Heavy-flavor electron-muon correlations in p+p and d+Au collisions at sNN−−−−√=200 GeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Heavy-flavor electron-muon correlations in p+p and d+Au collisions at sNN−−−−√=200 GeV Adare, A.; Afanasiev, S.; Aidala, C.; Ajitanand, N. N.; Akiba, Y.; Al-Bataineh, H.; Alexander, J.; Angerami, A.; Aoki, K.; Apadula, N.; Kim, Dong Jo; Rak, Jan Adare, A., Afanasiev, S., Aidala, C., Ajitanand, N.N., Akiba, Y., Al-Bataineh, H., Alexander, J., Angerami, A., Aoki, K., Apadula, N., Kim, D. J., & Rak, J. (2014). Heavyflavor electron-muon correlations in p+p and d+Au collisions at sNN−−−−√=200 GeV. Physical Review C, 89(3), Article 034915. https://doi.org/10.1103/PhysRevC.89.034915 2014 PHYSICAL REVIEW C 89, 034915 (2014) Heavy-flavor electron-muon correlations in p+pand d+Au collisions at √sNN =200 GeV A. Adare,14 S. Afanasiev,31 C. Aidala,44,45 N. N. Ajitanand,63 Y. Akiba,57,58 H. Al-Bataineh,51 J. Alexander,63 A. Angerami,15 K. Aoki,36,57 N. Apadula,64 L. Aphecetche,65 Y. Aramaki,13,57 J. Asai,57 E. T. Atomssa,37 R. Averbeck,64 T. C. Awes,53 B. Azmoun,8V. Babintsev,25 M. Bai,7G. Baksay,21 L. Baksay,21 A. Baldisseri,17 K. N. Barish,9P. D. Barnes,40,* B. Bassalleck,50 A. T. Basye,1S. Bathe,6,9,58 S. Batsouli,53 V. Baublis,56 C. Baumann,46 A. Bazilevsky,8S. Belikov,8,* R. Belmont,69 R. Bennett,64 A. Berdnikov,60 Y. Berdnikov,60 J. H. Bhom,73 A. A. Bickley,14 D. S. Blau,35 J. G. Boissevain,40 J. S. Bok,73 H. Borel,17 K. Boyle,64 M. L. Brooks,40 H. Buesching,8V. Bumazhnov,25 G. Bunce,8,58 S. Butsyk,40 C. M. Camacho,40 S. Campbell,64 A. Caringi,47 B. S. Chang,73 W. C. Chang,2J.-L. Charvet,17 C.-H. Chen,64 S. Chernichenko,25 C. Y. Chi,15 M. Chiu,8,26 I. J. Choi,73 J. B. Choi,11 R. K. Choudhury,5P. Christiansen,42 T. Chujo,68 P. Chung,63 A. Churyn,25 O. Chvala,9V. Cianciolo,53 Z. Citron,64 B. A. Cole,15 Z. Conesa del Valle,37 M. Connors,64 P. Constantin,40 M. Csan´ ad,19 T. Cs ¨ org˝ o,72 T. Dahms,64 S. Dairaku,36,57 I. Danchev,69 K. Das,22 A. Datta,44 G. David,8 M. K. Dayananda,23 A. Denisov,25 D. d’Enterria,37 A. Deshpande,58,64 E. J. Desmond,8K. V. Dharmawardane,51 O. Dietzsch,61 A. Dion,29,64 M. Donadelli,61 O. Drapier,37 A. Drees,64 K. A. Drees,7A. K. Dubey,71 J. M. Durham,40,64 A. Durum,25 D. Dutta,5V. Dzhordzhadze,9L. D’Orazio,43 S. Edwards,22 Y. V. Efremenko,53 F. Ellinghaus,14 T. Engelmore,15 A. Enokizono,39,53 H. En’yo,57,58 S. Esumi,68 K. O. Eyser,9B. Fadem,47 D. E. Fields,50,58 M. Finger,10 M. Finger, Jr.,10 F. Fleuret,37 S. L. Fokin,35 Z. Fraenkel,71,*J. E. Frantz,52,64 A. Franz,8A. D. Frawley,22 K. Fujiwara,57 Y. Fukao,36,57 T. Fusayasu,49 I. Garishvili,66 A. Glenn,14,39 H. Gong,64 M. Gonin,37 J. Gosset,17 Y. Goto,57,58 R. Granier de Cassagnac,37 N. Grau,3,15 S. V. Greene,69 G. Grim,40 M. Grosse Perdekamp,26,58 T. Gunji,13 H.- ˚ A. Gustafsson,42,*A. Hadj Henni,65 J. S. Haggerty,8K. I. Hahn,20 H. Hamagaki,13 J. Hamblen,66 R. Han,55 J. Hanks,15 E. P. Hartouni,39 K. Haruna,24 E. Haslum,42 R. Hayano,13 X. He,23 M. Heffner,39 T. K. Hemmick,64 T. Hester,9J. C. Hill,29 M. Hohlmann,21 W. Holzmann,15,63 K. Homma,24 B. Hong,34 T. Horaguchi,13,24,57,67 D. Hornback,66 S. Huang,69 T. Ichihara,57,58 R. Ichimiya,57 H. Iinuma,36,57 Y. Ikeda,68 K. Imai,30,36,57 J. Imrek,18 M. Inaba,68 D. Isenhower,1M. Ishihara,57 T. Isobe,13,57 M. Issah,63,69 A. Isupov,31 D. Ivanischev,56 Y. Iwanaga,24 B. V. Jacak,64 J. Jia,8,15,63 X. Jiang,40 J. Jin,15 B. M. Johnson,8T. Jones,1K. S. Joo,48 D. Jouan,54 D. S. Jumper,1F. Kajihara,13 S. Kametani,57 N. Kamihara,58 J. Kamin,64 J. H. Kang,73 J. Kapustinsky,40 K. Karatsu,36,57 M. Kasai,57,59 D. Kawall,44,58 M. Kawashima,57,59 A. V. Kazantsev,35 T. Kempel,29 A. Khanzadeev,56 K. M. Kijima,24 J. Kikuchi,70 A. Kim,20 B. I. Kim,34 D. H. Kim,48 D. J. Kim,32,73 E. Kim,62 E.-J. Kim,11 S. H. Kim,73 Y.- J. K im ,26 E. Kinney,14 K. Kiriluk,14 ´ A. Kiss,19 E. Kistenev,8J. Klay,39 C. Klein-Boesing,46 D. Kleinjan,9L. Kochenda,56 B. Komkov,56 M. Konno,68 J. Koster,26 A. Kozlov,71 A. Kr´ al,16 A. Kravitz,15 G. J. Kunde,40 K. Kurita,57,59 M. Kurosawa,57 M. J. Kweon,34 Y. Kwon,66,73 G. S. Kyle,51 R. Lacey,63 Y. S. La i, 15 J. G. Lajoie,29 D. Layton,26 A. Lebedev,29 D. M. Lee,40 J. Lee,20 K. B. Lee,34 K. S. Lee,34 T. Lee,62 M. J. Leitch,40 M. A. L. Leite,61 B. Lenzi,61 X. Li,12 P. Lichtenwalner,47 P. Liebing,58 L. A. Linden Levy,14 T. Liˇ ska,16 A. Litvinenko,31 H. Liu,40,51 M. X. Liu,40 B. Love,69 D. Lynch,8C. F. Maguire,69 Y. I. Makdisi,7A. Malakhov,31 M. D. Malik,50 V. I. Manko,35 E. Mannel,15 Y. Mao,55,57 L. Maˇ sek,10,28 H. Masui,68 F. Matathias,15 M. McCumber,64 P. L. McGaughey,40 D. McGlinchey,14,22 N. Means,64 B. Meredith,26 Y. Miake,68 T. Mibe,33 A. C. Mignerey,43 P. Mikeˇ s,28 K. Miki,57,68 A. Milov,8M. Mishra,4J. T. Mitchell,8A. K. Mohanty,5H. J. Moon,48 Y. Morino,13 A. Morreale,9 D. P. Morrison,8,†T. V. Moukhanova,35 D. Mukhopadhyay,69 T. Murakami,36 J. Murata,57,59 S. Nagamiya,33 J. L. Nagle,14,‡ M. Naglis,71 M. I. Nagy,19,72 I. Nakagawa,57,58 Y. Nakamiya,24 K. R. Nakamura,36,57 T. Nakamura,24,57 K. Nakano,57,67 S. Nam,20 J. Newby,39 M. Nguyen,64 M. Nihashi,24 T. Niida,68 R. Nouicer,8A. S. Nyanin,35 C. Oakley,23 E. O’Brien,8 S. X. Oda,13 C. A. Ogilvie,29 M. Oka,68 K. Okada,58 Y. Onuki,57 A. Oskarsson,42 M. Ouchida,24,57 K. Ozawa,13 R. Pak,8 A. P. T. Palounek,40 V. Pantuev,27,64 V. Papavassiliou,51 I. H. Park,20 J. Park,62 S. K. Park,34 W. J. Park,34 S. F. Pate,51 H. Pei,29 J.-C. Peng,26 H. Pereira,17 V. Peresedov,31 D. Yu. Peressounko,35 R. Petti,64 C. Pinkenburg,8R. P. Pisani,8M. Proissl,64 M. L. Purschke,8A. K. Purwar,40 H. Qu,23 J. Rak,32,50 A. Rakotozafindrabe,37 I. Ravinovich,71 K. F. Read,53,66 S. Rembeczki,21 K. Reygers,46 V. Riabov,56 Y. Riabov,56 E. Richardson,43 D. Roach,69 G. Roche,41 S. D. Rolnick,9M. Rosati,29 C. A. Rosen,14 S. S. E. Rosendahl,42 P. Rosnet,41 P. Rukoyatkin,31 P. Ru ˇ ziˇ cka,28 V. L. Rykov,57 B. Sahlmueller,46,64 N. Saito,33,36,57,58 T. Sakaguchi,8S. Sakai,68 K. Sakashita,57,67 V. Samsonov,56 S. Sano,13,70 T. Sato,68 S. Sawada,33 K. Sedgwick,9J. Seele,14 R. Seidl,26,58 A. Yu. Semenov,29 V. Semenov,25,27 R. Seto,9D. Sharma,71 I. Shein,25 T.-A. Shibata,57,67 K. Shigaki,24 M. Shimomura,68 K. Shoji,36,57 P. Shukla,5A. Sickles,8C. L. Silva,29,61 D. Silvermyr,53 C. Silvestre,17 K. S. Sim,34 B. K. Singh,4C. P. Singh,4V. Singh,4M. Sluneˇ cka,10 A. Soldatov,25 R. A. Soltz,39 W. E. Sondheim,40 S. P. Sorensen,66 I. V. Sourikova,8F. Staley,17 P. W. Stankus,53 E. Stenlund,42 M. Stepanov,51 A. Ster,72 S. P. Stoll,8T. Sugitate,24 C. Suire,54 A. Sukhanov,8J. Sziklai,72 E. M. Takagui,61 A. Taketani,57,58 R. Tanabe,68 Y. Tanaka,49 S. Taneja,64 K. Tanida,36,57,58,62 M. J. Tannenbaum,8S. Tarafdar,4A. Taranenko,63 P. Tarj ´ an,18 H. Themann,64 D. Thomas,1T. L. Thomas,50 M. Togawa,36,57,58 A. Toia,64 L. Tom´ aˇ sek,28 Y. Tomita,68 H. Torii,24,57 R. S. Towell,1V.-N. Tram,37 I. Tserruya,71 Y. Tsuchimoto,24 C. Vale,8,29 H. Valle,69 H. W. van Hecke,40 E. Vazquez-Zambrano,15 A. Veicht,26 J. Velkovska,69 R. V´ ertesi,18,72 A. A. Vinogradov,35 M. Virius,16 V. Vrba,28 E. Vznuzdaev,56 X. R. Wang,51 D. Watanabe,24 K. Watanabe,68 Y. Watanabe,57,58 F. Wei,29 R. Wei,63 J. Wessels,46 S. N. White,8D. Winter,15 C. L. Woody,8R. M. Wright,1M. Wysocki,14 W. Xie,58 Y. L. Yamaguchi,13,57,70 K. Yamaura,24 R. Yang,26 A. Yanovich,25 J. Ying,23 S. Yokkaichi,57,58 Z. You,55 G. R. Young,53 I. Younus,38,50 I. E. Yushmanov,35 W. A. Zajc,15 O. Zaudtke,46 C. Zhang,53 S. Zhou,12 and L. Zolin31 (PHENIX Collaboration) 0556-2813/2014/89(3)/034915(14) 034915-1 ©2014 American Physical Society A. ADARE et al. PHYSICAL REVIEW C 89, 034915 (2014) 1Abilene Christian University, Abilene, Texas 79699, USA 2Institute of Physics, Academia Sinica, Taipei 11529, Taiwan 3Department of Physics, Augustana College, Sioux Falls, South Dakota 57197, USA 4Department of Physics, Banaras Hindu University, Varanasi 221005, India 5Bhabha Atomic Research Centre, Bombay 400 085, India 6Baruch College, City University of New York, New York, New York 10010, USA 7Collider-Accelerator Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA 8Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA 9University of California - Riverside, Riverside, California 92521, USA 10Charles University, Ovocn´ y trh 5, Praha 1, 116 36 Prague, Czech Republic 11Chonbuk National University, Jeonju 561-756, Korea 12Science and Technology on Nuclear Data Laboratory, China Institute of Atomic Energy, Beijing 102413, P. R. China 13Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan 14University of Colorado, Boulder, Colorado 80309, USA 15Columbia University, New York, New York 10027 and Nevis Laboratories, Irvington, New York 10533, USA 16Czech Technical University, Zikova 4, 166 36 Prague 6, Czech Republic 17Dapnia, CEA Saclay, F-91191 Gif-sur-Yvette, France 18Debrecen University, H-4010 Debrecen, Egyetem t´ er 1, Hungary 19ELTE, E¨ otv¨ os Lor´ and University, H-1117 Budapest, P´ azm´ any P. s. 1/A, Hungary 20Ewha Womans University, Seoul 120-750, Korea 21Florida Institute of Technology, Melbourne, Florida 32901, USA 22Florida State University, Tallahassee, Florida 32306, USA 23Georgia State University, Atlanta, Georgia 30303, USA 24Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan 25IHEP Protvino, State Research Center of Russian Federation, Institute for High Energy Physics, Protvino 142281, Russia 26University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA 27Institute for Nuclear Research of the Russian Academy of Sciences, prospekt 60-letiya Oktyabrya 7a, Moscow 117312, Russia 28Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic 29Iowa State University, Ames, Iowa 50011, USA 30Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan 31Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia 32Helsinki Institute of Physics and University of Jyv¨ askyl¨ a, P.O. Box 35, FI-40014 Jyv¨ askyl¨ a, Finland 33KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan 34Korea University, Seoul 136-701, Korea 35Russian Research Center “Kurchatov Institute”, Moscow 123098, Russia 36Kyoto University, Kyoto 606-8502, Japan 37Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS-IN2P3, Route de Saclay, F-91128 Palaiseau, France 38Physics Department, Lahore University of Management Sciences, Lahore, Pakistan 39Lawrence Livermore National Laboratory, Livermore, California 94550, USA 40Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 41LPC, Universit´ e Blaise Pascal, CNRS-IN2P3, Clermont-Fd, 63177 Aubiere Cedex, France 42Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden 43University of Maryland, College Park, Maryland 20742, USA 44Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003-9337, USA 45Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, USA 46Institut fur Kernphysik, University of Muenster, D-48149 Muenster, Germany 47Muhlenberg College, Allentown, Pennsylvania 18104-5586, USA 48Myongji University, Yongin, Kyonggido 449-728, Korea 49Nagasaki Institute of Applied Science, Nagasaki-shi, Nagasaki 851-0193, Japan 50University of New Mexico, Albuquerque, New Mexico 87131, USA 51New Mexico State University, Las Cruces, New Mexico 88003, USA 52Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA 53Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 54IPN-Orsay, Universite Paris Sud, CNRS-IN2P3, BP1, F-91406 Orsay, France 55Peking University, Beijing 100871, P. R. China 56PNPI, Petersburg Nuclear Physics Institute, Gatchina, Leningrad Region 188300, Russia 57RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan 58RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973-5000, USA 034915-2 HEAVY-FLAVOR ELECTRON-MUON CORRELATIONS IN . . . PHYSICAL REVIEW C 89, 034915 (2014) 59Physics Department, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan 60Saint Petersburg State Polytechnic University, St. Petersburg 195251, Russia 61Universidade de S˜ ao Paulo, Instituto de F´ ısica, Caixa Postal 66318, S˜ ao Paulo CEP05315-970, Brazil 62Seoul National University, Seoul, Korea 63Chemistry Department, Stony Brook University, SUNY, Stony Brook, New York 11794-3400, USA 64Department of Physics and Astronomy, Stony Brook University, SUNY, Stony Brook, New York 11794-3800, USA 65SUBATECH (Ecole des Mines de Nantes, CNRS-IN2P3, Universit´ e de Nantes) BP 20722-44307, Nantes, France 66University of Tennessee, Knoxville, Tennessee 37996, USA 67Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan 68Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan 69Vanderbilt University, Nashville, Tennessee 37235, USA 70Waseda University, Advanced Research Institute for Science and Engineering, 17 Kikui-cho, Shinjuku-ku, Tokyo 162-0044, Japan 71Weizmann Institute, Rehovot 76100, Israel 72Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Hungarian Academy of Sciences (Wigner RCP, RMKI) H-1525 Budapest 114, P. O. Box 49, Budapest, Hungary 73Yonsei University, IPAP, Seoul 120-749, Korea (Received 6 November 2013; published 31 March 2014) Background: Heavy-flavor modification in relativistic p(d)+Acollisions are sensitive to different kinds of strong-interaction physics ranging from modifications of the nuclear wave function to initialand final-state energy loss. Modifications to single heavy-flavor particles and their decay leptons at midrapidity and forward rapidity are well established at the Relativistic Heavy Ion Collider (RHIC). Purpose: This paper presents measurements of azimuthal correlations of electron-muon pairs produced from heavy-flavor decays, primarily c¯ c,in√sNN =200 GeV p+pand d+Au collision using the PHENIX detector at RHIC. The electrons are measured at midrapidity while the muons in the pair are measured at forward rapidity, defined as the direction of the deuteron beam, in order to utilize the deuteron to probe low-xpartons in the gold nucleus. Methods: This analysis uses the central spectrometer arms for electron identification and forward spectrometer arms for muon identification. Azimuthal correlations are built in all sign combinations for e-μpairs. Subtracting the like-sign yield from the unlike-sign yield removes the correlations from light flavor decays and conversions. Results: Comparing the p+presults with several different Monte Carlo event generators, we find the results are consistent with a total charm cross section σc¯ c=538 ±46 (stat) ±197 (data syst) ±174 (model syst) μb. These generators also indicate that the back-to-back peak at φ =πis 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 suppression factor for 2.7<φ<3.2radisJdA =0.433 ±0.087 (stat) ±0.135 (syst). Conclusions: The e-μpairs result from partons at xAu ∼10−2at Q2=10 GeV/c2at the edge of the shadowing region. The pair suppression indicates modification to c¯ cpairs for these kinematics in the cold nuclear medium at RHIC. DOI: 10.1103/PhysRevC.89.034915 PACS number(s): 25.75.Dw,25.75.Gz I. INTRODUCTION The study of open heavy-flavor production in relativistic p(d)+Acollisions is sensitive to different kinds of strong-interaction physics. Because the leading-order (LO) production mechanism is gluon fusion [1], open heavy-flavor production rates are directly related to modification of the gluon parton distribution function (PDF), i.e., shadowing or saturation [2]. Also, the initialand/or final-state partons can scatter and lose energy in the cold nuclear medium [3–5], thereby modifying and producing a nuclear modification *Deceased. †PHENIX Co-Spokesperson: [email protected]v ‡PHENIX Co-Spokesperson: [email protected] of open heavy-flavor production. Recently, the possibility of flow even in small collision systems such as p(d)+A has raised the question of modified charm momentum distributions [6]. Modification to heavy quark production rates and kinematics in d+Au collisions at the Relativistic Heavy Ion Collider (RHIC) is well established. Electron production from open heavy-flavor decay is enhanced [7], while J/ψ production [8] and ϒproduction [9] is suppressed at midrapidity. At positive rapidity, defined with the positive zaxis as the direction of the deuteron, there is a suppression of heavy-flavor decay muons [10] and a larger suppression of J/ψ [8]. While e-μcorrelations from open heavy-flavor decays have not been published at RHIC to date, correlations involving light flavor hadrons have shown modification in d+Au collisions at RHIC. A suppression has been observed of positive rapidity 034915-3 A. ADARE et al. PHYSICAL REVIEW C 89, 034915 (2014) π0mesons associated with midrapidity trigger hadrons, especially in the back-to-back peak at φ =π, indicating 2→2 scatterings [11,12]. This suppression increases as x, the fraction of the nucleon momentum carried by the gluon, decreases. These results are in quantitative agreement with energy-loss models [13] and saturation models [14–16]. This paper presents measurements of azimuthal correlations of electron-muon pairs produced from heavy-flavor decays, primarily c¯ c,inp+pand d+Au collisions using the PHENIX detector at RHIC. The heavy-flavor e-μcorrelations are free of backgrounds from other sources that contribute to other dilepton analyses (e+e−or μ+μ−), such as resonance decay and Drell-Yan. While analysis of dilepton mass and pT provides a way to separate charm and bottom contributions, the azimuthal correlations have an important advantage for studying the charm production process. The leading-order production, gg →Q¯ Qand q¯ q→Q¯ Q, will produce backto-back open heavy-flavor pairs that can semileptonically decay and produce azimuthally correlated e-μpairs. Next-toleading-order (NLO) processes like flavor excitation and gluon splitting produce much less correlated Q¯ Qand thus much less correlated e-μpairs. Therefore, modification to different portions of the azimuthal correlations can be attributed to modifications of c¯ cpairs from different production mechanisms. In energy loss models such as Ref. [13], a broadening of the back-to-back azimuthal correlation should accompany a suppression of the peak due to the multiple scattering that the incoming gluons and/or the outgoing c¯ cundergo in the cold nuclear medium. This paper is organized as follows. The PHENIX detector is outlined in Sec. II. Section III describes the details of the method used to measure the correlations, the background subtraction method, and the tests of the method. Section IV presents the results in p+pand compares them to Monte Carlo models. The d+Au results are presented and compared to the p+presults in Sec. IV B. Conclusions are given in Sec. V. II. PHENIX EXPERIMENT The PHENIX detector at RHIC is multipurposed and optimized for precision measurements of electromagnetic probes for relativistic hadronic and heavy-ion collisions. A complete overview of the detector can be found in Ref. [17]. The data presented here are from 2006 p+pand 2008 d+Au data taking at RHIC. Figure 1shows a schematic of the detector during those years. This analysis uses the central spectrometer arms for electron detection and the forward rapidity muon spectrometer arms, labeled North and South in Fig. 1, for muon identification. For the 2008 d+Au collisions, the deuteron beam moves toward the North arm, which defines positive rapidity for both p+pand d+Au. The forward produced muons come from a high-xparton in the deuteron interacting withalow-xparton in the gold. PYTHIA [18] indicates that the average xof a parton producing a heavy-flavor muon from 1<p μ T<6GeV/c in the forward muon spectrometer is about 5×10−3. This analysis focuses only on the muons measured in the North arm utilizing the deuteron beam as a probe of low-xpartons in the gold nucleus. The central spectrometer comprises two arms subtending π/2 in azimuth and covering |η|<0.35. Charged tracks are measured using a drift chamber (DC) and a set of multiwire proportional chambers with pad readout (PC1 and PC3). The DC measures the bend angle in the r-φplane due to a central magnetic field directed along the beam axis. PC1 is used to measure the longitudinal coordinate of the track. These tracks are then projected into PC3, where a hit is required to ensure high track quality. The momentum resolution of the tracks in this data is δp/p =1.10%⊕1.16%p, where pis the total momentum measured in GeV/c. Electrons can be identified from associated hits in the Ring Imaging ˇ Cerenkov (RICH) detector and the Electromagnetic Calorimeters (EMCal). Electrons above 17 MeV/c passing through the CO2-filled RICH will emit ˇ Cerenkov radiation. The EMCal comprises eight sectors, six of lead-scintillator and two of lead-glass, used to collect the energy from electron and photon showers. The nominal energy resolution for the lead-scintillator and lead-glass is 8.1% ±√E[GeV]⊕2.1% and 6.0% ±√E[GeV]⊕0.9% [19], respectively. The North muon spectrometer is located at 1.2<η<2.4 and covers 2πin azimuth. The spectrometer measures tracks in the muon tracker (MuTr) and the muon identifier (MuID). Prior to entering the muon arm, particles pass through approximately 20 cm of copper and 60 cm of iron. Particles that are not absorbed pass through the MuTr, which comprises three stations of cathode strip chambers with multiple ionization regions and located inside a radial magnetic field. After the MuTr, particles pass through the MuID, which comprises five alternating steel absorbers and MuID detector planes, called gaps, with Iarocci tubes. MuID roads reconstructed from MuID hits are projected back to MuTr tracks and to the measured vertex to provide the complete information for a track through the spectrometer. Trigger and global event characterization in p+pand d+Au are provided by the beam-beam counter (BBC). The BBC is a set of 64 hexagonal ˇ Cerenkov counters located from 3.0<|η|<3.9 and covering full azimuth. The vertex of the collision along the beam line (zvtx) is determined by the time difference between the BBCs on either side of the collision region. The minimum bias (MB) trigger requires that there is at least one hit in each of the BBCs. From Vernier scans and verified by Monte Carlo studies, the BBC MB trigger is sensitive to 55 ±5% of the p+pinelastic cross section and 88 ±4% of the d+Au inelastic cross section [20]. The trigger used for this analysis is a combination of the BBC trigger and a deep muon trigger. The deep muon trigger requires three or more MuID gaps with a signal in both the xand ydirection tubes and that the last pair of hits be in the last (fifth gap) or next to last gap (fourth gap). After quality cuts and requiring a vertex within 25 cm of the z=0 vertex, an integrated luminosity of 2.1 pb−1in p+p and a p+pequivalent of 7.7 pb−1in d+Au was sampled. III. ANALYSIS The primary goal of this analysis is to identify p+p(d+Au) →c¯ c+X→e±μ∓+X, (1) 034915-4 HEAVY-FLAVOR ELECTRON-MUON CORRELATIONS IN . . . PHYSICAL REVIEW C 89, 034915 (2014) FIG. 1. (Color online) A schematic view of the PHENIX detector during the 2008 d+Au data taking. (a) Beam view of the central spectrometer arms. (b) Longitudinal view including the global event and triggering detectors, as well as the muon spectrometer arms. The configurations of the central spectrometer and muon arms were the same for the 2006 p+pdata taking. where the opposite-sign electron-muon pair is from the c¯ cpair decay. A. Particle identification 1. Muon identification Real muons with total momentum less than ≈2.7 GeV/c are stopped in the muon arm before reaching the fifth (and last) gap. We apply an additional cut on muons with pT<1GeV/c to avoid a region with larger backgrounds and near the acceptance edge. Single muon candidates are constructed from MuID roads projected and matched to MuTr tracks. Cuts on MuID roads and MuTr tracks are designed to reject hadrons that mimic a muon signal and to reject tracks that did not originate from the collision vertex. For the MuID roads, at least three of five gaps with x-yhit information are required, including a pair of hits in the fifth gap. These MuID roads must project back near the nominal vertex position, thus selecting muons that do not typically come from beam-related backgrounds. For the MuTr tracks, cuts that reject hadrons are 034915-5 A. ADARE et al. PHYSICAL REVIEW C 89, 034915 (2014) detailed in Ref. [21]. The MuID roads are then projected and matched to MuTr tracks at the first MuID gap. An identified muon candidate is the closest MuTr track that matches a MuID road within at least 10◦in slope and 10 cm in distance. Muon candidates are further restricted to 1.4<η<2.1. During both the p+pand d+Au data taking periods, there were backgrounds primarily from beam-related particles interacting with material in the accelerator upstream of PHENIX, which varied throughout the running period. Collimators were used in the accelerator to reduce this background, but it was not totally eliminated. However, restricting the ηrange of the muon candidates helped to minimize this background. The analysis was divided into several run groups to assess this and other similar systematic errors. The fully corrected yields for the different run groups varied within 2%. 2. Electron identification Electrons with pT>0.5GeV/c are identified by matching a track in DC, PC1, and PC3 to a signal in the RICH and a cluster in the EMCal. The relevant details on measuring electrons in PHENIX are given in Ref. [22]. For this analysis, the projected track must match within 3σin position to a cluster in the EMCal. Clusters are also required to have a matching profile, when compared to an electromagnetic shower shape profile at the measured energy. Once a track matches both the RICH and the EMCal, an E/p cut is applied, where it is required that the energy measured in the the EMCal Ebe approximately equal to the reconstructed track momentum p. This is sufficient to remove most combinatorial matches and background from real electrons resulting from long-lived particle decays occurring near the DC, which have mismeasured momentum. A cut of −2σto +3σfrom the mean E/p in the p+pdata and −1.5σto +3σfrom the mean in the d+Au data is applied. The asymmetry of the cuts is due to the dominance of backgrounds below 2 or 1.5σof the mean. The tighter cut in the d+Au data was necessary because of the increased background from the hadron blind detector (HBD) support material not present during 2006 data taking. B. Acceptance and efficiencies After particle identification cuts have been applied to an event, all pairs of identified electrons and muons are formed in each of the four charge-sign combinations. The fully corrected invariant-pair yield, calculated for each sign combination, is [23] d3N dyμdyedφ =c NMB evt yeyμφbin dφMix(φ) 2π ×Neμ(φ) Mixeμ(φ,e,μ),(2) where NMB evt is the number of sampled BBC triggered events; cis the MB trigger bias accounting for events missed by the BBC trigger [20]; yeand yμare the rapidity ranges of the electrons and muons, respectively; Neμ(φ) is the inclusive electron-muon pair yield; and Mixeμ(φ,e,μ)isthe mixed-event electron-muon pair distribution. The two-particle acceptance times efficiency is corrected by the mixed-event technique, where electrons from one event are paired with muons from a different event. Pools of inclusive electrons and muons are kept in 2.5-cm-wide z-vertex bins and, in the case of d+Au, 10%-wide centrality bins. When mixing events, the pair distribution is weighted by the yand φ-averaged efficiency of each particle, eand μ. Both eand μwere determined by generating single electrons and single muons with a flat distribution in pT,φ, |ye|<0.5or1.4<y μ<2.2 and collision z-vertex location and running them through a GEANT-3simulation of the PHENIX detector. The output was weighted with the PHENIX single lepton pTspectra and then subjected to the same analysis cuts applied to the data. The efficiency is defined as the ratio of particles reconstructed through the analysis to the number simulated. These simulations demonstrated that e and μare independent of the zposition of the event vertex, eis independent of η, and μhas a slight ηdependence. Pair yields are reported with the average pseudorapidity ημ, which include the ηdependence of both single inclusive muons and the single-particle efficiency. C. Background subtraction Inclusive muon and electron candidates come from both heavyand light-flavor decays and from misidentified hadrons. The fully corrected inclusive electron-muon pair yield for each sign combinations can be written as Neμ(φ)=Neμ H(φ)+Neμ LH (φ)+Neμ L(φ).(3) Here Neμ indicates the fully corrected inclusive pair yield defined in Eq. (2); Neμ H(φ) is the fully corrected pair yield produced from a heavy-flavor pair decay; Neμ LH (φ)isthe fully corrected pair yield from correlating a heavy-flavor decay product with a light flavor decay product; and Neμ L(φ) is the fully corrected pair yield from correlating pairs of light-flavor decay products or misidentified hadrons. Pairs from the semileptonic decay of a c¯ cpair have opposite signs. Equation (3) can be decomposed into its likeand unlike-sign pieces as follows: Neμ like(φ)=Neμ LH,like(φ)+Neμ L,like(φ) Neμ unlike(φ)=Neμ H,unlike(φ)+Neμ LH,unlike(φ) +Neμ L,unlike(φ).(4) While semileptonic decays of b¯ bcan also produce both likeand unlike-sign e-μsignals, in this analysis, PYTHIA indicates that only about 1% of the final heavy-flavor e-μpair yield is from b¯ band is neglected. If we assume muon (electron) candidates from light flavors are not charge correlated with electron (muon) candidates from light flavors, then Neμ L,like(φ)=Neμ L,unlike(φ).(5) If only one of the pair is from heavy flavor, then, again, we assume they are not charge correlated and Neμ LH,like(φ)=Neμ LH,unlike(φ).(6) Therefore, the heavy-flavor e-μsignal distribution is the difference between the unlike-sign and the like-sign inclusive 034915-6 HEAVY-FLAVOR ELECTRON-MUON CORRELATIONS IN . . . PHYSICAL REVIEW C 89, 034915 (2014) (rad)φΔ 024 ) -1 ) (radφΔd e dy μ N/(dy 3 d 0 0.05 0.1 0.15 0.2 0.25 -6 10× (a) p+p (rad)φΔ -1 0 1 2 3 4 (unlike-like)/like 0 0.5 1 (rad)φΔ 024 ) -1 ) (radφΔd e dy μ N/(dy 3 d 0 0.5 1 1.5 2 2.5 -6 10× p+p unlike-sign p+p like-sign d+Au unlike-sign d+Au like-sign (b) d+Au (rad)φΔ -1 0 1 2 3 4 ( unlike-like)/like 0 0.1 0.2 0.3 FIG. 2. (Color online) The fully corrected inclusive like-sign (e±-μ±) and unlike-sign (e±-μ∓) distributions for (a) p+pand (b) d+Au as a function of φ. The inset shows the unlike-like difference divided by the like-sign distribution, which is the heavy-flavor signal-to-background in the inclusive unlike-sign distribution. correlations as follows: Neμ H(φ)=Neμ unlike(φ)−Neμ like(φ).(7) Figure 2shows the fully corrected inclusive like-sign [Neμ like(φ)] and unlike-sign [Neμ unlike(φ)] e-μpair distributions in p+pand d+Au. The inset figures show the signalto-background distributions given the assumptions above. We have checked the like-sign subtraction method using PYTHIA leading-order quantum chromodynamics (QCD) events. With all events containing a heavy quark in the final state removed, the pair yields as a function of φ for like-sign and unlike-sign electron-muon pairs were the same within 3% over all φ. While this corroborates the basic idea of the subtraction, the assumption was further tested with data. In the following sections we detail the results of different methods to tag electrons and muons from light flavor decay to examine the validity of Eq. (7) and to quantify the systematic uncertainty of the method. The general method is to use a sample of single electrons paired with single muons, where one or both are likely from light-hadron decays. If the method is correct, the like-sign subtraction should produce no correlation at all. If there are statistically significant correlations after like-sign subtraction, these are subtracted from the final e-μ pair yield and uncertainties on the residual correlation strength (GeV/c) z p 23456 Counts/Bin 100 200 300 400 500 600 3 10× FIG. 3. The distribution of pzfor tracks that stop in the next-tolast MuID gap (fourth gap). The peak at lower pzis due to muons, while the broad distribution is from hadrons that punch through the absorber to the fourth gap. The solid line is a two-Gaussian fit to this distribution with the solid line indicating the hadronic background in the muon peak region. are propagated as a systematic uncertainty on the final pair yield. If no statistically significant yield is found after like-sign subtraction, the statistical uncertainty on the zero yield is propagated as the systematic uncertainty. 1. Correlations between inclusive electrons and punch-through hadrons that fake single muons One source of background to the single muons is from hadrons that penetrate to the fifth gap, called punch-through hadrons. After single-particle cuts there is some small fraction (roughly 1 of every 250 [24]) of candidate tracks with pT>1GeV/c that are hadrons that punch through. While this represents an irreducible background to the single muons, we can obtain a clean sample of hadrons that punch through and stop in the fourth gap of the MuID. Figure 3shows the pzdistribution of muon candidates that stop in the fourth gap. The peak at 2.3 GeV/c is composed of muons that have insufficient energy to penetrate further. The broader portion of the distribution comprises light hadrons that are not stopped by the upstream absorber materials but are subsequently absorbed in the steel just after the fourth gap, thus not leaving a hit in the fifth gap. We identify punch-through hadrons as having stopped in the fourth gap with pzlarger than 3 GeV/c. Figure 4shows the fully corrected like-sign-subtracted pair yield of central-arm electrons and the punch-through hadrons in the muon arms for both p+pand d+Au collisions. If both the likeand unlike-sign pair yields were dominantly from light-hadron decays, the like-sign subtraction should produce zero pair yield. To determine the magnitude of the residual correlation strength after like-sign subtraction, the p+pdata were fitted with a flat line. This is shown as the solid line in Fig. 4(a). The fit uncertainty is shown as the shaded band around the solid line. The flat fit in p+phad a χ2per number of degrees of freedom (NDF) of 22.7/24 and gave a value that was nonzero with greater than 1σsignificance. This means there is yield in the final e-μcorrelations from these punchthrough hadrons. The fitted yield was subtracted from the final 034915-7 A. ADARE et al. PHYSICAL REVIEW C 89, 034915 (2014) (rad)φΔ 024 ) -1 ) (radφΔd e dy μ N/(dy 3 d -30 -20 -10 0 10 20 30 40 50 -9 10× -punch-through hadrons ± e (a) p+p (rad)φΔ 024 ) -1 ) (radφΔd e dy μ N/(dy 3 d -0.2 -0.1 0 0.1 0.2 -6 10× (b) d+Au FIG. 4. The fully corrected like-sign-subtracted electron plus punch-through hadron pair yield in (a) p+pand (b) d+Au collisions. The (a) solid line and (b) solid curve are the fitted yields that are removed from the inclusive electron-muon pair correlation. The shaded bands indicate the fit uncertainty that is propagated as a systematic uncertainty in the final pair yield. In (a) p+p, the fit is a flat line with χ2/NDF =22.7/24. In (b) d+Au, the fit is a flat line and a Gaussian centered at πwith χ2/NDF =26.3/22. pair yield and its uncertainty was propagated as a systematic uncertainty on the final pair yield. For the d+Au case, we fitted the residual correlation to a flat line and found reasonable agreement with a χ2/NDF of 30.9/24 or a pvalue of 14%. However, there is a possible excess of counts near φ =π, which when included as a Gaussian component fixed at φ = πand the width and yield as free parameters, a slightly better χ2/NDF of 26.3/22 or a pvalue of 26% was found. If there is any correlated yield beyond a pedestal, it would show up in the back-to-back peak. Therefore, we subtract the Gaussian fit, shown as the solid curve in Fig. 4from the final pair yield, and propagate the uncertainty on the fit, shown as the shaded region around the solid curve, to the systematic uncertainty in the final pair yield. Two additional corrections to this data are applied before subtraction from the final pair yield. Because the punchthrough hadrons are measured in the fourth gap, the yields need to be scaled to match the rate of hadrons at the last gap. The rate of hadrons at the fifth gap was determined by using pion and kaon NLO perturbative QCD spectra [25] and passing them through a GEANT-3model of the PHENIX muon arms. The MuID absorber steel cross section was modified until there was agreement between data and the simulation for the rate of punch-through hadrons in the third and fourth gap. We extrapolated to the fifth gap and find the rate of hadrons is 2.81 ±0.30 times the rate of punch-through hadrons in the fourth gap [21].The3GeV/c pzcut removes some fraction of the punch-through hadrons. Based on the two-component fit to the pzdistribution shown in Fig. 3, the yield is scaled up to account for those hadrons rejected by the pzcut. In the end, the pair yield uncertainty is 2.17×10−9(rad)−1in p+p.In d+Au there is a φ-independent uncertainty on the final pair yield that is 1.42 ×10−8(rad)−1and the Gaussian uncertainty that ranges from 0 to 6.30 ×10−8(rad)−1. 2. Correlations between inclusive electrons and light-hadron decay muons One source of real muons is from decays of light hadrons before and in the absorber material. These decay muons are predominantly from pions and kaons that are either directly produced in the collision or the result of low-mass resonance decays. The observed rate of muons into the North arm is higher, when the collision vertex is farther from the spectrometer arm. Because heavy-flavor decays (including Drell-Yan, heavy quarkonia, etc.) have a much shorter cτ than light flavor decays, heavy-flavor decay muons have a much weaker vertex dependence. Therefore, we assume there are two components to the muon rate: a component that follows the primary vertex distribution, attributable to heavy-flavor decays, and a component that folds the linear component due to light-hadron decays with the primary vertex distribution. Muons from events that are near the detector (0 <z vtx < 30 cm) and far from the detector (−30 <z vtx <0 cm), where zvtx is the measured collisions vertex, are separately correlated with central arm electrons. Because the signal heavy-flavor muons follow the primary collision vertex distribution, subtracting the near-vertex pair yield from the far-vertex pair yield, should remove these and only residual correlations from decay muons should be present. The pair yields in p+p and d+Au after subtracting nearand far-vertex muons and after like-sign subtraction are shown in Fig. 5.Thed+Au correlations are consistent with a flat line with zero yield with aχ2/NDF of 18.0/24. The p+pdata are not exactly flat at zero yield. However, this shape is not seen in d+Au and is not symmetric about φ =0orφ =π,soitisnotrelated to physics. Therefore, we fit with a flat line that results in zero correlation yield and a χ2/NDF of 27.1/24 corresponding to a pvalue of 30%. The fits are shown in Fig. 5as solid lines and shaded bands, indicating the statistical uncertainties. These uncertainties were propagated into the systematic uncertainties of the final pair yields. To propagate the uncertainties, additional corrections are needed. First, in the far-near subtraction, some fraction of the decay muons are removed. Second, light-hadron decays outside the ±30-cm vertex cut are not counted in the subtraction. To account for both effects, a fit to the vertex dependence of the muon yield is extrapolated to a point one interaction length inside the absorber, a distance of about 56 cm from the nominal zvertex and about 16 cm into the absorber. It is assumed that the decay contribution to the muons is negligible at that point, 034915-8