$J/\psi$ production from proton-proton collisions at $\sqrt s$=200 GeV

arXiv:hep-ex/0307019v1 8 Jul 2003

J/ψ production from proton-proton collisions at

√

s = 200 GeV

S.S. Adler,5 _{S. Afanasiev,}17 _{C. Aidala,}5 _{N.N. Ajitanand,}43 _{Y. Akiba,}20, 38 _{J. Alexander,}43 _{R. Amirikas,}12 L. Aphecetche,45 _{S.H. Aronson,}5 _{R. Averbeck,}44_{T.C. Awes,}35 _{R. Azmoun,}44 _{V. Babintsev,}15 _{A. Baldisseri,}10

K.N. Barish,6_{P.D. Barnes,}27_{B. Bassalleck,}33_{S. Bathe,}30 _{S. Batsouli,}9 _{V. Baublis,}37 _{A. Bazilevsky,}39, 15 S. Belikov,16, 15 _{Y. Berdnikov,}40 _{S. Bhagavatula,}16_{J.G. Boissevain,}27 _{H. Borel,}10 _{S. Borenstein,}25 _{M.L. Brooks,}27 D.S. Brown,34_{N. Bruner,}33_{D. Bucher,}30 _{H. Buesching,}30 _{V. Bumazhnov,}15_{G. Bunce,}5, 39 _{J.M. Burward-Hoy,}26, 44

S. Butsyk,44 _{X. Camard,}45 _{J.-S. Chai,}18_{P. Chand,}4 _{W.C. Chang,}2 _{S. Chernichenko,}15_{C.Y. Chi,}9 _{J. Chiba,}20 M. Chiu,9 _{I.J. Choi,}52 _{J. Choi,}19 _{R.K. Choudhury,}4 _{T. Chujo,}5 _{V. Cianciolo,}35 _{Y. Cobigo,}10 _{B.A. Cole,}9 P. Constantin,16 _{D.G. d’Enterria,}45 _{G. David,}5 _{H. Delagrange,}45_{A. Denisov,}15 _{A. Deshpande,}39_{E.J. Desmond,}5 O. Dietzsch,41 _{O. Drapier,}25_{A. Drees,}44_{K.A. Drees,}5 _{R. du Rietz,}29 _{A. Durum,}15 _{D. Dutta,}4 _{Y.V. Efremenko,}35 K. El Chenawi,49_{A. Enokizono,}14_{H. En’yo,}38, 39 _{S. Esumi,}48_{L. Ewell,}5_{D.E. Fields,}33, 39 _{F. Fleuret,}25 _{S.L. Fokin,}23

B.D. Fox,39 _{Z. Fraenkel,}51 _{J.E. Frantz,}9_{A. Franz,}5 _{A.D. Frawley,}12_{S.-Y. Fung,}6 _{S. Garpman,}29,∗ _{T.K. Ghosh,}49 A. Glenn,46 _{G. Gogiberidze,}46 _{M. Gonin,}25 _{J. Gosset,}10 _{Y. Goto,}39 _{R. Granier de Cassagnac,}25 _{N. Grau,}16

S.V. Greene,49 _{M. Grosse Perdekamp,}39 _{W. Guryn,}5 _{H.-˚A. Gustafsson,}29 _{T. Hachiya,}14 _{J.S. Haggerty,}5 H. Hamagaki,8 _{A.G. Hansen,}27 _{E.P. Hartouni,}26_{M. Harvey,}5 _{R. Hayano,}8 _{X. He,}13 _{M. Heffner,}26 _{T.K. Hemmick,}44

J.M. Heuser,44 _{M. Hibino,}50 _{J.C. Hill,}16_{W. Holzmann,}43 _{K. Homma,}14 _{B. Hong,}22 _{A. Hoover,}34_{T. Ichihara,}38, 39 V.V. Ikonnikov,23_{K. Imai,}24, 38 _{D. Isenhower,}1 _{M. Ishihara,}38_{M. Issah,}43 _{A. Isupov,}17_{B.V. Jacak,}44_{W.Y. Jang,}22

Y. Jeong,19_{J. Jia,}44 _{O. Jinnouchi,}38 _{B.M. Johnson,}5 _{S.C. Johnson,}26 _{K.S. Joo,}31 _{D. Jouan,}36 _{S. Kametani,}8, 50 N. Kamihara,47, 38 _{J.H. Kang,}52 _{S.S. Kapoor,}4_{K. Katou,}50 _{S. Kelly,}9 _{B. Khachaturov,}51 _{A. Khanzadeev,}37

J. Kikuchi,50 _{D.H. Kim,}31 _{D.J. Kim,}52 _{D.W. Kim,}19 _{E. Kim,}42 _{G.-B. Kim,}25 _{H.J. Kim,}52 _{E. Kistenev,}5 A. Kiyomichi,48 _{K. Kiyoyama,}32 _{C. Klein-Boesing,}30_{H. Kobayashi,}38, 39 _{L. Kochenda,}37 _{V. Kochetkov,}15 D. Koehler,33_{T. Kohama,}14 _{M. Kopytine,}44 _{D. Kotchetkov,}6 _{A. Kozlov,}51 _{P.J. Kroon,}5 _{C.H. Kuberg,}1, 27 K. Kurita,39 _{Y. Kuroki,}48 _{M.J. Kweon,}22 _{Y. Kwon,}52 _{G.S. Kyle,}34 _{R. Lacey,}43 _{V. Ladygin,}17 _{J.G. Lajoie,}16

A. Lebedev,16, 23 _{S. Leckey,}44_{D.M. Lee,}27 _{S. Lee,}19 _{M.J. Leitch,}27 _{X.H. Li,}6 _{H. Lim,}42 _{A. Litvinenko,}17 M.X. Liu,27 _{Y. Liu,}36 _{C.F. Maguire,}49_{Y.I. Makdisi,}5 _{A. Malakhov,}17_{V.I. Manko,}23 _{Y. Mao,}7, 38 _{G. Martinez,}45

M.D. Marx,44 _{H. Masui,}48 _{F. Matathias,}44 _{T. Matsumoto,}8, 50 _{P.L. McGaughey,}27 _{E. Melnikov,}15_{F. Messer,}44 Y. Miake,48 _{J. Milan,}43 _{T.E. Miller,}49 _{A. Milov,}44, 51 _{S. Mioduszewski,}5 _{R.E. Mischke,}27 _{G.C. Mishra,}13

J.T. Mitchell,5 _{A.K. Mohanty,}4 _{D.P. Morrison,}5_{J.M. Moss,}27 _{F. M¨uhlbacher,}44 _{D. Mukhopadhyay,}51 M. Muniruzzaman,6 _{J. Murata,}38, 39 _{S. Nagamiya,}20_{J.L. Nagle,}9 _{T. Nakamura,}14_{B.K. Nandi,}6 _{M. Nara,}48 J. Newby,46 _{P. Nilsson,}29 _{A.S. Nyanin,}23 _{J. Nystrand,}29_{E. O’Brien,}5 _{C.A. Ogilvie,}16_{H. Ohnishi,}5, 38 _{I.D. Ojha,}49, 3

K. Okada,38 _{M. Ono,}48 _{V. Onuchin,}15 _{A. Oskarsson,}29 _{I. Otterlund,}29 _{K. Oyama,}8 _{K. Ozawa,}8 _{D. Pal,}51 A.P.T. Palounek,27 _{V.S. Pantuev,}44_{V. Papavassiliou,}34_{J. Park,}42 _{A. Parmar,}33 _{S.F. Pate,}34 _{T. Peitzmann,}30

J.-C. Peng,27 _{V. Peresedov,}17 _{C. Pinkenburg,}5 _{R.P. Pisani,}5 _{F. Plasil,}35 _{M.L. Purschke,}5 _{A.K. Purwar,}44 J. Rak,16 _{I. Ravinovich,}51_{K.F. Read,}35, 46 _{M. Reuter,}44 _{K. Reygers,}30_{V. Riabov,}37, 40 _{Y. Riabov,}37 _{G. Roche,}28

A. Romana,25 _{M. Rosati,}16 _{P. Rosnet,}28 _{S.S. Ryu,}52 _{M.E. Sadler,}1_{N. Saito,}38, 39 _{T. Sakaguchi,}8, 50 _{M. Sakai,}32 S. Sakai,48 _{V. Samsonov,}37 _{L. Sanfratello,}33 _{R. Santo,}30 _{H.D. Sato,}24, 38_{S. Sato,}5, 48 _{S. Sawada,}20_{Y. Schutz,}45 V. Semenov,15_{R. Seto,}6_{M.R. Shaw,}1, 27 _{T.K. Shea,}5 _{T.-A. Shibata,}47, 38 _{K. Shigaki,}14, 20_{T. Shiina,}27 _{C.L. Silva,}41 D. Silvermyr,27, 29 _{K.S. Sim,}22 _{C.P. Singh,}3 _{V. Singh,}3_{M. Sivertz,}5 _{A. Soldatov,}15 _{R.A. Soltz,}26 _{W.E. Sondheim,}27

S.P. Sorensen,46 _{I.V. Sourikova,}5_{F. Staley,}10 _{P.W. Stankus,}35 _{E. Stenlund,}29 _{M. Stepanov,}34 _{A. Ster,}21 S.P. Stoll,5 _{T. Sugitate,}14 _{J.P. Sullivan,}27 _{E.M. Takagui,}41 _{A. Taketani,}38, 39 _{M. Tamai,}50 _{K.H. Tanaka,}20

Y. Tanaka,32 _{K. Tanida,}38 _{M.J. Tannenbaum,}5 _{P. Tarj´an,}11 _{J.D. Tepe,}1, 27 _{T.L. Thomas,}33 _{J. Tojo,}24, 38 H. Torii,24, 38 _{R.S. Towell,}1_{I. Tserruya,}51_{H. Tsuruoka,}48_{S.K. Tuli,}3 _{H. Tydesj¨o,}29 _{N. Tyurin,}15_{H.W. van Hecke,}27

J. Velkovska,5, 44 _{M. Velkovsky,}44 _{L. Villatte,}46 _{A.A. Vinogradov,}23 _{M.A. Volkov,}23 _{E. Vznuzdaev,}37 X.R. Wang,13 _{Y. Watanabe,}38, 39 _{S.N. White,}5 _{F.K. Wohn,}16 _{C.L. Woody,}5 _{W. Xie,}6 _{Y. Yang,}7_{A. Yanovich,}15

S. Yokkaichi,38, 39 _{G.R. Young,}35 _{I.E. Yushmanov,}23 _{W.A. Zajc,}9,† _{C. Zhang,}9_{S. Zhou,}7, 51 _{and L. Zolin}17 (PHENIX Collaboration)

1_{Abilene Christian University, Abilene, TX 79699, USA} 2_{Institute of Physics, Academia Sinica, Taipei 11529, Taiwan} 3_{Department of Physics, Banaras Hindu University, Varanasi 221005, India}

4_{Bhabha Atomic Research Centre, Bombay 400 085, India} 5_{Brookhaven National Laboratory, Upton, NY 11973-5000, USA}

6_{University of California - Riverside, Riverside, CA 92521, USA} 7_{China Institute of Atomic Energy (CIAE), Beijing, People´s Republic of China}

8_{Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan} 9_{Columbia University, New York, NY 10027 and Nevis Laboratories, Irvington, NY 10533, USA}

10_{Dapnia, CEA Saclay, F-91191, Gif-sur-Yvette, France} 11_{Debrecen University, H-4010 Debrecen, Egyetem t´er 1, Hungary}

12_{Florida State University, Tallahassee, FL 32306, USA} 13_{Georgia State University, Atlanta, GA 30303, USA}

14_{Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan} 15_{Institute for High Energy Physics (IHEP), Protvino, Russia}

16_{Iowa State University, Ames, IA 50011, USA}

17_{Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia} 18_{KAERI, Cyclotron Application Laboratory, Seoul, South Korea} 19_{Kangnung National University, Kangnung 210-702, South Korea}

20_{KEK, High Energy Accelerator Research Organization, Tsukuba-shi, Ibaraki-ken 305-0801, Japan} 21_{KFKI Research Institute for Particle and Nuclear Physics (RMKI), H-1525 Budapest 114, POBox 49, Hungary}

22_{Korea University, Seoul, 136-701, Korea}

23_{Russian Research Center “Kurchatov Institute”, Moscow, Russia} 24_{Kyoto University, Kyoto 606, Japan}

25_{Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128, Palaiseau, France} 26_{Lawrence Livermore National Laboratory, Livermore, CA 94550, USA}

27_{Los Alamos National Laboratory, Los Alamos, NM 87545, USA}

28_{LPC, Universit´e Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France} 29_{Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden} 30_{Institut fuer Kernphysik, University of Muenster, D-48149 Muenster, Germany}

31_{Myongji University, Yongin, Kyonggido 449-728, Korea}

32_{Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan} 33_{University of New Mexico, Albuquerque, NM, USA}

34_{New Mexico State University, Las Cruces, NM 88003, USA} 35_{Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA}

36_{IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406, Orsay, France} 37_{PNPI, Petersburg Nuclear Physics Institute, Gatchina, Russia}

38_{RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, JAPAN} 39_{RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973-5000, USA}

40_{St. Petersburg State Technical University, St. Petersburg, Russia}

41_{Universidade de S˜ao Paulo, Instituto de F´ısica, Caixa Postal 66318, S˜ao Paulo CEP05315-970, Brazil} 42_{System Electronics Laboratory, Seoul National University, Seoul, South Korea}

43_{Chemistry Department, Stony Brook University, SUNY, Stony Brook, NY 11794-3400, USA} 44_{Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, NY 11794, USA} 45_{SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universit´e de Nantes) BP 20722 - 44307, Nantes, France}

46_{University of Tennessee, Knoxville, TN 37996, USA}

47_{Department of Physics, Tokyo Institute of Technology, Tokyo, 152-8551, Japan} 48_{Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan}

49_{Vanderbilt University, Nashville, TN 37235, USA} 50_{Waseda University, Advanced Research Institute for Science and}

Engineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan

51_{Weizmann Institute, Rehovot 76100, Israel} 52_{Yonsei University, IPAP, Seoul 120-749, Korea}

(Dated: November 1, 2018)

J/ψ production has been measured in proton-proton collisions at √s = 200 GeV over a wide rapidity and transverse momentum range by the PHENIX experiment at RHIC. Distributions of the rapidity and transverse momentum, along with measurements of the mean transverse momentum and total production cross section are presented and compared to available theoretical calculations. The total J/ψ cross section is 3.99 ± 0.61(stat) ± 0.58(sys) ± 0.40(abs) µb. The mean transverse momentum is 1.80 ± 0.23(stat) ± 0.16(sys) GeV/c.

PACS numbers: 13.85.Ni, 13.20.Fc, 14.40.Gx, 25.75.Dw

Understanding J/ψ production mechanisms requires data over a large range of collision energies and with broad coverage in rapidity and transverse momentum (pT). Existing data at lower energies from fixed

tar-get hadron experiments yield total cross sections and mean pT (hpTi) values in the energy range √s = 7 -38.8 GeV [1]. Limited kinematic coverage in collider ex-periments [1, 2, 3] has so far meant that total cross

sec-tions and mean pT values could not be measured. The systematic study of J/ψ production at Relativistic Heavy Ion Collider (RHIC) energies with wide pT and rapid-ity coverage should therefore provide crucial tests of J/ψ production models. In addition, the RHIC proton-proton results provide a baseline for studying cold and hot nu-clear matter in proton-nucleus and nucleus-nucleus colli-sions using J/ψ yields as a probe.

Intense theoretical interest in the J/ψ production mechanism was stimulated when the Color Singlet Model (CSM) was found [4] to dramatically underpredict the high pT CDF prompt J/ψ and ψ(2S) cross sections [3]. Attention turned toward models in which color octet c¯c states can also contribute to the J/ψ yield. The Color Octet Model (COM), which is based on the Non-Relativistic QCD model [5], has been successful in re-producing the high pT CDF prompt J/ψ cross sections, as has the more phenomenological Color Evaporation Model [6].

In this paper we report results of the first measure-ments of pp → J/ψ+X at RHIC, made at√s = 200 GeV by the PHENIX experiment. The data yield the first to-tal cross sections for J/ψ production beyond fixed target energies, and the first measurement of hpTi beyond√s = 63 GeV. They will constrain models in the lower pT region where gluon fusion is expected to dominate (at pT beyond about 5 GeV/c, the direct J/ψ production cross section is expected to be dominated by fragmentation of high pT gluons [7]).

The PHENIX experiment [8] detects electrons in the pseudo-rapidity range |η| ≤ 0.35 in two central spectrom-eter arms covering ∆φ = 90◦_{, and forward rapidity muons} in two muon arms covering ∆φ = 360◦_{. Only one muon} arm, covering 1.2 < η < 2.2, was operational for this data set. Electrons are identified by matching charged particle tracks to energy deposits in the Electromagnetic Calorimeter (EMC) and to rings in the Ring Imaging

ˇ

Cerenkov Detector (RICH), which has a threshold of 4.7 GeV/c for pions. Muons are identified by finding deeply penetrating roads in the Muon Identifier (MuID) and matching them to tracks in the Muon Tracker (MuTr).

The data were recorded during the 2001/2002 pp run at √

s = 200 GeV. After quality assurance and vertex cuts (± 35 cm for ee and ± 38 cm for µµ), 67 nb−1 _{were used} for the J/ψ → µ+_µ− _{analysis, and 82 nb}−1 _{for J/ψ →} e+_e−_{. The minimum bias interaction trigger required} at least one hit on each side of the interaction vertex in the Beam-Beam counter (BBC). Minimum bias trigger rates varied from 5 to 30 kHz. Events containing J/ψ decays were selected using level-1 triggers in coincidence with the minimum bias interaction trigger. The J/ψ → e+_e−_{trigger required a minimum energy deposit of either} 0.75 GeV in a 2 × 2 tile of EMC towers or 2.1 GeV in a 4 × 4 tile. The J/ψ → µ+_µ− _{trigger required at} least two deeply penetrating roads in separate azimuthal quadrants of the MuID [9].

2 3 4 0 5 10 15 20

)

Inv. mass (GeV/c

unlike-sign

like-sign

Counts

ee

)

Inv. mass (GeV/c

unlike-sign

like-sign

µ

µ

FIG. 1: The invariant mass spectra for dielectron and dimuon pairs. Unlike-sign pairs are shown as solid lines, the sum of like-sign pairs as dashed lines.

J/ψ yields in the central arms were obtained by re-constructing electron-positron pairs. Electron candi-dates were charged particle tracks that were associated with a RICH ring (≥ 2 hit phototubes) and an EMC hit (±4σ position association cut), and which satisfied 0.5 < E/p < 1.5, where E is the EMC cluster energy and p is the reconstructed track momentum. A 5 GeV/c upper limit on electron momentum prevented charged pi-ons from firing the RICH.

J/ψ yields in the muon arm were obtained by recon-structing µ+_µ− _{pairs. Muon tracks were reconstructed} by finding a track seed in the MuID and matching it to clusters of hits in each of the three MuTr stations. The momentum was determined by fitting, with a correction for energy loss, the MuID and MuTr hit positions and the BBC vertex position. Each track was required to pass 5σ cuts on the χ2 _{from the track fit and on the radial} distance of the fitted track from the measured z-vertex position.

Unsign pairs and, for background estimation, like-sign pairs satisfying the above conditions were combined to form invariant mass spectra. Simulations show that the acceptance for like-sign and unlike-sign pairs is the same to within a few percent for invariant masses above 1 GeV/c2 _{for electrons. In Fig. 1, unsign and} like-sign invariant mass spectra from the entire pp data set are shown together. For electrons, the net yield inside 2.8 -3.4 GeV/c2_{is 46, for muons inside 2.71 - 3.67 GeV/c}2_{it is} 65. For electrons, the peak width is 110 MeV/c2_{and the} centroid agrees well with the PDG value [10]. For muons, the width is 160 MeV/c2_{. The muon peak centroid is} higher than the PDG value by about 3%, consistent with the uncertainty in the muon magnetic field calibration.

The J/ψ cross sections were determined from the mea-sured yields using

Bll d2_σ J/ψ dydpT = NJ/ψ (R Ldt) ∆y ∆pT 1 ǫbiasǫlvl1 1 A ǫrec

where NJ/ψis the measured J/ψ yield, R

Ldt is the inte-grated luminosity measured by the minimum bias trigger, Bll is the branching fraction for the J/ψ to either e+e− or µ+_µ−_{pairs (PDG average value 5.9% [10]), ǫ}

biasis the minimum bias trigger efficiency for an event containing a J/ψ, ǫlvl1 is the level-1 trigger efficiency for detecting a J/ψ, and A ǫrec is the acceptance times reconstruction efficiency for a J/ψ.

The integrated luminosity can be written asR Ldt = NMB/σBBC, where NMB is the number of minimum bias triggers and σBBC is the minimum bias trigger cross sec-tion. Using a van der Meer scan measurement, σBBC was determined to be 21.8 ± 2.1 mb [11]. We have estimated ǫbias in two ways. First, the minimum bias trigger efficiency for J/ψ events from a simulation study using Pythia [12] (with the GRV94NLO parton dis-tribution functions (PDFs)) was 0.74, with no pT de-pendence. Good agreement is observed in the dNch/dη distribution between Pythia simulations and measure-ments [13] for events involving high pT π0 production. Second, we measured [11] our minimum bias trigger effi-ciency for high pT π0 production using events recorded with a high pT EMC trigger. The efficiency of 0.75 ± 3% is constant within uncertainties over the measured range of 1.5 < pT < 9 GeV/c. We chose to use our measured trigger efficiency from high pT π0events when calculating the J/ψ cross section, assuming that the trigger efficiency is the same for both processes.

For the electron analysis, A ǫrec was determined as a function of pT using a full GEANT simulation of single J/ψ events with flat distributions in dN/dy (|y| < 0.6), pT (pT < 6 GeV/c) and collision vertex (|z| < 35 cm). The GEANT simulations were tuned to match real detec-tor responses for single electrons. The reconstruction effi-ciency calculations used a typical dead channel map. An average correction for run-to-run variations in detector active area was determined from single electron yields. An estimate of the systematic uncertainty in A ǫrecdue to z vertex dependence of the acceptance, momentum resolution effects, the pair mass cut, and electron identifi-cation cuts is given in Table I, along with the uncertainty in the yield due to Drell-Yan and correlated charm decay contributions. The efficiency ǫlvl1of the level-1 J/ψ trig-gers in the central arms was determined as a function of pT by using a software trigger-emulator to analyze simu-lated single J/ψ events. The results are shown in Table I. The trigger emulator was tuned by analyzing simulated single electrons and comparing with the real single elec-tron trigger efficiency.

For the muon arm, A ǫrecǫlvl1 was determined as an average within each rapidity and pT bin, using a full GEANT simulation with J/ψ events generated by Pythia (with GRV94LO PDFs). The Pythia J/ψ ra-pidity and pTdistributions are very similar to those of the real data, so that bin averaging effects should be approx-imately accounted for by this procedure. The simulated

TABLE I: Table of quantities and their systematic error esti-mates. Ranges are given for pTdependent quantities. For the

µ+

µ−_{case the values of Aǫ}

recand ǫlvl1are combined. The

ab-solute cross section normalization uncertainty from ǫbias and

R

Ldt is kept separate and is labeled (abs). Quantity e+ e− _µ+ µ− Yield _{± 5% ± 5%} Aǫrec 0.026-0.010 ± 13% 0.038 - 0.017 ± 13% ǫlvl1 (2×2) 0.87-0.90 ± 5% (4×4) 0.30-0.74 ± 36% ǫbias 0.75 ± 3% 0.75 ± 3% R Ldt 82 nb−1 ± 9.6% 67 nb−1 ± 9.6% Total ± 15%(sys) ± 10%(abs) ± 14%(sys) ±10%(abs)

0 2 4 6 10-3 10-2 10-1 1 10

(GeV/c)

p

-e

e

→

ψ

J/

(GeV/c)

p

-µ

µ

→

ψ

J/

FIG. 2: The J/ψ pT distributions for the dielectron

and dimuon measurements, with statistical uncertainties. The solid line is a phenomenological fit of the form 1/(2πpT) dσ/dpT = A (1 + (pT/B)2)−6. The dashed line is

an exponential fit. The CSM (dot-dashed) and COM (dotted) calculations are from [15].

events were reconstructed using the same reconstruction software and cuts as for the real data, assuming nomi-nal detector efficiencies and typical realistic dead channel and dead high-voltage maps. Each event had to pass the simulated dimuon trigger. The systematic error includes discrepancies between Monte Carlo and real detector re-sponse, run to run variations in the detector state, and uncertainties in the Pythia distributions. The results, integrated over the rapidity range of the muon arm, are shown in Table I, along with an estimate of the system-atic error on the yield due to the background subtraction technique. In both the electron and muon cases the J/ψ polarization was assumed to be zero, since existing J/ψ polarization measurements are consistent with zero at low pT [14]. The effect of the unknown J/ψ polarization has not been included in the systematic error.

The pT distributions for J/ψ → e+e− and J/ψ → µ+_µ− _{are shown in Fig. 2, with predictions [15] from the} COM. Predictions of the CSM, which greatly underpre-dicts the cross sections, are also shown. These predictions

are limited to pT > 2 GeV/c because parton intrinsic transverse momentum (kT) broadening is not accounted for properly in the calculation. The COM calculations do not include fragmentation contributions, which become important at around 5 GeV/c [3]. Calculations cover-ing all pT and including fragmentation contributions are needed. The solid lines are a phenomenological fit of a form that has been shown to fit J/ψ data well at fixed target energies [16]. The dashed line is an exponential fit. The phenomenological fits yield hpTi values of 1.85 ± 0.46(stat) ± 0.16(sys) GeV/c (central arm) and 1.78 ± 0.27(stat) ± 0.16(sys) GeV/c (muon arm), with a com-bined value of 1.80 ± 0.23(stat) ± 0.16(sys) GeV/c. The systematic uncertainties were estimated from the spread in hpTi from a weighted mean of the binned data, the phenomenological fit, and the exponential fit. An ad-ditional 3% was assigned to the muon hpTi due to the uncertainty in momentum scale.

The J/ψ rapidity distribution obtained by combining the dielectron and dimuon measurements is shown in Fig. 3, with the muon arm data divided into two ra-pidity bins. The COM curves are theoretical shape pre-dictions [9] using the same models as are discussed in connection with Fig. 4b, except that they are normal-ized to our data to make the shape comparison clearer. Since gluon fusion is the dominant process in all of the models, the rapidity shape depends mostly on the gluon distribution function and is not very sensitive to the pro-duction model. Most of the available PDFs are consistent with the data, and improved statistical precision will be needed to constrain them. A Pythia calculation that reproduces the shape of our data best is also shown in Fig. 3. Normalizing this to the data, the total cross sec-tion was determined to be 3.99 ± 0.61(stat) ± 0.58(sys) ± 0.40(abs) µb. The quoted systematic error of 14% was estimated by setting the measured cross sections all to their upper systematic error limits or all to their lower systematic error limits and noting how the cross section changed. The variation in the total cross section ex-tracted if we use the same procedure with different PDF choices and models was estimated to be small ( 3%).

A comparison is made in Fig. 4a of the present hpTi value with values from previous experiments [1]. There are no theoretical predictions that we can compare with the hpTi measurements. The total J/ψ cross section determined in this analysis is shown in Fig. 4b, along with cross sections determined by lower energy experi-ments [1] and predictions from the COM [9] using two different PDFs. The√s dependence of the cross section is sensitive to the factorization scale Q, since the shape of the PDFs depend on Q. The values of Q (3.1 GeV for GRV98NLO and 2.3 GeV for MRST2001NLO) were chosen to give good agreement with the data. The to-tal cross section normalization was obtained using color octet matrix elements from [17], but has large theo-retical uncertainties associated with the charm quark

-3 -2 -1 0 1 2 3 0 20 40 60

(nb)

dy

σ

d

B

rapidity

FIG. 3: The central rapidity point is from J/ψ → e+_e−_{, the}

others from J/ψ → µ+_µ−_{. The brackets represent systematic}

uncertainties. All curves have their overall normalization fit-ted to the data. The Pythia shape was used to determine the cross section. There is an overall 10% absolute normalization error not shown.

(GeV) s 10 102 > (GeV/c)_T <p 0 0.5 1 1.5 2 2.5 ) s Fit (p + q ln PHENIX (a) (GeV) s 10 102 b) µ ( σ 10-4 10-3 10-2 10-1 1 10 COM(GRV98NLO) COM(MRST2001NLO) PHENIX (b)

FIG. 4: (a) The present J/ψ mean pT value compared with

previous measurements at lower energy. The linear fit param-eters are p = 0.53, q = 0.19. (b) The present J/ψ total cross section compared with previous measurements at other values of√s. The curves are discussed in the text.

mass and the renormalization scale. The renormaliza-tion scale was taken to be equal to the quark mass Mc, and their values (1.48 GeV for GRV98NLO and 1.55 GeV for MRST2001NLO) were chosen to give good agreement with the data. The CEM is also able to describe the to-tal cross section data [6]. All measurements and models include feed-down from the χc and the ψ′ to the J/ψ. We estimate [18] that B decay feed-down contributes less than 4% to the J/ψ total cross section at√s = 200 GeV. In summary, we have presented the first pp → J/ψ +X measurements from RHIC, obtained at √s = 200 GeV. The rapidity distributions, pT distributions, hpTi and total cross sections have been presented and compared with available model calculations. The transverse mo-mentum distributions above 2 GeV/c are reasonably well described by the COM. With the present statistical pre-cision, our rapidity distribution shape is consistent with

most of the available PDFs. COM calculations are able to reproduce the√s dependence of the cross section us-ing color octet matrix elements found in the literature, with a reasonable choice of QCD parameters.

RHIC is expected to have proton-proton runs with en-hanced luminosity at√s = 200 and 500 GeV in the near future. The increased luminosity will improve the statis-tical precision and pT reach of the PHENIX data, and will ultimately make it possible to measure the J/ψ polariza-tion, which has been an important test for models [14].

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), CNRS-IN2P3 and CEA (France), BMBF, DAAD, and AvH (Germany), OTKA (Hungary), DAE and DST (India), ISF (Israel), KRF and CHEP (Korea), RMIST, RAS, and RMAE, (Russia), VR and KAW (Sweden), U.S. CRDF for the FSU, Hungarian NSF-OTKA-MTA, and US-Israel BSF.

∗ _Deceased

† _{PHENIX Spokesperson:zajc@nevis.columbia.edu}

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