Observation of D_{s}^{\pm}/D^0 enhancement in Au+Au collisions at \sqrt{s_{_{\rm NN}}} = 200 GeV
STAR Collaboration, J. Adam, L. Adamczyk, J. R. Adams, J. K. Adkins, G. Agakishiev, M. M. Aggarwal, Z. Ahammed, I. Alekseev, D. M. Anderson, A. Aparin, E. C. Aschenauer, M. U. Ashraf, F. G. Atetalla, A. Attri, G. S. Averichev, V. Bairathi, K. Barish, A. Behera, R. Bellwied, A. Bhasin, J. Bielcik, J. Bielcikova, L. C. Bland, I. G. Bordyuzhin, J. D. Brandenburg, A. V. Brandin, J. Butterworth, H. Caines, M. Calderón de la Barca Sánchez, D. Cebra, I. Chakaberia, P. Chaloupka, B. K. Chan, F-H. Chang, Z. Chang, N. Chankova-Bunzarova, A. Chatterjee, D. Chen, J. Chen, J. H. Chen, X. Chen, Z. Chen, J. Cheng, M. Cherney, M. Chevalier, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, M. Csanád, M. Daugherity, T. G. Dedovich, I. M. Deppner, A. A. Derevschikov, L. Didenko, X. Dong, J. L. Drachenberg, J. C. Dunlop, T. Edmonds, N. Elsey, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, A. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, P. Federic, J. Fedorisin, C. J. Feng, Y. Feng, P. Filip, E. Finch, Y. Fisyak, A. Francisco, C. Fu, L. Fulek, C. A. Gagliardi, T. Galatyuk, F. Geurts, N. Ghimire, A. Gibson, K. Gopal, X. Gou, D. Grosnick, W. Guryn, A. I. Hamad, A. Hamed, S. Harabasz, J. W. Harris, S. He, W. He, X. H. He, Y. He, S. Heppelmann, S. Heppelmann, N. Herrmann, E. Hoffman, et al. (272 additional authors not shown)
OObservation of D ± s /D enhancement in Au+Au collisions at √ s NN = 200 GeV J. Adam, L. Adamczyk, J. R. Adams, J. K. Adkins, G. Agakishiev, M. M. Aggarwal, Z. Ahammed, I. Alekseev,
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D. M. Anderson, A. Aparin, E. C. Aschenauer, M. U. Ashraf, F. G. Atetalla, A. Attri, G. S. Averichev, V. Bairathi, K. Barish, A. Behera, R. Bellwied, A. Bhasin, J. Bielcik, J. Bielcikova, L. C. Bland, I. G. Bordyuzhin, J. D. Brandenburg, A. V. Brandin, J. Butterworth, H. Caines, M. Calder´on de la Barca S´anchez, D. Cebra, I. Chakaberia,
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P. Chaloupka, B. K. Chan, F-H. Chang, Z. Chang, N. Chankova-Bunzarova, A. Chatterjee, D. Chen, J. Chen, J. H. Chen, X. Chen, Z. Chen, J. Cheng, M. Cherney, M. Chevalier, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, M. Csan´ad, M. Daugherity, T. G. Dedovich, I. M. Deppner, A. A. Derevschikov, L. Didenko, X. Dong, J. L. Drachenberg, J. C. Dunlop, T. Edmonds, N. Elsey, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, A. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, P. Federic, J. Fedorisin, C. J. Feng, Y. Feng, P. Filip, E. Finch, Y. Fisyak, A. Francisco, C. Fu, L. Fulek, C. A. Gagliardi, T. Galatyuk, F. Geurts, N. Ghimire, A. Gibson, K. Gopal, X. Gou, D. Grosnick, W. Guryn, A. I. Hamad, A. Hamed, S. Harabasz, J. W. Harris, S. He, W. He, X. H. He, Y. He, S. Heppelmann, S. Heppelmann, N. Herrmann, E. Hoffman, L. Holub, Y. Hong, S. Horvat, Y. Hu, H. Z. Huang, S. L. Huang, T. Huang, X. Huang, T. J. Humanic, P. Huo, G. Igo, ∗ D. Isenhower, W. W. Jacobs, C. Jena, A. Jentsch, Y. Ji, J. Jia,
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K. Jiang, S. Jowzaee, X. Ju, E. G. Judd, S. Kabana, M. L. Kabir, S. Kagamaster, D. Kalinkin, K. Kang, D. Kapukchyan, K. Kauder, H. W. Ke, D. Keane, A. Kechechyan, M. Kelsey, Y. V. Khyzhniak, D. P. Kiko(cid:32)la, C. Kim, B. Kimelman, D. Kincses, T. A. Kinghorn, I. Kisel, A. Kiselev, M. Kocan, L. Kochenda, L. K. Kosarzewski, L. Kramarik, P. Kravtsov, K. Krueger, N. Kulathunga Mudiyanselage, L. Kumar, S. Kumar, R. Kunnawalkam Elayavalli, J. H. Kwasizur, R. Lacey, S. Lan, J. M. Landgraf, J. Lauret, A. Lebedev, R. Lednicky, J. H. Lee, Y. H. Leung, C. Li, C. Li, W. Li, W. Li, X. Li, Y. Li, Y. Liang, R. Licenik, T. Lin, Y. Lin, M. A. Lisa, F. Liu, H. Liu, P. Liu, P. Liu, T. Liu, X. Liu, Y. Liu, Z. Liu, T. Ljubicic, W. J. Llope, R. S. Longacre, N. S. Lukow, S. Luo, X. Luo, G. L. Ma, L. Ma, R. Ma, Y. G. Ma, N. Magdy, R. Majka, ∗ D. Mallick, S. Margetis, C. Markert, H. S. Matis, J. A. Mazer, N. G. Minaev, S. Mioduszewski, B. Mohanty, I. Mooney, Z. Moravcova, D. A. Morozov, M. Nagy, J. D. Nam, Md. Nasim, K. Nayak, D. Neff, J. M. Nelson, D. B. Nemes, M. Nie, G. Nigmatkulov, T. Niida, L. V. Nogach, T. Nonaka, A. S. Nunes, G. Odyniec, A. Ogawa, S. Oh, V. A. Okorokov, B. S. Page, R. Pak, A. Pandav, Y. Panebratsev, B. Pawlik, D. Pawlowska, H. Pei, C. Perkins, L. Pinsky, R. L. Pint´er, J. Pluta, B. R. Pokhrel, J. Porter, M. Posik, N. K. Pruthi, M. Przybycien, J. Putschke, H. Qiu, A. Quintero, S. K. Radhakrishnan, S. Ramachandran, R. L. Ray, R. Reed, H. G. Ritter, O. V. Rogachevskiy, J. L. Romero, L. Ruan, J. Rusnak, N. R. Sahoo, H. Sako, S. Salur, J. Sandweiss, ∗ S. Sato, W. B. Schmidke, N. Schmitz, B. R. Schweid, F. Seck, J. Seger, M. Sergeeva, R. Seto, P. Seyboth, N. Shah, E. Shahaliev, P. V. Shanmuganathan, M. Shao, A. I. Sheikh, W. Q. Shen, S. S. Shi, Y. Shi, Q. Y. Shou, E. P. Sichtermann, R. Sikora, M. Simko, J. Singh, S. Singha, N. Smirnov, W. Solyst, P. Sorensen, H. M. Spinka, ∗ B. Srivastava, T. D. S. Stanislaus, M. Stefaniak, D. J. Stewart, M. Strikhanov, B. Stringfellow, A. A. P. Suaide, M. Sumbera, B. Summa, X. M. Sun, X. Sun, Y. Sun, Y. Sun, B. Surrow, D. N. Svirida, P. Szymanski, A. H. Tang, Z. Tang, A. Taranenko, T. Tarnowsky, J. H. Thomas, A. R. Timmins, D. Tlusty, M. Tokarev, C. A. Tomkiel, S. Trentalange, R. E. Tribble, P. Tribedy, S. K. Tripathy, O. D. Tsai, Z. Tu, T. Ullrich, D. G. Underwood, I. Upsal,
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G. Van Buren, J. Vanek, A. N. Vasiliev, I. Vassiliev, F. Videbæk, S. Vokal, S. A. Voloshin, F. Wang, G. Wang, J. S. Wang, P. Wang, Y. Wang, Y. Wang, Z. Wang, J. C. Webb, P. C. Weidenkaff, L. Wen, G. D. Westfall, H. Wieman, S. W. Wissink, R. Witt, Y. Wu, Z. G. Xiao, G. Xie, W. Xie, H. Xu, N. Xu, Q. H. Xu, Y. F. Xu, Y. Xu, Z. Xu, Z. Xu, C. Yang, Q. Yang, S. Yang, Y. Yang, Z. Yang, Z. Ye, Z. Ye, L. Yi, K. Yip, Y. Yu, H. Zbroszczyk, W. Zha, C. Zhang, D. Zhang, S. Zhang, S. Zhang, X. P. Zhang, Y. Zhang, Y. Zhang, Z. J. Zhang, Z. Zhang, Z. Zhang, J. Zhao, C. Zhong, C. Zhou, X. Zhu, Z. Zhu, M. Zurek, and M. Zyzak (STAR Collaboration) a r X i v : . [ h e p - e x ] J a n Abilene Christian University, Abilene, Texas 79699 AGH University of Science and Technology, FPACS, Cracow 30-059, Poland Alikhanov Institute for Theoretical and Experimental Physics NRC ”Kurchatov Institute”, Moscow 117218, Russia Argonne National Laboratory, Argonne, Illinois 60439 American University of Cairo, New Cairo 11835, New Cairo, Egypt Brookhaven National Laboratory, Upton, New York 11973 University of California, Berkeley, California 94720 University of California, Davis, California 95616 University of California, Los Angeles, California 90095 University of California, Riverside, California 92521 Central China Normal University, Wuhan, Hubei 430079 University of Illinois at Chicago, Chicago, Illinois 60607 Creighton University, Omaha, Nebraska 68178 Czech Technical University in Prague, FNSPE, Prague 115 19, Czech Republic Technische Universit¨at Darmstadt, Darmstadt 64289, Germany ELTE E¨otv¨os Lor´and University, Budapest, Hungary H-1117 Frankfurt Institute for Advanced Studies FIAS, Frankfurt 60438, Germany Fudan University, Shanghai, 200433 University of Heidelberg, Heidelberg 69120, Germany University of Houston, Houston, Texas 77204 Huzhou University, Huzhou, Zhejiang 313000 Indian Institute of Science Education and Research (IISER), Berhampur 760010 , India Indian Institute of Science Education and Research (IISER) Tirupati, Tirupati 517507, India Indian Institute Technology, Patna, Bihar 801106, India Indiana University, Bloomington, Indiana 47408 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000 University of Jammu, Jammu 180001, India Joint Institute for Nuclear Research, Dubna 141 980, Russia Kent State University, Kent, Ohio 44242 University of Kentucky, Lexington, Kentucky 40506-0055 Lawrence Berkeley National Laboratory, Berkeley, California 94720 Lehigh University, Bethlehem, Pennsylvania 18015 Max-Planck-Institut f¨ur Physik, Munich 80805, Germany Michigan State University, East Lansing, Michigan 48824 National Research Nuclear University MEPhI, Moscow 115409, Russia National Institute of Science Education and Research, HBNI, Jatni 752050, India National Cheng Kung University, Tainan 70101 Nuclear Physics Institute of the CAS, Rez 250 68, Czech Republic Ohio State University, Columbus, Ohio 43210 Institute of Nuclear Physics PAN, Cracow 31-342, Poland Panjab University, Chandigarh 160014, India Pennsylvania State University, University Park, Pennsylvania 16802 NRC ”Kurchatov Institute”, Institute of High Energy Physics, Protvino 142281, Russia Purdue University, West Lafayette, Indiana 47907 Rice University, Houston, Texas 77251 Rutgers University, Piscataway, New Jersey 08854 Universidade de S˜ao Paulo, S˜ao Paulo, Brazil 05314-970 University of Science and Technology of China, Hefei, Anhui 230026 Shandong University, Qingdao, Shandong 266237 Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800 Southern Connecticut State University, New Haven, Connecticut 06515 State University of New York, Stony Brook, New York 11794 Instituto de Alta Investigaci´on, Universidad de Tarapac´a, Arica 1000000, Chile Temple University, Philadelphia, Pennsylvania 19122 Texas A&M University, College Station, Texas 77843 University of Texas, Austin, Texas 78712 Tsinghua University, Beijing 100084 University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan United States Naval Academy, Annapolis, Maryland 21402 Valparaiso University, Valparaiso, Indiana 46383 Variable Energy Cyclotron Centre, Kolkata 700064, India Warsaw University of Technology, Warsaw 00-661, Poland Wayne State University, Detroit, Michigan 48201 Yale University, New Haven, Connecticut 06520 (Dated: January 29, 2021)We report on the first measurement of charm-strange meson D ± s production at midrapidity inAu+Au collisions at √ s NN = 200 GeV from the STAR experiment. The yield ratio between strange( D ± s ) and nonstrange ( D ) open-charm mesons is presented and compared to model calculations. Asignificant enhancement, relative to a PYTHIA model calculation of p + p collisions, is observed in the D ± s /D yield ratio in Au+Au collisions over a large range of collision centralities. Model calculationsincorporating strangeness enhancement and coalescence hadronization in the quark-gluon plasma(QGP) medium qualitatively reproduce the data. The transverse momentum integrated yield ratioof D ± s /D (0.42 ± ± Under extremely high temperatures and energy den-sities in ultrarelativistic heavy-ion collisions a new stateof matter, the quark-gluon plasma (QGP), is formed inwhich quarks and gluons are the degrees of freedom [1, 2].Since heavy quark masses are larger than the typical tem-perature of the fireball formed in nuclear collisions atthe Relativistic Heavy Ion Collider (RHIC), it is difficultto produce heavy quarks via thermal processes. There-fore, heavy quarks are predominantly produced via initialhard scatterings, and their production cross sections canbe evaluated by perturbative quantum chromodynamics(pQCD) [3, 4].Charm quarks are produced before the QGP is cre-ated, and they subsequently experience the whole evolu-tion of the QGP matter. Since their thermal relaxationtime ( ∼ c ) is comparable to the lifetime of theQGP, they carry information about the transport prop-erties of the medium. During the cooling down of thehot and dense medium, the charm quarks can hadronizeinto different species of open-charm hadrons, e.g. , D , D ± , D ± s and Λ c . The hadronization mechanisms ofthese open-charm hadrons are of particular interest. In p + p / e + e / p + e collisions, charm-hadron production athigh transverse momentum ( p T ) is well described byquark-fragmentation models, including PYTHIA [5]. Inthe QGP medium, D ± s mesons, which consist of a c (¯ c )quark and an ¯ s ( s ) quark, are argued to be formed byrecombination of charm quarks and strange quarks (co-alescence hadronization) [6–9]. Support for the coales-cence hadronization picture has been observed in a recentSTAR measurement of Λ c baryon production in Au+Aucollisions at √ s NN = 200 GeV [10].Enhancement of strangeness production in high-energyheavy-ion collisions has been observed in experiments atSPS, RHIC and LHC energies [11–14], and it is believedto be one piece of evidence of QGP formation. The prob-ability of the hadronization of a charm (anticharm) quarkcombining with an antistrange (strange) quark to forma D + s ( D − s ) meson is expected to be increased due tothe enhanced surrounding strange quark density. The p T spectra of D mesons have been measured previously bySTAR with a high precision provided by the Heavy Fla-vor Tracker (HFT) [15, 16]. These results provide a good reference for the study of D ± s enhancement through com-parison of yields of D ± s and D mesons as a function of p T in different collision centralities. Comparing the D ± s / D yield ratio in heavy-ion collisions with that in proton-proton collisions helps us understand the QGP mediumeffect and hadronization mechanisms [17].Various D ± s measurements have been carried out by the LHC exper-iments [18–20]. Those measurements show evidence ofan enhancement of the D ± s / D yield ratio, however withlarge uncertainties.In this letter, we report on the first measurement of D ± s production over a transverse momentum range of1 < p T < c , in Au+Au collisions at a center-of-mass energy of √ s NN = 200 GeV. The measurementwas performed via invariant-mass reconstruction of thehadronic decay channel, D + s → φ + π + → K + + K − + π + (branching ratio 2.27% ± | y | <
1) and comparing it to aMonte Carlo Glauber simulation [26]. The tracks used in ) (GeV/c – p - K + K Invariant Mass M ) C oun t s ( pe r M e V / c = 200 GeV NN sAu+Au < 8 GeV/c T – s D – D (a) (GeV/c) T Transverse Momentum p ) / G e V d y ( c T dp T p p N / d - - - - +X – s D fi STAR Au+Au = 200 GeV NN s · Levy fit (b)
FIG. 1. (a) Invariant mass distribution M K + K − π ± in 0–80% Au+Au collisions at √ s NN = 200 GeV. The solid line depicts afit with two Gaussian functions representing D ± s and D ± candidates plus a linear function for background. (b) D ± s invariantyield as a function of p T in various centrality bins of Au+Au collisions at √ s NN = 200 GeV. Vertical bars and brackets (forall figures) on data points represent statistical and systematic uncertainties, respectively. Solid and dashed lines depict Levyfunction fits to the p T spectrum in each centrality bin. D ± s meson reconstruction are those with at least 20 hitsrecorded by the TPC, one hit in each PXL layer, andone hit in either IST or SSD. Those tracks must also bewithin pseudorapidity | η | < p T minimum( p kaon T > c and p pion T > c ). The distanceof closest approach (DCA) between tracks and the pri-mary vertex is required to be larger than 60 µ m in orderto reduce the combinatorial background. The pions andkaons are identified by selecting tracks with | nσ π | < | nσ K | <
2, respectively. The quantity nσ x is definedas the number of standard deviations of the measuredionization energy loss in the TPC ( dE/dx ) relative tothe theoretical value [27]. If Time-Of-Flight (TOF) [28]information is available, 1 /β is required to be less than 3Gaussian standard deviations relative to the theoreticalvalue, in addition to the dE/dx cuts.Since the decay length of φ mesons is negligible com-pared to the vertex resolution, the D ± s mesons are re-garded as decaying into K + K − π ± at a single secondaryvertex. The invariant mass of K + K − pairs is requiredto be within 1.011 and 1.027 GeV/ c , to exclude the re-constructed D ± s from the other decay channels. To im-prove the significance of the reconstructed D ± s , a machinelearning algorithm, the Boosted Decision Tree (BDT)from the Toolkit for MultiVariate Analysis (TMVA) [29]was employed. The BDT classifier was obtained by train-ing the signal sample from a data-driven simulation (de-scribed elsewhere [16]) and a background sample fromwrong-sign combinations of KKπ triplets. The variablescharacterizing the D ± s decay topology such as the DCAbetween the primary vertex and the daughter tracks, theDCA between decay daughters ( K + K − π ± ), and the de-cay length were used as input variables to the BDT classi-fier. The cut on the BDT response evaluated by the BDT classifier was optimized to have the best signal signifi-cance for the number of signal and background counts indata. Figure 1 (a) shows the invariant mass distributionof M K + K − π ± candidates in 0–80% collision centrality.The solid line depicts a fit with two Gaussian functionsrepresenting the D ± s and D ± signals plus a linear functionfor the background. The D ± s candidates are extracted bysumming the right-sign distribution within 3 standarddeviations of the D ± s Gaussian fit mean. The combina-torial background is calculated by integrating the linearfunction within the same range. The raw-signal yieldsare then obtained by subtracting the background fromthe D ± s candidates.The efficiency of D ± s reconstruction is evaluated bythe data-driven simulation validated in the D spectrameasurement with the HFT at STAR [16]. The D ± s mesons are generated by Monte Carlo with uniform ra-pidity and p T distributions weighted according to the D yields, and the decay kinematics from the PYTHIA pack-age [5]. The efficiency is calculated based on: the accep-tance cuts ( | η | < D ± s ( D + s + D − s ) invariant yield ((1/2 πp T ) d N / dp T dy ) ob-tained as the average raw-signal yield per event, scaledby the inverse of the reconstruction efficiency as well as bythe decay branching ratio from the Particle Data Group(PDG) [21], is calculated for each centrality and p T bin.The systematic uncertainties are estimated from theraw-yield extraction and efficiency calculation. The sys-tematic uncertainty due to methods of raw-yield extrac-tion was calculated to be 2-10%, depending on p T andcentrality, by changing the fitting ranges and functiontypes for the background estimate. The uncertaintiesfrom the TPC efficiency and PID efficiency are evaluatedto be ∼
9% and ∼ ∼
3% [16]. The uncer-tainty from the TPC-to-HFT matching and topologicalcut efficiency is obtained by changing the default BDT re-sponse cut to be higher and lower so that the reconstruc-tion efficiency varies by ∼
50% relative to the nominalone [16]. The maximum variation of the invariant yieldsfrom the default BDT cut and the varied BDT cut afterremoving the variation from statistical fluctuation [32]is treated as systematic uncertainty (2-20%). The effi-ciencies are evaluated using p T distributions weighted by D ± s and D yields. The difference is also included in thesystematic uncertainties (1-20%). The feed-down contri-bution from bottom hadrons to the D s measurement isevaluated to be 2-7% for 2.5 < p T < c and 7-10%for 5 < p T < c for different collision centralities.In the D s /D yield ratio, the feed-down contribution par-tially cancels, leaving 2-6% contribution at 2.5 < p T < c , and it is less than 1% for the other p T bins. Thefeed-down contribution is included in the systematic un-certainties. The final systematic uncertainty is the squareroot of the quadratic sum of each source. Finally, the un-certainty from the decay branching ratio is considered asa global normalization uncertainty ( ∼ D ± s invariant yield.The distributions of the D ± s invariant yields as a func-tion of p T in different collision centralities are shown inFig. 1 (b). The statistical and systematic uncertaintieson data points are denoted by vertical bars (smaller thanthe marker size when not visible) and brackets, respec-tively. Solid and dashed lines depict Levy-function [33]fits to the p T spectrum in each centrality bin. The ra-tios of the invariant yield of D ± s over that of D as afunction of p T in Au+Au collisions at √ s NN = 200 GeVare shown in Fig. 2. The correlated systematic uncer-tainties from the tracking efficiency correction going intoboth D ± s and D partially cancel in the ratio. Figure2 (a) shows the D s /D yield ratio as a function of p T for different collision centralities compared to that froma PYTHIA (version 8.2, Monash tune) model calculationof p + p collisions at the same energy. It is observed thatthe D s /D ratio in Au+Au collisions shows a large en-hancement (about 1.5-2 times) relative to the PYTHIAcalculation in p + p collisions, and weak centrality and p T dependencies within the uncertainties. This indicatesthat the hadronization of charm quarks is different inheavy-ion collisions compared to p + p collisions.Figure 2 (b) compares the present STAR results withthe D s /D yield ratio from Pb+Pb collisions at √ s NN =5.02 TeV (open circles) in 0–10% collision centrality [18]and p + p collisions at √ s = 7 TeV [34] (solid triangles). (GeV/c) T Transverse Momentum p ) D + ) / ( D - s + D + s ( D NN s (a) (GeV/c) T Transverse Momentum p ) D + ) / ( D - s + D + s ( D (b) STAR Au+Au 200 GeV (0-10%)ALICE Pb+Pb 5.02 TeV (0-10%)ALICE p+p 7 TeVPYTHIA p+p 200 GeV 7 TeV
FIG. 2. (a) D s /D yield ratio as a function of p T in variouscentrality bins of Au+Au collisions at √ s NN = 200 GeV, com-pared to a PYTHIA calculation for p + p collisions at the sameenergy. (b) STAR measurement of D s /D yield ratio (blacksolid points) as a function of p T in 0–10% central Au+Aucollisions at √ s NN = 200 GeV, compared to ALICE measure-ments in Pb+Pb collisions at √ s NN = 5.02 TeV (open circles)and in p + p collisions at √ s = 7 TeV (solid triangles) as wellas to PYTHIA calculations for p + p collisions at 200 GeV and7 TeV (green and purple curves). It shows that the ratio in p + p collisions can be well de-scribed by PYTHIA, and STAR measurements are com-patible with the ALICE results [18] in Pb+Pb collisionswithin uncertainties in the overlapping p T region for 0–10% collision centrality.Figure 3 shows the D s /D yield ratio as a function of p T compared to coalescence model calculations for dif-ferent collision centralities. These models assume that D ± s mesons are formed by the recombination of charmquarks with enhanced strange quarks in the QGP. TheTsinghua model [35] is the only one of these models thathas sequential coalescence ( D ± s mesons hadronize ear-lier than D ). The calculations from ‘Tsinghua (seq.coal.)’ and ‘Catania (coal.)’ [36] only include coales-cence hadronization. The calculations from ‘Catania(coal.+frag.)’, ‘He/Rapp’ [37] and ‘Cao/Ko’ [38] con-tain both coalescence and fragmentation hadronization ofcharm quarks. For 0–10% collision centrality, the calcula-tions which include hadronization through coalescence ofcharm quarks, from He/Rapp, Cao/Ko, Catania (coal.)and Tsinghua (seq. coal.) describe the general enhance-ment of the data relative to PYTHIA p + p , althoughthere is some tension with data for p T < c . TheTsinghua model describes the data for the 20–40% cen- (GeV/c) T Transverse Momentum p ) D + ) / ( D - s + D + s ( D NN s (a)0-10%: Tsinghua (seq. coal.)Catania (coal.)Catania (coal.+frag.)Cao,Ko (GeV/c) T Transverse Momentum p ) D + ) / ( D - s + D + s ( D Au+Au, Au+Au, (b)
Tsinghua (seq. coal.), 20-40%Tsinghua (seq. coal.), 40-80%PYTHIA p+p
FIG. 3. (a) D s /D yield ratio as a function of p T compared tovarious model calculations from He/Rapp (0–20%), Tsinghua,Catania and Cao/Ko in 0–10% centrality bin of Au+Au col-lisions, and PYTHIA prediction in p + p collisions at √ s NN =200 GeV. (b) D s /D yield ratio as a function of p T comparedto model calculations from Tsinghua in 20–40% (solid circles)and 40–80% (open circles) centrality bins of Au+Au collisionsat √ s NN = 200 GeV. trality, as shown in the bottom panel of Fig. 3. For the40–80% centrality class the model calculation is close tothe PYTHIA p + p prediction. The Tsinghua model con-siders the centrality dependence based on the degree ofcharm-quark thermalization, which underestimates thedata in peripheral collisions. Overall, these comparisonsindicate that coalescence hadronization plays an impor-tant role in charm-quark hadronization in the surround-ing QGP medium.In order to extract the p T -integrated cross sections themeasured D ± s invariant yields are extrapolated to p T =0 at midrapidity. The measured D yields (for p T < c ) are scaled by the D s /D ratio, calculated in thecoalescence models shown in Fig. 3, and the resulting D s spectrum is fit with a Levy function. The average ofcross sections from different model estimates is treated asthe central value of D ± s total cross section, and the max-imum difference from central value and model estimatesis included in the systematic uncertainty. The total crosssections per nucleon-nucleon collision, dσ / dy | y =0 , are es-timated to be 14 ± ± µb for 0–10%centrality, 18 ± ± µb for 10–40% centrality and 15 ± ± µb for 40–80% centrality. The p T -integrated D s /D yield ratio is 0.42 ± ± T ch = 160 MeV, µ B =21.9 MeV and strangeness fugacity γ s = 1.0, is ∼ > part
1) in Au+Au collisions at √ s NN = 200 GeV. A clearenhancement of the D s /D yield ratio is found comparedto calculations from PYTHIA in p + p collisions at thesame collision energy. The enhancement can be qual-itatively described by model calculations incorporatingstrangeness enhancement and coalescence hadronizationof charm quarks. The p T -integrated D s /D ratio is com-patible with the prediction from a statistical hadroniza-tion model. These results suggest that recombination ofcharm quarks with strange quarks in the QGP plays animportant role in charm-quark hadronization.We thank the RHIC Operations Group and RCF atBNL, the NERSC Center at LBNL, and the Open Sci-ence Grid consortium for providing resources and sup-port. This work was supported in part by the Officeof Nuclear Physics within the U.S. DOE Office of Sci-ence, the U.S. National Science Foundation, the Min-istry of Education and Science of the Russian Federa-tion, National Natural Science Foundation of China, Chi-nese Academy of Science, the Ministry of Science andTechnology of China and the Chinese Ministry of Educa-tion, the Higher Education Sprout Project by Ministryof Education at NCKU, the National Research Founda-tion of Korea, Czech Science Foundation and Ministryof Education, Youth and Sports of the Czech Republic,Hungarian National Research, Development and Innova-tion Office, New National Excellency Programme of theHungarian Ministry of Human Capacities, Departmentof Atomic Energy and Department of Science and Tech-nology of the Government of India, the National ScienceCentre of Poland, the Ministry of Science, Education andSports of the Republic of Croatia, RosAtom of Russia andGerman Bundesministerium fur Bildung, Wissenschaft,Forschung and Technologie (BMBF), Helmholtz Associ-ation, Ministry of Education, Culture, Sports, Science,and Technology (MEXT) and Japan Society for the Pro-motion of Science (JSPS). ∗ Deceased[1] S. A. Bass, M. Gyulassy, H. Stoecker and W. Greiner, J.Phys. G , R1 (1999).[2] J. Adams et al. (STAR Collaboration), Nucl. Phys. A , 102 (2005).[3] S. Wicks et al., Nucl. Phys. A , 426 (2007).[4] N. Armesto et al.,
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