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ψ production and hadron vn in Cu + Au collisions in PHENIX
Available online at www.sciencedirect.com
Nuclear Physics A 904–905 (2013) 507c–510c www.elsevier.com/locate/nuclphysa
Forward/backward J/ψ production and hadron vn in Cu+Au collisions in PHENIX Richard S. Hollis (for the PHENIX Collaboration)1 University of California, Riverside, CA 92521
Abstract Cu+Au collisions provide a test for theories trying to describe heavy-ion data by changing the initial conditions and introducing distinct asymmetries into the initial geometry of the collision system. We present the first results from the PHENIX collaboration from these asymmetric collisions. The measured hadron v1 is found to be large at midrapidity, whilst v3 is found to be small when the reaction plane is determined from the spectators. In the forward region, the J/ψ is found to be more suppressed in the Cu-going direction compared to the Au-going direction.
1. Introduction The flexibility of RHIC to collide different nuclei provides experiments with a rich resource to systematically test models and scaling behaviors. During Run 12 (2012) at the Relativistic Heavy-Ion Collider, Cu+Au collision data were recorded using the PHENIX detector. The interest in asymmetric collisions, such as Cu+Au, can be illustrated using a Glauber Model [1], where the initial geometrical overlap properties of the system are explored. Figure 1 shows such a calculation which illustrates the participant overlap density for mid-central collisions (left-most panel), the position of singly-interacting nucleons (also mid-central), and the position of spectators in very central collisions (right). In the first of these figures, a natural v3 structure is clearly evident in the initial geometry which could translate into a stronger v3 signal in such collisions. In Au+Au collisions, a finite v3 signal is observed, which is thought to originate from fluctuations in the initial participant locations at the point of collision. From the azimuthal distribution of singly-interacting participants in the Cu+Au overlap area (center) an asymmetric (or v1 -like) elliptic flow dependence is prominent. Finally, in very central collisions (right-most illustration), the Cu-nucleus is entirely consumed within the Au-nucleus, due to the larger size of the latter. From the data collected in the PHENIX detector, several measurements were made to elucidate the azimuthal dependence of particle production at midrapidity and forward/backward differences using J/ψ’s. 2. Bulk particle production and flow To examine global properties of this new system, we first estimated the collision centrality from the energy signals in the Beam-Beam Counters (BBCs, covering 3.1<|η|<3.9). The BBC’s 1A
list of members of the PHENIX Collaboration and acknowledgments can be found at the end of this issue.
0375-9474/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysa.2013.02.064
508c
R.S. Hollis / Nuclear Physics A 904–905 (2013) 507c–510c
Figure 1: Illustration of the initial geometry of Cu+Au collisions, derived from a Glauber model simulation [1]. The left panel shows the participant density (log-z scale) for mid-central collisions. The center panel shows the relative location of singly-interacting nucleons at the same centrality. The right panel shows the spectator nucleons for very central collisions.
were also employed to trigger the collisions in the minimum bias data sample for use in the global analyses. The data were divided into nine centrality bins for this analysis (0-10%, 1020%, ..., 80-93%). Reconstruction was limited to collisions closest to the center of the detector (zvtx = ±10 cm). A correlation of the BBCs and the Cu-going Zero-Degree Calorimeters shows that the number of spectator neutrons falls to zero in very central (large BBC signal) events. Using this event selection, the energy density and number of charged particles emitted at midrapidity were measured. The energy density, proportional to the amount of transverse energy produced, is found to be higher in this system, as compared to symmetric systems. For the most central collisions, the Cu nucleus is completely encapsulated within the Au nucleus. As the highest number of nucleons are found toward the center of the nucleus, it may be expected that the energy density should be higher in such cases. For the more differential analysis of the azimuthal dependence of particle production a reaction plane is estimated using the Shower-Max Detector in the Zero-Degree Calorimeters (SMD). This reaction plane is measured using the impact position of spectator neutrons and is, therefore, decoupled from fluctuations introduced in reaction plane angles calculated using collision products. 3
×10
Centrality:30~40%
195
p :2~3 GeV/c T
dN / dΔ φ
190
185
χ2 / ndf 180
175
170
-1
29.84 / 21
const
1.88e+05 ± 8.67e+01
v1
0.003122 ± 0.000327
v2
0.004562 ± 0.000326
v3
0.0002057 ± 0.0003262 0
1
Δφ=φ-Ψ
2
3
Figure 2: Azimuthal dependence of charged-particle production at midrapidity, relative to the spectator reaction plane. The data are selected from the 30-40% centrality class, with momenta 2
4
1,SMD
Figure 2 shows the measured azimuthal dependence of charged-particles near midrapidity. This data represents all charged hadrons at midrapidity selected from the 30-40% centrality bin at 2
R.S. Hollis / Nuclear Physics A 904–905 (2013) 507c–510c
509c
features are apparent. Firstly, the estimated magnitude of the v1 signal is large, and has the same order of magnitude as the elliptic flow, v2 . This is surprising as in symmetric collisions a v1 signal is consistent with zero at midrapidity2 . In this representation, the Δφ = 0 corresponds to the side of the collision in which the Au-nucleus resides. From the initial motivation for studying such asymmetric collisions, one may expect an asymmetric Δφ dependence. The second striking feature is the absence of v3 for this particular measurement. A finite v3 may still be evident when the reaction plane is determined from a collision-product reaction plane – which may be different from the SMD-plane – and necessarily maximizes the apparent v3 contribution event-by-event. This may simply suggest that v3 is purely a consequence of fluctuations. A further discussion on the azimuthal dependencies in Cu+Au collisions can be found in Ref [2]. 3. J/ψ production at forward rapidities In symmetric A+A collisions, particles which are dominantly from hard-scattering processes (large momenta and/or those which contain a heavy quark) have been shown to be suppressed relative to their expected production rate. This expectation is based on the yield obtained from pp interactions scaled by the number of binary (nucleon-nucleon) collisions estimated from a Glauber calculation. This is usually expressed as the nuclear modification factor, of the particle, see for example [3]. For central events, only one-fifth of the expected number of particles are observed. We surmise that the rest are suppressed by an energy loss to the plasma. The production of J/ψ is dominated by hard-interaction gluon-gluon fusion, and thus provides a tool with which to study hard processes in heavy-ion collisions. From earlier studies, the production rates of J/ψ’s are known to be modified by both cold and hot nuclear matter effects. Cold nuclear matter (CNM) effects include shadowing (and antishadowing), and Cronin Enhancement [4] which can be evaluated in systems such as d+Au collisions, where such effects are active, but the hot nuclear matter effects are absent. CNM effects (dependent on the kinematic region) can cause a suppression or an enhancement. Hot nuclear matter, or final state effects, encompass a wide variety of modification possibilities, most notably color screening in the hot, dense medium created in the collision. The current interpretation of A+A collision data accounts for both cold and hot nuclear matter effects. The true relative contribution is difficult to disentangle, so colliding different systems (at the same energy) provides additional insight into the relative importance of such mechanisms. In PHENIX, J/ψ → μμ candidate events were triggered by a coincidence of a BBC collision vertex within |z|<30 cm and two hits in the same muon-identifier detectors (North or South arms). This trigger was necessary to capture all forward J/ψ candidates as the luminosity was sufficiently high and all minimum bias events could not be recorded. The sample was normalized per minimum bias event – correcting for the necessary minimum-bias scale-down factor. Forward J/ψ mesons are reconstructed using a combination of the forward muon-tracker and the muon-identifier. The former provides kinematic information (pT , η, y) of the μ which passed through the active elements. The muon-identifier consists of a series of thick iron plates and active read-out. Light hadrons are rejected, if a signal is detected in all active elements of the muon-identifier. After this requirement, light hadrons still out-number muons. A large combinatorial background is present from all possible pairs. To further reduce the combinatorial background, track-quality variables and tracker-identifier matching are used to reject light hadrons which typically have broader residual distributions. 2 more
precisely a node at η = 0 with negative (positive) values for η>0 (η<0)
510c
R.S. Hollis / Nuclear Physics A 904–905 (2013) 507c–510c
RAA
The extracted J/ψ → μμ signal is measured by subtracting the combinatorial background using an event-mixing technique, where real μμ correlations are necessarily broken. A doubleGaussian fit to the mass spectrum in the vicinity of the J/ψ mass is used to determine the final raw number of J/ψ candidates. Data are corrected for acceptance and detector/triggering inefficiency using simulated J/ψ decays (from Pythia) embedded into real events at various centralities. To form the nuclear modification factor, RAB , in each arm, the yield in pp collisions was calculated using data from 2006 and 2008.
J/ ψ→μμ
CuAu (gl. sys. 7.1%)
Cu-going 1.2
1
Au-going Cu-going 0-5% central
Figure 3: Nuclear modification factor of J/ψ at forward (1.2
AuAu (gl. sys. 9.2%) CuCu (gl. sys. 8.0%)
0.5
00 100
200
300
400
Number of Participants
Figure 3 shows the RAA measured in Au+Au and Cu+Cu collisions along with that measured in Cu+Au system (colored circles, cross). The suppression observed on the Au-going side is in agreement with that seen in the symmetric systems. However, a larger suppression is observed on the Cu-going side, which can be partially explained considering initial state effects [5]. Final state effects must account for the additional forward/backward difference. For example, if a particle is produced on the Cu-going side it may have to propagate through a higher energy density, as approximated by the number of particles measured in the BBCs on the Cu-going versus Au-going directions. 4. Summary From early results in Cu+Au collision in PHENIX we have observed a number of differences from symmetric collisions. A large v1 , taken with respect to the spectator plane is observed which is comparable to the magnitude of the v2 signal, whilst an expected propagation of the initialgeometry v3 is not seen. The nuclear modification of J/ψ is found to be larger on the Cu-going side as compared to the Au-going side and that observed in Au+Au and Cu+Cu collisions. References [1] [2] [3] [4] [5]
B. Alver et al., arXiv:0805.4411v1. S. Huang et al., published in this edition. A. Adare et al., Phys. Rev. Lett. 98 (2007) 232301. J. W. Cronin et al., Phys. Rev. D11 (1975) 3105. A new calculation based on J. L. Nagle et al., arXiv:1011.4534
Nuclear Physics A 904–905 (2013) 507c–510c www.elsevier.com/locate/nuclphysa
Forward/backward J/ψ production and hadron vn in Cu+Au collisions in PHENIX Richard S. Hollis (for the PHENIX Collaboration)1 University of California, Riverside, CA 92521
Abstract Cu+Au collisions provide a test for theories trying to describe heavy-ion data by changing the initial conditions and introducing distinct asymmetries into the initial geometry of the collision system. We present the first results from the PHENIX collaboration from these asymmetric collisions. The measured hadron v1 is found to be large at midrapidity, whilst v3 is found to be small when the reaction plane is determined from the spectators. In the forward region, the J/ψ is found to be more suppressed in the Cu-going direction compared to the Au-going direction.
1. Introduction The flexibility of RHIC to collide different nuclei provides experiments with a rich resource to systematically test models and scaling behaviors. During Run 12 (2012) at the Relativistic Heavy-Ion Collider, Cu+Au collision data were recorded using the PHENIX detector. The interest in asymmetric collisions, such as Cu+Au, can be illustrated using a Glauber Model [1], where the initial geometrical overlap properties of the system are explored. Figure 1 shows such a calculation which illustrates the participant overlap density for mid-central collisions (left-most panel), the position of singly-interacting nucleons (also mid-central), and the position of spectators in very central collisions (right). In the first of these figures, a natural v3 structure is clearly evident in the initial geometry which could translate into a stronger v3 signal in such collisions. In Au+Au collisions, a finite v3 signal is observed, which is thought to originate from fluctuations in the initial participant locations at the point of collision. From the azimuthal distribution of singly-interacting participants in the Cu+Au overlap area (center) an asymmetric (or v1 -like) elliptic flow dependence is prominent. Finally, in very central collisions (right-most illustration), the Cu-nucleus is entirely consumed within the Au-nucleus, due to the larger size of the latter. From the data collected in the PHENIX detector, several measurements were made to elucidate the azimuthal dependence of particle production at midrapidity and forward/backward differences using J/ψ’s. 2. Bulk particle production and flow To examine global properties of this new system, we first estimated the collision centrality from the energy signals in the Beam-Beam Counters (BBCs, covering 3.1<|η|<3.9). The BBC’s 1A
list of members of the PHENIX Collaboration and acknowledgments can be found at the end of this issue.
0375-9474/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysa.2013.02.064
508c
R.S. Hollis / Nuclear Physics A 904–905 (2013) 507c–510c
Figure 1: Illustration of the initial geometry of Cu+Au collisions, derived from a Glauber model simulation [1]. The left panel shows the participant density (log-z scale) for mid-central collisions. The center panel shows the relative location of singly-interacting nucleons at the same centrality. The right panel shows the spectator nucleons for very central collisions.
were also employed to trigger the collisions in the minimum bias data sample for use in the global analyses. The data were divided into nine centrality bins for this analysis (0-10%, 1020%, ..., 80-93%). Reconstruction was limited to collisions closest to the center of the detector (zvtx = ±10 cm). A correlation of the BBCs and the Cu-going Zero-Degree Calorimeters shows that the number of spectator neutrons falls to zero in very central (large BBC signal) events. Using this event selection, the energy density and number of charged particles emitted at midrapidity were measured. The energy density, proportional to the amount of transverse energy produced, is found to be higher in this system, as compared to symmetric systems. For the most central collisions, the Cu nucleus is completely encapsulated within the Au nucleus. As the highest number of nucleons are found toward the center of the nucleus, it may be expected that the energy density should be higher in such cases. For the more differential analysis of the azimuthal dependence of particle production a reaction plane is estimated using the Shower-Max Detector in the Zero-Degree Calorimeters (SMD). This reaction plane is measured using the impact position of spectator neutrons and is, therefore, decoupled from fluctuations introduced in reaction plane angles calculated using collision products. 3
×10
Centrality:30~40%
195
p :2~3 GeV/c T
dN / dΔ φ
190
185
χ2 / ndf 180
175
170
-1
29.84 / 21
const
1.88e+05 ± 8.67e+01
v1
0.003122 ± 0.000327
v2
0.004562 ± 0.000326
v3
0.0002057 ± 0.0003262 0
1
Δφ=φ-Ψ
2
3
Figure 2: Azimuthal dependence of charged-particle production at midrapidity, relative to the spectator reaction plane. The data are selected from the 30-40% centrality class, with momenta 2
4
1,SMD
Figure 2 shows the measured azimuthal dependence of charged-particles near midrapidity. This data represents all charged hadrons at midrapidity selected from the 30-40% centrality bin at 2
R.S. Hollis / Nuclear Physics A 904–905 (2013) 507c–510c
509c
features are apparent. Firstly, the estimated magnitude of the v1 signal is large, and has the same order of magnitude as the elliptic flow, v2 . This is surprising as in symmetric collisions a v1 signal is consistent with zero at midrapidity2 . In this representation, the Δφ = 0 corresponds to the side of the collision in which the Au-nucleus resides. From the initial motivation for studying such asymmetric collisions, one may expect an asymmetric Δφ dependence. The second striking feature is the absence of v3 for this particular measurement. A finite v3 may still be evident when the reaction plane is determined from a collision-product reaction plane – which may be different from the SMD-plane – and necessarily maximizes the apparent v3 contribution event-by-event. This may simply suggest that v3 is purely a consequence of fluctuations. A further discussion on the azimuthal dependencies in Cu+Au collisions can be found in Ref [2]. 3. J/ψ production at forward rapidities In symmetric A+A collisions, particles which are dominantly from hard-scattering processes (large momenta and/or those which contain a heavy quark) have been shown to be suppressed relative to their expected production rate. This expectation is based on the yield obtained from pp interactions scaled by the number of binary (nucleon-nucleon) collisions estimated from a Glauber calculation. This is usually expressed as the nuclear modification factor, of the particle, see for example [3]. For central events, only one-fifth of the expected number of particles are observed. We surmise that the rest are suppressed by an energy loss to the plasma. The production of J/ψ is dominated by hard-interaction gluon-gluon fusion, and thus provides a tool with which to study hard processes in heavy-ion collisions. From earlier studies, the production rates of J/ψ’s are known to be modified by both cold and hot nuclear matter effects. Cold nuclear matter (CNM) effects include shadowing (and antishadowing), and Cronin Enhancement [4] which can be evaluated in systems such as d+Au collisions, where such effects are active, but the hot nuclear matter effects are absent. CNM effects (dependent on the kinematic region) can cause a suppression or an enhancement. Hot nuclear matter, or final state effects, encompass a wide variety of modification possibilities, most notably color screening in the hot, dense medium created in the collision. The current interpretation of A+A collision data accounts for both cold and hot nuclear matter effects. The true relative contribution is difficult to disentangle, so colliding different systems (at the same energy) provides additional insight into the relative importance of such mechanisms. In PHENIX, J/ψ → μμ candidate events were triggered by a coincidence of a BBC collision vertex within |z|<30 cm and two hits in the same muon-identifier detectors (North or South arms). This trigger was necessary to capture all forward J/ψ candidates as the luminosity was sufficiently high and all minimum bias events could not be recorded. The sample was normalized per minimum bias event – correcting for the necessary minimum-bias scale-down factor. Forward J/ψ mesons are reconstructed using a combination of the forward muon-tracker and the muon-identifier. The former provides kinematic information (pT , η, y) of the μ which passed through the active elements. The muon-identifier consists of a series of thick iron plates and active read-out. Light hadrons are rejected, if a signal is detected in all active elements of the muon-identifier. After this requirement, light hadrons still out-number muons. A large combinatorial background is present from all possible pairs. To further reduce the combinatorial background, track-quality variables and tracker-identifier matching are used to reject light hadrons which typically have broader residual distributions. 2 more
precisely a node at η = 0 with negative (positive) values for η>0 (η<0)
510c
R.S. Hollis / Nuclear Physics A 904–905 (2013) 507c–510c
RAA
The extracted J/ψ → μμ signal is measured by subtracting the combinatorial background using an event-mixing technique, where real μμ correlations are necessarily broken. A doubleGaussian fit to the mass spectrum in the vicinity of the J/ψ mass is used to determine the final raw number of J/ψ candidates. Data are corrected for acceptance and detector/triggering inefficiency using simulated J/ψ decays (from Pythia) embedded into real events at various centralities. To form the nuclear modification factor, RAB , in each arm, the yield in pp collisions was calculated using data from 2006 and 2008.
J/ ψ→μμ
CuAu (gl. sys. 7.1%)
Cu-going 1.2
1
Au-going Cu-going 0-5% central
Figure 3: Nuclear modification factor of J/ψ at forward (1.2
AuAu (gl. sys. 9.2%) CuCu (gl. sys. 8.0%)
0.5
00 100
200
300
400
Number of Participants
Figure 3 shows the RAA measured in Au+Au and Cu+Cu collisions along with that measured in Cu+Au system (colored circles, cross). The suppression observed on the Au-going side is in agreement with that seen in the symmetric systems. However, a larger suppression is observed on the Cu-going side, which can be partially explained considering initial state effects [5]. Final state effects must account for the additional forward/backward difference. For example, if a particle is produced on the Cu-going side it may have to propagate through a higher energy density, as approximated by the number of particles measured in the BBCs on the Cu-going versus Au-going directions. 4. Summary From early results in Cu+Au collision in PHENIX we have observed a number of differences from symmetric collisions. A large v1 , taken with respect to the spectator plane is observed which is comparable to the magnitude of the v2 signal, whilst an expected propagation of the initialgeometry v3 is not seen. The nuclear modification of J/ψ is found to be larger on the Cu-going side as compared to the Au-going side and that observed in Au+Au and Cu+Cu collisions. References [1] [2] [3] [4] [5]
B. Alver et al., arXiv:0805.4411v1. S. Huang et al., published in this edition. A. Adare et al., Phys. Rev. Lett. 98 (2007) 232301. J. W. Cronin et al., Phys. Rev. D11 (1975) 3105. A new calculation based on J. L. Nagle et al., arXiv:1011.4534