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Quarkonia results from PHENIX
Available online at www.sciencedirect.com
ScienceDirect Nuclear Physics A 932 (2014) 508–515 www.elsevier.com/locate/nuclphysa
Quarkonia results from PHENIX D. McGlinchey for the PHENIX Collaboration University of Colorado at Boulder, United States Received 18 April 2014; received in revised form 30 September 2014; accepted 1 October 2014 Available online 8 October 2014
Abstract Measuring the production of multiple quarkonia states in different collision systems and over a range of energies is necessary to disentangle the effects of production in a nuclear target from those due to a hot medium. Measurements by PHENIX of J /ψ production in A + A collisions show similar levels of modification over a range of collision systems as well as energies, which might be explained by an interplay between suppression and regeneration in hot nuclear matter. Measurements of J /ψ and ψ production in √ d + Au collisions at sNN = 200 GeV have presented a number of interesting puzzles. The pT dependence of J /ψ suppression is found to be different at backward (Au-going) rapidities compared to mid and forward (d-going) rapidities. While this is unexplained by models, comparisons to the modification of heavy flavor muons show a similar discrepancy between backward and forward rapidities. The increased suppression of ψ production compared to J /ψ production in central events is incompatible with either nPDF modification or nuclear breakup. However, an approximate scaling of the relative modification of the ψ to the J /ψ with charged particle multiplicity between SPS p + A, PHENIX d + Au and SPS A + A is also observed, perhaps indicating that interactions with final state partons or hadrons may be important. © 2014 CERN. Published by Elsevier B.V. All rights reserved.
Keywords: Heavy quarkonium; Quark–gluon plasma
1. Introduction Measurements of different quarkonia states in A + A collisions allow us to probe the temperature and Debye screening length of the produced medium, as well as effects like the regeneration of quarkonia (see, for example, Ref. [1] for a review of heavy-quarkonia results). The PHENIX Collaboration has measured J /ψ production in Au + Au [2,3] and Cu + Cu [4] collisions at http://dx.doi.org/10.1016/j.nuclphysa.2014.10.001 0375-9474/© 2014 CERN. Published by Elsevier B.V. All rights reserved.
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√ Fig. 1. J /ψRAA as a function of Npart for Au + Au [3] and Cu + Cu [4] collisions at sNN = 200 GeV, and U + U √ collisions at sNN = 193 GeV (left). J /ψRAA as a function of Npart measured with ALICE and PHENIX [10] (right).
√ √ s = 200 GeV, U + U collisions at sNN = 193 GeV, as well as Au + Au collisions at √ NN sNN = 39 and 62.4 GeV [5]. The study of quarkonia in small systems like d + Au and p + p is important not only as a baseline for understanding the medium effects in A + A collisions, but also for understanding a number of interesting nuclear effects. The measurement of the rapidity (y) and transverse mo√ mentum (pT ) dependence of quarkonia production in p(d) + A collisions at sNN = 200 GeV sets constraints on the modification of the gluon distribution in nuclei, parameterized by nuclear parton distribution functions (nPDF). The nuclear breakup effect on quarkonia due to collisions with nucleons can be studied by measuring the modification of quarkonia as a function of the time spent in the nucleus, which depends on the collision energy and the rapidity at which the quarkonia are measured. It is also interesting to investigate what effect the possible formation of a medium produced in small systems, hinted at by a number of recent measurements in d + Au collisions at RHIC [6] and p + Pb collisions at the LHC [7,8], may have on quarkonia production. To investigate these effects, the PHENIX Collaboration has measured both J /ψ and ψ √ production in d + Au collisions at sNN = 200 GeV [9]. 2. Quarkonia in A + A collisions One of the hallmarks of RHIC is its ability to collide various nucleus species over a wide range of center-of-mass energies. This ability has been used to map out J /ψ production in collisions between different nuclei over a range of collision energies. Fig. 1 (left) compares the J /ψ nuclear √ modification factor, RAA , in Au + Au [3] and Cu + Cu [4] collisions at sNN = 200 GeV with √ new preliminary results in U + U collisions at sNN = 193 GeV, all at 1.2 < |y| < 2.2, as a function of collision centrality. The results show a similar trend between all three systems. A similar suppression is also observed when comparing the modification of J /ψ production over a range of energies, as shown in Fig. 2 (left), which shows a comparison of J /ψRAA in √ Au + Au collisions at sNN = 34, 62.4 and 200 GeV [5]. Why is the modification so similar across more than a factor of 5 increase in collision energy? One possible explanation to this puzzle is a change in the contributions of direct suppression and regeneration in the produced medium. Calculations by Rapp et al. [5], indicate that though the direct suppression increases by ∼50% between 39 and 200 GeV, the regeneration appears to increase by a similar amount. The interplay between these two effects also provides a possible explanation of the differences in suppression observed between RHIC and the LHC, shown in
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√ Fig. 2. J /ψRAA as a function of Npart for Au + Au collisions at sNN = 34, 62.4, and 200 GeV [5] (left). The theory curves are from Ref. [11]. J /ψ nuclear modification factor RCP (0–20%/60–88%) vs. the centrality integrated RdAu (0–100%) [12], where points with the same marker represent a rapidity sub-interval within the range quoted in the figures legend (right).
Fig. 1 (right), where measurements at 2.76 TeV show less suppression in central events compared to measurements at 200 GeV. At 2.76 TeV, the number of cc pairs produced in each collision becomes large. Therefore, even though the suppression of direct J /ψ production may be comparable, or even greater, than that at RHIC, the regeneration effect could be significantly larger at the LHC, causing the decrease in overall suppression observed at the LHC compared to RHIC. 3. Quarkonia in d + Au collisions A second contribution to the above puzzle comes from the modification of the quarkonia kinematic distribution not related to the presence of a hot medium, due to what are often termed Cold Nuclear Matter (CNM) effects. CNM effects on quarkonia have been studied extensively in fixed target p + A experiments at FNAL, SPS, and HERA (see [1] for a review), and the study of small systems in general has become a large focus in recent years at hadron colliders through the study of d + Au collisions at RHIC and now p + Pb collisions at the LHC. PHENIX has measured J /ψ [12,13] production as a function of pT , y, and centrality and ψ [9] production √ as a function of centrality in d + Au collisions at sNN = 200 GeV. In the PHENIX experiment, the centrality is determined using the charge collected in the Au-going beam–beam counter, and Ncoll is determined using a Glauber + NBD method with corrections for auto-correlation biases inherent in the centrality determination [14]. The measurement of J /ψ modification in d + Au collisions as a function of rapidity and centrality shows a significant suppression at forward (d-going) rapidity, but a measurement consistent with no modification at backward (Au-going) rapidity. It was found that nPDF modification using EPS09 [15] with a linear dependence on the nuclear thickness plus a constant nuclear breakup factor could not simultaneously reproduce the RdAu for peripheral (60–88%), and central (0–20%), collisions and the nuclear modification factor RCP (0–20%/60–88%). Further, Fig. 2 (right) compares the RCP with the centrality integrated RdAu , and shows that any model using a linear or exponential dependence of the nuclear modification on the nuclear thickness will not simultaneously describe the rapidity and centrality dependence of the data [12].
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Fig. 3. J /ψRdAu as a function of pT [13] (left). Comparison of the μHF and J /ψRdAu vs. pT at −2.2 < y < −1.2 and 1.2 < y < 2.2 [16] (right).
√ The pT dependence of J /ψ modification has also been measured in d + Au at sNN = 200 GeV [13]. Shown in Fig. 3 (left) is a comparison of the J /ψRdAu at backward, mid, and forward rapidity as a function of pT . The mid and forward rapidity data show a moderate suppression at low pT which gradually decreases to RdAu ∼ 1 around 3–4 GeV/c. This dependence is consistent with predictions including nPDF modification and nuclear breakup [13]. The backward rapidity data, however, show a very different trend which is unexplained by current models. The backward rapidity is similarly suppressed at low-pT , however the RdAu increases rapidly with pT , crossing 1 between 1 and 2 GeV/c. Unlike the mid and forward rapidities, the backward rapidity suggests an enhancement at larger pT . It is instructive to compare these results with the measurements of heavy flavor muons (μHF ) from the same data set. Fig. 3 (right) compares the backward and forward rapidity J /ψRdAu with the μHF RdAu [16] over the same rapidity ranges for central d + Au collisions. At forward rapidity the J /ψ and μHF RdAu are similar, suggesting that the same underlying mechanism is likely causing the modification. At backward rapidity the μHF RdAu shows significantly greater enhancement than that of the J /ψ at lower pT . A comparison between the midrapidity J /ψ and heavy flavor electron modification [17] shows a similar relation to that at backward rapidity. This may indicate that different effects begin to dominate the open heavy flavor and quarkonia production when moving to mid and backward rapidities. One effect that could contribute to this difference at mid and backward rapidity is the nuclear breakup effect. Fig. 4 shows the J /ψ nuclear breakup cross section as a function of time spent in the nucleus (τ ), estimated from the PHENIX data using a simultaneous fit to the y and centrality dependence with a model containing nPDF modification and nuclear breakup. When compared to similar calculations from lower energy data a common scaling emerges for τ > 0.4 fm/c. At RHIC, this scaling occurs at backward, and possibly mid, rapidity, and is in line with the scenario of an increase in the nuclear breakup effect for an expanding cc pair traversing the nucleus. At forward rapidity, where the J /ψ and μHF RdAu values are comparable, this breaks down. Given that, at forward rapidity, τ is estimated to be even shorter than the expected formation time of the cc pair, nuclear breakup of the J /ψ may not be an effective mechanism to explain the observed modification of J /ψ production, which thus might point to a possible modification of the underlying charm quark kinematics.
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Fig. 4. J /ψ nuclear breakup cross section as a function of estimated time spent in the nucleus (τ ) [18].
Fig. 5. J /ψ and ψ RdAu as a function of Ncoll [9] (left). Relative modification of the ψ to the J /ψ as a function of estimated time spent in the nucleus (τ ) [9] (right).
The high statistics d + Au data sample taken in 2008 made the measurement of the ψ at midrapidity possible for the first time at RHIC in nuclear collisions. Fig. 5 (left) shows the √ ψ RdAu at midrapidity as a function of Ncoll for d + Au collisions at sNN = 200 GeV [9] compared to the inclusive J /ψRdAu under the same conditions. The ψ RdAu shows a suppression which increases strongly with increasing Ncoll . The inclusive J /ψRdAu , on the other hand, shows only a moderate increase in suppression with increasing Ncoll . Indeed while the J /ψ and ψ RdAu values are consistent for peripheral and mid–peripheral collisions, for the most central collisions the ψ RdAu is about 3 times smaller than that of the J /ψ . This result is unexplained by either nPDF modification, or an increase in the nuclear breakup of the ψ due to an expanding cc pair, a model that was successful in explaining low energy results from E866/NuSea [19] and NA50 [20]. Fig. 5 shows the relative modification of the ψ to the J /ψ as a function of τ for E866, NA50, and PHENIX results. The E866 and NA50 data are consistent with unity up to 0.1 fm/c, which coincides roughly with the J /ψ formation time of ∼0.15 fm/c [21], and are well reproduced by calculations by Arleo et al. [22]. PHENIX data, on the other hand, show an indication for a departure from this trend, below even the expected cc formation time of ∼0.05 fm/c [21]. To achieve this large difference between the ψ and J /ψ suppression in central events, an initial state effect that differs at very short time scales, or perhaps a long range final state ef-
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Fig. 6. Relative modification of the ψ to the J /ψ as a function of the charged particle multiplicity at midrapidity as calculated within HIJING [9].
fect, such as interactions with co-moving hadrons, are needed. The latter possibility is tested by plotting the relative modification as a function of the charged particle multiplicity at midrapidity, as calculated within HIJING, in Fig. 6. An approximate scaling is observed as a function of dNch /dη between low energy p + A, RHIC d + Au and SPS A + A collisions, indicating that perhaps interactions with final state hadrons may play a role, even in smaller colliding systems. 4. Summary PHENIX has measured quarkonia production for several collision systems, namely p + p, d + Au, Cu + Cu, Au + Au, and U + U, as well as a range of energies for Au + Au collisions √ at sNN = 39, 62.4 and 200 GeV. Similar suppression of J /ψ production is observed in Au + Au collisions at 1.2 < |y| < 2.2 √ √ from sNN = 39 to sNN = 200 GeV. This similarity may be due to an increase in suppression with increasing energy being offset by a corresponding increase in the effect of regeneration. A better understanding of the CNM effects at all 3 center of mass energies is likely needed to gain a more precise understanding of this interplay. The modification of inclusive J /ψ production in d + Au collisions shows a number of interesting features. The first is the observation that any model which is less than quadratically dependent on the nuclear thickness will be unable to simultaneously explain the rapidity and centrality dependence of the measured RdAu . The pT dependence of the J /ψRdAu at backward rapidity is found to be different than that observed at either mid or forward rapidities. While this difference is unexplained by current models, it is interesting to note that the J /ψ and μHF RdAu show similar behaviors at forward rapidities, but different behavior at backward rapidity, where the μHF shows a larger enhancement in production compared to the J /ψ . The comparison of ψ modification to that of the J /ψ in d + Au collisions shows a much greater level of suppression of the ψ in central events. This increase in suppression is unexplained by nPDF modification. Nor can it be explained by nuclear breakup. The short time spent traversing the Au nucleus at midrapidity at RHIC energies would not lead to stronger breakup for the ψ than the J /ψ . However, when plotting the relative modification of the ψ to the J /ψ as a function of dNch /dη at midrapidity, an approximate scaling is observed between results from
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low energy p + A, RHIC d + Au, and low energy A + A collisions, indicating the possibility that interactions with final state hadrons may play an important role in quarkonia production, even in small systems. It is important to understand how the interpretation of these results affect, or are affected by, our growing understanding of small collision systems, as well as other probes, such as new open heavy flavor results [23] which show a smooth transition between enhancement to suppression when moving from d + Au, to Cu + Cu, to Au + Au collisions. In order to disentangle the multitude of physics effects probed by quarkonia production we must continue to study all quarkonia states over a broad range of collision systems and energy, including lower energy fixed target experiments, and the higher energies reached at the LHC. Future possibilities at RHIC are bright, with the possibility of p + A running in the near future, and the ability to measure the ψ at backward and forward rapidity at PHENIX with the new forward vertex detector, as well as far future measurements with the proposed sPHENIX upgrade [24]. References [1] N. Brambilla, S. Eidelman, B. Heltsley, R. Vogt, G. Bodwin, et al., Heavy quarkonium: progress, puzzles, and opportunities, Eur. Phys. J. C 71 (2011) 1534. √ [2] A. Adare, et al., J /ψ production vs centrality, transverse momentum, and rapidity in Au + Au collisions at sNN = 200 GeV, Phys. Rev. Lett. 98 (2007) 232301. √ [3] A. Adare, et al., J /ψ suppression at forward rapidity in Au + Au collisions at sNN = 200 GeV, Phys. Rev. C 84 (2011) 054912. √ [4] A. Adare, et al., J /ψ production in sNN = 200 GeV Cu + Cu collisions, Phys. Rev. Lett. 101 (2008) 122301. √ [5] A. Adare, et al., J /ψ suppression at forward rapidity in Au + Au collisions at sNN = 39 and 62.4 GeV, Phys. Rev. C 86 (2012) 064901. √ [6] A. Adare, et al., Quadrupole anisotropy in dihadron azimuthal correlations in central d + Au collisions at sNN = 200 GeV, Phys. Rev. Lett. 111 (2013) 212301. √ [7] G. Aad, et al., Observation of associated near-side and away-side long-range correlations in sNN = 5.02 TeV proton–lead collisions with the ATLAS detector, Phys. Rev. Lett. 110 (2013) 182302. √ [8] B. Abelev, et al., Long-range angular correlations on the near and away side in p–Pb collisions at sNN = 5.02 TeV, Phys. Lett. B 719 (2013) 29–41. √ [9] A. Adare, et al., Nuclear modification of ψ , χc and J /ψ production in d + Au collisions at sNN = 200 GeV, Phys. Rev. Lett. 111 (2013) 202301. √ [10] B. Abelev, et al., J /ψ suppression at forward rapidity in Pb–Pb collisions at sNN = 2.76 TeV, Phys. Rev. Lett. 109 (2012) 072301. [11] X. Zhao, R. Rapp, Charmonium in medium: from correlators to experiment, Phys. Rev. C 82 (2010) 064905. [12] A. Adare, et al., Cold nuclear matter effects on J /ψ yields as a function of rapidity and nuclear geometry in √ deuteron–gold collisions at sNN = 200 GeV, Phys. Rev. Lett. 107 (2011) 142301. [13] A. Adare, et al., Transverse-momentum dependence of the J /ψ nuclear modification in d + Au collisions at √ sNN = 200 GeV, Phys. Rev. C 87 (3) (2013) 034904. [14] A. Adare, et al., Centrality categorization for Rp(d)+A in high-energy collisions, arXiv:1310.4793. [15] K.J. Eskola, H. Paukkunen, C.A. Salgado, EPS09 – a new generation of NLO and LO nuclear parton distribution functions, J. High Energy Phys. 0904 (2009) 065. [16] A. Adare, et al., Cold-nuclear-matter effects on heavy-quark production at forward and backward rapidity in d + Au √ collisions at sNN = 200 GeV, arXiv:1310.1005. [17] A. Adare, S. Afanasiev, C. Aidala, N. Ajitanand, Y. Akiba, et al., Cold-nuclear-matter effects on heavy-quark pro√ duction in d + Au collisions at sNN = 200 GeV, arXiv:1208.1293. [18] D. McGlinchey, A. Frawley, R. Vogt, Impact parameter dependence of the nuclear modification of J /ψ production √ in d + Au collisions at SNN = 200 GeV, Phys. Rev. C 87 (5) (2013) 054910. [19] M.J. Leitch, et al., Measurement of J /ψ and ψ suppression in p A collisions at 800-GeV/c, Phys. Rev. Lett. 84 (2000) 3256–3260. [20] B. Alessandro, et al., J /ψ and ψ production and their normal nuclear absorption in proton–nucleus collisions at 400 GeV, Eur. Phys. J. C 48 (2006) 329. [21] H. Satz, Colour deconfinement and quarkonium binding, J. Phys. G 32 (2006) R25.
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[22] F. Arleo, P.B. Gossiaux, T. Gousset, J. Aichelin, Charmonium suppression in p–A collisions, Phys. Rev. C 61 (2000) 054906. √ [23] A. Adare, et al., System-size dependence of open-heavy-flavor production in nucleus–nucleus collisions at sNN = 200 GeV, arXiv:1310.8286. [24] C. Aidala, N. Ajitanand, Y. Akiba, Y. Akiba, R. Akimoto, et al., sPHENIX: an upgrade concept from the PHENIX Collaboration, arXiv:1207.6378.
ScienceDirect Nuclear Physics A 932 (2014) 508–515 www.elsevier.com/locate/nuclphysa
Quarkonia results from PHENIX D. McGlinchey for the PHENIX Collaboration University of Colorado at Boulder, United States Received 18 April 2014; received in revised form 30 September 2014; accepted 1 October 2014 Available online 8 October 2014
Abstract Measuring the production of multiple quarkonia states in different collision systems and over a range of energies is necessary to disentangle the effects of production in a nuclear target from those due to a hot medium. Measurements by PHENIX of J /ψ production in A + A collisions show similar levels of modification over a range of collision systems as well as energies, which might be explained by an interplay between suppression and regeneration in hot nuclear matter. Measurements of J /ψ and ψ production in √ d + Au collisions at sNN = 200 GeV have presented a number of interesting puzzles. The pT dependence of J /ψ suppression is found to be different at backward (Au-going) rapidities compared to mid and forward (d-going) rapidities. While this is unexplained by models, comparisons to the modification of heavy flavor muons show a similar discrepancy between backward and forward rapidities. The increased suppression of ψ production compared to J /ψ production in central events is incompatible with either nPDF modification or nuclear breakup. However, an approximate scaling of the relative modification of the ψ to the J /ψ with charged particle multiplicity between SPS p + A, PHENIX d + Au and SPS A + A is also observed, perhaps indicating that interactions with final state partons or hadrons may be important. © 2014 CERN. Published by Elsevier B.V. All rights reserved.
Keywords: Heavy quarkonium; Quark–gluon plasma
1. Introduction Measurements of different quarkonia states in A + A collisions allow us to probe the temperature and Debye screening length of the produced medium, as well as effects like the regeneration of quarkonia (see, for example, Ref. [1] for a review of heavy-quarkonia results). The PHENIX Collaboration has measured J /ψ production in Au + Au [2,3] and Cu + Cu [4] collisions at http://dx.doi.org/10.1016/j.nuclphysa.2014.10.001 0375-9474/© 2014 CERN. Published by Elsevier B.V. All rights reserved.
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√ Fig. 1. J /ψRAA as a function of Npart for Au + Au [3] and Cu + Cu [4] collisions at sNN = 200 GeV, and U + U √ collisions at sNN = 193 GeV (left). J /ψRAA as a function of Npart measured with ALICE and PHENIX [10] (right).
√ √ s = 200 GeV, U + U collisions at sNN = 193 GeV, as well as Au + Au collisions at √ NN sNN = 39 and 62.4 GeV [5]. The study of quarkonia in small systems like d + Au and p + p is important not only as a baseline for understanding the medium effects in A + A collisions, but also for understanding a number of interesting nuclear effects. The measurement of the rapidity (y) and transverse mo√ mentum (pT ) dependence of quarkonia production in p(d) + A collisions at sNN = 200 GeV sets constraints on the modification of the gluon distribution in nuclei, parameterized by nuclear parton distribution functions (nPDF). The nuclear breakup effect on quarkonia due to collisions with nucleons can be studied by measuring the modification of quarkonia as a function of the time spent in the nucleus, which depends on the collision energy and the rapidity at which the quarkonia are measured. It is also interesting to investigate what effect the possible formation of a medium produced in small systems, hinted at by a number of recent measurements in d + Au collisions at RHIC [6] and p + Pb collisions at the LHC [7,8], may have on quarkonia production. To investigate these effects, the PHENIX Collaboration has measured both J /ψ and ψ √ production in d + Au collisions at sNN = 200 GeV [9]. 2. Quarkonia in A + A collisions One of the hallmarks of RHIC is its ability to collide various nucleus species over a wide range of center-of-mass energies. This ability has been used to map out J /ψ production in collisions between different nuclei over a range of collision energies. Fig. 1 (left) compares the J /ψ nuclear √ modification factor, RAA , in Au + Au [3] and Cu + Cu [4] collisions at sNN = 200 GeV with √ new preliminary results in U + U collisions at sNN = 193 GeV, all at 1.2 < |y| < 2.2, as a function of collision centrality. The results show a similar trend between all three systems. A similar suppression is also observed when comparing the modification of J /ψ production over a range of energies, as shown in Fig. 2 (left), which shows a comparison of J /ψRAA in √ Au + Au collisions at sNN = 34, 62.4 and 200 GeV [5]. Why is the modification so similar across more than a factor of 5 increase in collision energy? One possible explanation to this puzzle is a change in the contributions of direct suppression and regeneration in the produced medium. Calculations by Rapp et al. [5], indicate that though the direct suppression increases by ∼50% between 39 and 200 GeV, the regeneration appears to increase by a similar amount. The interplay between these two effects also provides a possible explanation of the differences in suppression observed between RHIC and the LHC, shown in
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√ Fig. 2. J /ψRAA as a function of Npart for Au + Au collisions at sNN = 34, 62.4, and 200 GeV [5] (left). The theory curves are from Ref. [11]. J /ψ nuclear modification factor RCP (0–20%/60–88%) vs. the centrality integrated RdAu (0–100%) [12], where points with the same marker represent a rapidity sub-interval within the range quoted in the figures legend (right).
Fig. 1 (right), where measurements at 2.76 TeV show less suppression in central events compared to measurements at 200 GeV. At 2.76 TeV, the number of cc pairs produced in each collision becomes large. Therefore, even though the suppression of direct J /ψ production may be comparable, or even greater, than that at RHIC, the regeneration effect could be significantly larger at the LHC, causing the decrease in overall suppression observed at the LHC compared to RHIC. 3. Quarkonia in d + Au collisions A second contribution to the above puzzle comes from the modification of the quarkonia kinematic distribution not related to the presence of a hot medium, due to what are often termed Cold Nuclear Matter (CNM) effects. CNM effects on quarkonia have been studied extensively in fixed target p + A experiments at FNAL, SPS, and HERA (see [1] for a review), and the study of small systems in general has become a large focus in recent years at hadron colliders through the study of d + Au collisions at RHIC and now p + Pb collisions at the LHC. PHENIX has measured J /ψ [12,13] production as a function of pT , y, and centrality and ψ [9] production √ as a function of centrality in d + Au collisions at sNN = 200 GeV. In the PHENIX experiment, the centrality is determined using the charge collected in the Au-going beam–beam counter, and Ncoll is determined using a Glauber + NBD method with corrections for auto-correlation biases inherent in the centrality determination [14]. The measurement of J /ψ modification in d + Au collisions as a function of rapidity and centrality shows a significant suppression at forward (d-going) rapidity, but a measurement consistent with no modification at backward (Au-going) rapidity. It was found that nPDF modification using EPS09 [15] with a linear dependence on the nuclear thickness plus a constant nuclear breakup factor could not simultaneously reproduce the RdAu for peripheral (60–88%), and central (0–20%), collisions and the nuclear modification factor RCP (0–20%/60–88%). Further, Fig. 2 (right) compares the RCP with the centrality integrated RdAu , and shows that any model using a linear or exponential dependence of the nuclear modification on the nuclear thickness will not simultaneously describe the rapidity and centrality dependence of the data [12].
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Fig. 3. J /ψRdAu as a function of pT [13] (left). Comparison of the μHF and J /ψRdAu vs. pT at −2.2 < y < −1.2 and 1.2 < y < 2.2 [16] (right).
√ The pT dependence of J /ψ modification has also been measured in d + Au at sNN = 200 GeV [13]. Shown in Fig. 3 (left) is a comparison of the J /ψRdAu at backward, mid, and forward rapidity as a function of pT . The mid and forward rapidity data show a moderate suppression at low pT which gradually decreases to RdAu ∼ 1 around 3–4 GeV/c. This dependence is consistent with predictions including nPDF modification and nuclear breakup [13]. The backward rapidity data, however, show a very different trend which is unexplained by current models. The backward rapidity is similarly suppressed at low-pT , however the RdAu increases rapidly with pT , crossing 1 between 1 and 2 GeV/c. Unlike the mid and forward rapidities, the backward rapidity suggests an enhancement at larger pT . It is instructive to compare these results with the measurements of heavy flavor muons (μHF ) from the same data set. Fig. 3 (right) compares the backward and forward rapidity J /ψRdAu with the μHF RdAu [16] over the same rapidity ranges for central d + Au collisions. At forward rapidity the J /ψ and μHF RdAu are similar, suggesting that the same underlying mechanism is likely causing the modification. At backward rapidity the μHF RdAu shows significantly greater enhancement than that of the J /ψ at lower pT . A comparison between the midrapidity J /ψ and heavy flavor electron modification [17] shows a similar relation to that at backward rapidity. This may indicate that different effects begin to dominate the open heavy flavor and quarkonia production when moving to mid and backward rapidities. One effect that could contribute to this difference at mid and backward rapidity is the nuclear breakup effect. Fig. 4 shows the J /ψ nuclear breakup cross section as a function of time spent in the nucleus (τ ), estimated from the PHENIX data using a simultaneous fit to the y and centrality dependence with a model containing nPDF modification and nuclear breakup. When compared to similar calculations from lower energy data a common scaling emerges for τ > 0.4 fm/c. At RHIC, this scaling occurs at backward, and possibly mid, rapidity, and is in line with the scenario of an increase in the nuclear breakup effect for an expanding cc pair traversing the nucleus. At forward rapidity, where the J /ψ and μHF RdAu values are comparable, this breaks down. Given that, at forward rapidity, τ is estimated to be even shorter than the expected formation time of the cc pair, nuclear breakup of the J /ψ may not be an effective mechanism to explain the observed modification of J /ψ production, which thus might point to a possible modification of the underlying charm quark kinematics.
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Fig. 4. J /ψ nuclear breakup cross section as a function of estimated time spent in the nucleus (τ ) [18].
Fig. 5. J /ψ and ψ RdAu as a function of Ncoll [9] (left). Relative modification of the ψ to the J /ψ as a function of estimated time spent in the nucleus (τ ) [9] (right).
The high statistics d + Au data sample taken in 2008 made the measurement of the ψ at midrapidity possible for the first time at RHIC in nuclear collisions. Fig. 5 (left) shows the √ ψ RdAu at midrapidity as a function of Ncoll for d + Au collisions at sNN = 200 GeV [9] compared to the inclusive J /ψRdAu under the same conditions. The ψ RdAu shows a suppression which increases strongly with increasing Ncoll . The inclusive J /ψRdAu , on the other hand, shows only a moderate increase in suppression with increasing Ncoll . Indeed while the J /ψ and ψ RdAu values are consistent for peripheral and mid–peripheral collisions, for the most central collisions the ψ RdAu is about 3 times smaller than that of the J /ψ . This result is unexplained by either nPDF modification, or an increase in the nuclear breakup of the ψ due to an expanding cc pair, a model that was successful in explaining low energy results from E866/NuSea [19] and NA50 [20]. Fig. 5 shows the relative modification of the ψ to the J /ψ as a function of τ for E866, NA50, and PHENIX results. The E866 and NA50 data are consistent with unity up to 0.1 fm/c, which coincides roughly with the J /ψ formation time of ∼0.15 fm/c [21], and are well reproduced by calculations by Arleo et al. [22]. PHENIX data, on the other hand, show an indication for a departure from this trend, below even the expected cc formation time of ∼0.05 fm/c [21]. To achieve this large difference between the ψ and J /ψ suppression in central events, an initial state effect that differs at very short time scales, or perhaps a long range final state ef-
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Fig. 6. Relative modification of the ψ to the J /ψ as a function of the charged particle multiplicity at midrapidity as calculated within HIJING [9].
fect, such as interactions with co-moving hadrons, are needed. The latter possibility is tested by plotting the relative modification as a function of the charged particle multiplicity at midrapidity, as calculated within HIJING, in Fig. 6. An approximate scaling is observed as a function of dNch /dη between low energy p + A, RHIC d + Au and SPS A + A collisions, indicating that perhaps interactions with final state hadrons may play a role, even in smaller colliding systems. 4. Summary PHENIX has measured quarkonia production for several collision systems, namely p + p, d + Au, Cu + Cu, Au + Au, and U + U, as well as a range of energies for Au + Au collisions √ at sNN = 39, 62.4 and 200 GeV. Similar suppression of J /ψ production is observed in Au + Au collisions at 1.2 < |y| < 2.2 √ √ from sNN = 39 to sNN = 200 GeV. This similarity may be due to an increase in suppression with increasing energy being offset by a corresponding increase in the effect of regeneration. A better understanding of the CNM effects at all 3 center of mass energies is likely needed to gain a more precise understanding of this interplay. The modification of inclusive J /ψ production in d + Au collisions shows a number of interesting features. The first is the observation that any model which is less than quadratically dependent on the nuclear thickness will be unable to simultaneously explain the rapidity and centrality dependence of the measured RdAu . The pT dependence of the J /ψRdAu at backward rapidity is found to be different than that observed at either mid or forward rapidities. While this difference is unexplained by current models, it is interesting to note that the J /ψ and μHF RdAu show similar behaviors at forward rapidities, but different behavior at backward rapidity, where the μHF shows a larger enhancement in production compared to the J /ψ . The comparison of ψ modification to that of the J /ψ in d + Au collisions shows a much greater level of suppression of the ψ in central events. This increase in suppression is unexplained by nPDF modification. Nor can it be explained by nuclear breakup. The short time spent traversing the Au nucleus at midrapidity at RHIC energies would not lead to stronger breakup for the ψ than the J /ψ . However, when plotting the relative modification of the ψ to the J /ψ as a function of dNch /dη at midrapidity, an approximate scaling is observed between results from
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low energy p + A, RHIC d + Au, and low energy A + A collisions, indicating the possibility that interactions with final state hadrons may play an important role in quarkonia production, even in small systems. It is important to understand how the interpretation of these results affect, or are affected by, our growing understanding of small collision systems, as well as other probes, such as new open heavy flavor results [23] which show a smooth transition between enhancement to suppression when moving from d + Au, to Cu + Cu, to Au + Au collisions. In order to disentangle the multitude of physics effects probed by quarkonia production we must continue to study all quarkonia states over a broad range of collision systems and energy, including lower energy fixed target experiments, and the higher energies reached at the LHC. Future possibilities at RHIC are bright, with the possibility of p + A running in the near future, and the ability to measure the ψ at backward and forward rapidity at PHENIX with the new forward vertex detector, as well as far future measurements with the proposed sPHENIX upgrade [24]. References [1] N. Brambilla, S. Eidelman, B. Heltsley, R. Vogt, G. Bodwin, et al., Heavy quarkonium: progress, puzzles, and opportunities, Eur. Phys. J. C 71 (2011) 1534. √ [2] A. Adare, et al., J /ψ production vs centrality, transverse momentum, and rapidity in Au + Au collisions at sNN = 200 GeV, Phys. Rev. Lett. 98 (2007) 232301. √ [3] A. Adare, et al., J /ψ suppression at forward rapidity in Au + Au collisions at sNN = 200 GeV, Phys. Rev. C 84 (2011) 054912. √ [4] A. Adare, et al., J /ψ production in sNN = 200 GeV Cu + Cu collisions, Phys. Rev. Lett. 101 (2008) 122301. √ [5] A. Adare, et al., J /ψ suppression at forward rapidity in Au + Au collisions at sNN = 39 and 62.4 GeV, Phys. Rev. C 86 (2012) 064901. √ [6] A. Adare, et al., Quadrupole anisotropy in dihadron azimuthal correlations in central d + Au collisions at sNN = 200 GeV, Phys. Rev. Lett. 111 (2013) 212301. √ [7] G. Aad, et al., Observation of associated near-side and away-side long-range correlations in sNN = 5.02 TeV proton–lead collisions with the ATLAS detector, Phys. Rev. Lett. 110 (2013) 182302. √ [8] B. Abelev, et al., Long-range angular correlations on the near and away side in p–Pb collisions at sNN = 5.02 TeV, Phys. Lett. B 719 (2013) 29–41. √ [9] A. Adare, et al., Nuclear modification of ψ , χc and J /ψ production in d + Au collisions at sNN = 200 GeV, Phys. Rev. Lett. 111 (2013) 202301. √ [10] B. Abelev, et al., J /ψ suppression at forward rapidity in Pb–Pb collisions at sNN = 2.76 TeV, Phys. Rev. Lett. 109 (2012) 072301. [11] X. Zhao, R. Rapp, Charmonium in medium: from correlators to experiment, Phys. Rev. C 82 (2010) 064905. [12] A. Adare, et al., Cold nuclear matter effects on J /ψ yields as a function of rapidity and nuclear geometry in √ deuteron–gold collisions at sNN = 200 GeV, Phys. Rev. Lett. 107 (2011) 142301. [13] A. Adare, et al., Transverse-momentum dependence of the J /ψ nuclear modification in d + Au collisions at √ sNN = 200 GeV, Phys. Rev. C 87 (3) (2013) 034904. [14] A. Adare, et al., Centrality categorization for Rp(d)+A in high-energy collisions, arXiv:1310.4793. [15] K.J. Eskola, H. Paukkunen, C.A. Salgado, EPS09 – a new generation of NLO and LO nuclear parton distribution functions, J. High Energy Phys. 0904 (2009) 065. [16] A. Adare, et al., Cold-nuclear-matter effects on heavy-quark production at forward and backward rapidity in d + Au √ collisions at sNN = 200 GeV, arXiv:1310.1005. [17] A. Adare, S. Afanasiev, C. Aidala, N. Ajitanand, Y. Akiba, et al., Cold-nuclear-matter effects on heavy-quark pro√ duction in d + Au collisions at sNN = 200 GeV, arXiv:1208.1293. [18] D. McGlinchey, A. Frawley, R. Vogt, Impact parameter dependence of the nuclear modification of J /ψ production √ in d + Au collisions at SNN = 200 GeV, Phys. Rev. C 87 (5) (2013) 054910. [19] M.J. Leitch, et al., Measurement of J /ψ and ψ suppression in p A collisions at 800-GeV/c, Phys. Rev. Lett. 84 (2000) 3256–3260. [20] B. Alessandro, et al., J /ψ and ψ production and their normal nuclear absorption in proton–nucleus collisions at 400 GeV, Eur. Phys. J. C 48 (2006) 329. [21] H. Satz, Colour deconfinement and quarkonium binding, J. Phys. G 32 (2006) R25.
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