- Email: [email protected]
Recent measurements of quarkonium production in p+p and A + A collisions from the STAR experiment
Available online at www.sciencedirect.com
ScienceDirect Nuclear Physics A 931 (2014) 596–600 www.elsevier.com/locate/nuclphysa
Recent measurements of quarkonium production in p + p and A + A collisions from the STAR experiment Wangmei Zha a,b,1 for the STAR Collaboration a Department of Modern Physics, University of Science and Technology of China, Hefei City, Anhui Province, China b Brookhaven National Laboratory, Upton, NY, USA
Received 31 July 2014; received in revised form 25 August 2014; accepted 26 August 2014 Available online 1 September 2014
Abstract We report the measurements by the STAR Collaboration of J /ψ √ invariant yields as a function of transverse momentum at mid-rapidity (|y| < 1.0) in p + p collisions at s = 500 GeV, in Au + Au collisions at √ √ sNN = 39, 62.4 and 200 GeV, and in U + U collisions at sNN = 193 GeV. The centrality, beam energy and collision system dependences of J /ψ production and nuclear modification factors are discussed. We √ also report the ratio of ψ(2S) to J /ψ yields in p + p collisions at s = 500 GeV and Υ production in √ U + U collisions at sNN = 193 GeV. © 2014 Elsevier B.V. All rights reserved. Keywords: Heavy-ion collisions; Quark–gluon plasma; Quarkonium production
1. Introduction The Relativistic Heavy Ion Collider (RHIC) was built to search for the quark–gluon plasma (QGP) and to study its properties in laboratory through high-energy heavy-ion collisions. Quarkonium suppression in heavy-ion collisions due to color screening of the quark and antiquark potential in a deconfined medium has been proposed as a signature of QGP formation [1]. However, other mechanisms, such as cold nuclear matter effects [2,3] and charm quark recombination [4–7], are likely to contribute to the observed modification of quarkonium production 1 Correspondence to: Department of Modern Physics, University of Science and Technology of China, Hefei City, Anhui Province, China.
http://dx.doi.org/10.1016/j.nuclphysa.2014.08.087 0375-9474/© 2014 Elsevier B.V. All rights reserved.
W. Zha / Nuclear Physics A 931 (2014) 596–600
597
Fig. 1. (Color online.) The J /ψ invariant cross section (left) and the ratio of ψ(2S) to J /ψ (right) versus transverse √ momentum in p + p collisions at s = 500 GeV, compared to results from other experiments.
in heavy-ion collisions. Measurements of quarkonium invariant yields in different collision energies, collision systems, and centralities can shed new light on the interplay of these mechanisms of quarkonium production and medium properties. The interpretation of quarkonium suppression in heavy-ion collisions also requires a good understanding of its production mechanisms in p + p collisions. In this article, we report the measurements of J /ψ invariant yield as a function √ of transverse momentum (pT ) at mid-rapidity (|y| < 1.0) in Au + Au collisions at sNN = 39, √ The new measurements 62.4, 200 GeV and in U + U collisions at sNN = 193 GeV from STAR. √ of the J /ψ pT spectrum, ψ(2S) to J /ψ ratio in p + p collisions at s = 500 GeV, and Υ √ nuclear modification factor in U + U collisions at sNN = 193 GeV are also presented. 2. Experiment and analysis The STAR experiment is a large-acceptance multi-purpose detector which covers full azimuth and a pseudorapidity range of |η| < 1. The p + p, Au + Au and U + U data presented in this paper were recorded in 2010, 2011 and 2012, respectively. The data were obtained using a minimumbias (MB) trigger. To improve statistics, the Υ analysis in U + U and the J /ψ analysis at high-pT in p + p and U + U collisions used a EMC trigger, which required the energy deposited in a Barrel ElectroMagnetic Calorimeter (BEMC) tower to be above a certain threshold. In this analysis, the quarkonium is reconstructed via its decay into a di-electron pair, J /ψ(Υ ) → e+ + e− . The electron identification and quarkonium reconstruction techniques are similar to those shown in [8] and [9]. 3. J /ψ and ψ(2S) production in p + p collisions at
√ s = 500 GeV
√ The invariant cross section for inclusive J /ψ production in p + p collisions at s = 500 GeV is shown as a function of pT in the left panel of Fig. 1. Precise measurements are done up to pT = 20 GeV/c. With the large accumulated luminosity and excellent electron identification capabilities, the first ψ(2S) production in p + p 500 GeV collisions has been measured at STAR. The right panel of Fig. 1 shows the result of the √ ψ(2S) to J /ψ yield ratio at high-pT (pT > 4 GeV/c) at mid-rapidity in p + p collisions at s = 500 GeV from STAR. Also shown are the results from HERA-B, PHENIX and CDF Collaborations [10–12]. All of the measurements seem to follow an increasing trend with pT . No significant beam energy dependence is observed.
598
W. Zha / Nuclear Physics A 931 (2014) 596–600
√ Fig. 2. (Color online.) (Left) J /ψ invariant yields in the 0–60% most central Au + Au collision at sNN = 39, 62.4 and 200 GeV as a function of pT . (Right) J /ψ RCP results with respect to peripheral 40–60% Au + Au collisions at √ sNN = 39, 62.4 and 200 GeV as a function of the number of participating nucleons.
4. J /ψ production in Au + Au collisions at
√ sNN = 39, 62.4 and 200 GeV
The left panel of Fig. 2 shows the J /ψ invariant yields as a function of pT for the 0–60% most √ central Au + Au collisions at sNN = 39, 62.4 and 200 GeV. The J /ψ invariant yields are larger for higher center-of-mass energy as expected. The RCP as a function of the average number of √ participants (Npart ) for Au + Au collisions at sNN = 39, 62.4 and 200 GeV are shown in the right panel of Fig. 2. The 40–60% centrality is selected as the reference in the RCP calculation. √ Significant suppression is observed in central Au + Au collisions at sNN = 62.4 GeV, which √ √ is on the same level than in Au + Au collisions at sNN = 200 GeV. For collisions at sNN = 39 GeV, due to the large statistical uncertainty in the peripheral bin, the RCP is consistent with √ no suppression as well as consistent with the results at sNN = 62.4 and 200 GeV. The left panel √ of Fig. 3 shows the RAA of J /ψ as a function of Npart for Au + Au collisions at sNN = 39, 62.4 and 200 GeV. We use the Color Evaporation Model (CEM) prediction as p + p reference baseline for 39 and 62.4 GeV, since there are no available reference data at RHIC for these two energies and results from other experiments [13–17] seem to be inconsistent with √ one another. CEM calculations describe the pT and rapidity distribution in p + p collisions at s = 200 GeV [18]. Significant suppression of J /ψ production is observed in Au + Au collisions from 39 to 200 GeV. No significant energy dependence is observed for RAA within uncertainties. The theoretical calculations [19], which include initial production and regeneration, describe the data within uncertainties. As the collision energy increases the QGP temperature also should increase, thus the J /ψ color screening (initial suppression) will become more significant. However, in this theoretical calculation, the regeneration contribution increases with collision energy due to the increase of the charm pair production, and nearly compensates the additional suppression arising from higher temperature. The right panel of Fig. 3 shows the J /ψ RAA as a function of pT . The trend is similar for different collision energies. 5. J /ψ and Υ production in U + U collisions at
√ sNN = 193 GeV
√ The pT dependence of J /ψ RAA in U + U collisions at sNN = 193 GeV is shown in the left √ panel √ of Fig. 4. The pT spectra in U + U collisions at sNN = 193 GeV are related to collisions at s = 200 GeV to obtain RAA . The results are compared to those in Au + Au collisions at
W. Zha / Nuclear Physics A 931 (2014) 596–600
599
√ Fig. 3. (Color online.) (Left) J /ψ RAA results as a function of Npart in Au + Au collisions at sNN = 39, 62.4 and √ 200 GeV. The theoretical curves are from Ref. [19]. (Right) J /ψ RAA results in 0–60% Au + Au collisions at sNN = 39, 62.4 and 200 GeV as a function of pT .
√ Fig. 4. (Color online.) (Left) J /ψ RAA results in U + U collisions at sNN = 193 GeV as a function of pT , compared √ to results in Au + Au collisions at sNN = 200 GeV. (Right) The Υ RAA results as a function of Npart in U + U √ √ collisions at sNN = 193 GeV, compared to the results in Au + Au, d + Au collisions at sNN = 200 GeV and model calculations.
√ sNN = 200 GeV [9], and they show a similar suppression pattern within uncertainties. The √ Υ RAA in U + U collisions at sNN = 193 GeV is shown as a function of Npart in the right panel √ of Fig. 4. The results in Au + Au and d + Au collisions at sNN = 200 GeV [20] are also shown. A significant suppression is observed in central events, similar to that in Au + Au collisions. The results are compared to different model predictions [21,22] and disfavor Strickland model A, which is based on the heavy quark free energy scenario. 6. Summary √ We report the recent J /ψ and ψ(2S) measurements in p + p collisions at s = 500 GeV. The ratio of ψ(2S) to J /ψ is consistent with the results at other energies. The J /ψ production √ in Au + Au from sNN = 39–200 GeV is also presented. Significant suppression of J /ψ pro-
600
W. Zha / Nuclear Physics A 931 (2014) 596–600
duction is observed for these three energies. No significant energy dependence of nuclear modification factor (both for RAA and RCP ) is found within uncertainties. Model calculations, which include direct suppression and regeneration, describe the centrality dependence of J /ψ RAA within uncertainties. The nuclear modification factors of J /ψ and Υ in U + U collisions at √ s = 193 GeV are reported, and are found to be similar to those in Au + Au collisions at √ NN sNN = 200 GeV. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
T. Matsui, H. Saltz, Phys. Lett. B 178 (1986) 416. M.J. Leitch, Nucl. Phys. A 782 (2007) 319. K.J. Eskola, Nucl. Phys. A 910–911 (2013) 163–170. L. Grandchamp, R. Rapp, Nucl. Phys. A 709 (2002) 415. P.J. Petreczky, J. Phys. G 37 (2010) 094009. P. Braun-Munzinger, J. Stachel, Phys. Lett. B 490 (2000) 196. R.L. Thews, M. Schroedter, J. Rafelski, Phys. Rev. C 63 (2001) 054905. C.B. Powell, J. Phys. Conf. Ser. 455 (2013) 012038. L. Adamczyk, et al., Phys. Lett. B 722 (2013) 55. I. Abt, et al., Eur. Phys. J. C 49 (2007) 545. A. Adare, et al., Phys. Rev. D 85 (2012) 092004. F. Abe, et al., Phys. Rev. Lett. 79 (1997) 572. T. Alexopoulos, et al., Phys. Rev. D 55 (1997) 3927. M.H. Schub, et al., Phys. Rev. D 52 (1995) 1307. A. Gribushin, et al., Phys. Rev. D 62 (2000) 012001. A.G. Clark, et al., Nucl. Phys. B 142 (1978) 29. C. Kourkounelis, et al., Phys. Lett. B 91 (1980) 481. R. Nelson, R. Vogt, et al., Phys. Rev. C 87 (2013) 014908. X. Zhao, R. Rapp, Phys. Rev. C 82 (2010) 064905. L. Adamczyk, et al., Phys. Lett. B 735 (2014) 127. M. Strickland, D. Bazow, Nucl. Phys. A 879 (2012) 25. A. Emerick, X. Zhao, R. Rapp, Eur. Phys. J. A 48 (2012) 72.
ScienceDirect Nuclear Physics A 931 (2014) 596–600 www.elsevier.com/locate/nuclphysa
Recent measurements of quarkonium production in p + p and A + A collisions from the STAR experiment Wangmei Zha a,b,1 for the STAR Collaboration a Department of Modern Physics, University of Science and Technology of China, Hefei City, Anhui Province, China b Brookhaven National Laboratory, Upton, NY, USA
Received 31 July 2014; received in revised form 25 August 2014; accepted 26 August 2014 Available online 1 September 2014
Abstract We report the measurements by the STAR Collaboration of J /ψ √ invariant yields as a function of transverse momentum at mid-rapidity (|y| < 1.0) in p + p collisions at s = 500 GeV, in Au + Au collisions at √ √ sNN = 39, 62.4 and 200 GeV, and in U + U collisions at sNN = 193 GeV. The centrality, beam energy and collision system dependences of J /ψ production and nuclear modification factors are discussed. We √ also report the ratio of ψ(2S) to J /ψ yields in p + p collisions at s = 500 GeV and Υ production in √ U + U collisions at sNN = 193 GeV. © 2014 Elsevier B.V. All rights reserved. Keywords: Heavy-ion collisions; Quark–gluon plasma; Quarkonium production
1. Introduction The Relativistic Heavy Ion Collider (RHIC) was built to search for the quark–gluon plasma (QGP) and to study its properties in laboratory through high-energy heavy-ion collisions. Quarkonium suppression in heavy-ion collisions due to color screening of the quark and antiquark potential in a deconfined medium has been proposed as a signature of QGP formation [1]. However, other mechanisms, such as cold nuclear matter effects [2,3] and charm quark recombination [4–7], are likely to contribute to the observed modification of quarkonium production 1 Correspondence to: Department of Modern Physics, University of Science and Technology of China, Hefei City, Anhui Province, China.
http://dx.doi.org/10.1016/j.nuclphysa.2014.08.087 0375-9474/© 2014 Elsevier B.V. All rights reserved.
W. Zha / Nuclear Physics A 931 (2014) 596–600
597
Fig. 1. (Color online.) The J /ψ invariant cross section (left) and the ratio of ψ(2S) to J /ψ (right) versus transverse √ momentum in p + p collisions at s = 500 GeV, compared to results from other experiments.
in heavy-ion collisions. Measurements of quarkonium invariant yields in different collision energies, collision systems, and centralities can shed new light on the interplay of these mechanisms of quarkonium production and medium properties. The interpretation of quarkonium suppression in heavy-ion collisions also requires a good understanding of its production mechanisms in p + p collisions. In this article, we report the measurements of J /ψ invariant yield as a function √ of transverse momentum (pT ) at mid-rapidity (|y| < 1.0) in Au + Au collisions at sNN = 39, √ The new measurements 62.4, 200 GeV and in U + U collisions at sNN = 193 GeV from STAR. √ of the J /ψ pT spectrum, ψ(2S) to J /ψ ratio in p + p collisions at s = 500 GeV, and Υ √ nuclear modification factor in U + U collisions at sNN = 193 GeV are also presented. 2. Experiment and analysis The STAR experiment is a large-acceptance multi-purpose detector which covers full azimuth and a pseudorapidity range of |η| < 1. The p + p, Au + Au and U + U data presented in this paper were recorded in 2010, 2011 and 2012, respectively. The data were obtained using a minimumbias (MB) trigger. To improve statistics, the Υ analysis in U + U and the J /ψ analysis at high-pT in p + p and U + U collisions used a EMC trigger, which required the energy deposited in a Barrel ElectroMagnetic Calorimeter (BEMC) tower to be above a certain threshold. In this analysis, the quarkonium is reconstructed via its decay into a di-electron pair, J /ψ(Υ ) → e+ + e− . The electron identification and quarkonium reconstruction techniques are similar to those shown in [8] and [9]. 3. J /ψ and ψ(2S) production in p + p collisions at
√ s = 500 GeV
√ The invariant cross section for inclusive J /ψ production in p + p collisions at s = 500 GeV is shown as a function of pT in the left panel of Fig. 1. Precise measurements are done up to pT = 20 GeV/c. With the large accumulated luminosity and excellent electron identification capabilities, the first ψ(2S) production in p + p 500 GeV collisions has been measured at STAR. The right panel of Fig. 1 shows the result of the √ ψ(2S) to J /ψ yield ratio at high-pT (pT > 4 GeV/c) at mid-rapidity in p + p collisions at s = 500 GeV from STAR. Also shown are the results from HERA-B, PHENIX and CDF Collaborations [10–12]. All of the measurements seem to follow an increasing trend with pT . No significant beam energy dependence is observed.
598
W. Zha / Nuclear Physics A 931 (2014) 596–600
√ Fig. 2. (Color online.) (Left) J /ψ invariant yields in the 0–60% most central Au + Au collision at sNN = 39, 62.4 and 200 GeV as a function of pT . (Right) J /ψ RCP results with respect to peripheral 40–60% Au + Au collisions at √ sNN = 39, 62.4 and 200 GeV as a function of the number of participating nucleons.
4. J /ψ production in Au + Au collisions at
√ sNN = 39, 62.4 and 200 GeV
The left panel of Fig. 2 shows the J /ψ invariant yields as a function of pT for the 0–60% most √ central Au + Au collisions at sNN = 39, 62.4 and 200 GeV. The J /ψ invariant yields are larger for higher center-of-mass energy as expected. The RCP as a function of the average number of √ participants (Npart ) for Au + Au collisions at sNN = 39, 62.4 and 200 GeV are shown in the right panel of Fig. 2. The 40–60% centrality is selected as the reference in the RCP calculation. √ Significant suppression is observed in central Au + Au collisions at sNN = 62.4 GeV, which √ √ is on the same level than in Au + Au collisions at sNN = 200 GeV. For collisions at sNN = 39 GeV, due to the large statistical uncertainty in the peripheral bin, the RCP is consistent with √ no suppression as well as consistent with the results at sNN = 62.4 and 200 GeV. The left panel √ of Fig. 3 shows the RAA of J /ψ as a function of Npart for Au + Au collisions at sNN = 39, 62.4 and 200 GeV. We use the Color Evaporation Model (CEM) prediction as p + p reference baseline for 39 and 62.4 GeV, since there are no available reference data at RHIC for these two energies and results from other experiments [13–17] seem to be inconsistent with √ one another. CEM calculations describe the pT and rapidity distribution in p + p collisions at s = 200 GeV [18]. Significant suppression of J /ψ production is observed in Au + Au collisions from 39 to 200 GeV. No significant energy dependence is observed for RAA within uncertainties. The theoretical calculations [19], which include initial production and regeneration, describe the data within uncertainties. As the collision energy increases the QGP temperature also should increase, thus the J /ψ color screening (initial suppression) will become more significant. However, in this theoretical calculation, the regeneration contribution increases with collision energy due to the increase of the charm pair production, and nearly compensates the additional suppression arising from higher temperature. The right panel of Fig. 3 shows the J /ψ RAA as a function of pT . The trend is similar for different collision energies. 5. J /ψ and Υ production in U + U collisions at
√ sNN = 193 GeV
√ The pT dependence of J /ψ RAA in U + U collisions at sNN = 193 GeV is shown in the left √ panel √ of Fig. 4. The pT spectra in U + U collisions at sNN = 193 GeV are related to collisions at s = 200 GeV to obtain RAA . The results are compared to those in Au + Au collisions at
W. Zha / Nuclear Physics A 931 (2014) 596–600
599
√ Fig. 3. (Color online.) (Left) J /ψ RAA results as a function of Npart in Au + Au collisions at sNN = 39, 62.4 and √ 200 GeV. The theoretical curves are from Ref. [19]. (Right) J /ψ RAA results in 0–60% Au + Au collisions at sNN = 39, 62.4 and 200 GeV as a function of pT .
√ Fig. 4. (Color online.) (Left) J /ψ RAA results in U + U collisions at sNN = 193 GeV as a function of pT , compared √ to results in Au + Au collisions at sNN = 200 GeV. (Right) The Υ RAA results as a function of Npart in U + U √ √ collisions at sNN = 193 GeV, compared to the results in Au + Au, d + Au collisions at sNN = 200 GeV and model calculations.
√ sNN = 200 GeV [9], and they show a similar suppression pattern within uncertainties. The √ Υ RAA in U + U collisions at sNN = 193 GeV is shown as a function of Npart in the right panel √ of Fig. 4. The results in Au + Au and d + Au collisions at sNN = 200 GeV [20] are also shown. A significant suppression is observed in central events, similar to that in Au + Au collisions. The results are compared to different model predictions [21,22] and disfavor Strickland model A, which is based on the heavy quark free energy scenario. 6. Summary √ We report the recent J /ψ and ψ(2S) measurements in p + p collisions at s = 500 GeV. The ratio of ψ(2S) to J /ψ is consistent with the results at other energies. The J /ψ production √ in Au + Au from sNN = 39–200 GeV is also presented. Significant suppression of J /ψ pro-
600
W. Zha / Nuclear Physics A 931 (2014) 596–600
duction is observed for these three energies. No significant energy dependence of nuclear modification factor (both for RAA and RCP ) is found within uncertainties. Model calculations, which include direct suppression and regeneration, describe the centrality dependence of J /ψ RAA within uncertainties. The nuclear modification factors of J /ψ and Υ in U + U collisions at √ s = 193 GeV are reported, and are found to be similar to those in Au + Au collisions at √ NN sNN = 200 GeV. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
T. Matsui, H. Saltz, Phys. Lett. B 178 (1986) 416. M.J. Leitch, Nucl. Phys. A 782 (2007) 319. K.J. Eskola, Nucl. Phys. A 910–911 (2013) 163–170. L. Grandchamp, R. Rapp, Nucl. Phys. A 709 (2002) 415. P.J. Petreczky, J. Phys. G 37 (2010) 094009. P. Braun-Munzinger, J. Stachel, Phys. Lett. B 490 (2000) 196. R.L. Thews, M. Schroedter, J. Rafelski, Phys. Rev. C 63 (2001) 054905. C.B. Powell, J. Phys. Conf. Ser. 455 (2013) 012038. L. Adamczyk, et al., Phys. Lett. B 722 (2013) 55. I. Abt, et al., Eur. Phys. J. C 49 (2007) 545. A. Adare, et al., Phys. Rev. D 85 (2012) 092004. F. Abe, et al., Phys. Rev. Lett. 79 (1997) 572. T. Alexopoulos, et al., Phys. Rev. D 55 (1997) 3927. M.H. Schub, et al., Phys. Rev. D 52 (1995) 1307. A. Gribushin, et al., Phys. Rev. D 62 (2000) 012001. A.G. Clark, et al., Nucl. Phys. B 142 (1978) 29. C. Kourkounelis, et al., Phys. Lett. B 91 (1980) 481. R. Nelson, R. Vogt, et al., Phys. Rev. C 87 (2013) 014908. X. Zhao, R. Rapp, Phys. Rev. C 82 (2010) 064905. L. Adamczyk, et al., Phys. Lett. B 735 (2014) 127. M. Strickland, D. Bazow, Nucl. Phys. A 879 (2012) 25. A. Emerick, X. Zhao, R. Rapp, Eur. Phys. J. A 48 (2012) 72.