nucl

-ex]

7 O

ct 2

010

Suppression of away-side jet fragments with respect to the reaction plane in Au+Au

collisions at√sNN

= 200 GeV

A. Adare,12 S. Afanasiev,27 C. Aidala,40 N.N. Ajitanand,57 Y. Akiba,51, 52 H. Al-Bataineh,46 J. Alexander,57

K. Aoki,33, 51 L. Aphecetche,59 Y. Aramaki,11 J. Asai,51 E.T. Atomssa,34 R. Averbeck,58 T.C. Awes,47 B. Azmoun,6

V. Babintsev,23 M. Bai,5 G. Baksay,19 L. Baksay,19 A. Baldisseri,15 K.N. Barish,7 P.D. Barnes,36 B. Bassalleck,45

A.T. Basye,1 S. Bathe,7 S. Batsouli,47 V. Baublis,50 C. Baumann,41 A. Bazilevsky,6 S. Belikov,6, ∗ R. Belmont,63

R. Bennett,58 A. Berdnikov,54 Y. Berdnikov,54 A.A. Bickley,12 J.G. Boissevain,36 J.S. Bok,66 H. Borel,15 K. Boyle,58

M.L. Brooks,36 H. Buesching,6 V. Bumazhnov,23 G. Bunce,6, 52 S. Butsyk,36 C.M. Camacho,36 S. Campbell,58

B.S. Chang,66 W.C. Chang,2 J.-L. Charvet,15 C.-H. Chen,58 S. Chernichenko,23 C.Y. Chi,13 M. Chiu,6, 24 I.J. Choi,66

R.K. Choudhury,4 P. Christiansen,38 T. Chujo,62 P. Chung,57 A. Churyn,23 O. Chvala,7 V. Cianciolo,47 Z. Citron,58

B.A. Cole,13 M. Connors,58 P. Constantin,36 M. Csanad,17 T. Csorgo,30 T. Dahms,58 S. Dairaku,33, 51 I. Danchev,63

K. Das,20 A. Datta,40 G. David,6 A. Denisov,23 D. d’Enterria,34 A. Deshpande,52, 58 E.J. Desmond,6 O. Dietzsch,55

A. Dion,58 M. Donadelli,55 O. Drapier,34 A. Drees,58 K.A. Drees,5 A.K. Dubey,65 J.M. Durham,58 A. Durum,23

D. Dutta,4 V. Dzhordzhadze,7 S. Edwards,20 Y.V. Efremenko,47 F. Ellinghaus,12 T. Engelmore,13 A. Enokizono,35

H. En’yo,51, 52 S. Esumi,62 K.O. Eyser,7 B. Fadem,42 D.E. Fields,45, 52 M. Finger, Jr.,8 M. Finger,8 F. Fleuret,34

S.L. Fokin,32 Z. Fraenkel,65, ∗ J.E. Frantz,58 A. Franz,6 A.D. Frawley,20 K. Fujiwara,51 Y. Fukao,33, 51 T. Fusayasu,44

I. Garishvili,60 A. Glenn,12 H. Gong,58 M. Gonin,34 J. Gosset,15 Y. Goto,51, 52 R. Granier de Cassagnac,34

N. Grau,13 S.V. Greene,63 M. Grosse Perdekamp,24, 52 T. Gunji,11 H.-A. Gustafsson,38, ∗ A. Hadj Henni,59

J.S. Haggerty,6 K.I. Hahn,18 H. Hamagaki,11 J. Hamblen,60 J. Hanks,13 R. Han,49 E.P. Hartouni,35 K. Haruna,22

E. Haslum,38 R. Hayano,11 M. Heffner,35 T.K. Hemmick,58 T. Hester,7 X. He,21 J.C. Hill,26 M. Hohlmann,19

W. Holzmann,13, 57 K. Homma,22 B. Hong,31 T. Horaguchi,11, 22, 51, 61 D. Hornback,60 S. Huang,63 T. Ichihara,51, 52

R. Ichimiya,51 J. Ide,42 H. Iinuma,33, 51 Y. Ikeda,62 K. Imai,33, 51 J. Imrek,16 M. Inaba,62 D. Isenhower,1

M. Ishihara,51 T. Isobe,11 M. Issah,57, 63 A. Isupov,27 D. Ivanischev,50 B.V. Jacak,58, † J. Jia,6, 13, 57 J. Jin,13

B.M. Johnson,6 K.S. Joo,43 D. Jouan,48 D.S. Jumper,1 F. Kajihara,11 S. Kametani,51 N. Kamihara,52 J. Kamin,58

J.H. Kang,66 J. Kapustinsky,36 K. Karatsu,33 D. Kawall,40, 52 M. Kawashima,53,51 A.V. Kazantsev,32 T. Kempel,26

A. Khanzadeev,50 K.M. Kijima,22 J. Kikuchi,64 B.I. Kim,31 D.H. Kim,43 D.J. Kim,28, 66 E.J. Kim,9 E. Kim,56

S.H. Kim,66 Y.J. Kim,24 E. Kinney,12 K. Kiriluk,12 A. Kiss,17 E. Kistenev,6 J. Klay,35 C. Klein-Boesing,41

L. Kochenda,50 B. Komkov,50 M. Konno,62 J. Koster,24 D. Kotchetkov,45 A. Kozlov,65 A. Kral,14 A. Kravitz,13

G.J. Kunde,36 K. Kurita,53, 51 M. Kurosawa,51 M.J. Kweon,31 Y. Kwon,60, 66 G.S. Kyle,46 R. Lacey,57 Y.S. Lai,13

J.G. Lajoie,26 D. Layton,24 A. Lebedev,26 D.M. Lee,36 J. Lee,18 K.B. Lee,31 K. Lee,56 K.S. Lee,31 T. Lee,56

M.J. Leitch,36 M.A.L. Leite,55 E. Leitner,63 B. Lenzi,55 P. Liebing,52 L.A. Linden Levy,12 T. Liska,14

A. Litvinenko,27 H. Liu,36, 46 M.X. Liu,36 X. Li,10 B. Love,63 R. Luechtenborg,41 D. Lynch,6 C.F. Maguire,63

Y.I. Makdisi,5 A. Malakhov,27 M.D. Malik,45 V.I. Manko,32 E. Mannel,13 Y. Mao,49, 51 L. Masek,8, 25 H. Masui,62

F. Matathias,13 M. McCumber,58 P.L. McGaughey,36 N. Means,58 B. Meredith,24 Y. Miake,62 A.C. Mignerey,39

P. Mikes,8, 25 K. Miki,62 A. Milov,6 M. Mishra,3 J.T. Mitchell,6 A.K. Mohanty,4 Y. Morino,11 A. Morreale,7

D.P. Morrison,6 T.V. Moukhanova,32 D. Mukhopadhyay,63 J. Murata,53, 51 S. Nagamiya,29 J.L. Nagle,12 M. Naglis,65

M.I. Nagy,17 I. Nakagawa,51,52 Y. Nakamiya,22 T. Nakamura,22, 29 K. Nakano,51, 61 J. Newby,35 M. Nguyen,58

T. Niita,62 R. Nouicer,6 A.S. Nyanin,32 E. O’Brien,6 S.X. Oda,11 C.A. Ogilvie,26 K. Okada,52 M. Oka,62 Y. Onuki,51

A. Oskarsson,38 M. Ouchida,22 K. Ozawa,11 R. Pak,6 A.P.T. Palounek,36 V. Pantuev,58 V. Papavassiliou,46

I.H. Park,18 J. Park,56 S.K. Park,31 W.J. Park,31 S.F. Pate,46 H. Pei,26 J.-C. Peng,24 H. Pereira,15 V. Peresedov,27

D.Yu. Peressounko,32 C. Pinkenburg,6 R.P. Pisani,6 M. Proissl,58 M.L. Purschke,6 A.K. Purwar,36 H. Qu,21

J. Rak,28, 45 A. Rakotozafindrabe,34 I. Ravinovich,65 K.F. Read,47, 60 S. Rembeczki,19 K. Reygers,41 V. Riabov,50

Y. Riabov,50 E. Richardson,39 D. Roach,63 G. Roche,37 S.D. Rolnick,7 M. Rosati,26 C.A. Rosen,12

S.S.E. Rosendahl,38 P. Rosnet,37 P. Rukoyatkin,27 P. Ruzicka,25 V.L. Rykov,51 B. Sahlmueller,41 N. Saito,29, 33, 51, 52

T. Sakaguchi,6 S. Sakai,62 K. Sakashita,51, 61 V. Samsonov,50 S. Sano,11, 64 T. Sato,62 S. Sawada,29 K. Sedgwick,7

J. Seele,12 R. Seidl,24 A.Yu. Semenov,26 V. Semenov,23 R. Seto,7 D. Sharma,65 I. Shein,23 T.-A. Shibata,51, 61

K. Shigaki,22 M. Shimomura,62 K. Shoji,33, 51 P. Shukla,4 A. Sickles,6 C.L. Silva,55 D. Silvermyr,47 C. Silvestre,15

K.S. Sim,31 B.K. Singh,3 C.P. Singh,3 V. Singh,3 M. Slunecka,8 A. Soldatov,23 R.A. Soltz,35 W.E. Sondheim,36

S.P. Sorensen,60 I.V. Sourikova,6 N.A. Sparks,1 F. Staley,15 P.W. Stankus,47 E. Stenlund,38 M. Stepanov,46

A. Ster,30 S.P. Stoll,6 T. Sugitate,22 C. Suire,48 A. Sukhanov,6 J. Sziklai,30 E.M. Takagui,55 A. Taketani,51, 52

R. Tanabe,62 Y. Tanaka,44 K. Tanida,33, 51, 52, 56 M.J. Tannenbaum,6 S. Tarafdar,3 A. Taranenko,57 P. Tarjan,16

H. Themann,58 T.L. Thomas,45 M. Togawa,33,51 A. Toia,58 L. Tomasek,25 Y. Tomita,62 H. Torii,22, 51

2

R.S. Towell,1 V-N. Tram,34 I. Tserruya,65 Y. Tsuchimoto,22 C. Vale,6, 26, 26 H. Valle,63 H.W. van Hecke,36

E. Vazquez-Zambrano,13 A. Veicht,24 J. Velkovska,63 R. Vertesi,16, 30 A.A. Vinogradov,32 M. Virius,14 V. Vrba,25

E. Vznuzdaev,50 X.R. Wang,46 D. Watanabe,22 K. Watanabe,62 Y. Watanabe,51, 52 F. Wei,26 R. Wei,57

J. Wessels,41 S.N. White,6 D. Winter,13 J.P. Wood,1 C.L. Woody,6 R.M. Wright,1 M. Wysocki,12 W. Xie,52

Y.L. Yamaguchi,11, 64 K. Yamaura,22 R. Yang,24 A. Yanovich,23 J. Ying,21 S. Yokkaichi,51, 52 G.R. Young,47

I. Younus,45 Z. You,49 I.E. Yushmanov,32 W.A. Zajc,13 O. Zaudtke,41 C. Zhang,47 S. Zhou,10 and L. Zolin27

(PHENIX Collaboration)1Abilene Christian University, Abilene, Texas 79699, USA

2Institute of Physics, Academia Sinica, Taipei 11529, Taiwan3Department of Physics, Banaras Hindu University, Varanasi 221005, India

4Bhabha Atomic Research Centre, Bombay 400 085, India5Collider-Accelerator Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA

6Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA7University of California - Riverside, Riverside, California 92521, USA

8Charles University, Ovocny trh 5, Praha 1, 116 36, Prague, Czech Republic9Chonbuk National University, Jeonju, 561-756, Korea

10China Institute of Atomic Energy (CIAE), Beijing, People’s Republic of China11Center 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, USA13Columbia University, New York, New York 10027 and Nevis Laboratories, Irvington, New York 10533, USA

14Czech Technical University, Zikova 4, 166 36 Prague 6, Czech Republic15Dapnia, CEA Saclay, F-91191, Gif-sur-Yvette, France

16Debrecen University, H-4010 Debrecen, Egyetem ter 1, Hungary17ELTE, Eotvos Lorand University, H - 1117 Budapest, Pazmany P. s. 1/A, Hungary

18Ewha Womans University, Seoul 120-750, Korea19Florida Institute of Technology, Melbourne, Florida 32901, USA

20Florida State University, Tallahassee, Florida 32306, USA21Georgia State University, Atlanta, Georgia 30303, USA

22Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan23IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, 142281, Russia

24University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA25Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic

26Iowa State University, Ames, Iowa 50011, USA27Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia

28Helsinki Institute of Physics and University of Jyvaskyla, P.O.Box 35, FI-40014 Jyvaskyla, Finland29KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan

30KFKI Research Institute for Particle and Nuclear Physics of the Hungarian Academyof Sciences (MTA KFKI RMKI), H-1525 Budapest 114, POBox 49, Budapest, Hungary

31Korea University, Seoul, 136-701, Korea32Russian Research Center “Kurchatov Institute”, Moscow, Russia

33Kyoto University, Kyoto 606-8502, Japan34Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128, Palaiseau, France

35Lawrence Livermore National Laboratory, Livermore, California 94550, USA36Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

37LPC, Universite Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France38Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden

39University of Maryland, College Park, Maryland 20742, USA40Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003-9337, USA

41Institut fur Kernphysik, University of Muenster, D-48149 Muenster, Germany42Muhlenberg College, Allentown, Pennsylvania 18104-5586, USA

43Myongji University, Yongin, Kyonggido 449-728, Korea44Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan

45University of New Mexico, Albuquerque, New Mexico 87131, USA46New Mexico State University, Las Cruces, New Mexico 88003, USA47Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

48IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406, Orsay, France49Peking University, Beijing, People’s Republic of China

50PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad region, 188300, Russia51RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan

52RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973-5000, USA53Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan

54Saint Petersburg State Polytechnic University, St. Petersburg, Russia

3

55Universidade de Sao Paulo, Instituto de Fısica, Caixa Postal 66318, Sao Paulo CEP05315-970, Brazil56Seoul National University, Seoul, Korea

57Chemistry Department, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA58Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA59SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universite de Nantes) BP 20722 - 44307, Nantes, France

60University of Tennessee, Knoxville, Tennessee 37996, USA61Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan

62Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan63Vanderbilt University, Nashville, Tennessee 37235, USA

64Waseda University, Advanced Research Institute for Science andEngineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan

65Weizmann Institute, Rehovot 76100, Israel66Yonsei University, IPAP, Seoul 120-749, Korea

(Dated: July 31, 2012)

Pair correlations between large transverse momentum neutral pion triggers (pT = 4–7 GeV/c) andcharged hadron partners (pT = 3–7 GeV/c) in central (0–20%) and midcentral (20–60%) Au+Aucollisions are presented as a function of trigger orientation with respect to the reaction plane. Theparticles are at larger momentum than where jet shape modifications have been observed, and thecorrelations are sensitive to the energy loss of partons traveling through hot dense matter. An out-of-plane trigger particle produces only 26±20% of the away-side pairs that are observed opposite ofan in-plane trigger particle. In contrast, near-side jet fragments are consistent with no suppressionor dependence on trigger orientation with respect to the reaction plane. These observations arequalitatively consistent with a picture of little near-side parton energy loss either due to surface biasor fluctuations and increased away-side parton energy loss due to a long path through the medium.The away-side suppression as a function of reaction-plane angle is shown to be sensitive to both theenergy loss mechanism in and the space-time evolution of heavy-ion collisions.

PACS numbers: 25.75.Nq,25.75.Bh

I. INTRODUCTION

Collisions of heavy nuclei at the Relativistic HeavyIon Collider have created matter with energy densitiesexceeding the predicted threshold for deconfinement ofcolor charge into a hot dense plasma [1]. In this quarkgluon plasma(QGP), quarks and gluons are not boundwithin hadronic states and the matter behaves collec-tively. Comparisons with hydrodynamic simulations in-dicate rapid thermalization of the colliding system intoa hot dense nuclear medium. The produced medium af-fords an opportunity to study the properties of a newphase of quantum chromodynamics (QCD) in an extremeenvironment.

Hard scattering with large momentum exchange be-tween partons in the incoming nuclei is well-describedby perturbative QCD (pQCD). The scattered partonsemerge back-to-back in azimuth in the plane transverseto the beam direction, and fragment into a pair of cor-related cones of high momentum particles, referred to asjets. The study of jets and their hadronic fragments inheavy-ion collisions provides insight into the properties ofhot dense nuclear matter. Measurements of single hightransverse momentum (pT ) particles [2] and correlationsbetween high-pT particles [3–5] have demonstrated that

∗Deceased†PHENIX Spokesperson: jacak@skipper.physics.sunysb.edu

the fast partons embedded in the produced medium dis-sipate a large amount of their initial kinetic energy.

In this paper, we present angular correlations of hadronpairs with both hadrons in the midrapidity range |η| <0.35. Fragments from the same jet form a peak at smallrelative azimuthal angle (∆φ), i.e. the near-side peak.Pairs composed of one fragment from each jet will ap-pear in an away-side peak at ∆φ ∼ π. Past measure-ments [3–5] for hadrons >∼ 5 GeV/c have shown that theaway-side correlations peak is suppressed relative to base-line measurements in p+p collisions. The suppression ofthe away-side jet is a signature of parton energy loss in-side the medium. The same measurements show thatnear-side jet fragments at large momentum are not sup-pressed. This feature of the data is understood to resultfrom the requirement of a large momentum particle inthe final state, which creates a bias towards small energyloss either by the preferential selection of hard scatter-ings near the medium surface [6] or due to fluctuationsin energy loss [7].

The detailed mechanism by which partons lose en-ergy when passing through a deconfined medium arenot yet fully understood. In pQCD descriptions of theparton-medium interaction the predicted parton energyloss should scale as the path length squared [8]. Incompeting anti-de-Sitter space/conformal field theory de-scriptions characterizing a strongly coupled medium, theenergy loss scales as the path length cubed [8]. The vari-ation in azimuthal angle of the away-side jet suppressionwith respect to the reaction plane (φs) is sensitive to

4

the total amount of energy lost by the away-side partonalong long paths (out-of-plane) or short paths (in-plane)through the medium. The degree to which the away-sidejet suppression varies will be determined in part by thepath-length scale of energy loss.

Single particle observables at high pT , such as thenuclear suppression with respect to the reaction plane(RAA(φs)) or the azimuthal anisotropy (i.e. v2) [9], arealso sensitive to this path-length variation of energy loss.Current pQCD calculations predict a lower v2 than isfound in the data and may imply a larger than pathlength squared dependence to energy loss [10, 11]. Thereaction-plane dependence of the back-to-back jets pro-vides an additional test on the path length dependencein that the two particle observable selects a different dis-tribution of hard-scattering locations and should probelonger paths through the medium than single particleobservables. The path-length dependence of both singleand two particle observables have already been studiedthrough selection of the collision centrality [4, 9]. How-ever, centrality selection varies not only the path length,but also other important properties of the medium (e.g.the overall energy density). Selection with respect tothe reaction plane more directly varies the path lengths,while leaving the other medium properties unchanged.

In addition to the uncertainties associated with the en-ergy loss mechanisms, many of the details within hydro-dynamic simulations of heavy-ion collisions have not beenfully constrained and tested by experiment. For instance,one such uncertainty is the geometrical description of theenergy deposited by the colliding nuclei which could con-tribute to the degree of away-side suppression variationwith respect to the reaction plane. Two competing de-scriptions, the Glauber model [12] and the Color GlassCondensate [13], predict different azimuthal distributionsof matter with respect to the reaction plane. Thus thetwo descriptions give different in-plane and out-of-planepath lengths through the medium. These descriptionsare also used as different starting points to the hydro-dynamic evolution of the medium. Other model uncer-tainties include, but are not limited to, the extent of ge-ometry fluctuations, the time required for thermalizationinto a hydrodynamic medium, the characteristics of thephase transition to confined hadrons, and the conditionsunder which those hadrons become free-streaming parti-cles into the vacuum. These ambiguities in the propermodeling of heavy-ion collisions can result in significantuncertainty in the extracted properties of the medium,such as the shear viscosity [14].

In midcentral collisions (the middle 20–60% of the totalcross section) the variation of the away-side suppressionis expected to be largest as the collision zone is the mostanisotropic. In contrast, central collisions are much moreisotropic and so provide a sample of events with smallanisotropy expected in the away-side suppression. Forinstance, in the Glauber model, midcentral events willhave a root-mean-square thickness through the mediumof 3.2 fm in the in-plane direction versus 4.8 fm in the

out-of-plane direction, which is a 50% variation in pathlength. However, for central 0–20% collisions, the pathlength through the medium varies from 5.0 fm in the in-plane direction to 5.8 fm in the out-of-plane direction,which is a much smaller 16% variation. It is notable thatthe thickness through the medium in midcentral colli-sions changes more with respect to the reaction planethan it does between central and midcentral collisionswhere the away-side suppression at large momentum isalready known to vary [4]. Also worth noting is that thelargest thickness in midcentral collisions is comparable tothe shortest thickness in central collisions.

Any prediction for the away-side suppression with re-spect to the reaction plane will be a convolution of theenergy loss and a description of the space-time evolu-tion of the medium. In the limit where the medium isnever fully opaque to fast partons and the energy lost bythe typical parton is some fraction of its initial energy,the away-side suppression will increase with angle withrespect to the reaction plane. This results because theaverage path length through the medium of the recoilparton is longer when out of the reaction plane. It ispossible, in the extreme limit of a medium with a largeopaque core and thin transparent corona, the away-sidesuppression could instead weaken as the trigger particleorientation varies from the in-plane to out-of-plane direc-tions. The weakening in the thin corona scenario resultsfrom two effects; a larger relative number of scatteringcenters producing a pair of back-to-back final-state par-ticles in the out-of-plane direction, but also the variationof the trigger particle multiplicity by angle with respectto the reaction plane. However, it is worth noting thata large core and thin corona is an extreme configuration.Variations within more realistic models of the away-sidesuppression will be intermediate between these extremescenarios.

In this paper, we present azimuthal correlation mea-surements between pairs of neutral pion trigger particles(t) within ptT = 4–7 GeV/c and charged hadron associ-ated partner particles (a) within paT = 3–4, 4–5, and 5–7GeV/c. These combinations of final-state particle mo-mentum ranges have previously been shown to be dom-inated by jet fragmentation as they are above medium-induced two-particle correlations which contribute sig-nificantly at lower momenta [4, 5]. The low momentumstructures (the “ridge” and “shoulder”) may be the resultof parton-medium interactions (e.g. [15–18]) or globalcorrelations from fluctuating initial conditions [19, 20].These fluctuations have substantially less impact at largepair momentum where the background contribution be-comes small. In this study, as illustrated in Fig. 1, us-ing only particles at large pair momentum, the away-side(∆φ ≈ π) suppression by trigger particle orientation withrespect to the reaction plane (φs = φt − ψ) is presentedas a probe of both the mechanism of parton energy lossand the space-time evolution of matter created by thecollisions of large nuclei.

A previous measurement [21] by the STAR collabo-

5

FIG. 1: (Color online) Definition of azimuthal angles. Triggerand associated partner particles are measured at φt and φa,respectively. The trigger particle orientations are taken withrespect to the reaction plane, φs = φt

− ψ. The relativeazimuthal separation of the trigger particle and the associatedpartner particle is ∆φ = φt

− φa.

ration for 20–60% centrality between 4–6 GeV/c triggerparticles and 2 < paT < ptT GeV/c partner particles fortwo 45◦ wide in-plane and out-of-plane selections indi-cated an increased suppression of the away-side jet forthe out-of-plane, but with little significance due to largeunderlying event subtraction uncertainties. The new re-sults presented in our paper have sufficient statistics tospecify a trend in the away-side suppression in midcentralcollisions at larger momentum where subtraction uncer-tainties are negligible.

II. EXPERIMENT

The results presented here are based on 3.4 billionminimum-bias Au+Au events recorded by the PHENIXdetector in 2007. Comparisons to p+p collisions use pre-viously published measurements from data recorded in2006 [5]. Collision centrality was determined by divi-sion into percentile of the integrated charge collected bybeam-beam counters (BBC) [22] located at |η| between3.0 and 3.9. The timing between the arrival of chargedparticles in the north and south BBC was used to re-construct the event position along the collision axis (z-vertex), and to restrict the event sample to ±30 cm ofthe nominal interaction point of the two beams. Theorientation of reaction-plane azimuthal angle (ψ) is re-constructed event-by-event using the quadrupole compo-nent (v2) of the charge in the Reaction Plane (RXPN)detector [23], located at |η| between 1.0 and 2.8. The res-olution of the RXPN detector is highest for midcentralcollisions (∼20%) where both the quadrupole componentand the detector occupancy are large. The set of resolu-tion corrections, ∆n : n ∈ {2, 4, 6, 8}, for single particle

anisotropies, vn, where:

vn =vobsn

∆n

(1)

are estimated from correlations between the independentnorth (ψN ) and south (ψS) RXPN reaction-plane recon-structions [24, 25]. A single fit parameter (x) is mappedinto the resolution corrections via:

∆n =1

2

√πxe−

x2

2

(

In/2−1

2

(

x2

2

)

+ In/2−1

2

(

x2

2

))

(2)

The fit parameter is extracted from the correlations via:

C(ψN − ψS) =1

2e−

x2

2

2

π

(

1 +x2

2

)

+z(

I0 (z) + L0 (z))

+x2

2

(

I1 (z) + L1 (z))

(3)

where

z =x2

2cos (ψN − ψS) (4)

The set of functions, I2k and L2k, are the even-orderedmodified Bessel functions and the modified Struve func-tions respectively. The extracted values used to correctthe measured second-order azimuthal anisotropy, are ∆2

= 0.66(4) and 0.66(3) for 0–20% and 20–60% collisions,respectively. A 10% systematic uncertainty in 0–5% col-lisions and 5% elsewhere accounts for non-flow contri-butions to the resolution corrections [23]. The similarvalues are a result of the peak in reaction-plane resolu-tion appearing near 20% centrality. A direct inspectionof these reaction-plane distributions for events contain-ing a photon above 1 GeV/c did not reveal significantcontributions from jets.Neutral pion trigger particles are reconstructed from

photon clusters measured by either lead-glass or lead-scintillator electromagnetic calorimeters (EMCal) in thetwo central arms of PHENIX, in total covering |η| < 0.35and 2× 90◦ in azimuth [26]. Clusters are subject to cutsbased on the known response of the EMCal, includingnoisy and low-response towers, as well as shower shapecuts. Neutral pions are identified through the 2γ de-cay channel by pairing all photons within an event. In-correct pairings between photons create a broad com-binatorial background under the π0 mass peak. Thisbackground is minimized by requiring the reconstructedmass to lie near the π0 mass peak. This requirement was0.125–0.160 MeV/c2 for central events, but was relaxedto 0.120–0.165 MeV/c2 in midcentral events where thecombinatorial background is lower. Since combinatorialpairs are more often made with the abundant photonsfound at low energy, the energy asymmetry of the decay(|E1 − E2|/(E1 + E2)) was restricted to be less than 0.5for 0–5% central events. This was also relaxed slowly for

6

more peripheral events until all pairs with asymmetriesless than 0.7 were accepted. The tightness of the cutswas used to control the rate of combinatorial pairingssuch that π0 trigger particles have a signal-to-backgroundratio averaged over the mass window of 4:1 in central col-lisions and 10:1 in midcentral collisions.Charged hadron partner particles are reconstructed in

the central arms using the drift chambers (DC) with hitassociation requirements in two layers of multi-wire pro-portional chambers with pad readout (PC1 and PC3),achieving a momentum resolution, ∆p/p, of 0.7%⊕1.1%p(GeV/c). Only tracks with unambiguous and distin-guishable DC and PC1 hit information are used. Pro-jections of these tracks are required to match a PC3 hitwithin a ±2σ proximity window to reduce backgroundfrom conversion and decay products. A track associationto a signal in the Ring Imaging Cerenkov detector is usedto reject electrons for partner selections below 5 GeV/cwhere little signal is produced by charged pions.

III. PAIR ANALYSIS

Within an event, all pairs formed from π0 trigger par-ticles (ptT = 4–7 GeV/c) and three sets of charged hadronassociated partner particles (paT = 3–4, 4–5, 5–7 GeV/c)are measured. Two centrality classes are used: a centralselection of 0–20% collisions and a midcentral selectionof 20–60% collisions. Trigger particles are separated intosix 15◦ bins in azimuthal angle with respect to the re-action plane, φs = φt − ψ. The angular resolution ofthe measured reaction plane, at approximately 25◦, islarger than this binning; consequently, significant smear-ing takes place between neighboring trigger orientationbins. Pairs within PHENIX are collected at different ef-ficiencies due to the non-uniform central arm acceptance.The relative pair efficiencies are corrected by mixed pairdistributions in which trigger and partner particles aredrawn from different events of the same class (bins of5% centrality, 5 cm z-vertex). The resulting acceptance-corrected distributions are reported as correlation func-tions, C(∆φ), which are defined as:

C(∆φ) =

dntasame

d∆φ

dntamix

d∆φ

∫ dntamix

d∆φd∆φ

∫ dntasame

d∆φd∆φ

(5)

where nta is the number of measured pairs per event foreither the same or mixed events, as indicated. Double-struck notation (n) is used here to indicate measuredquantities. Representative correlation functions for in-plane and out-of-plane trigger particle orientations areshown in Fig. 2. The full set of the measured correla-tion functions used in this analysis is shown in Figs. 13and 14. Note that these distributions are not correctedfor reaction-plane resolution.These inclusive pairs are assumed to correlate in one

of two ways. (1) Two particles within the same event

FIG. 2: (Color online) Correlation functions for the most in-plane, φs=0–15◦, (solid squares) and out-of-plane, φs=75-90◦,(open squares) trigger π0 orientations in central 0–20% andmidcentral 20–60% collisions, left and right columns respec-tively, for 3–4 GeV/c partner hadrons. Expected underlyingevent contributions are shown as solid curves (see text fordetails).

may correlate trivially by participation in the same col-lision geometry. These pairs produce an azimuthal an-gular correlation from the the single particle anisotropywith respect to the reaction plane. (2) Two particlesmay also correlate with each other via the same hard-scattering process. These particles will be fragments fromthe same (di)jet. To separate the jet particle pairs fromthe other background pairs, the two-source assumptionis expressed as [27]:

C (∆φ) = J (∆φ)

+ b0

(

1 +β

αcos (2∆φ) +

γ

αcos (4∆φ)

)

(6)

where the jet contribution to the correlation function iscontained in J(∆φ). The remaining harmonic terms de-scribe the background contribution which is complicatedby the trigger particle binning with respect to the re-action plane. The background modulation coefficients(α, β, γ) are calculated via:

α = 1 + 2vt2 cos (2φs)sin (2c)

2c∆2

+ 2vt2 cos (4φs)∆4 (7)

β = 2vt2va2 + 2va2

(

1 + vt4)

cos (2φs)sin (2c)

2c∆2

+ 2vt2va2 cos (4φs)

sin (4c)

4c∆4

7

+ 2va2vt4 cos (6φs)

sin (6c)

6c∆6 (8)

γ = 2vt4va4 + 2va4

(

1 + vt2)

cos (4φs)sin (4c)

4c∆4

+ 2vt2va4

(

cos (2φs)sin (2c)

2c∆2

+ cos (6φs)sin (6c)

6c∆6

)

+ 2vt4va4 cos (8φs)

sin (8c)

8c∆8. (9)

This description of the background accounts for the trig-ger particle binning and reaction-plane resolution effectson the background shape [28]. The trigger particle ori-entation appears explicitly in terms of the bin center, φs,and width, c. Single particle anisotropy values, v2 andv4, were measured by correlating the trigger and partnerparticles with respect to the reaction plane, such that:

C(φ− ψ) = 1 + 2vobs2 cos(2(φ− ψ))

+ 2vobs4 cos(4(φ− ψ)) (10)

where the observed anisotropies are corrected for thereaction-plane resolution as described previously inEquation 1. Given sufficient detector resolution and nar-rowness of the trigger particle orientation binning, thesign of the cos(2∆φ) term in Eq. 6 will flip sign betweenin-plane and out-of-plane bins as shown in Fig. 2. This ef-fect is expected as selecting out-of-plane trigger particlesshould decrease the likelihood of finding a second back-ground particle nearby. The same is not true for parti-cles correlated via hard scattering. Both the second- andfourth-order anisotropy of the background correlationshave been considered as the finite fourth-order contribu-tions were determined to be non-negligible for some trig-ger particle orientations. Likewise, higher-order terms inthe reaction-plane resolution correction are also included.The uncertainties on the reaction-plane resolution cor-

rections (∆n) and the observed anisotropies (vobs2 ,vobs4 )are propagated separately as they impact the away-sidesuppression with respect to the reaction plane in char-acteristically different ways. The uncertainty in thereaction-plane resolution corrections is fully correlatedbetween trigger orientations. For instance, this uncer-tainty increases (or decreases) both the extracted in-plane and out-of-plane jet yields at ∆φ = π. However,the uncertainty in the observed anisotropies is fully anti-correlated between trigger orientations. Thus, this un-certainty increases the extracted in-plane yield while de-creasing the out-of-plane yield (or vice versa). At largemomentum, both of these subtraction uncertainties aresmall and always dominated by other sources.The subtraction procedure was also examined for con-

tamination of the jet correlations by fakes in the chargedtracking, which become significant at large pT . The fakehigh pT tracks are present only in the partner sample andare largely uncorrelated with trigger particle for the part-ner pT presented here. Thus the fake tracks, which are

FIG. 3: Correlation functions for trigger π0s averaged overall trigger orientations in central 0–20% and midcentral 20–60% collisions, left and right columns respectively, for 3–4GeV/c partner hadrons. Expected average underlying eventcontributions are shown as solid curves.

already less influential in events with a high pT π0, aresubtracted with other uncorrelated pairs as part of thebackground contribution, so long as the anisotropies aremeasured with the same particle cuts. The subtractionrobustness against tracking fakes at high pT was checkedby repeating the procedure with a 3σ PC3 matching re-quirement.By taking the trigger particle orientation as φs = π/4,

the bin width as c = π/2, and by truncating higherthan second-order terms, the functional form of the back-ground in Eq. 6 reduces to the vt2×va2 modulation used inprevious trigger particle orientation averaged results suchas those found in [4]. This property is demonstrated inFig. 3 where the trigger particles from all orientations areconsidered.The background level, b0, is determined using the zero

yield at minimum (ZYAM) method [27]. At high-pT , thewell-separated near- and away-side jets provide a largeangular region at mid-∆φ angles with negligible jet sig-nal. This allows the ZYAM level to be found with neg-ligible bias and sufficient statistics despite the lower ef-ficiency PHENIX has for collecting pairs near 90◦. TheZYAM uncertainty was estimated through simulation ofthe statistical uncertainties as has been described in [29].The jet contribution, J(∆φ), is then reported as a per-

trigger azimuthal yield such that:

1

nt

dntajet

d∆φ=

1

ǫan

tasame

n

t∫

d∆φJ(∆φ). (11)

The efficiency-corrected single particle and pair rates arent and nta respectively. The single particle partner effi-ciency, ǫa, is estimated in simulations of detector accep-tance and occupancy as was done in [5]. By design, thetrigger particle efficiency cancels in the ratio.

IV. RESULTS

Central events, 0–20% collisions, are analyzed as across check against experimental artifacts in midcentral

8

collisions since they have a smaller away-side jet yield.Thus, the central events should exhibit a smaller triggerparticle angle variation, require a larger reaction-planeresolution correction, a larger event correlation subtrac-tion, and have increased background in π0 identification.Representative per-trigger azimuthal yields in central col-lisions for each of the partner momentum selections forthe most in-plane and most out-of-plane trigger particleselections are shown in Fig. 4. Figure 11 shows the fullset of the measured per-trigger azimuthal yields used inthis analysis for central collisions. The most in-plane andmost out-of-plane trigger-particle orientations select theshortest and longest average path lengths through themedium, respectively, and thus may be expected to havethe maximimum differences.On the near-side, a jet distribution is clearly observed

for each selection. A direct comparison between the mostin-plane and most out-of-plane trigger shows no signif-icant variation. The measurement at mid-∆φ demon-strates the good agreement resulting from correct de-scription of the underlying event correlations. On theaway-side, the jet yield is small due to medium sup-pression and the statistical precision suffers once thepairs are divided among the various trigger particle ori-entations. No evidence of experimental artifacts suchas over-subtraction or incorrect description of the back-ground is seen, despite the challenging analysis environ-ment present in central collisions.Integrated near- and away-side per-trigger yields (Y )

are calculated within angular ∆φ windows, as indicatedin Fig. 4, approximating the 2σ width of the jet distri-butions measured in the trigger particle orientation av-eraged results. The near-side azimuthal integration win-dows are ∆φ < π/9 (< 3π/18) for paT > 4 GeV/c (< 4GeV/c). Similarly, the away-side azimuthal integrationswindows are π−∆φ < 3π/18 (< 2π/9) for paT > 4 GeV/c(< 4 GeV/c). Use of these windows corresponds to anassumption that the jet distributions do not widen sig-nificantly at high pT , as a function of the trigger particleorientation with respect to the reaction plane. This as-sumption is supported by the absence of significant cen-trality dependence in jet correlation widths (<∼ 20%) forparticles at high pT [5]. Within statistical uncertaintiesa constant jet width is consistent with the data. Inte-grated yields as a function of trigger particle orientationfor both the near- and away-side are then corrected forthe reaction-plane resolution. The resolution correctionis applied such that:

Y (φs) =1 + 2

(

vobs,Y2 /∆2

)

cos (2φs)

1 + 2vobs,Y2 cos (2φs)Ymeas(φs). (12)

where Y and Ymeas are the corrected and uncorrectedyields, respectively. The value of vobs,Y2 is the observedsecond-order anisotropy of integrated per-trigger yieldwith respect to the reaction plane and is determined byfitting the trigger particle orientation dependence of eachYmeas(φs) measurement individually. This procedure is

the similar to the correction of reaction-plane resolutionon single particles, here applied to integrated per-triggerpair yields.

The corrected per-trigger yields (Y ) are reported asthe nuclear jet suppression with respect to p+p collisions,IAA = YA+A/Yp+p. The result for central collisions isshown in Fig. 5. The variation of the fit used in the reso-lution correction is the dominant source of φs-correlateduncertainty, having larger impact than the insignificantevent anisotropy uncertainties. In the case of zero signalvariation with reaction plane orientation, the correctionbecomes completely correlated with statistical scatter inthe uncorrected measurement. Thus, the φs-correlatedsystematic uncertainty from the resolution correction isconservatively treated as correlated with the statisticaluncertainty when computing the final significance of themeasured trends. For the same reason, this source ofsystematic uncertainty has little correlation between thecentrality and momentum selections.

For central events the near-side suppression is consis-tent with a constant as a function of φs within the sta-tistical and φs-correlated systematic uncertainties. Thevalues are also consistent with no suppression when con-sidering the global scale uncertainty that appears on thetrigger particle orientation averaged IAA. On the away-side, there is significant suppression in central events, asevidenced by the trigger particle averaged IAA, but thestatistical precision with which to determine the φs vari-ation is limited.

Mid-central events, 20–60% collisions, have greater ec-centricity and could be expected to show correspond-ingly larger trigger particle orientation dependence dueto path-length variation through the collision zone. Thesame set of representative per-trigger azimuthal yields isshown in Fig. 6 for the midcentral selection. The full setat midcentrality is shown in Fig. 12. Again, the near-sidejets for the most in-plane and most out-of-plane triggerparticle orientations are consistent with each other, a di-rect indication of little variation with respect to the re-action plane. The mid-∆φ are also in agreement withzero, as before, further demonstrating that the under-lying event flow correlations are well described by Equa-tions 6-9. In contrast to the near-side, the away-side mea-surements (see insets in Fig. 6) change between the in-plane and out-of-plane trigger particle orientations withthe latter having consistently smaller yield for all partnermomenta.

The integrated near- and away-side per-trigger jetyields for midcentral collisions are shown in Fig. 7. Thenear-side jet is essentially flat, with negligible suppres-sion (IAA(φs) = 1). The away-side jet yield is increas-ingly suppressed with increasing φs. This falling trendresults in only small associated particle yield remainingfor out-of-plane trigger particle orientations.

In order to quantify the variation and significance ofthe trigger particle orientation dependencies shown inFigs. 5 and 7, the ratio of the out-of-plane to in-planesuppression (IoutAA/I

inAA) is constructed. In the ratio, the

9

FIG. 4: (Color online) Per-trigger azimuthal jet yields forthe most in-plane, φs=0–15◦ (solid circles) and out-of-plane,φs=75–90◦ (open circles) trigger particle selections in central0–20% collisions for various partner momenta. Insets showaway-side region on a zoomed scale. Bars indicate statisticaluncertainties. Underlying event modulation systematic un-certainties are represented by bands through the points whilethe corresponding normalization uncertainties are shown asdashed lines around zero. Near- and away-side jet yield inte-gration windows are indicated with arrows.

FIG. 5: (Color online) Nuclear jet suppression factor, IAA,by angle with respect to the reaction plane, φs, for near- andaway-side angular selections, circles and squares respectively,in central 0–20% collisions for various partner momenta. Barsindicate statistical uncertainties. The shaded band shows thesystematic uncertainty on the reaction-plane resolution un-smearing correction. Solid points show trigger particle angleaveraged results and the global scale uncertainty.

global scale uncertainties on each measurement cancel.The IAA values at φs = 0◦ (I inAA) and at 90◦ (IoutAA) are es-timated by both linear and flow-like cosine fits to the trig-ger particle angle measurements and evaluation at theseangles. The reported ratios are therefore independent ofthe chosen binning with respect to the reaction plane andthe values do not rely heavily on the assumed functionalform of the dependence. The best-fit was determined by

χ2 minimization in which:

χ2 =∑ (yi + ǫsysσsys,i − f (φs))

2

σ2i (ǫsys)

+ ǫ2sys (13)

where ǫsys is ±1 for the ±1σsys variation of the φs-correlated systematic error [30]. As discussed above, thesystematic uncertainty is conservatively treated as fullycorrelated with the statistical uncertainty. The differ-ence between linear and cosine fits provides only a smallsource of additional uncertainty due to the unknown true

10

FIG. 6: (Color online) Per-trigger azimuthal jet yields forthe most in-plane, φs=0–15◦, (solid circles) and out-of-plane,φs=75–90◦, (open circles) trigger particle selections in mid-central 20–60% collisions for various partner momenta. In-sets show away-side region on a zoomed scale. Bars indicatestatistical uncertainties. Underlying event modulation sys-tematic uncertainties are represented by bands through thepoints while the corresponding normalization uncertaintiesare shown as dashed lines around zero. Near- and away-sidejet yield integration windows are indicated with arrows.

FIG. 7: (Color online) Nuclear jet suppression factor, IAA,by angle with respect to the reaction plane, φs, for near-and away-side angular selections, circles and squares respec-tively, in midcentral 20–60% collisions for various partner mo-menta. Bars indicate statistical uncertainties. The shadedband shows the systematic uncertainty on the reaction-planeresolution unsmearing correction. Solid points show triggerparticle angle averaged results and the global scale uncer-tainty.

functional form. The resulting values of IoutAA/IinAA and

the total uncertainty are shown in Figs. 8 and 9. Theaverage value of IoutAA/I

inAA across partner momentum is

constructed by weighting the individual measurementsby the p+p per-trigger yields [5]. In general, the data arewell fit by both the linear and cosine functions, giving rea-sonable χ2. No evidence is seen for systematic deviationsfrom either fit within the sizable statistical uncertaintiesand both forms give similar goodness of fit values. Thesevalues appear along with the IoutAA/I

inAA ratios in Table I.

For both central and midcentral collisions, the near-

side jet yield is independent of trigger particle orientationwith respect to the reaction plane within one standarddeviation of the experimental uncertainties. These mea-surements are consistent with surface bias of the hardscattering center created by the requirement of a triggerparticle and a resulting short path length through thecollision zone traversed by the near-side parton. Cen-tral collisions have insufficient statistics to determine theaway-side variation.

However in midcentral collisions where the expecta-tion of surface bias would lead to a large variation in

11

TABLE I: Angle with respect to the reaction-plane dependence of the nuclear suppression factor, IAA, expressed as the ratiobetween in-plane and out-of-plane trigger particles from linear and cosine fits to the data (see text for details). The totaluncertainty taking into account the correlations between sources is reported.

Selection Near-side Away-side

linear cosine average linear cosine average

Cent paT IoutAA/Iin

AA χ2/dof IoutAA/Iin

AA χ2/dof IoutAA/Iin

AA χ2/dof IoutAA/Iin

AA χ2/dof

0–20% 3–4 0.95 ± 0.15 9.5/4 0.96 ± 0.15 10.0/4 0.96± 0.15 0.1 ± 0.7 5.0/4 0.2± 0.8 5.1/4 0.2± 0.8

4–5 0.92 ± 0.18 3.0/4 0.92 ± 0.16 3.0/4 0.92± 0.18 0.7 ± 1.3 9.0/4 0.6± 1.2 8.7/4 0.7± 1.3

5–7 1.15 ± 0.30 3.1/4 1.10 ± 0.26 3.3/4 1.13± 0.28 1.5 ± 2.0 2.0/4 1.3± 1.4 1.8/4 1.4± 1.7

3–7 — — — — 0.98± 0.11 — — — — 0.5± 0.6

20–60% 3–4 0.90 ± 0.14 5.0/4 0.92 ± 0.12 5.5/4 0.91± 0.13 0.15 ± 0.25 4.0/4 0.25 ± 0.38 5.5/4 0.20± 0.32

4–5 0.85 ± 0.17 1.2/4 0.88 ± 0.15 1.5/4 0.87± 0.16 0.20 ± 0.20 3.0/4 0.30 ± 0.35 4.0/4 0.25± 0.28

5–7 0.88 ± 0.28 0.5/4 0.88 ± 0.21 0.7/4 0.88± 0.25 0.40 ± 0.30 0.3/4 0.50 ± 0.30 0.5/4 0.45± 0.30

3–7 — — — — 0.89± 0.10 — — — — 0.26± 0.20

FIG. 8: (Color online) Angle with respect to the reaction-plane dependence of the nuclear suppression factor, IAA, ex-pressed as the ratio between in-plane and out-of-plane triggerparticles from fits to the data in central 0–20% collisions. Thebars represent total uncertainty taking into account the cor-relations between sources (see text for details).

the path length traversed by the away-side parton, themeasurements show a significant falling trend with in-creasing trigger particle angle with respect to the reac-tion plane. The suppression of away-side jet fragments inthe out-of-plane direction is larger than in the in-planedirection, the out-of-plane away-side jet peak having only(26±20)% of the yield of the in-plane direction. Thus thelarge variation by angle with respect to the reaction planeis significant. Assuming the modulation to be flow-like(dominated by the second-order variation), the suppres-

sion pattern implies vIAA

2 = 0.29+0.15−0.11. As the midcentral

away-side measurements are consistent between paT selec-tions within the stated uncertainties, the hint of a risingtrend in paT is not significant. The values quoted here areconsistent with those previously measured in [21] andprovide a factor four better constraint in the IoutAA/I

inAA

ratio.Recent single particle measurements of azimuthal

FIG. 9: (Color online) Angle with respect to the reaction-plane dependence of the nuclear suppression factor, IAA, ex-pressed as the ratio between in-plane and out-of-plane triggerparticles from fits to the data in midcentral 20–60% collisions.The bars represent total uncertainty taking into account thecorrelations between sources (see text for details).

anisotropy at high pT (6 − 9 GeV/c) found that v2 =0.13 ± 0.01 ± 0.01 [9]. Thus, the away-side per-triggeryields at high pT favor an anisotropy larger than thatmeasured for the single particles. However, the differenceis marginal and additional measurements will be neededto confirm.Shown in Fig. 10 are the results of a Monte-Carlo en-

ergy loss calculation from T. Renk [31, 32] using the time-space evolution provided by two different hydrodynamicsimulations [33, 34] and two initial state descriptions,Glauber and CGC. These particular combinations of ajet energy loss model and collision evolution together pre-dict less variation in the away-side suppression with re-spect to the reaction plane than is witnessed by the data.Variation of the initial geometry description within [33]between Glauber and CGC produces only small changesin the extracted IAA out-of-plane to in-plane ratio, in-dicating limited sensitivity to this model parameter of

12

FIG. 10: (Color online) Away-side IoutAA/Iin

AA ratio for mid-central, 20–60% collisions, from Fig. 9. The solid line showsthe results from an energy loss calculation [31, 32] using twohydrodynamic evolution models [33, 34]. The shaded bandshows the uncertainty that results from the selection of a par-ticular hydrodynamic evolution; the lower extent covering [33]and the upper covering [34]. Dotted lines show the uncer-tainty from the initial event geometry (Glauber or CGC) ascalculated within [33].

the reaction plane dependent dijet observable. However,other model parameters that vary between the two hy-drodynamic models (such as the thermalization time andfreeze-out temperature) were found to impact the away-side suppression anisotropy to a greater degree, indicat-ing sensitivity to simulation parameters that are not well-constrained by other measurements. Consequently, thesedata warrant more detailed study with various energyloss models, and also different space-time evolution mod-els.

V. SUMMARY

We have shown that away-side jet fragment suppres-sion increases substantially with increasing angle withrespect to the reaction plane in midcentral Au+Au col-lisions at

√sNN

= 200 GeV. The away-side yield in theout-of-plane orientation is reduced by a factor of ∼ 4 rel-ative to the in-plane direction. In contrast, the measurednear-side IAA is reaction plane independent, and consis-tent with no suppression. These results directly showthat the energy lost by fast partons in the hot nuclearmedium increases as their paths through the mediumbecome long. A theoretical description of these exper-imental data implementing an energy loss formalism anda time-space evolution of the collision should be sought inunion with other experimental quantities; such as RAA,IAA, and RAA(φs) [2, 4, 5, 25, 35]. Energy loss for-malisms that have successfully described the large mo-mentum RAA and IAA may be paired with a particulartime-space evolution in also describing the φs dependenceof these same quantities. As shown for the combination

above, the data presented here disagree with the presentcalculations. These data should play an important rolein constraining simulations of the space-time evolutionof heavy-ion collisions and the subsequent extraction ofmedium properties.

Acknowledgments

We thank the staff of the Collider-Accelerator andPhysics Departments at Brookhaven National Labora-tory and the staff of the other PHENIX participating in-stitutions for their vital contributions. We acknowledgesupport from the Office of Nuclear Physics in the Of-fice of Science of the Department of Energy, the NationalScience Foundation, a sponsored research grant from Re-naissance Technologies LLC, Abilene Christian Univer-sity Research Council, Research Foundation of SUNY,and Dean of the College of Arts and Sciences, Van-derbilt University (U.S.A), Ministry of Education, Cul-ture, Sports, Science, and Technology and the Japan So-ciety for the Promotion of Science (Japan), ConselhoNacional de Desenvolvimento Cientıfico e Tecnologicoand Fundacao de Amparo a Pesquisa do Estado de SaoPaulo (Brazil), Natural Science Foundation of China(People’s Republic of China), Ministry of Education,Youth and Sports (Czech Republic), Centre National

de la Recherche Scientifique, Commissariat a l’EnergieAtomique, and Institut National de Physique Nucleaireet de Physique des Particules (France), Ministry of In-dustry, Science and Tekhnologies, Bundesministerium furBildung und Forschung, Deutscher Akademischer Aus-tausch Dienst, and Alexander von Humboldt Stiftung(Germany), Hungarian National Science Fund, OTKA(Hungary), Department of Atomic Energy and Depart-ment of Science and Technology (India), Israel ScienceFoundation (Israel), National Research Foundation andWCU program of the Ministry Education Science andTechnology (Korea), Ministry of Education and Science,Russia Academy of Sciences, Federal Agency of AtomicEnergy (Russia), VR and the Wallenberg Foundation(Sweden), the U.S. Civilian Research and DevelopmentFoundation for the Independent States of the Former So-viet Union, the US-Hungarian Fulbright Foundation forEducational Exchange, and the US-Israel Binational Sci-ence Foundation.

Appendix: Appendix

The complete set of per-trigger yields and correlationfunctions used as source material for the analysis of thedependence of the away-side suppression on angle withrespect to the reaction plane are shown in Figs. 11–14.

13

FIG. 11: (Color online) Per-trigger azimuthal jet yields in central 0–20% collision for all trigger orientations with respect tothe reaction plane in 15◦ selections from the most in-plane to the most out-of-plane proceeding from left to right. Associatedpartner momentum selections from lower to higher paT are arranged from top to bottom. Bars indicate statistical uncertainties.Flow modulation systematic uncertainties are represented by bands through the points while flow normalization uncertaintiesare shown as dashed lines around zero.

FIG. 12: (Color online) Per-trigger azimuthal jet yields in midcentral 20–60% collision for all trigger orientations with respectto the reaction plane in 15◦ selections from the most in-plane to the most out-of-plane proceeding from left to right. Associatedpartner momentum selections from lower to higher paT are arranged from top to bottom. Bars indicate statistical uncertainties.Flow modulation systematic uncertainties are represented by bands through the points while flow normalization uncertaintiesare shown as dashed lines around zero.

14

FIG. 13: (Color online) Correlation functions in 0–20% central collisions for all trigger orientations with respect to the reactionplane in 15◦ selections from most in-plane to most out-of-plane proceeding from left to right. Associated partner momentumselections from lower to higher paT arranged from top to bottom. Expected flow contributions are shown as solid curves (seetext for details).

FIG. 14: (Color online) Correlation functions in 20–60% central collisions for all trigger orientations with respect to the reactionplane in 15◦ selections from most in-plane to most out-of-plane proceeding from left to right. Associated partner momentumselections from lower to higher paT arranged from top to bottom. Expected flow contributions are shown as solid curves (seetext for details).

15

[1] K. Adcox et al., Nucl. Phys. A757, 184 (2005).[2] S. S. Adler et al., Phys. Rev. C76, 034904 (2007).[3] J. Adams et al., Phys. Rev. Lett. 97, 162301 (2006).[4] A. Adare et al., Phys. Rev. C78, 014901 (2008).[5] A. Adare et al., Phys. Rev. Lett. 104, 252301 (2010).[6] C. Loizides, Eur. Phys. J. C49, 339 (2007).[7] T. Renk and K. J. Eskola, Phys. Rev. C 75, 054910

(2007).[8] F. Dominguez, C. Marquet, A. H. Mueller, B. Wu, and

B.-W. Xiao, Nucl. Phys. A811, 197 (2008).[9] A. Adare et al. (2010), 1006.3740.

[10] S. A. Bass, C. Gale, A. Majumder, C. Nonaka, G.-Y.Qin, T. Renk, and J. Ruppert, Phys. Rev. C 79, 024901(2009).

[11] S. Wicks, W. Horowitz, M. Djordjevic, and M. Gyulassy,Nucl. Phys. A784, 426 (2007).

[12] M. L. Miller, K. Reygers, S. J. Sanders, and P. Steinberg,Ann. Rev. Nucl. Part. Sci. 57, 205 (2007).

[13] H.-J. Drescher and Y. Nara, Phys. Rev. C 75, 034905(2007).

[14] M. Luzum and P. Romatschke, Phys. Rev. C78, 034915(2008).

[15] J. Casalderrey-Solana, E. V. Shuryak, and D. Teaney(2006), hep-ph/0602183.

[16] S. S. Gubser, S. S. Pufu, and A. Yarom, Phys. Rev. Lett.100, 012301 (2008).

[17] I. Vitev, Phys. Lett. B630, 78 (2005).[18] A. D. Polosa and C. A. Salgado, Phys. Rev. C75, 041901

(2007).[19] P. Sorensen, arXiv:0808.0503 [nucl-ex].[20] J. Takahashi et al. (2009), 0902.4870.[21] J. Adams et al., Phys. Rev. Lett. 93, 252301 (2004).[22] M. Allen et al., Nucl. Inst. and Meth., A 499, 549 (2003).[23] A. Adare et al., Phys. Rev. Lett. 105, 062301 (2010).[24] S. Afanasiev et al., Phys. Rev. C80, 024909 (2009).[25] S. Afanasiev et al., Phys. Rev. C80, 054907 (2009).[26] A. Adare et al., Phys. Rev. Lett. 101, 232301 (2008).[27] S. S. Adler et al., Phys. Rev. Lett. 97, 052301 (2006).[28] J. Bielcikova, S. Esumi, K. Filimonov, S. Voloshin, and

J. P. Wurm, Phys. Rev. C 69, 021901 (2004).[29] A. Sickles, M. P. McCumber, and A. Adare, Phys. Rev.

C81, 014908 (2010).[30] A. Adare et al., Phys. Rev. C77, 024912 (2008).[31] T. Renk, Phys. Rev. C78, 034904 (2008).[32] Renk, T., private communication.[33] C. Nonaka and S. A. Bass, Phys. Rev. C 75, 014902

(2007).[34] K. J. Eskola, H. Honkanen, H. Niemi, P. V. Ruuskanen,

and S. S. Rasanen, Phys. Rev. C 72, 044904 (2005).[35] A. Adare et al., Phys. Rev. C80, 024908 (2009).