Heavy-flavor electron-muon correlations in p + p and d +Au collisions at s NN − − − √ = 200 GeV
A. Adare, S. Afanasiev, C. Aidala, N.N. Ajitanand, Y. Akiba, H. Al-Bataineh, J. Alexander, A. Angerami, K. Aoki, N. Apadula, L. Aphecetche, Y. Aramaki, J. Asai, E.T. Atomssa, R. Averbeck, T.C. Awes, B. Azmoun, V. Babintsev, M. Bai, G. Baksay, L. Baksay, A. Baldisseri, K.N. Barish, P.D. Barnes, B. Bassalleck, A.T. Basye, S. Bathe, S. Batsouli, V. Baublis, C. Baumann, A. Bazilevsky, S. Belikov, R. Belmont, R. Bennett, A. Berdnikov, Y. Berdnikov, J.H. Bhom, A.A. Bickley, D.S. Blau, J.G. Boissevain, J.S. Bok, H. Borel, K. Boyle, M.L. Brooks, H. Buesching, V. Bumazhnov, G. Bunce, S. Butsyk, C.M. Camacho, S. Campbell, A. Caringi, B.S. Chang, W.C. Chang, J.-L. Charvet, C.-H. Chen, S. Chernichenko, C.Y. Chi, M. Chiu, I.J. Choi, J.B. Choi, R.K. Choudhury, P. Christiansen, T. Chujo, P. Chung, A. Churyn, O. Chvala, V. Cianciolo, Z. Citron, B.A. Cole, Z. Conesa del Valle, M. Connors, P. Constantin, M. Csanád, T. Csörgő, T. Dahms, S. Dairaku, I. Danchev, K. Das, A. Datta, G. David, M.K. Dayananda, A. Denisov, D. d'Enterria, A. Deshpande, E.J. Desmond, K.V. Dharmawardane, O. Dietzsch, A. Dion, M. Donadelli, O. Drapier, A. Drees, K.A. Drees, A.K. Dubey, J.M. Durham, A. Durum, D. Dutta, V. Dzhordzhadze, L. D'Orazio, S. Edwards, Y.V. Efremenko, et al. (349 additional authors not shown)
aa r X i v : . [ nu c l - e x ] N ov Heavy-flavor electron-muon correlations in p + p and d +Au collisionsat √ s NN = 200 GeV A. Adare, S. Afanasiev, C. Aidala,
44, 45
N.N. Ajitanand, Y. Akiba,
57, 58
H. Al-Bataineh, J. Alexander, A. Angerami, K. Aoki,
36, 57
N. Apadula, L. Aphecetche, Y. Aramaki,
13, 57
J. Asai, E.T. Atomssa, R. Averbeck, T.C. Awes, B. Azmoun, V. Babintsev, M. Bai, G. Baksay, L. Baksay, A. Baldisseri, K.N. Barish, P.D. Barnes, ∗ B. Bassalleck, A.T. Basye, S. Bathe,
6, 9, 58
S. Batsouli, V. Baublis, C. Baumann, A. Bazilevsky, S. Belikov, ∗ R. Belmont, R. Bennett, A. Berdnikov, Y. Berdnikov, J.H. Bhom, A.A. Bickley, D.S. Blau, J.G. Boissevain, J.S. Bok, H. Borel, K. Boyle, M.L. Brooks, H. Buesching, V. Bumazhnov, G. Bunce,
8, 58
S. Butsyk, C.M. Camacho, S. Campbell, A. Caringi, B.S. Chang, W.C. Chang, J.-L. Charvet, C.-H. Chen, S. Chernichenko, C.Y. Chi, M. Chiu,
8, 26
I.J. Choi, J.B. Choi, R.K. Choudhury, P. Christiansen, T. Chujo, P. Chung, A. Churyn, O. Chvala, V. Cianciolo, Z. Citron, B.A. Cole, Z. Conesa del Valle, M. Connors, P. Constantin, M. Csan´ad, T. Cs¨org˝o, T. Dahms, S. Dairaku,
36, 57
I. Danchev, K. Das, A. Datta, G. David, M.K. Dayananda, A. Denisov, D. d’Enterria, A. Deshpande,
58, 64
E.J. Desmond, K.V. Dharmawardane, O. Dietzsch, A. Dion,
29, 64
M. Donadelli, O. Drapier, A. Drees, K.A. Drees, A.K. Dubey, J.M. Durham,
40, 64
A. Durum, D. Dutta, V. Dzhordzhadze, L. D’Orazio, S. Edwards, Y.V. Efremenko, F. Ellinghaus, T. Engelmore, A. Enokizono,
39, 53
H. En’yo,
57, 58
S. Esumi, K.O. Eyser, B. Fadem, D.E. Fields,
50, 58
M. Finger, M. Finger, Jr., F. Fleuret, S.L. Fokin, Z. Fraenkel, ∗ J.E. Frantz,
52, 64
A. Franz, A.D. Frawley, K. Fujiwara, Y. Fukao,
36, 57
T. Fusayasu, I. Garishvili, A. Glenn,
14, 39
H. Gong, M. Gonin, J. Gosset, Y. Goto,
57, 58
R. Granier de Cassagnac, N. Grau,
3, 15
S.V. Greene, G. Grim, M. Grosse Perdekamp,
26, 58
T. Gunji, H.-˚A. Gustafsson, ∗ A. Hadj Henni, J.S. Haggerty, K.I. Hahn, H. Hamagaki, J. Hamblen, R. Han, J. Hanks, E.P. Hartouni, K. Haruna, E. Haslum, R. Hayano, X. He, M. Heffner, T.K. Hemmick, T. Hester, J.C. Hill, M. Hohlmann, W. Holzmann,
15, 63
K. Homma, B. Hong, T. Horaguchi,
13, 24, 57, 67
D. Hornback, S. Huang, T. Ichihara,
57, 58
R. Ichimiya, H. Iinuma,
36, 57
Y. Ikeda, K. Imai,
30, 36, 57
J. Imrek, M. Inaba, D. Isenhower, M. Ishihara, T. Isobe,
13, 57
M. Issah,
63, 69
A. Isupov, D. Ivanischev, Y. Iwanaga, B.V. Jacak, J. Jia,
8, 15, 63
X. Jiang, J. Jin, B.M. Johnson, T. Jones, K.S. Joo, D. Jouan, D.S. Jumper, F. Kajihara, S. Kametani, N. Kamihara, J. Kamin, J.H. Kang, J. Kapustinsky, K. Karatsu,
36, 57
M. Kasai,
57, 59
D. Kawall,
44, 58
M. Kawashima,
A.V. Kazantsev, T. Kempel, A. Khanzadeev, K.M. Kijima, J. Kikuchi, A. Kim, B.I. Kim, D.H. Kim, D.J. Kim,
32, 73
E. Kim, E.-J. Kim, S.H. Kim, Y.-J. Kim, E. Kinney, K. Kiriluk, ´A. Kiss, E. Kistenev, J. Klay, C. Klein-Boesing, D. Kleinjan, L. Kochenda, B. Komkov, M. Konno, J. Koster, A. Kozlov, A. Kr´al, A. Kravitz, G.J. Kunde, K. Kurita,
57, 59
M. Kurosawa, M.J. Kweon, Y. Kwon,
66, 73
G.S. Kyle, R. Lacey, Y.S. Lai, J.G. Lajoie, D. Layton, A. Lebedev, D.M. Lee, J. Lee, K.B. Lee, K.S. Lee, T. Lee, M.J. Leitch, M.A.L. Leite, B. Lenzi, X. Li, P. Lichtenwalner, P. Liebing, L.A. Linden Levy, T. Liˇska, A. Litvinenko, H. Liu,
40, 51
M.X. Liu, B. Love, D. Lynch, C.F. Maguire, Y.I. Makdisi, A. Malakhov, M.D. Malik, V.I. Manko, E. Mannel, Y. Mao,
55, 57
L. Maˇsek,
10, 28
H. Masui, F. Matathias, M. McCumber, P.L. McGaughey, D. McGlinchey,
14, 22
N. Means, B. Meredith, Y. Miake, T. Mibe, A.C. Mignerey, P. Mikeˇs, K. Miki,
57, 68
A. Milov, M. Mishra, J.T. Mitchell, A.K. Mohanty, H.J. Moon, Y. Morino, A. Morreale, D.P. Morrison, † T.V. Moukhanova, D. Mukhopadhyay, T. Murakami, J. Murata,
57, 59
S. Nagamiya, J.L. Nagle, ‡ M. Naglis, M.I. Nagy,
19, 72
I. Nakagawa,
Y. Nakamiya, K.R. Nakamura,
36, 57
T. Nakamura,
24, 57
K. Nakano,
57, 67
S. Nam, J. Newby, M. Nguyen, M. Nihashi, T. Niida, R. Nouicer, A.S. Nyanin, C. Oakley, E. O’Brien, S.X. Oda, C.A. Ogilvie, M. Oka, K. Okada, Y. Onuki, A. Oskarsson, M. Ouchida,
24, 57
K. Ozawa, R. Pak, A.P.T. Palounek, V. Pantuev,
27, 64
V. Papavassiliou, I.H. Park, J. Park, S.K. Park, W.J. Park, S.F. Pate, H. Pei, J.-C. Peng, H. Pereira, V. Peresedov, D.Yu. Peressounko, R. Petti, C. Pinkenburg, R.P. Pisani, M. Proissl, M.L. Purschke, A.K. Purwar, H. Qu, J. Rak,
32, 50
A. Rakotozafindrabe, I. Ravinovich, K.F. Read,
53, 66
S. Rembeczki, K. Reygers, V. Riabov, Y. Riabov, E. Richardson, D. Roach, G. Roche, S.D. Rolnick, M. Rosati, C.A. Rosen, S.S.E. Rosendahl, P. Rosnet, P. Rukoyatkin, P. Ruˇziˇcka, V.L. Rykov, B. Sahlmueller,
46, 64
N. Saito,
33, 36, 57, 58
T. Sakaguchi, S. Sakai, K. Sakashita,
57, 67
V. Samsonov, S. Sano,
13, 70
T. Sato, S. Sawada, K. Sedgwick, J. Seele, R. Seidl,
26, 58
A.Yu. Semenov, V. Semenov,
25, 27
R. Seto, D. Sharma, I. Shein, T.-A. Shibata,
57, 67
K. Shigaki, M. Shimomura, K. Shoji,
36, 57
P. Shukla, A. Sickles, C.L. Silva,
29, 61
D. Silvermyr, C. Silvestre, K.S. Sim, B.K. Singh, C.P. Singh, V. Singh, M. Sluneˇcka, A. Soldatov, R.A. Soltz, W.E. Sondheim, S.P. Sorensen, I.V. Sourikova, F. Staley, P.W. Stankus, E. Stenlund, M. Stepanov, A. Ster, S.P. Stoll, T. Sugitate, C. Suire, A. Sukhanov, J. Sziklai, E.M. Takagui, A. Taketani,
57, 58
R. Tanabe, Y. Tanaka, S. Taneja, K. Tanida,
36, 57, 58, 62
M.J. Tannenbaum, S. Tarafdar, A. Taranenko, P. Tarj´an, H. Themann, D. Thomas, T.L. Thomas, M. Togawa,
36, 57, 58
A. Toia, L. Tom´aˇsek, Y. Tomita, H. Torii,
24, 57
R.S. Towell, V-N. Tram, I. Tserruya, Y. Tsuchimoto, C. Vale,
8, 29
H. Valle, H.W. van Hecke, E. Vazquez-Zambrano, A. Veicht, J. Velkovska, R. V´ertesi,
18, 72
A.A. Vinogradov, M. Virius, V. Vrba, E. Vznuzdaev, X.R. Wang, D. Watanabe, K. Watanabe, Y. Watanabe,
57, 58
F. Wei, R. Wei, J. Wessels, S.N. White, D. Winter, C.L. Woody, R.M. Wright, M. Wysocki, W. Xie, Y.L. Yamaguchi,
13, 57, 70
K. Yamaura, R. Yang, A. Yanovich, J. Ying, S. Yokkaichi,
57, 58
Z. You, G.R. Young, I. Younus,
38, 50
I.E. Yushmanov, W.A. Zajc, O. Zaudtke, C. Zhang, S. Zhou, and L. Zolin (PHENIX Collaboration) Abilene Christian University, Abilene, Texas 79699, USA Institute of Physics, Academia Sinica, Taipei 11529, Taiwan Department of Physics, Augustana College, Sioux Falls, South Dakota 57197, USA Department of Physics, Banaras Hindu University, Varanasi 221005, India Bhabha Atomic Research Centre, Bombay 400 085, India Baruch College, City University of New York, New York, New York, 10010 USA Collider-Accelerator Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA University of California - Riverside, Riverside, California 92521, USA Charles University, Ovocn´y trh 5, Praha 1, 116 36, Prague, Czech Republic Chonbuk National University, Jeonju, 561-756, Korea Science and Technology on Nuclear Data Laboratory, China Institute of Atomic Energy, Beijing 102413, P. R. China Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan University of Colorado, Boulder, Colorado 80309, USA Columbia University, New York, New York 10027 and Nevis Laboratories, Irvington, New York 10533, USA Czech Technical University, Zikova 4, 166 36 Prague 6, Czech Republic Dapnia, CEA Saclay, F-91191, Gif-sur-Yvette, France Debrecen University, H-4010 Debrecen, Egyetem t´er 1, Hungary ELTE, E¨otv¨os Lor´and University, H - 1117 Budapest, P´azm´any P. s. 1/A, Hungary Ewha Womans University, Seoul 120-750, Korea Florida Institute of Technology, Melbourne, Florida 32901, USA Florida State University, Tallahassee, Florida 32306, USA Georgia State University, Atlanta, Georgia 30303, USA Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, 142281, Russia University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA Institute for Nuclear Research of the Russian Academy of Sciences, prospekt 60-letiya Oktyabrya 7a, Moscow 117312, Russia Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic Iowa State University, Ames, Iowa 50011, USA Advanced Science Research Center, Japan Atomic Energy Agency, 2-4Shirakata Shirane, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia Helsinki Institute of Physics and University of Jyv¨askyl¨a, P.O.Box 35, FI-40014 Jyv¨askyl¨a, Finland KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan Korea University, Seoul, 136-701, Korea Russian Research Center “Kurchatov Institute”, Moscow, 123098 Russia Kyoto University, Kyoto 606-8502, Japan Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128, Palaiseau, France Physics Department, Lahore University of Management Sciences, Lahore, Pakistan Lawrence Livermore National Laboratory, Livermore, California 94550, USA Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA LPC, Universit´e Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden University of Maryland, College Park, Maryland 20742, USA Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003-9337, USA Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, USA Institut fur Kernphysik, University of Muenster, D-48149 Muenster, Germany Muhlenberg College, Allentown, Pennsylvania 18104-5586, USA Myongji University, Yongin, Kyonggido 449-728, Korea Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan University of New Mexico, Albuquerque, New Mexico 87131, USA New Mexico State University, Las Cruces, New Mexico 88003, USA Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406, Orsay, France Peking University, Beijing 100871, P. R. China PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad region, 188300, Russia RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973-5000, USA Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan Saint Petersburg State Polytechnic University, St. Petersburg, 195251 Russia Universidade de S˜ao Paulo, Instituto de F´ısica, Caixa Postal 66318, S˜ao Paulo CEP05315-970, Brazil Seoul National University, Seoul, Korea Chemistry Department, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universit´e de Nantes) BP 20722 - 44307, Nantes, France University of Tennessee, Knoxville, Tennessee 37996, USA Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan Vanderbilt University, Nashville, Tennessee 37235, USA Waseda University, Advanced Research Institute for Science andEngineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan Weizmann Institute, Rehovot 76100, Israel Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, HungarianAcademy of Sciences (Wigner RCP, RMKI) H-1525 Budapest 114, POBox 49, Budapest, Hungary Yonsei University, IPAP, Seoul 120-749, Korea (Dated: October 1, 2018)We report e ± − µ ∓ pair yield from charm decay measured between midrapidity electrons ( | η | < . p T > . c ) and forward rapidity muons (1 . < η < . p T > . c ) as a functionof ∆ φ in both p + p and in d +Au collisions at √ s NN = 200 GeV. Comparing the p + p results withseveral different models, we find the results are consistent with a total charm cross section σ c ¯ c =538 ±
46 (stat) ±
197 (data syst) ±
174 (model syst) µ b. These generators also indicate that theback-to-back peak at ∆ φ = π is dominantly from the leading order contributions (gluon fusion),while higher order processes (flavor excitation and gluon splitting) contribute to the yield at all ∆ φ .We observe a suppression in the pair yield per collision in d +Au. We find the pair yield suppressionfactor for 2 . < ∆ φ < . J dA = 0.433 ± ± c ¯ c pairs. I. INTRODUCTION
The study of open heavy flavor production in rela-tivistic p ( d ) + A collisions is sensitive to different kindsof strong interaction physics. Because the leading or-der (LO) production mechanism is gluon fusion [1], openheavy flavor production rates are directly related to mod-ification of the gluon parton distribution function (PDF),i.e. shadowing or saturation [2]. Also, the initial and/orfinal state partons can scatter and lose energy in the coldnuclear medium [3–5], thereby modifying and producinga nuclear modification of open heavy flavor production.Recently, the possibility of flow even in small collisionsystems such as p ( d ) + A has raised the question of mod-ified charm momentum distributions [6]. ∗ Deceased † PHENIX Co-Spokesperson: [email protected] ‡ PHENIX Co-Spokesperson: [email protected]
Modification to heavy quark production rates and kine-matics in d +Au collisions at the Relativistic Heavy IonCollider (RHIC) is well established. Electron productionfrom open heavy flavor decay is enhanced [7], while J/ψ production is suppressed [8] at midrapidity. At positiverapidity, defined with the positive z axis as the directionof the deuteron, there is a suppression of heavy-flavordecay muons [9] and a larger suppression of J/ψ [8].While e – µ correlations from open heavy flavor decayshave not been published at RHIC to date, correlations in-volving light flavor hadrons have shown modification in d +Au collisions at RHIC. A suppression has been ob-served of positive rapidity π mesons associated withmidrapidity trigger hadrons, especially in the back-to-back peak at ∆ φ = π , indicating 2 → x , the fraction of the nu-cleon momentum carried by the gluon, decreases. Theseresults are in quantitative agreement with energy lossmodels [11] and saturation models [12–14].This paper presents measurements of azimuthal cor-relations of electron-muon pairs produced from heavyflavor decays, primarily c ¯ c , in p + p and d +Au colli-sions using the PHENIX detector at RHIC. The heavy-flavor e – µ correlations are free of backgrounds from othersources that contribute to other dilepton analyses ( e + e − or µ + µ − ), such as resonance decay and Drell-Yan. Whileanalysis of dilepton mass and p T provides a way to sepa-rate charm and bottom contributions, the azimuthal cor-relations have an important advantage for studying thecharm production process. The leading order produc-tion, gg → Q ¯ Q and q ¯ q → Q ¯ Q , will produce back-to-backopen heavy flavor pairs that can semileptonically decayand produce azimuthally correlated e – µ pairs. Next-to-leading order (NLO) processes like flavor excitation andgluon splitting produce much less correlated Q ¯ Q andthus much less correlated e – µ pairs. Therefore, mod-ification to different portions of the azimuthal correla-tions can be attributed to modifications of c ¯ c pairs fromdifferent production mechanisms. In energy loss modelssuch as Ref. [11], a broadening of the back-to-back az-imuthal correlation should accompany a suppression ofthe peak due to the multiple scattering that the incom-ing gluons and/or the outgoing c ¯ c undergo in the coldnuclear medium.This paper is organized as follows. The PHENIX de-tector is outlined in Section II. Section III describes thedetails of the method used to measure the correlations,the background subtraction method, and the tests of themethod. Section IV presents the results in p + p and com-pares them to Monte Carlo models. The d +Au resultsare presented and compared to the p + p results in Sec-tion IV B. Conclusions are given in Section V. II. PHENIX EXPERIMENT
The PHENIX detector at RHIC is multi-purposed andoptimized for precision measurements of electromagneticprobes for relativistic hadronic and heavy ion collisions.A complete overview of the detector can be found inRef. [15]. The data presented here are from 2006 p + p and 2008 d +Au data taking at RHIC. Figure 1 shows aschematic of the detector during those years. This analy-sis uses the central spectrometer arms for electron detec-tion and the forward rapidity muon spectrometer arms,labeled North and South in Fig. 1, for muon identifica-tion. For the 2008 d +Au collisions, the deuteron beammoves toward the North arm, which defines positive ra-pidity for both p + p and d +Au. The forward producedmuons come from a high- x parton in the deuteron inter-acting with a low- x parton in the gold. pythia [16] indi-cates that the average x of a parton producing a heavyflavor muon from 1 < p µT < c in the forward muonspectrometer is about 5 × − . This analysis focusesonly on the muons measured in the North arm utilizingthe deuteron beam as a probe of low- x partons in thegold nucleus.The central spectrometer comprises two arms subtend-ing π/ | η | < .
35. Charged
FIG. 1: (Color online) A schematic view of the PHENIX de-tector during the 2008 d +Au data taking. (a) beam view ofthe central spectrometer arms. (b) longitudinal view includ-ing the global event and triggering detectors, as well as themuon spectrometer arms. The configurations of the centralspectrometer and muon arms were the same for the 2006 p + p data taking. tracks are measured using a drift chamber (DC) and aset of multi-wire proportional chambers with pad read-out (PC1 and PC3). The DC measures the bend anglein the r − φ plane due to a central magnetic field di-rected along the beam axis. PC1 is used to measure thelongitudinal coordinate of the track. These tracks arethen projected into PC3, where a hit is required to en-sure high track quality. The momentum resolution ofthe tracks in this data is δp/p = 1 . ⊕ p , where p is the total momentum measured in GeV/ c . Electronscan be identified from associated hits in the Ring Imag-ing ˇCerenkov (RICH) detector and the ElectromagneticCalorimeters (EMCal). Electrons above 17 MeV/ c pass-ing through the CO -filled RICH will emit ˇCerenkov ra-diation. The EMCal comprises eight sectors, six of lead-scintillator and two of lead-glass, used to collect the en-ergy from electron and photon showers. The nominal en-ergy resolution for the lead-scintillator and lead-glass is8.1% ± p E [GeV] ⊕ ± p E [GeV] ⊕ . < η < . π in azimuth. The spectrometer measurestracks in the muon tracker (MuTr) and the muon iden-tifier (MuID). Prior to entering the muon arm, particlespass through approximately 20 cm of copper and 60 cmof iron. Particles that are not absorbed pass through theMuTr, which comprise three stations of cathode stripchambers with multiple ionization regions and locatedinside a radial magnetic field. After the MuTr, parti-cles pass through the MuID, which comprises five alter-nating steel absorbers and MuID detector planes, calledgaps, with Iarocci tubes. MuID roads reconstructed fromMuID hits are projected back to MuTr tracks and to themeasured vertex to provide the complete information fora track through the spectrometer.Trigger and global event characterization in p + p and d +Au are provided by the beam-beam counter (BBC).The BBC is a set of 64 hexagonal ˇCerenkov counterslocated from 3 . < | η | < . z vtx ) isdetermined by the time difference between the BBCs oneither side of the collision region. The minimum bias(MB) trigger requires that there is at least one hit ineach of the BBCs. From Vernier scans and verified byMonte Carlo studies, the BBC MB trigger is sensitive to55 ±
5% of the p + p inelastic cross section and 88 ±
4% ofthe d +Au inelastic cross section[18]. The trigger usedfor this analysis is a combination of the BBC trigger anda deep muon trigger. The deep muon trigger requiresthree or more MuID gaps with a signal in both the x and y direction tubes and that the last pair of hits be in thelast (5th gap) or next to last gap (4th gap).After quality cuts and requiring a vertex within 25 cmof the z =0 vertex, an integrated luminosity of 2.1 pb − in p + p and a p + p -equivalent of 7.7 pb − in d +Au wassampled. III. ANALYSIS
The primary goal of this analysis is to identify p + p ( d + Au) → c ¯ c + X → e ± µ ∓ + X, (1)where the opposite sign electron-muon pair is from the c ¯ c pair decay. A. Particle Identification
1. Muon Identification
Only muon candidates with p T > c are usedin the analysis because real muons with total momen-tum less than about 2.7 GeV/ c are stopped in the muonarm before reaching the 5th (and last) gap. Single muoncandidates are constructed from MuID roads projectedand matched to MuTr tracks. Cuts on MuID roads andMuTr tracks are designed to reject hadrons that mimic a muon signal and to reject tracks that did not origi-nate from the collision vertex. For the MuID roads, atleast three of five gaps with x − y hit information arerequired, including a pair of hits in the 5th gap. TheseMuID roads must project back near the nominal vertexposition. Those muons that do not typically come frombeam-related backgrounds. For the MuTr tracks, cutsthat reject hadrons are detailed in Ref. [19]. The MuIDroads are then projected and matched to MuTr tracksat the 1st MuID gap. An identified muon candidate isthe closest MuTr track that matches a MuID road withinat least 10 ◦ in slope and 10 cm in distance. Muon can-didates are further restricted to 1 . < η < .
1. Dur-ing both the p + p and d +Au data taking periods, therewere backgrounds primarily from beam-related particlesinteracting with material in the accelerator upstream ofPHENIX. Collimators were used in the accelerator to re-duce this background but it was not totally eliminated.Restricting the η range of the muon candidates helpedminimize this background.
2. Electron Identification
Electrons with p T > . c are identified bymatching a track in DC, PC1, and PC3 to a signal in theRICH and a cluster in the EMCal. The relevant detailson measuring electrons in PHENIX are given in Ref. [20].For this analysis, the projected track must match within3 σ in position to a cluster in the EMCal. Clusters are alsorequired to have a matching profile, when compared toan electromagnetic shower shape profile at the measuredenergy. Once a track matches both the RICH and theEMCal, an E/p cut is applied, where it is required thatthe energy measured in the the EMCal E be approxi-mately equal to the reconstructed track momentum p .This is sufficient to remove most combinatorial matchesand background from real electrons resulting from long-lived particle decays occurring near the DC, which havemismeasured momentum. A cut of -2 σ to +3 σ from themean E/p in the p + p data and -1.5 σ to +3 σ from themean in the d +Au data is applied. The asymmetry ofthe cuts is due to the dominance of backgrounds below 2or 1.5 σ of the mean. The tighter cut in the d +Au datawas necessary because of the increased background fromthe hadron blind detector (HBD) support material notpresent during 2006 data taking. B. Acceptance and Efficiencies
After particle identification cuts have been applied toan event, all pairs of identified electrons and muons areformed in each of the four charge-sign combinations. Thefully corrected invariant-pair yield, calculated for eachsign combination, is [21] d Ndy µ dy e d ∆ φ = cN MBevt ∆ y e ∆ y µ ∆ φ bin × R d ∆ φ Mix(∆ φ )2 π N eµ (∆ φ )Mix e µ (∆ φ, ǫ e , ǫ µ ) (2)where N MBevt is the number of sampled BBC triggeredevents, c is the MB trigger bias accounting for eventsmissed by the BBC trigger[18], ∆ y e and ∆ y µ are the ra-pidity ranges of the electrons and muons, respectively, N eµ (∆ φ ) is the inclusive electron-muon pair yield, andMix eµ (∆ φ, ǫ e , ǫ µ ) is the mixed-event electron-muon pairdistribution. The two-particle acceptance and efficiencyis corrected by the mixed-event technique, where elec-trons from one event are paired with muons from a dif-ferent event. Pools of inclusive electrons and muons arekept in bins 2.5 cm-wide z vertex bins, and, in the caseof d +Au, 10%-wide centrality bins. When mixing events,the pair distribution is weighted by the y - and φ -averagedefficiency of each particle, ǫ e and ǫ µ .Both ǫ e and ǫ µ were determined by generating singleelectrons and single muons with a flat distribution in p T , φ , | y e | < . . < y µ < . z -vertexlocation and running them through a geant -3 simulationof the PHENIX detector. The output was subjected tothe same analysis cuts applied to the data. The efficiencyis defined as the ratio of particles reconstructed throughthe analysis to the number simulated. These simulationsdemonstrated that ǫ e and ǫ µ are independent of the z -position of the event vertex, ǫ e is independent of η and ǫ µ has a slight η -dependence. Pair yields are reported withthe average pseudorapidity h η µ i , which include the η -dependence of both single inclusive muons and the singleparticle efficiency. C. Background Subtraction
Inclusive muon and electron candidates come fromboth heavy- and light-flavor decays and from misidenti-fied hadrons. The fully-corrected inclusive electron-muonpair yield for each sign combinations can be written as N eµ (∆ φ ) = N eµH (∆ φ ) + N eµLH (∆ φ ) + N eµL (∆ φ ) . (3)Here N eµ indicates the fully-corrected inclusive pairyield defined in Eq. 2; N eµH (∆ φ ) is the fully-corrected pairyield produced from a heavy flavor pair decay; N eµLH (∆ φ )is the fully-corrected pair yield from correlating a heavyflavor decay product with a light flavor decay product;and N eµL (∆ φ ) is the fully-corrected pair yield from corre-lating pairs of light-flavor decay products or misidentifiedhadrons. Pairs from the semileptonic decay of a c ¯ c pairhave opposite signs. Eq. 3 can be decomposed into itslike- and unlike-sign pieces: N eµ like (∆ φ ) = N eµLH, like (∆ φ ) + N eµL, like (∆ φ ) N eµ unlike (∆ φ ) = N eµH, unlike (∆ φ ) + N eµLH, unlike (∆ φ ) + N eµL, unlike (∆ φ ) . (4)While semileptonic decays of b ¯ b can also produce bothlike- and unlike-sign e – µ signals, in this analysis, pythia indicates that only about 1% of the final heavy-flavor e – µ pair yield is from b ¯ b and is neglected. If we assume muon(electron) candidates from light flavors are not charge-correlated with electron (muon) candidates from lightflavors, then N eµL, like (∆ φ ) = N eµL, unlike (∆ φ ) . (5)If only one of the pair is from heavy flavor, then again,we assume they are not charge correlated, and N eµLH, like (∆ φ ) = N eµLH, unlike (∆ φ ) . (6)Therefore, the heavy flavor e – µ signal distributions isthe difference between the unlike-sign and the like-signinclusive correlations: N eµH (∆ φ ) = N eµ unlike (∆ φ ) − N eµ like (∆ φ ) . (7)Figure 2 shows the fully-corrected inclusive like-sign( N eµ like (∆ φ )) and unlike-sign ( N eµ unlike (∆ φ )) e – µ pair dis-tributions in p + p and d +Au. The inset figures show thesignal-to-background distributions given the assumptionsabove.We have checked the like-sign subtraction methodusing pythia leading order quantum chromodynamics(QCD) events. With all events containing a heavy quarkin the final state removed, the pair yields as a functionof ∆ φ for like-sign and unlike-sign electron-muon pairswere the same within 3% over all ∆ φ .While this corroborates the basic idea of the subtrac-tion, the assumption was further tested with data. In thefollowing sections we detail the results of different meth-ods to tag electrons and muons from light flavor decay toexamine the validity of Eq. 7 and to quantify the system-atic uncertainty of the method. The general method is touse a sample of single electrons paired with single muons,where one or both are likely from light-hadron decays. Ifthe method is correct, the like-sign subtraction shouldproduce no correlation at all. If there are statisticallysignificant correlations after like-sign subtraction, theseare subtracted from the final e – µ pair yield and uncer-tainties on the residual correlation strength are propa-gated as a systematic uncertainty on the final pair yield.If no statistically significant yield is found after like-signsubtraction, the statistical uncertainty on the zero yieldis propagated as the systematic uncertainty.
1. Correlations between inclusive electrons andpunch-through hadrons that fake single muons
One source of background to the single muons is fromhadrons that penetrate to the 5th gap, called punch- (rad) φ∆ ) - ) ( r a d φ ∆ d e d y µ N / ( d y d -6 × unlike-sign like-sign a) p+p (rad) φ∆ -1 0 1 2 3 4 ( un li ke - li ke ) /li ke (rad) φ∆ ) - ) ( r a d φ ∆ d e d y µ N / ( d y d -6 × unlike-sign like-signb) d+Au (rad) φ∆ -1 0 1 2 3 4 ( un li ke - li ke ) /li ke FIG. 2: (Color online) The fully-corrected inclusive like-sign( e ± − µ ± ) and unlike-sign ( e ± − µ ∓ ) distributions for (a) p + p and (b) d +Au, as a function of ∆ φ . The inset showsthe unlike-like difference divided by the like-sign distribution,which is the heavy flavor signal-to-background in the inclusiveunlike-sign distribution. through hadrons. After single particle cuts there is somesmall fraction (roughly 1 out of every 250 [22]) of candi-date tracks with p T > c that are hadrons thatpunch through. While this represents an irreduciblebackground to the single muons, we can obtain a cleansample of hadrons that punch through and stop in the4th gap of the MuID. Fig. 3 shows the p z distributionof muon candidates that stop in the 4th gap. The peakat 2.3 GeV is composed of muons that have insufficientenergy to penetrate further. The broader portion of thedistribution comprises light hadrons that are not stoppedby the upstream absorber materials but are subsequentlyabsorbed in the steel just after the 4th gap, thus notleaving a hit in the 5th gap. We identify punch-throughhadrons as having stopped in the 4th gap with p z largerthan 3 GeV.Fig. 4 shows the fully corrected like-sign subtractedpair yield of central-arm electrons and the punch-throughhadrons in the muon arms for both p + p and d +Au col-lisions. If both the like- and unlike-sign pair yields weredominantly from light hadron decays, the like-sign sub- (GeV/c) z p C oun t s / B i n × FIG. 3: The distribution of p z for tracks that stop in thenext-to-last MuID gap (4th gap). The peak at lower p z is dueto muons, while the broad distribution is from hadrons thatpunch through the absorber to the 4th gap. The solid lineis a two-Gaussian fit to this distribution with the solid lineindicating the hadronic background in the muon peak region. traction should produce zero pair yield. To determinethe magnitude of the residual correlation strength afterlike-sign subtraction, the p + p data were fitted with aflat line. This is shown as the solid line in Fig. 4a. Thefit uncertainty is shown as the shaded band around thesolid line. The flat fit in p + p had a χ /NDF of 22.7/24and gave a value that was nonzero with greater than 1 σ significance. This means there is yield in the final e – µ correlations from these punch-through hadrons. The fit-ted yield was subtracted from the final pair yield and itsuncertainty was propagated as a systematic uncertaintyon the final pair yield. For the d +Au case, we fittedthe residual correlation to a flat line and found reason-able agreement with a χ /NDF of 30.9/24 or a p -value of14%. However, there is a possible excess of counts near∆ φ = π , which when included as a Gaussian componentfixed at ∆ φ = π and the width and yield as free parame-ters, a slightly better χ /NDF of 26.3/22 or a p -value of26% was found. If there is any correlated yield beyonda pedestal, it would show up in the back-to-back peak.Therefore, we subtract the Gaussian fit, shown as thesolid line in Fig. 4 from the final pair yield and propa-gate the uncertainty, shown as the shaded region aroundthe solid line, on the fit to the systematic uncertainty inthe final pair yield.Two additional corrections to this data are applied be-fore subtraction from the final pair yield. Because thepunch-through hadrons are measured in the 4th gap, theyields need to be scaled to match the rate of hadrons atthe last gap. The rate of hadrons at the 5th gap wasdetermined by using pion and kaon NLO perturbativeQCD spectra [23] and passing them through a geant -3model of the PHENIX muon arms. The MuID absorbersteel cross section was modified until there was agree-ment between data and the simulation for the rate of (rad) φ∆ - ( r a d ) φ ∆ d e d y µ N / d y d -30-20-1001020304050 -9 × -punchthrough hadrons ± e a) p+p (rad) φ∆ - ( r a d ) φ ∆ d e d y µ N / d y d -0.2-0.100.10.2 -6 × b) d+Au FIG. 4: The fully-corrected like-sign-subtracted electron pluspunch-through hadron pair yield in (a) p + p and (b) d +Aucollisions. The line indicates the fitted yield that is removedfrom the inclusive electron-muon pair correlation. The shadedband indicates the fit uncertainty that is propagated as asystematic uncertainty in the final pair yield. In p + p the fitis is a flat line with χ /NDF = 22.7/24. In d +Au it is a flatline and a Gaussian centered at π with χ /NDF = 26.3/22. punch-through hadrons in the 3rd and 4th gap. We ex-trapolated to the 5th gap and find the rate of hadronsis 2.81 ± c p z cut removes somefraction of the punch-through hadrons. Based on thetwo-component fit to the p z distribution shown in Fig. 3,the yield is scaled up to account for those hadrons re-jected by the p z cut. In the end, the pair yield uncer-tainty is 2.17 × − (rad) − in p + p . In d +Au there is a∆ φ -independent uncertainty on the final pair yield thatis 1.42 × − (rad) − and the Gaussian uncertainty thatranges from 0 to 6.30 × − (rad) − .
2. Correlations Between Inclusive Electrons andLight-Hadron Decay Muons
One source of real muons is from decays of lighthadrons, predominantly charged pions and kaons, before the absorber material. The observed rate of muons intothe North arm is higher, when the collision vertex is far-ther from the spectrometer arm. Because heavy flavordecays (including Drell-Yan, heavy quarkonia, etc.) havea much shorter cτ than light flavor decays, heavy fla-vor decay muons have a much weaker vertex dependence.Therefore, we assume there are two components to themuon rate: a component that follows the primary vertexdistribution, attributable to heavy flavor decays, and acomponent that folds the linear component due to lighthadron decays with the primary vertex distribution. (rad) φ∆ - ( r a d ) φ ∆ d e d y µ N / d y d -0.2-0.100.10.2 -6 × -light hadron decay muons ± e a) p+p (rad) φ∆ - ( r a d ) φ ∆ d e d y µ N / d y d -80-60-40-20020406080 -9 × b) d+Au FIG. 5: The fully-corrected like-sign-subtracted and near-far vertex-subtracted (see text) muon-decay ∆ φ pair yield in(a) p + p and (b) d +Au collisions. Both are consistent with noresidual correlation after like-sign subtraction. The solid linesand shaded bands indicate the flat line fits and their uncer-tainty with χ /NDF of 27.1/24 and 18.0/24 in p + p and d +Au,respectively. Muons that are near the detector (0 < z vtx <
30 cm)and far from the detector ( − < z vtx < z vtx is the measured collisions vertex, are separately cor-related with central arm electrons. Because the signalheavy flavor muons follow the primary collision vertexdistribution, subtracting the near-vertex pair yield fromthe far-vertex pair yield, should remove these and onlyresidual correlations from decay muons should be present.The pair yields in p + p and d +Au after subtracting near-and far-vertex muons and after like-sign subtraction areshown in Fig. 5. The d +Au correlations are consistentwith a flat line with zero yield with a χ /NDF of 18.0/24.The p + p data seems to have a residual shape. However,this shape is asymmetric about ∆ φ of zero and is notphysical. Therefore, we fit with a flat line that results inzero correlation yield and a χ /NDF of 27.1/24. The fitsare shown in Fig. 5 as solid lines and shaded bands indi-cating the statistical uncertainties. These uncertaintieswere propagated into the systematic uncertainties of thefinal pair yields.To propagate the uncertainties, additional correctionsare needed. First, in the far-near subtraction, some frac-tion of the decay muons are removed. Second, lighthadron decays outside the ±
30 cm vertex cut are notcounted in the subtraction. To account for both effects,a fit to the vertex dependence of the muon yield is ex-trapolated to a point one interaction length inside theabsorber, a distance of about 56 cm from the nominal z vertex and about 16 cm into the absorber. It is assumedthat the decay contribution to the muons is negligibleat that point, which fixes the fraction of muons that arefrom light decays within the measured vertex window ofthe analysis. Under this assumption, only 22% of thedecay muons are measured within the vertex window af-ter the like-sign subtraction. The fit uncertainties areincreased to account for those muons not measured. Thefinal systematic uncertainties on the final pair yield are1.13 × − (rad) − and 5.05 × − , independent of ∆ φ for p + p and d +Au, respectively.
3. Correlations Between Photonic Electrons and InclusiveMuons
Electrons can result from light hadron decays throughinternal and external photon conversions. The dominantphotonic source of electrons are from π decays. We as-sume that, if we measure the π -decay electrons correla-tions with muons, this will represent the other photonicsources (such as η and ω decay) in shape and yield. Totag decay or converted electrons, we construct the invari-ant mass distribution of all pairs of electrons and pho-tons in an event. Electrons paired with photons withinthe π mass peak are then correlated with muon can-didates. The signal-to-background of pairs in the π mass range is about one. To remove correlations fromcombinatorial electron-photon pairs that fall within the π mass window, muon candidates were also correlatedwith the e – γ pairs in a “sideband” π mass region from0.2–0.4 GeV/ c . After scaling by the appropriate signal-to-background under the π mass region, the “sideband”correlations were subtracted from the in-mass electron-muon correlations for each of the e – µ charge types.Fig. 6 shows the “sideband”-subtracted and like-signsubtracted correlation between electrons tagged in the π mass region with muons from p + p and d +Au data.Flat fits to these correlations produced a yield consis- tent with zero with χ /NDF of 33.2/24 and 20.2/24 in p + p and d +Au data, respectively. The statistical un-certainty from the fitted yield to the π -tagged correla-tions is a factor of 10 smaller than the other backgroundcorrelations after accounting for reconstruction efficiencyand additional sources of photonic electrons. This un-certainty is negligible compared to those from the muonbackgrounds. (rad) φ∆ - ( r a d ) φ ∆ d e d y µ N / d y d -10-50510 -9 × ± µ - ± photonic e a) p+p (rad) φ∆ - ( r a d ) φ ∆ d e d y µ N / d y d -20-1001020 -9 × b) d+Au FIG. 6: The fully-corrected like-sign-subtracted photonicelectron-muon ∆ φ pair yield in (a) p + p and (b) d +Au colli-sions. Both are consistent with no residual correlation afterlike-sign subtraction. The solid lines and shaded bands indi-cate the flat line fits and their uncertainties with χ /NDF of33.2/24 and 20.2/24 in p + p and d +Au, respectively. D. Systematic Uncertainties
In this analysis there are three general types of uncer-tainty that we identify as type A, point-to-point uncor-related, type B, point-to-point but correlated, and typeC, total normalization uncertainty. Except for statisticaluncertainties there are no type A uncertainties in thisanalysis.The type B uncertainties are from the subtraction ofknown backgrounds discussed in Section III C. The fully0
TABLE I: Table of type B and type C systematic uncertain-ties for p + p and d +Au collision data. The uncertainties onthe muon and electron cuts are highly correlated between p + p and d +Au. Type Description p + p d +AuB ∆ φ dependent – 0%—6.30 × − (rad) − B punch-through 2 . × − (rad) − . × − (rad) − B decay muons 1 . × − (rad) − . × − (rad) − C muon cuts 7.8% 8.3%C electron cuts 8.3% 9.3%C muon efficiency 2.2% 2.2%C electron efficiency 1.0% 1.0%C trigger efficiency 11.1% 4.2%C total 16.1% 13.4% corrected pair yield uncertainties in p + p are 2 . × − (rad) − and 1 . × − (rad) − from punch-throughhadron and decay hadron subtraction uncertainties, re-spectively. These values are independent of ∆ φ . In d +Au the flat-line fit contributions to the systematicuncertainty are 1 . × − (rad) − and 5 . × − (rad) − from punch-through hadron and decay hadronsubtraction uncertainties, respectively. The additionaluncertainty from the Gaussian fit to the punch-throughhadron correlations resulted in a ∆ φ -dependent uncer-tainty ranging in absolute value of 0 at ∆ φ ∼ . × − (rad) − at ∆ φ ∼ π . The type-B systematicsare summarized in Table I.The type C uncertainties are attributable to severalsources and are given in Table I. One source of systematicuncertainty is evaluated by tightening the single particlecuts for this analysis. Each single particle cut was tight-ened independently and the analysis, including reeval-uation of the single particle efficiency, was performed.The uncertainty from each of the individual single par-ticle cuts was combined using the correlation amongstthe cuts. The values of these are different in p + p and d +Au data, because of the higher backgrounds in d +Aucollisions. However, these uncertainties are highly cor-related between p + p and d +Au, because the same cutsare applied to both data sets. Another source of uncer-tainty is in the evaluation of the single particle efficien-cies. The single particles were generated flat in p T andthen weighted to match the measured PHENIX heavyflavor lepton spectra [24]. For the uncertainty deter-mination, the single particle efficiency was re-evaluatedwithout the weighting applied. This was estimated tobe 1.0% for the electrons and 0.8% for the muons. Formuons there is an additional 2.0% uncertainty due to therun-by-run variation in muon acceptance. The final por-tion of the type C systematic uncertainty is due to thetrigger efficiency. To evaluate this uncertainty, the datawere analyzed for several data-taking periods defined bythe muon trigger performance. The difference in fully-corrected yields between data sets was taken to be the uncertainty in the muon trigger efficiency. This is com-bined with the uncertainties in the bias factor c in Eq. 2.The total uncertainty for the trigger is 11.1% for p + p and 4.2% for d +Au. As indicated in Table I, combiningall Type C uncertainties gives 16.1% for p + p data and13.4% for d +Au data. IV. RESULTSA. Pair Yields for p + p data and Comparison withMonte Carlo Generators The fully-corrected like-sign subtracted e – µ pair yieldas a function of ∆ φ for electrons with p T > . c and | η | < .
5, with opposite-signed forward muons with p T > . c and 1 . < η < .
1, in p + p is shownin Fig. 7. The average muon η in these correlations is1.75. The error bars are statistical uncertainties only,while the boxes are the type B systematic uncertainties.We note that the distribution has two components: anonzero continuum as well as a back-to-back peak near∆ φ = π . (rad) φ∆ ) - ) ( r a d φ ∆ d e d y µ N / ( d y d -9 × =200 GeVsp+p |<0.5 η >0.5 GeV/c, | T : p ± e <2.1 η >1 GeV/c, 1.4< T : p ± µ FIG. 7: (Color Online) The fully-corrected like-sign-subtracted heavy flavor e – µ pair yield in p + p . The error barsare statistical only. The boxes show the type B systematicuncertainty from the punch-through hadron and light hadrondecay muon background subtraction. The 16.1% type C sys-tematic uncertainty is not shown. To interpret these data, we compare the p + p resultsto several different Monte Carlo generators, pythia , powheg [25], and MC@NLO[26].The pythia MB QCD events were generated to modelthe LO gluon fusion process and also model next-to-leading order processes, like flavor excitation and gluonsplitting. Events with a c ¯ c pair and an electron and amuon in the measured kinematic range as the correcteddata ( p eT > . c and | η e | < . p µT > c and1 . < η µ < .
1) were correlated and a like-sign subtrac-tion was performed. An overall scale factor was used to fitthe pythia curve to the p + p data. In the fit, the χ was1 (rad) φ∆ ) - ) ( r a d φ ∆ d e d y µ N / ( d y d -9 × =200 GeVsp+p POWHEGPYTHIAPYTHIA (NO LO)MC@NLO
FIG. 8: (Color online) Comparison of the measured p + p pairyield ([red] points) with heavy flavor production in powheg ([blue] dashed line), pythia ([black] solid line) and MC@NLO([green] long dashed line). The e – µ pair yield from the subsetof pythia events, when the c ¯ c is not produced at the eventvertex is plotted as the dotted [black] line. Each Monte Carlocurve was scaled by a single parameter to match the observedyield. The resulting cross sections are consistent with thepreviously measured PHENIX results (see Table II). calculated for different scale parameters using the statis-tical error on the p + p data. We report the cross sectionfor the scale factor that minimizes that χ and reporta statistical error on the cross section as the value thatchanges the χ by one unit. To evaluate the systematicuncertainty on the cross section, the p + p data were in-creased and decreased by their combined type B and typeC systematic uncertainty and the process to determinethe scale factor by finding a minimum χ using the sta-tistical uncertainty in the data was repeated. We find the pythia correlation is consistent with the p + p data witha c ¯ c cross section of σ c ¯ c = 340 ± ± µ bwith a χ /NDF of 20.5/24. This is shown as the solidcurve in Fig. 8.The other model comparisons are from NLO genera-tors, powheg and MC@NLO. Events were generated toproduce the hard scattering heavy flavor event vertex andthen interfaced to pythia , which performed the fragmen-tation and underlying event generation. The qualitativefeatures of the data are present in these correlations: thecontinuum and the back-to-back peak. As described forthe pythia fit, a single scale parameter was used to cal-culate a χ between the generated e – µ correlations andthe data using the data’s statistical uncertainty. The re-sulting best fits for powheg and MC@NLO are shownin Fig. 8 as the short dashed and the long dashed lines,respectively. The extracted cross sections are σ c ¯ c = 511 ±
44 (stat) ±
198 (syst) µ b with χ /NDF of 23.5/24 for powheg and σ c ¯ c = 764 ±
64 (stat) ±
284 (syst) µ b with χ /NDF of 19.2/24 for [email protected] combine the cross sections from the three mod-els and report a measured cross section of σ c ¯ c = 538 ±
46 (stat) ±
197 (data syst) ±
174 (model syst). Thecentral value of the cross section is the average of thethree model cross sections, while the model systematicuncertainty is the standard deviation of the three modelcross sections. This value can be compared with previousPHENIX measurements. From the heavy flavor electronspectra at midrapidity, PHENIX found σ c ¯ c = 567 ± ±
224 (syst) [24] and from the dielectron massspectrum at midrapidity, PHENIX extracted σ c ¯ c = 554 ± ±
142 (data syst) ±
200 (model syst) [27].Within the data systematics the value extracted here isconsistent with previously published PHENIX results.Using the pythia event record, it is possible to sepa-rate the c ¯ c production into an LO component, where the gg ( q ¯ q ) → c ¯ c and a component from the pythia modelof NLO mechanisms of flavor excitation and gluon split-ting, where the c ¯ c pair is produced in the initial or final-state shower. The “ pythia (NO LO)” dashed line inFig. 8 shows the correlations from the sample of pro-duced pythia events, where the c ¯ c were not generatedin the primary event vertex of pythia . The back-to-backpeak at ∆ φ = π is dominated by the LO gluon fusion pro-cess while the continuum is due to the correlations fromthe higher order processes. From an accounting from pythia , we find that 32% of the e – µ pair yield resultsfrom gluon fusion, consistent with the expectations fromcharm production [1].Throughout the analysis it has been assumed thatsemileptonic c ¯ c decay is the dominant contribution tothe correlations. However, b ¯ b semileptonic decays wouldproduce a signal in both the like- and the unlike-signpair distributions. Up to four semileptonic decays canoccur where b -quarks semileptonically decay to c -quarks,which subsequently semileptonically decay. We have used pythia and powheg to check these contribution frombottom. In both cases, for electrons and muons in thekinematic region that we measure, the bottom contri-bution is about a factor of 100 below the charm yield.This is further corroborated by the PHENIX heavy flavorelectron measurements that show that bottom becomessignificant only at p T above 3 GeV/ c [28]. In this anal-ysis only 3% of the sampled electrons have a p T above3 GeV/ c , so we expect that the bottom contribution isnegligible in this measurement especially compared to thebackground subtraction systematic uncertainties. B. Yields in d + Au and Comparison to p + p The fully-corrected like-sign subtracted pair yield as afunction of ∆ φ for electrons with p eT > . c and | η e | < . p µT > . c and1 . < η µ < . d +Au, corresponding to thetotal inelastic cross section, is shown in Fig. 9. A nonzerocorrelations strength is observed. However, unlike the p + p data, there is a much less distinct back-to-back peaknear ∆ φ of π . Fig. 10 shows the overlay of the p + p and d +Au pair correlations. The p + p pair correlations are2 TABLE II: Table of measured c ¯ c cross sections from previous PHENIX analysis and from Monte Carlo generators comparedto the e – µ correlations in this analysis.description σ c ¯ c ( µ b) pythia e – µ ± ± powheg e – µ ± ± e – µ ± ± e – µ ± ± ± e ± [24] 567 ± ± e + e − ) [27] 554 ± ± ± (rad) φ∆ -1 0 1 2 3 4 ) - ) ( r a d φ ∆ d e d y µ N / ( d y d -6 × NN s13.4% Scale Uncertainty |<0.5 η >0.5 GeV/c, | T : p ± e <2.1 η >1 GeV/c, 1.4< T : p ± µ FIG. 9: (Color Online) The fully corrected like-sign-subtracted heavy flavor e – µ pair yield in d +Au. The errorbars are statistical only. The boxes show the type B sys-tematic uncertainty from the punch-through hadron and lighthadron decay muon background subtraction. The 13.4% typeC systematic uncertainty is not shown. scaled by the d +Au h N coll i = 7 . ± .
43 [18]. The peakin d +Au is suppressed compared to p + p , indicating amedium modification to the yield per collision in d +Au.To quantify the difference between p + p and d +Auyields, we calculate the ratio J dA defined as the ratioof a pair yield in d +Au to the N coll -scaled pair yield in p + p . J dA = d + Au pair yield h N coll i p + p pair yield . (8)Any deviation from unity of this ratio would indicatemodification to the yield. When taking this ratio sev-eral systematic uncertainties common to p + p and d +Aucancel. These are dominantly from identical cuts usedin the analyses with the same systematic uncertainties.The noncanceling type C systematic uncertainties in the p + p and d +Au yields are 7.7% and 8.9%, respectively.Fig. 11a shows a plot of J dA as a function of ∆ φ forall bins in ∆ φ . The bars are statistical uncertainties andthe type B systematic uncertainties are plotted as boxes. (rad) φ∆ ) - ) ( r a d φ ∆ d e d y µ N / ( d y d -6 × = 200 GeV NN s|<0.5 η >0.5 GeV/c, | T : p ± e <2.1 η >1 GeV/c, 1.4< T : p ± µ >-scaled p+p coll 10 GeV , on the edge of the shadowingregion. As discussed in Section IV A, the back-to-backpeak is dominated by leading order gluon fusion, whilethe continuum is dominated by other processes like fla-vor excitation and gluon splitting. The observed back-to-back peak and pedestal in p + p and d +Au should helplead to an understanding of the mechanism or mecha-nisms responsible for the modification. For example, theback-to-back peak is dominated by low- x gluons partici-pating in the hard scattering, whereas the continuum hasa larger contribution of quarks participating in the hardscattering. Quarks are probably less shadowed than glu-ons at the x and Q where this analysis is measured. It ispossible that there are kinematic differences between thefinal state charm quarks in the peak and the continuum.These differences could affect the amount of final stateenergy loss and multiple scattering that modify the mea-sured pair yields. It may be possible to combine these results with other cold nuclear matter charm measure-ments to disentangle the effects of shadowing, saturation,and energy loss. V. SUMMARY AND CONCLUSIONS We presented PHENIX results for heavy flavor pro-duction of azimuthally-correlated unlike sign e – µ pairsin p + p and d +Au collisions at √ s NN of 200 GeV. The p + p yield shows a nonzero continuum as well as a back-to-back peak structure centered at ∆ φ = π . When com-pared with several models, we find the charm cross sec-tion σ c ¯ c = 538 ± 46 (stat) ± 197 (data syst) ± µ b. This is also consistent with previouslymeasured c ¯ c cross sections at this center of mass energy.In d +Au collisions a yield reduction in the back-to-backpeak is observed, where we measure J dA (2 . < ∆ φ < . ± ± c ¯ c correla-tions. Such a suppression could arise due to nuclear PDFshadowing, saturation of the gluon wavefunction in theAu nucleus, or initial/final state energy loss and multiplescattering. 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 Cient´ıfico e Tecnol´ogicoand Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜aoPaulo (Brazil), Natural Science Foundation of China(P. R. China), Ministry of Education, Youth and Sports(Czech Republic), Centre National de la Recherche Sci-entifique, Commissariat `a l’´Energie Atomique, and Insti-tut National de Physique Nucl´eaire et de Physique desParticules (France), Bundesministerium f¨ur Bildung undForschung, Deutscher Akademischer Austausch Dienst,and Alexander von Humboldt Stiftung (Germany), Hun-garian National Science Fund, OTKA (Hungary), De-partment of Atomic Energy and Department of Scienceand Technology (India), Israel Science Foundation (Is-rael), National Research Foundation and WCU programof the Ministry Education Science and Technology (Ko-rea), Physics Department, Lahore University of Manage-ment Sciences (Pakistan), Ministry of Education and Sci-4ence, Russian Academy of Sciences, Federal Agency ofAtomic Energy (Russia), VR and 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. [1] N. Brambilla, S. Eidelman, B. Heltsley, R. Vogt, G. Bod-win, et al. , Eur. Phys. J. C , 1534 (2011).[2] F. Gelis, E. Iancu, J. Jalilian-Marian, and R. Venu-gopalan, Ann. Rev. Nucl. Part. Sci. , 463 (2010).[3] X.-N. Wang and X.-f. Guo, Nucl. Phys. A , 788(2001).[4] L. Frankfurt and M. Strikman, Phys. Lett. B , 412(2007).[5] I. Vitev, Phys. Rev. C , 064906 (2007).[6] A. M. Sickles, arXiv:1309.6924.[7] A. Adare et al. (PHENIX Collaboration), Phys. Rev.Lett. , 242301 (2012).[8] A. Adare et al. , Phys. Rev. C , 034904 (2013).[9] A. Adare et al. (PHENIX Collaboration),arXiv:1310.1005.[10] A. Adare et al. (PHENIX Collaboration), Phys. Rev.Lett. , 172301 (2011).[11] Z.-B. Kang, I. Vitev, and H. Xing, Phys. Rev. D ,054024 (2012).[12] A. Stasto, B.-W. Xiao, and F. Yuan, Phys. Lett. B ,430 (2012).[13] J. Jalilian-Marian and A. H. Rezaeian, Phys. Rev. D ,034016 (2012).[14] T. Lappi and H. Mantysaari, Nucl. Phys. A , 51(2013).[15] K. Adcox et al. (PHENIX Collaboration), Nucl. Instrum.Methods A , 469 (2003). [16] T. Sjostrand, S. Mrenna, and P. Z. Skands, PYTHIA 6.4Physics and Manual , J. High Energy Phys. (2006)026.[17] L. Aphecetche et al. (PHENIX Collaboration), Nucl. In-strum. Methods A , 521 (2003).[18] A. Adare et al. (PHENIX Collaboration),arXiv:1310.4793.[19] S. Adler et al. (PHENIX Collaboration), Phys. Rev. D , 092002 (2007).[20] A. Adare et al. (PHENIX Collaboration), Phys. Rev. C , 044905 (2011).[21] S. Adler et al. (PHENIX Collaboration), Phys. Rev. C , 054903 (2006).[22] D. Hornback, Ph.D. thesis, University of Tennessee(2007).[23] W. Vogelsang, private communication.[24] A. Adare et al. (PHENIX Collaboration), Phys. Rev.Lett. , 252002 (2006).[25] S. Frixione, P. Nason, and G. Ridolfi, J. High EnergyPhys. (2007) 126.[26] S. Frixione and B. R. Webber, J. High Energy Phys. (2002) 029.[27] A. Adare et al. (PHENIX Collaboration), Phys. Rev. D , 012003 (2009).[28] A. Adare et al. (PHENIX Collaboration), Phys. Rev.Lett.103