Energy dependence of π0 production in Cu+Cu collisions at √sNN = 22.4, 62.4, and 200 GeV A. Adare,12 S. Afanasiev,26 C. Aidala,13,37 N.N. Ajitanand,54 Y. Akiba,48,49 H. Al-Bataineh,43 J. Alexander,54 K. Aoki,31,48 L. Aphecetche,56 R. Armendariz,43 S.H. Aronson,7 J. Asai,48,49 E.T. Atomssa,32 R. Averbeck,55 T.C. Awes,44 B. Azmoun,7 V. Babintsev,22 M. Bai,6 G. Baksay,18 L. Baksay,18 A. Baldisseri,15 K.N. Barish,8 P.D. Barnes,34 B. Bassalleck,42 A.T. Basye,1 S. Bathe,8 S. Batsouli,44 V. Baublis,47 C. Baumann,38 A. Bazilevsky,7 S. Belikov,7, R. Bennett,55 A. Berdnikov,51 Y. Berdnikov,51 A.A. Bickley,12 J.G. Boissevain,34 H. Borel,15 ∗ K. Boyle,55 M.L. Brooks,34 H. Buesching,7 V. Bumazhnov,22 G. Bunce,7,49 S. Butsyk,34,55 C.M. Camacho,34 S. Campbell,55 B.S. Chang,63 W.C. Chang,2 J.-L. Charvet,15 S. Chernichenko,22 J. Chiba,27 C.Y. Chi,13 M. Chiu,23 I.J. Choi,63 R.K. Choudhury,4 T. Chujo,59,60 P. Chung,54 A. Churyn,22 V. Cianciolo,44 Z. Citron,55 C.R. Cleven,20 8 B.A. Cole,13 M.P. Comets,45 P. Constantin,34 M. Csana´d,17 T. Cs¨orgo˝,28 T. Dahms,55 S. Dairaku,31,48 K. Das,19 0 G. David,7 M.B. Deaton,1 K. Dehmelt,18 H. Delagrange,56 A. Denisov,22 D. d’Enterria,13,32 A. Deshpande,49,55 0 E.J. Desmond,7 O. Dietzsch,52 A. Dion,55 M. Donadelli,52 O. Drapier,32 A. Drees,55 K.A. Drees,6 A.K. Dubey,62 2 A. Durum,22 D. Dutta,4 V. Dzhordzhadze,8 Y.V. Efremenko,44 J. Egdemir,55 F. Ellinghaus,12 W.S. Emam,8 n T. Engelmore,13 A. Enokizono,33 H. En’yo,48,49 S. Esumi,59 K.O. Eyser,8 B. Fadem,39 D.E. Fields,42,49 a J M. Finger,Jr.,9,26 M. Finger,9,26 F. Fleuret,32 S.L. Fokin,30 Z. Fraenkel,62 J.E. Frantz,55 A. Franz,7 A.D. Frawley,19 9 K. Fujiwara,48 Y. Fukao,31,48 T. Fusayasu,41 S. Gadrat,35 I. Garishvili,57 A. Glenn,12 H. Gong,55 M. Gonin,32 2 J. Gosset,15 Y. Goto,48,49 R. Granier de Cassagnac,32 N. Grau,13,25 S.V. Greene,60 M. Grosse Perdekamp,23,49 T. Gunji,11 H.-˚A. Gustafsson,36 T. Hachiya,21 A. Hadj Henni,56 C. Haegemann,42 J.S. Haggerty,7 H. Hamagaki,11 ] x R. Han,46 H. Harada,21 E.P. Hartouni,33 K. Haruna,21 E. Haslum,36 R. Hayano,11 M. Heffner,33 T.K. Hemmick,55 e T. Hester,8 X. He,20 H. Hiejima,23 J.C. Hill,25 R. Hobbs,42 M. Hohlmann,18 W. Holzmann,54 K. Homma,21 - l B. Hong,29 T. Horaguchi,11,48,58 D. Hornback,57 S. Huang,60 T. Ichihara,48,49 R. Ichimiya,48 Y. Ikeda,59 c u K. Imai,31,48 J. Imrek,16 M. Inaba,59 Y. Inoue,50,48 D. Isenhower,1 L. Isenhower,1 M. Ishihara,48 T. Isobe,11 n M. Issah,54 A. Isupov,26 D. Ivanischev,47 B.V. Jacak,55, J. Jia,13 J. Jin,13 O. Jinnouchi,49 B.M. Johnson,7 † [ K.S. Joo,40 D. Jouan,45 F. Kajihara,11 S. Kametani,11,48,61 N. Kamihara,48,49 J. Kamin,55 M. Kaneta,49 1 J.H. Kang,63 H. Kanou,48,58 J. Kapustinsky,34 D. Kawall,37,49 A.V. Kazantsev,30 A. Khanzadeev,47 K.M. Kijima,21 v J. Kikuchi,61 B.I. Kim,29 D.H. Kim,40 D.J. Kim,63 E. Kim,53 S.H. Kim,63 E. Kinney,12 K. Kiriluk,12 A. Kiss,17 5 5 E. Kistenev,7 A. Kiyomichi,48 J. Klay,33 C. Klein-Boesing,38 L. Kochenda,47 V. Kochetkov,22 B. Komkov,47 5 M. Konno,59 J. Koster,23 D. Kotchetkov,8 A. Kozlov,62 A. Kr´al,14 A. Kravitz,13 J. Kubart,9,24 G.J. Kunde,34 4 N. Kurihara,11 K. Kurita,50,48 M. Kurosawa,48 M.J. Kweon,29 Y. Kwon,57,63 G.S. Kyle,43 R. Lacey,54 Y.-S. Lai,13 . 1 Y.S. Lai,13 J.G. Lajoie,25 D. Layton,23 A. Lebedev,25 D.M. Lee,34 K.B. Lee,29 M.K. Lee,63 T. Lee,53 M.J. Leitch,34 0 M.A.L. Leite,52 B. Lenzi,52 P. Liebing,49 T. Liˇska,14 A. Litvinenko,26 H. Liu,43 M.X. Liu,34 X. Li,10 B. Love,60 8 0 D. Lynch,7 C.F. Maguire,60 Y.I. Makdisi,6,7 A. Malakhov,26 M.D. Malik,42 V.I. Manko,30 E. Mannel,13 : Y. Mao,46,48 L. Maˇsek,9,24 H. Masui,59 F. Matathias,13 M. McCumber,55 P.L. McGaughey,34 N. Means,55 v i B. Meredith,23 Y. Miake,59 P. Mikeˇs,9,24 K. Miki,59 T.E. Miller,60 A. Milov,7,55 S. Mioduszewski,7 M. Mishra,3 X J.T. Mitchell,7 M. Mitrovski,54 A.K. Mohanty,4 Y. Morino,11 A. Morreale,8 D.P. Morrison,7 T.V. Moukhanova,30 ar D. Mukhopadhyay,60 J. Murata,50,48 S. Nagamiya,27 Y. Nagata,59 J.L. Nagle,12 M. Naglis,62 M.I. Nagy,17 I. Nakagawa,48,49 Y. Nakamiya,21 T. Nakamura,21 K. Nakano,48,58 J. Newby,33 M. Nguyen,55 T. Niita,59 B.E. Norman,34 R. Nouicer,5 A.S. Nyanin,30 E. O’Brien,7 S.X. Oda,11 C.A. Ogilvie,25 H. Ohnishi,48 H. Okada,31,48 K. Okada,49 M. Oka,59 O.O. Omiwade,1 Y. Onuki,48 A. Oskarsson,36 M. Ouchida,21 K. Ozawa,11 R. Pak,5,7 D. Pal,60 A.P.T. Palounek,34 V. Pantuev,55 V. Papavassiliou,43 J. Park,53 W.J. Park,29 S.F. Pate,43 H. Pei,25 J.-C. Peng,23 H. Pereira,15 V. Peresedov,26 D.Yu. Peressounko,30 C. Pinkenburg,7 M.L. Purschke,7 A.K. Purwar,34 H. Qu,20 J. Rak,42 A. Rakotozafindrabe,32 I. Ravinovich,62 K.F. Read,44,57 S. Rembeczki,18 M. Reuter,55 K. Reygers,38 V. Riabov,47 Y. Riabov,47 D. Roach,60 G. Roche,35 S.D. Rolnick,8 A. Romana,32, M. Rosati,25 ∗ S.S.E. Rosendahl,36 P. Rosnet,35 P. Rukoyatkin,26 P. Ruˇziˇcka,24 V.L. Rykov,48 B. Sahlmueller,38 N. Saito,31,48,49 T. Sakaguchi,7 S. Sakai,59 K. Sakashita,48,58 H. Sakata,21 V. Samsonov,47 S. Sato,27 T. Sato,59 S. Sawada,27 K. Sedgwick,8 J. Seele,12 R. Seidl,23 A.Yu. Semenov,25 V. Semenov,22 R. Seto,8 D. Sharma,62 I. Shein,22 A. Shevel,47,54 T.-A. Shibata,48,58 K. Shigaki,21 M. Shimomura,59 K. Shoji,31,48 P. Shukla,4 A. Sickles,7,55 C.L. Silva,52 D. Silvermyr,44 C. Silvestre,15 K.S. Sim,29 B.K. Singh,3 C.P. Singh,3 V. Singh,3 S. Skutnik,25 M. Sluneˇcka,9,26 A. Soldatov,22 R.A. Soltz,33 W.E. Sondheim,34 S.P. Sorensen,57 I.V. Sourikova,7 F. Staley,15 2 P.W. Stankus,44 E. Stenlund,36 M. Stepanov,43 A. Ster,28 S.P. Stoll,7 T. Sugitate,21 C. Suire,45 A. Sukhanov,5 J. Sziklai,28 T. Tabaru,49 S. Takagi,59 E.M. Takagui,52 A. Taketani,48,49 R. Tanabe,59 Y. Tanaka,41 K. Tanida,48,49 M.J. Tannenbaum,7 A. Taranenko,54 P. Tarj´an,16 H. Themann,55 T.L. Thomas,42 M. Togawa,31,48 A. Toia,55 J. Tojo,48 L. Toma´ˇsek,24 Y. Tomita,59 H. Torii,21,48 R.S. Towell,1 V-N. Tram,32 I. Tserruya,62 Y. Tsuchimoto,21 C. Vale,25 H. Valle,60 H.W. vanHecke,34 A. Veicht,23 J. Velkovska,60 R. Vertesi,16 A.A. Vinogradov,30 M. Virius,14 V. Vrba,24 E. Vznuzdaev,47 M. Wagner,31,48 D. Walker,55 X.R. Wang,43 Y. Watanabe,48,49 F. Wei,25 J. Wessels,38 S.N. White,7 D. Winter,13 C.L. Woody,7 M. Wysocki,12 W. Xie,49 Y.L. Yamaguchi,61 K. Yamaura,21 R. Yang,23 A. Yanovich,22 Z. Yasin,8 J. Ying,20 S. Yokkaichi,48,49 G.R. Young,44 I. Younus,42 I.E. Yushmanov,30 W.A. Zajc,13 O. Zaudtke,38 C. Zhang,44 S. Zhou,10 J. Zim´anyi,28, and L. Zolin26 ∗ (PHENIX Collaboration) 1Abilene Christian University, Abilene, TX 79699, U.S. 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 5Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. 6Collider-Accelerator Department, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. 7Physics Department, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. 8University of California - Riverside, Riverside, CA 92521, U.S. 9Charles University, Ovocn´y trh 5, Praha 1, 116 36, Prague, Czech Republic 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, CO 80309, U.S. 13Columbia University, New York, NY 10027 and Nevis Laboratories, Irvington, NY 10533, U.S. 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 18Florida Institute of Technology, Melbourne, FL 32901, U.S. 19Florida State University, Tallahassee, FL 32306, U.S. 20Georgia State University, Atlanta, GA 30303, U.S. 21Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan 22IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, 142281, Russia 23University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S. 24Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic 25Iowa State University, Ames, IA 50011, U.S. 26Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia 27KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan 28KFKI Research Institute for Particle and Nuclear Physics of the Hungarian Academy of Sciences (MTA KFKI RMKI), H-1525 Budapest 114, POBox 49, Budapest, Hungary 29Korea University, Seoul, 136-701, Korea 30Russian Research Center “Kurchatov Institute”, Moscow, Russia 31Kyoto University, Kyoto 606-8502, Japan 32Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128, Palaiseau, France 33Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S. 34Los Alamos National Laboratory, Los Alamos, NM 87545, U.S. 35LPC, Universit´e Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France 36Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden 37Department of Physics, University of Massachusetts, Amherst, MA 01003-9337, U.S. 38Institut fu¨r Kernphysik, University of Muenster, D-48149 Muenster, Germany 39Muhlenberg College, Allentown, PA 18104-5586, U.S. 40Myongji University, Yongin, Kyonggido 449-728, Korea 41Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan 42University of New Mexico, Albuquerque, NM 87131, U.S. 43New Mexico State University, Las Cruces, NM 88003, U.S. 44Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S. 45IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406, Orsay, France 46Peking University, Beijing, People’s Republic of China 47PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad region, 188300, Russia 48RIKEN, The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan 49RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S. 50Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan 3 51Saint Petersburg State Polytechnic University, St. Petersburg, Russia 52Universidade de S˜ao Paulo, Instituto de F´ısica, Caixa Postal 66318, S˜ao Paulo CEP05315-970, Brazil 53System Electronics Laboratory, Seoul National University, Seoul, Korea 54Chemistry Department, Stony Brook University, Stony Brook, SUNY, NY 11794-3400, U.S. 55Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, NY 11794, U.S. 56SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universit´e de Nantes) BP 20722 - 44307, Nantes, France 57University of Tennessee, Knoxville, TN 37996, U.S. 58Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan 59Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan 60Vanderbilt University, Nashville, TN 37235, U.S. 61Waseda University, Advanced Research Institute for Science and Engineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan 62Weizmann Institute, Rehovot 76100, Israel 63Yonsei University, IPAP, Seoul 120-749, Korea (Dated: February 4, 2008) Neutral pion transverse momentum (pT) spectra at midrapidity (y 0.35) were measured in Cu+Cucollisionsat√sNN =22.4,62.4,and200GeV.Relativetoπ0|yi|el≤dsinp+pcollisionsscaled by the number of inelastic nucleon-nucleon collisions (Ncoll) the π0 yields for pT > 2 GeV/c in centralCu+Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhanceme∼nt is observed at 22.4 GeV. A comparison with a jet quenching model suggests that final state parton energy- loss dominates in central Cu+Cu collisions at 62.4 GeV and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclearmodifications in theinitial state alone. PACSnumbers: 25.75.Dw The measurement of particle yields at high transverse collisions is particularly useful because hadron suppres- momentum(p >2GeV/c)hasplayedakeyroleinchar- sion in Au+Au collisions is observed for rather periph- T acterizing the m∼edium created in nucleus-nucleus colli- eralcollisionswithanumber ofparticipatingnucleonsof sionsattheRelativisticHeavyIonCollider(RHIC)[1,2]. N 50 100 [5]. This N range can be studied part part ∼ − Hadronsproduced atsufficiently highp result fromthe withreduceduncertaintiesinN withthesmaller63Cu T coll interaction of quarks and gluons with high momentum nucleus. transfer (“hard scattering”) which can be described by A critical parameter in jet quenching models is the perturbative quantum-chromodynamics (pQCD). These initial color-charge density of the medium. By studying hadrons are produced as particle jets in the fragmenta- Cu+Cu collisions in the range √sNN 20 200 GeV ∼ − tion of the scattered partons. A scattered parton prop- this parameter can be varied with essentially no change agating through a quark-gluon plasma, a thermalized in transverse size and shape of the reaction zone. More- medium in which quarks and gluons are not confined over, the enhancement of hadron yields due to multiple in hadrons, loses energy (“jet-quenching”) resulting in soft scattering of the incoming partons (“nuclear k ” or T hadron yields at high pT being suppressed [3]. Such a “Cronin enhancement”) is expected to increase towards suppression was indeed observed in central Au+Au col- smaller √sNN [8], thus the interplay between this en- lisions at √sNN =130 and200 GeV at RHIC, providing hancementandthesuppressionduetopartonenergy-loss evidence for largecolor-chargedensities in these systems can be studied. [4, 5, 6]. Inthisletterwepresentinvariantπ0 yieldsforCu+Cu Characteristicpropertiesofthesuppressionofhadrons collisions at √sNN = 22.4,62.4, and 200 GeV. Refer- at high-p , e.g., the dependence on p and centrality, ence data for p+p collisions at √s = 62.4 GeV and T T were studied in detail in Au+Au collisions at √sNN = 200 GeV were taken with the same experiment [13, 14]. 200 GeV [5]. However,the energy dependence of hadron At √sNN =22.4 GeV p+p referencedata wereobtained productioninA+Acollisionsaspredictedbyjetquench- from a parameterization of the world’s data on π0 pro- ing models [7, 8, 9] is not well constrained by mea- duction [15]. surements. Work in this direction was presented in Neutral pions were measured via their π0 γγ decay → [10, 11, 12]. To study the energy dependence of jet- branchwith the electromagneticcalorimeter (EMCal)of quenching it is desirable to measure identified particles thePHENIXexperiment[16]. TheEMCalcomprisestwo in the same colliding system over a large √sNN range calorimeter types: 6 sectors of a lead scintillator sam- and to compare to p+p reference data measured in the pling calorimeter (PbSc) and 2 sectors of a lead glass same experimental setup. Identified particles provide an Cherenkov calorimeter (PbGl). Each sector is located advantage over unidentified hadrons in that the inter- 5m from the beamline and subtends η < 0.35 in ∼ | | pretation is not complicated by the different contribu- pseudorapidity and ∆ϕ = 22.5 in azimuth. Owing to ◦ tions from baryons and mesons. The study of Cu+Cu thePbSc(PbGl)granularityof∆η ∆ϕ=0.011 0.011 × × 4 TABLEI:Datasetspresentedinthispaperwiththenumber TABLEII:GlauberMonteCarlocalculationsforCu+Cucol- of analyzed events. For the ERT triggered data the number lisions at 22.4, 62.4, and 200 GeV. The N systematic un- coll of equivalent minimum bias eventsis given. certaintyat 62.4 and200 GeV is 12%, almost independent ∼ ofN . At22.4 GeVtherelativeuncertaintyofN canbe coll coll system √sNN εtrig NeMvtB NeEvRtT (Nesvatmpled) parameterized as 0.094+0.173e−0.0272Ncoll. Cu+Cu 22.4 GeV 75 90% 5.8 106 — − · 22.4GeV 62.4GeV 200GeV Cu+Cu 62.4 GeV (88 4)% 192 106 — Cu+Cu 200 GeV (94±2)% 794·106 15.5 106 (4720 106) hNparti hNcolli hNparti hNcolli hNparti hNcolli ± · · · 0-10% 92.2 140.7 93.3 152.3 98.2 182.7 10-20% 67.8 93.3 71.1 105.5 73.6 121.1 20-30% 48.3 59.7 51.3 67.8 53.0 76.1 (0.008 0.008)theprobabilitythatthetwophotonshow- 30-40% 34.1 38.0 36.2 42.6 37.3 47.1 × ers from a π0 decay result in partially overlapping clus- 40-50% 23.1 22.9 24.9 26.2 25.4 28.1 ters is negligible up to a π0 p of 12 GeV/c (15 GeV/c). T 50-60% 15.5 13.9 16.1 15.0 16.7 16.2 The energy calibration of the EMCal was corroborated 60-70% — — — — 10.4 9.0 bythepositionoftheπ0 invariantmasspeak,theenergy 70-80% — — — — 6.4 4.9 depositedbyminimumionizingchargedparticlestravers- 80-94% — — — — 3.6 2.4 ing the EMCal (PbSc), and the correlation between the 60-88% — — 7.0 5.5 — — measuredmomenta of electronand positron tracksiden- tified by the ring-imaging Cherenkov detector and the associated energy deposited in the EMCal. These stud- ies showedthat the accuracyof the energy measurement was better than 1.5%. Neutral pions yields were measured on a statistical The total number of analyzed Cu+Cu events for the basis by calculating the invariant mass of all photon three energies is shown in Table I. The minimum bias pairs in a given event and counting those within the (MB) trigger for all reaction systems was provided by π0 mass range. The background of combinatorial pairs Beam-Beam-Counters (BBC’s) located at 3.0 < η < was calculated by pairing photon hits from different 3.9. Thereactionvertexalongthebeamaxis,det∼erm|i|ne∼d events. Only photon pairs with an energy asymmetry from the arrival time differences in the BBC’s, was re- E E /(E +E ) < 0.7 were accepted. The raw π0 1 2 1 2 | − | quired to be in the range z 30 cm. An additional yields were correctedfor the geometricalacceptance and | | ≤ hardware trigger (ERT) on high-pT photons/electrons reconstruction efficiency. The latter takes into account wasemployedinCu+Cuat√sNN =200GeV.Thistrig- the loss of π0’s due to photon identification cuts, the en- ger was based on the analog energy signal measured in ergyasymmetrycut, inactivedetectorareas,andphoton overlapping4 4towersoftheEMCalincoincidencewith conversions. Moreover, it corrects the distortion of the × the MB trigger condition. The ERT reached a efficiency π0 spectrum which results from the finite energy resolu- plateau for photon energies E >4 GeV. tion in conjunction with the steeply falling spectra and The centrality selection in∼Cu+Cu at √sNN = showeroverlapeffects. ForCu+Cuat√sNN =200GeV 200 GeV and √sNN = 62 GeV was based on the the transition between the minimum bias and the ERT charge signal of the BBC’s which is proportional to the sample occurs at pT = 8 GeV/c. The final spectra were charged particle multiplicity in the respective pseudora- calculatedastheweightedaverageofthePbScandPbGl pidity range. The BBC trigger efficiency (ε ) for these results, which agree well within the uncertainties. trig systemswasdeterminedwiththeaidoftheHIJINGevent The main systematic uncertainties of the π0 spec- generator and a full GEANT simulation of the BBC re- tra result from the π0 peak extraction, the reconstruc- sponse (see Table I). At √sNN = 22.4 GeV centrality tion efficiency, and the EMCal energy calibration. For classes were defined based on the charged particle mul- p > 2 GeV/c the peak extraction uncertainty is 4% T tiplicity measuredwith the pad chamber (PC1) detector for∼all systems, approximately independent of p .∼The T (η <0.35). ThemeasuredPC1multiplicitydistribution uncertaintyinthereconstructionefficiencywasestimated | | wasaccuratelyreproducedinaGlauberMonteCarlocal- to be 15% for the three Cu+Cu analyses. It includes ∼ culation [17] and centrality classes were determined by uncertainties of the photon identification cuts, the en- identical cuts on the measured and simulated PC1 mul- ergy resolution, and the modeling of shower overlap ef- tiplicities. The estimatedBBC triggerefficiency givenin fects. TheuncertaintyintheEMCalenergyscaleof1.5% Table I results from a comparison of the simulated and translatesintoanuncertaintyintheyieldsthatincreases themeasuredPC1multiplicitydistributions. Theresults from 8% at p = 3 GeV/c to 15% at p = 6 GeV/c. T T ∼ of the Glauber calculation [17] for Cu+Cu collisions at The high-p part of the spectra in Cu+Cu at 200 GeV T 22.4, 62.4, and 200 GeV using inelastic cross sections of measuredwiththeERTtriggerissubjecttoanadditional 32.3,35.6,and42mb, respectively,aregiveninTable II. uncertainty of 10% related to the ERT trigger efficiency 5 2)eV 1 a) 2)eV 10 b) 2)eV 1014 c) G G G 2/c 10-1 Cu+Cu, 200 GeV, 0-10% 3/c p+p, 200 GeV 3/c 2Nd1 ( dydp N pevtTT1100--53 CCuu++CCuu,, 6222..44 GGeeVV,, 00--1100%% 3sd (mb E 3pd1100--31 pp++pp,, 6222..44 GGeeVV para. 3sdneff (mb E V)3pd 1100181 p2 10-5 Ge /s 10-7 10-7 ( 105 p+p, 200 GeV, neff = 6.3 p+p, 62.4 GeV, n = 6.2 eff p+p, 22.4 GeV, n = 5.8 10-9 10-9 102 eff 0 5 10 15 0 5 10 15 10-2 10-1 p (GeV/c) p (GeV/c) x T T T FIG. 1: Invariant π0 yields in central Cu+Cu collisions (a) and invariant π0 cross sections in p+p collisions (b) at √sNN = 22.4,62.4,200 GeV [13, 14]. The error bars represent the quadratic sum of the statistical and total systematic uncertainties. Plotted as a function of xT=2pT/√s (c) thep+p data exhibit an approximate xT scaling. and normalization. isshowninFig.2. The suppressionat62.4GeV(R AA PHENIXhasnotyetacquiredap+pdatasetat√s= 0.6 for p > 3 GeV/c) and 200 GeV (R 0.5 0.≈6 T AA 22.4 GeV. Therefore world data on charged and neutral for p > 3∼GeV/c) is consistent with expecta≈tions−from T pionproductionintherange21.7 √s 23.8GeVwere parton∼energy-loss. TheR >1inCu+Cuat22.4GeV AA scaled to √s = 22.4 GeV and fit≤in the≤range 0 < p < is similar to the enhancementby a factor 1.5 (at p T T ∼ ≈ 7 GeV/c with Ed3σ/dp3 = A(eapT+b)n(√s/2 ∼p )∼m 3 GeV/c) observed in p+W relative to p+Be collisions T − where A=1.22 10−17 mbGeV−2c3, a=0.053 GeV−1c, at √sNN = 19.4 GeV and 23.8 GeV [20]. For a similar · b= 0.884,n= 15.25,andm=4.653[15]. Thescaling number of participants the R in Cu+Cu at 22.4 GeV AA − − correction was determined with a next-to-leading-order agrees with the R in Pb+Pb collisions at 17.3 GeV AA QCD calculation. The scaling correction was largest for [12]. √s=23.8GeVandreducedthesespectraby∼30%[15]. For pT > 3 GeV/c the measured nuclear modification Theparameterizationisconsistentwithin 25%withthe ± factors at∼62.4, and 200 GeV are consistent with a nu- existingπ0andπ measurementswithoutdiscerniblep - ± T merically evaluated parton energy-loss model described dependent systematic deviations. in [21, 22] as indicated by the comparison in Fig. 2. The π0 p spectra for p+p and central Cu+Cu colli- T This calculation takes into account shadowing from co- sions(0−10%ofσiCnuel+Cu)at√sNN =22.4,62.4[13],and herent final state interactions in nuclei [23], Cronin en- 200 GeV [6] are shown in Fig. 1a and 1b. At sufficiently hancement [24], initial state parton energy-loss in cold high p where pion production in p+p collisions is dom- T nuclear matter [25], and final state parton energy-loss inated by fragmentation of jets, QCD predicts a scaling in dense partonic matter [9, 21, 22]. The Cronin en- law √sneff(xT,√s)Ed3σ/dp3 = G(xT) with a universal hancement measured in p+A collisions is described well function G(xT) where xT = 2pT/√s [18]. Fig. 1c shows by this model [24]. The initial gluon rapidity density thatsuchascalinginxT isindeedobservedforp+pcolli- dNg/dy which characterizes the medium was not fit to sionsat22.4,62.4,and200GeV,consistentwithprevious the R values, but instead was constrained by mea- AA observations [19]. The xT values at which the universal sured charged-particle multiplicities and the assumption cisudrvoemGin(axtTe)dibsyrehaacrhdedprioncdeiscsaetsefothraptTp>art2icGleeVpr/ocdfuocrttiohne wofitpharκto=n-h3a/d2ron30d%ualaitnyd(ddηN/gd/ydy =1.2κadtη/adllyednNecrhg/iedsη) three considered energies. ∼ [21, 22]. The a±verage fractional en≡ergy losses ∆E/E for Nucleareffects onhigh-pT π0 productioncanbequan- a quark (gluon) with E = 6 GeV corresponding to the tified with the nuclear modification factor dNg/dy ranges in Fig. 2 are 0.13 0.19 (0.29 0.42), − − 0.16 0.20(0.35 0.44),0.20 0.28(0.44 0.63)incentral R (p ) = (1/NAevAt)d2NAA/dpTdy (1) Cu+−Cucollision−sat22.4,62.−4,and200G−eV,respectively AA T T d2σ /dp dy h ABi × pp T [22]. For Cu+Cu at √sNN = 22.4 GeV the calculation isalsoshownwithoutfinalstatepartonenergy-loss. The where T = N /σinel. In the absence of nuclear effectshRABi= 1hfocrollpi >pp2 GeV/c where pions result measurementis consistentwith this calculationbut does AA T not rule out a scenario with parton energy-loss. fromhardscatteringproc∼esses. R (p )forthe0 10% AA T − mostcentralCu+Cucollisionsat22.4,62.4,and200GeV Fig. 3 shows that the π0 suppression in the range 6 RAA Vnoit eevn,e rsgNyN =lo 2s2s.4 GeV, sNN = 22.4 GeV V/c) s = 62.4 GeV e NN G 2 VVVsiiitttNeeeNvvv =,,, 226202200..044 G GGGeeeeVVVV, ,,2 11537505 < << d ddNNN g gg/d//ddyyy < << 3 12785055 < 3.5 T1.5 1.5 p Cu+Cu, 0-10% most central < 5 1 2. 1 > ( A A R < 0.5 0.5 Cu+Cu sNN = 22.4 GeV: data Vitev sNN = 62.4 GeV: data Vitev sNN = 200 GeV: data Vitev 0 0 0 5 10 15 0 50 100 p (GeV/c) N T part FIG.2: Measuredπ0RAA asafunctionofpTforthe0 10% FIG. 3: The average RAA in the interval 2.5 < pT < − mostcentralCu+Cucollisionsat√sNN =22.4,62.4,200GeV 3.5 GeV/c as a function of centrality for Cu+Cu collisions incomparison toajetquenchingcalculation [21,22]. Theer- at √sNN = 22.4,62.4, and 200 GeV. The shaded bands rep- ror bars in thisfigure (and in Fig. 3) represent thequadratic resentjetquenchingcalculationsatthreediscretecentralities sumofthestatisticaluncertaintiesandthepoint-to-pointun- (Npart 10,50,100) [21, 22]. The boxes around unity repre- ∼ correlatedandcorrelatedsystematicuncertainties. Theboxes sent the normalization and N uncertainties for a typical coll h i aroundunityindicateuncertaintiesrelated to N andab- N uncertaintyof 12%. coll coll h i solute normalization. The bands for the theory calculation correspond to the assumed range of the initial gluon density dNg/dy. The thin solid line is a calculation without parton energy-loss for central Cu+Cu at √sNN =22.4 GeV. vailoverthe Croninenhancementbetween√sNN =22.4 and 62.4 GeV. We thank the staff of the Collider-Accelerator and 2.5 < pT < 3.5 GeV/c increases towards more central Physics Departments at BNL for their vital contribu- Cu+Cu collisions for √sNN = 62.4, 200 GeV. On the tions. We thankIvanVitevforprovidingthe jetquench- otherhand, RAA at√sNN =22.4GeVremainsapproxi- ing calculations. We acknowledge support from the Of- mately constantas a function of Npart, suggesting either fice of Nuclear Physics in DOE Office of Science and that the Cronin enhancement depends only weakly on NSF (U.S.A.), MEXT and JSPS (Japan), CNPq and centralityorthatinthis energyrangepartonenergy-loss FAPESP(Brazil),NSFC(China),MSMT(CzechRepub- isoffsetbythelargereffectofCroninenhancementovera lic), IN2P3/CNRS, and CEA (France), BMBF, DAAD, broadrangeofcentrality. Itappearsfromthesedatathat and AvH (Germany), OTKA (Hungary), DAE (India), inCu+Cucollisionsbetween√sNN =22.4and62.4GeV ISF(Israel),KRFandKOSEF(Korea),MES,RAS,and parton energy-loss will start to prevail over the Cronin FAAE(Russia),VRandKAW(Sweden),U.S.CRDFfor enhancement, resulting in a net suppression. the FSU, US-Hungary Fulbright, and US-Israel BSF. In summary, for the first time π0 p spectra for the T same nuclear colliding system (Cu+Cu) were measured inthesameexperimentalsetupoverawiderangeofener- gies (√sNN = 22.4,62.4, and 200 GeV). Nuclear effects were studied using measured p+p π0 reference spectra ∗ Deceased † PHENIXSpokesperson: [email protected] from PHENIX at 62.4 and 200 GeV, and a parameteri- [1] K. Adcox et al., Nucl. Phys. A757, 184 (2005). zation of world data at 22.4 GeV. High-p π0 yields in T [2] K. Adcox et al., (in preparation) (2008). central Cu+Cu collisions at 62.4 GeV and 200 GeV are [3] M. Gyulassy and M. Plumer, Phys. Lett. B243, 432 suppressed,suggestingthatpartonenergy-lossisasignif- (1990). icant effect in these systems. At 22.4 GeV π0 yields for [4] K. Adcox et al., Phys. Rev.Lett. 88, 022301 (2002). p >2GeV/carenotsuppressed. TheR measuredin [5] S. S. Adleret al., Phys.Rev.Lett. 91, 072301 (2003). T AA cen∼tralCu+Cuat22.4GeVisconsistentwithCroninen- [6] J. Adamset al., Phys. Rev.Lett. 91, 172302 (2003). [7] X.-N.Wang, Phys. Rev.C61, 064910 (2000). hancementalonebutdoesnotruleoutpartonenergy-loss [8] I. Vitev and M. Gyulassy, Phys. Rev. Lett. 89, 252301 effects. Themeasurementsofhigh-p π0 productionover T (2002). a factor 10 in center-of-mass energy presented in this [9] I. Vitev,Phys. Lett. B606, 303 (2005). ∼ letterprovideauniqueconstraintforjet-quenchingmod- [10] B. Alveret al., Phys. Rev.Lett. 96, 212301 (2006). elsanddemonstratethatpartonenergy-lossstartstopre- [11] B. I.Abelev et al. (2007), nucl-ex/0703040. 7 [12] M.M.Aggarwal etal.(2007), arXiv:0708.2630 [nucl-ex]. Phys. Rev.D11, 1199 (1975). [13] A.Adare et al., (in preparation) (2008). [19] S. S. Adleret al., Phys.Rev. C69, 034910 (2004). [14] A.Adare et al., Phys. Rev. D76, 051106 (2007). [20] D. Antreasyan et al., Phys.Rev. D19, 764 (1979). [15] F. Arleo and D. d’Enterria, in preparation (2008). [21] I. Vitev,Phys. Lett. B639, 38 (2006). [16] L. Aphecetche et al., Nucl. Instrum. Meth. A499, 521 [22] I. Vitev,(private communication) (2007). (2003). [23] J.-w. Qiu and I. Vitev,Phys.Lett. B632, 507 (2006). [17] M.L.Miller,K.Reygers,S.J.Sanders,andP.Steinberg, [24] I. Vitev,Phys. Lett. B562, 36 (2003). Ann.Rev.Nucl.Part. Sci. 57, 205 (2007). [25] I. Vitev,Phys. Rev. C75, 064906 (2007). [18] R.F.Cahalan,K.A.Geer,J.B.Kogut,andL.Susskind,