- Email: [email protected]
backward asymmetry in vn measured in Cu + Au collisions at sNN=200 GeV at RHIC-PHENIX
Available online at www.sciencedirect.com
Nuclear Physics A 967 (2017) 912–915 www.elsevier.com/locate/nuclphysa
Forward/backward √ asymmetry in vn measured in Cu+Au collisions at sNN = 200 GeV at RHIC-PHENIX Hiroshi Nakagomi for the PHENIX Collaboration Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Abstract We present measurements of second and third order azimuthal anisotropies for charged hadrons produced at forward √ and backward pseudorapidity (3 < |η| <3.9) in Cu+Au collisions at sNN = 200 GeV. The azimuthal anisotropy strength vn with respect to the event plane is examined as a function of the local charged particle multiplicity dNch /dη. Higher values of v2 and v3 are observed on the Au-going side (−3.9 < η < −3) than on the Cu-going side (3 < η < 3.9). In order to understand how the spatial anisotropy εn of the initial geometry at forward/backward η influences the measured vn , we divided the values of vn at forward/backward η by η-symmetric and asymmetric εn and found that universal behavior of vn /εn as a function of local dNch /dη is achieved when η-symmetric values of εn are used. Keywords: forward η, backward η, azimuthal anisotropy, v2 , v3
1. Introduction In relativistic heavy ion collisions, measurements of azimuthal anisotropies of particle production have been used to gain insight into the initial energy density distribution in the collisions and the bulk properties of the quark gluon plasma (QGP) that evolves from these initial conditions. Until now, the PHENIX measurements have been restricted to mid-rapidity [1, 2], and these measurements have been used to constrain the initial conditions and viscosity-over-entropy density ratio η/s of the QGP. There are several candidate models used to describe the initial energy density distribution in the transverse plane, such as Monte Carlo Glauber model [3], Monte Carlo Kharzeev-Levin-Nardi model [4], and IPGlasma model [5]. However, presently there is no first principle understanding of the initial conditions in the longitudinal direction. Recently, almost rapidity independent (boost invariant) initial conditions have been used in many hydrodynamical models and been successful in describing a large amount of experimental data. However, recent measurements by the CMS collaboration suggest that there are longitudinal anisotropic flow fluctuations that may be driven by rapidity-dependent event plane orientation or/and rapidity asymmetric vn [6]. Theoretically, the possible explanation of this breaking of the boost invariance is that it could result from longitudinal initial energy density fluctuations [7, 8, 9]. Event-by-event, the number of participant nucleons and the participant geometries in the forward-going nucleus and in the backward-going nucleus may be asymmetric due to these fluctuations. Therefore, an η-dependent event plane angle may result, which gives rise to forward/backward flow fluctuations. http://dx.doi.org/10.1016/j.nuclphysa.2017.05.018 0375-9474/© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
H. Nakagomi / Nuclear Physics A 967 (2017) 912–915
913
In 2012, Cu+Au collisions were delivered at RHIC and recorded by the PHENIX experiment. In Cu+Au collisions, a forward/backward asymmetry of vn is expected, because due to the asymmetric collision species different geometries and number of participants would be present in the forward/backward direction. Reference [10] shows the eccentricities of the Au geometry and the Cu geometry in Monte Carlo Glauber model and predicts the eccentricity of the Au geometry is higher than that of the Cu geometry. Therefore the measurements of vn at forward/backward η in Cu+Au collisions may provide a information about the initial geometry at forward/backward η. 2. Experiment and analysis method Experimental events are triggered by two sets of beam beam counters (BBC) [11] placed at 144 cm from the nominal interaction point. Each of the BBCs is composed of 64 Cherenkov radiators read out by photomultiplier tubes (PMTs), which are arranged radially around the beam line. The BBCs are also used in the determination of centrality, the z coordinate of the collision vertex, and the event plane angle. In all heavy ion collision systems, the centrality class is categorized based on the total charge measured in both of the BBCs. Charged particles emitted at mid-η are reconstructed using drift chambers and pad chambers that constitute the PHENIX central arm tracking systems (CNT) [11]. Both of these detectors cover azimuthal angle π/2 and the pseudorapidity range |η| < 0.35. The azimuthal anisotropies of the particle produced at forward/backward rapidity are measured with the event plane technique [12]. The event plane angle Ψobs n is determined on an event-by-event basis for each harmonic n. The magnitudes of the azimuthal anisotropies are evaluated as the coefficients vn in the Fourier expansion of the produced particle distributions with respect to the event plane, dN/dφ ∝ 1 + 2vn cos(n[φ − Ψn ]) (1) n=1
vn
= cos([φ − Ψn ]) obs = cos([φ − Ψobs n ])/Res{Ψn },
(2) (3)
where φ is the azimuthal angle of produced particle and Res{Ψn } = cos(n[Ψobs n − Ψn ]) is the event plane resolution. In Cu+Au collisions, the second order and third order event planes Ψ2 and Ψ3 are determined by using the CNT. The CNT event plane is determined with the tracks restricted to low pT ( less than 2 GeV/c ) in order to reduce jet contributions. To estimate the event plane resolutions of the CNT for second order and third order harmonics, we use a three sub-event method [12] with the combination of the event-plane correlations among BBCS (−3.9 < η < −3), CNTs and BBCN (3 < η < 3.9). The numerical formula for the estimation of the CNT event plane resolution with the three sub-event method is given as follows, obs obs obs cos(n[Ψobs n,CNT − Ψn,BBCS ])cos(n[Ψn,CNT − Ψn,BBCN ]) obs Res{Ψn,CNT } = (4) obs cos(n[Ψobs n,BBCS − Ψn,BBCN ]) In this study, the forward/backward vn is measured using the particles detected in the BBC. We first measure the vn using the azimuthal position of each of the PMTs. Then, we convert the measured vmes into n the true vtrue n . Since the BBC can not reconstruct charged particle tracks, the azimuthal anisotropy measured using the PMTs includes background contributions, such as secondary particles. To devise a correction that accounts for these background contributions, we rely on a GEANT simulation [13]. In this study, the systematic uncertainties from the measurements of the event plane and the input particle distributions in the simulation are estimated. The systematic uncertainties from the different choice of detectors used to determine the event plane are estimated by comparing the vn measured with respect to BBCS and BBCN. In the simulation study, we evaluate the systematic uncertainties by varying the input shapes of the particles’ pT , η, and vn (pT ) distributions.
H. Nakagomi / Nuclear Physics A 967 (2017) 912–915
914
0.06
Cu+Au 200 GeV Au-going
Cu+Au 200 GeV Au-going
0.01
Cu+Au 200 GeV Cu-going
sNN=200 GeV
0.02
Cu+Au 200 GeV Cu-going Charged hadron 3<|η|<3.9 sNN=200 GeV
v3
v2
0.04
Charged hadron 3<|η|<3.9
0 0 100 200 300 400 500 dNlocal /dη ch
0.005
0 0 100 200 300 400 500 dNlocal /dη ch
Fig. 1. Measurements of v2 and v3 for charged hadrons at forward/backward η (3 < |η| < 3.9) in Cu+Au √ collisions at sNN = 200 GeV as a function of the local charged particle multiplicity. The Au-going side (−3.9 < η < −3) and the Cu-going side (3 < η < 3.9) results are shown in blue and red markers, respectively.
3. Results and discussions Figure 1 shows the v2 and v3 values measured in the forward/backward direction (3 < |η| < 3.9) for charged hadrons as a function of local dNch /η in the corresponding forward/backward pseudorapidity (3 < |η| < 3.9) in Cu+Au collisions. The vn data on the Au-going side (−3.9 < η < −3) and Cu-going side (3 < η < 3.9) are shown with blue and red markers respectively, and the systematic uncertainties are shown with shaded boxes. The v2 values at both of Au-going and Cu-going directions decrease with increasing dNch /dη. On the other hand, the v3 values at both of Au-going and Cu-going directions decrease at low dNch /dη. In order to study how the participant eccentricity of the initial geometry, εn , influences the measured vn values at forward/backward η, we study the empirical relation among vn , εn and dNch /dη, vn /εn ∝ f (dNch /dη) used for the study of εn [14]. In addition to the standard definition of participant eccentricity that uses the location of all participant nucleons from both nuclei [14] and is symmetric in η, we define a η-dependent εn , which is based on the participants from the Cu nucleus in the forward direction (positive η) and the participants from the Au nucleus in the backward direction (negative η). We study ηasymmetric/symmetric εn . These eccentricities are estimated from a Monte Carlo Glauber model as follows: In η-asymmetric εn , Au-going side is given by participants in Au nuclei εn,Au , and Cu-going side is given by participants in Cu nuclei εn,Cu : εn,Au(Cu) =
rn cos(n[φAu(Cu) − Φn,Cu+Au ]) , rn
(5)
where Φn,Cu+Au is participant plane determined by all participants in both Au and Cu nuclei, φAu(Cu) are the azimuthal coordinates of the participants in the Au (Cu) nucleus, and r is their radial coordinate. Since the vn values are measured with respect to the single midrapidity event plane, the participant plane angle Φn,Cu+Au given by participants in both Au and Cu nuclei is used. In η-symmetric εn , general εn,Cu+Au given by participants in both Au and Cu nuclei is assumed at both of Au-going and Cu-going directions. Figure 2 shows the eccentricity-scaled vn at forward/backward η by the η-asymmetry of εn in Cu+Au collisions : the vn values measured on the Cu-going side and the Au-going side vn are scaled with the εn,Cu and the εn,Au respectively. For both of the second and third harmonic v2 and v3 results, the eccetricity-scaled values vn /εn,Au(Cu) are higher on the Cu-going side than on the Au-going side. Figure 3 shows the eccentricity-scaled vn at forward/backward η where the standard η-symmetric definition of εn,Cu+Au is used both on the Cu-going side, and on the Au-going side. The values of vn /εn , Cu+Au converge to a universal curve as a function of the local dNch /dη. This indicates that the εn,Cu+Au at forward/backward η is favored over a η-dependent definition and the difference of vn at forward/backward η is likely caused by the difference in the forward/backward dNch /dη.
H. Nakagomi / Nuclear Physics A 967 (2017) 912–915
0.2
Cu+Au Au-going ε2,Au-going = ε2,Au
Cu+Au Au-going ε3,Au-going = ε3,Au
Cu+Au Cu-going ε2,Cu-going = ε2,Cu
Cu+Au Cu-going ε3,Cu-going = ε3,Cu
0.15
0.05
v3/ε3
v2/ε2
0.1
0.05
Charged hadron 3<|η|<3.9
Charged hadron 3<|η|<3.9
sNN=200 GeV
sNN=200 GeV
0
0
0.2
200
dNlocal ch /dη
0
400
0
200
dNlocal ch /dη
Cu+Au Au-going ε2,Au-going = ε2,Cu+Au
Cu+Au Au-going ε3,Au-going = ε3,Cu+Au
Cu+Au Cu-going ε2,Cu-going = ε2,Cu+Au
Cu+Au Cu-going ε3,Cu-going = ε3,Cu+Au
0.15
0.05
v3/ε3
v2/ε2
0.1
0.05
Charged hadron 3<|η|<3.9
Charged hadron 3<|η|<3.9
0
200
dNlocal ch /dη
400
Fig. 2. Forward/backward (3 < |η| < 3.9) vn divided with η-asymmetric ε in Cu+Au collisions at √n sNN = 200 GeV. The vn measured on the Au-going side (−3.9 < η < −3) and the Cu-going side (3 < η < 3.9) are divided by the εn,Au given by the participants in the Au nucleus and the εn,Cu given by the participants in the Cu nucleus, respectively.
Fig. 3. Forward/backward (3 < |η| < 3.9) vn divided by η-symmetric εn in Cu+Au √ collisions at sNN = 200 GeV. The vn measured on both the Au-going side (−3.9 < η < −3) and the Cu-going side (3 < η < 3.9) are divided by the εn,Cu+Au given by the participants in both Cu and Au nuclei.
sNN=200 GeV
sNN=200 GeV
0
400
915
0
0
200
dNlocal ch /dη
400
4. Summary We have presented measurements of the azimuthal anisotropy strength vn for charged hadrons measured √ at forward/backward pseudorapidity in Cu+Au collisions at sNN = 200 GeV. Larger vn is observed on the Au-going side than on the Cu-going side. To understand the role of the initial geometry in the longitudinal direction in the observed η dependence of vn , we utilize two definitions of participant eccentricity: η-symmetric and η-asymmetric εn to scale the vn values. We find that vn /εn follows a universal trend as a function of the local charged-particle density when the scaled with η-independent eccentricity. The difference in the magnitude of vn between the Au-going side and the Cu-going side may be related to the different local charged-particle density at forward/backward pseudorapidity. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A. Adare et al. (PHENIX Collaboration) Phys. Rev. Lett. 107, 252301 (2011) A. Adare et al. (PHENIX Collaboration) Phys. Rev. C 94, 054910 (2016) M. L. Miller, K. Reygers, S. J. Sanders, and P. Steinberg, DOI: 10.1146/annurev.nucl.57.090506.123020 (2007) D. Kharzeev, M. Nardi, Phys. Lett. B 507, 121 (2001) B. Schenke, P. Tribedy and R. Venugopalan, Phys. Rev. Lett. 108, 252301 (2012) V. Khachatryan et al. (CMS Collaboration), Phys. Rev. C 92, 034911 (2015) L.-G. Pang, H. Petersen, G.-Y. Qin, V. Roy, and X.-N. Wang, Eur. Phys. J. A 52, 97 (2016) P. Bozek and W. Broniowski, Physics Lett. B 752, 206 (2016) B. Schenke and S. Schlichting, Physics Rev. C 94, 044907 (2016) H. Nakagomi for the PHENIX Collaboration, https://indico.cern.ch/event/433345/contributions/2358190/ poster presentation, Quark Matter 2017 K. Adcox, et al., PHENIX detector overview, Nucl. Instrum. Meth. A 499 469 (2003) A. M. Poskanzer and S. A. Voloshin, Phys. Rev. C 58, 1671 (1998) GEANT 3.2.1 manual (CERN, Geneva, 1993) B. Alver et al. (PHOBOS Collaboration) Phys. Rev. Lett. 98, 242302 (2007)
Nuclear Physics A 967 (2017) 912–915 www.elsevier.com/locate/nuclphysa
Forward/backward √ asymmetry in vn measured in Cu+Au collisions at sNN = 200 GeV at RHIC-PHENIX Hiroshi Nakagomi for the PHENIX Collaboration Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Abstract We present measurements of second and third order azimuthal anisotropies for charged hadrons produced at forward √ and backward pseudorapidity (3 < |η| <3.9) in Cu+Au collisions at sNN = 200 GeV. The azimuthal anisotropy strength vn with respect to the event plane is examined as a function of the local charged particle multiplicity dNch /dη. Higher values of v2 and v3 are observed on the Au-going side (−3.9 < η < −3) than on the Cu-going side (3 < η < 3.9). In order to understand how the spatial anisotropy εn of the initial geometry at forward/backward η influences the measured vn , we divided the values of vn at forward/backward η by η-symmetric and asymmetric εn and found that universal behavior of vn /εn as a function of local dNch /dη is achieved when η-symmetric values of εn are used. Keywords: forward η, backward η, azimuthal anisotropy, v2 , v3
1. Introduction In relativistic heavy ion collisions, measurements of azimuthal anisotropies of particle production have been used to gain insight into the initial energy density distribution in the collisions and the bulk properties of the quark gluon plasma (QGP) that evolves from these initial conditions. Until now, the PHENIX measurements have been restricted to mid-rapidity [1, 2], and these measurements have been used to constrain the initial conditions and viscosity-over-entropy density ratio η/s of the QGP. There are several candidate models used to describe the initial energy density distribution in the transverse plane, such as Monte Carlo Glauber model [3], Monte Carlo Kharzeev-Levin-Nardi model [4], and IPGlasma model [5]. However, presently there is no first principle understanding of the initial conditions in the longitudinal direction. Recently, almost rapidity independent (boost invariant) initial conditions have been used in many hydrodynamical models and been successful in describing a large amount of experimental data. However, recent measurements by the CMS collaboration suggest that there are longitudinal anisotropic flow fluctuations that may be driven by rapidity-dependent event plane orientation or/and rapidity asymmetric vn [6]. Theoretically, the possible explanation of this breaking of the boost invariance is that it could result from longitudinal initial energy density fluctuations [7, 8, 9]. Event-by-event, the number of participant nucleons and the participant geometries in the forward-going nucleus and in the backward-going nucleus may be asymmetric due to these fluctuations. Therefore, an η-dependent event plane angle may result, which gives rise to forward/backward flow fluctuations. http://dx.doi.org/10.1016/j.nuclphysa.2017.05.018 0375-9474/© 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
H. Nakagomi / Nuclear Physics A 967 (2017) 912–915
913
In 2012, Cu+Au collisions were delivered at RHIC and recorded by the PHENIX experiment. In Cu+Au collisions, a forward/backward asymmetry of vn is expected, because due to the asymmetric collision species different geometries and number of participants would be present in the forward/backward direction. Reference [10] shows the eccentricities of the Au geometry and the Cu geometry in Monte Carlo Glauber model and predicts the eccentricity of the Au geometry is higher than that of the Cu geometry. Therefore the measurements of vn at forward/backward η in Cu+Au collisions may provide a information about the initial geometry at forward/backward η. 2. Experiment and analysis method Experimental events are triggered by two sets of beam beam counters (BBC) [11] placed at 144 cm from the nominal interaction point. Each of the BBCs is composed of 64 Cherenkov radiators read out by photomultiplier tubes (PMTs), which are arranged radially around the beam line. The BBCs are also used in the determination of centrality, the z coordinate of the collision vertex, and the event plane angle. In all heavy ion collision systems, the centrality class is categorized based on the total charge measured in both of the BBCs. Charged particles emitted at mid-η are reconstructed using drift chambers and pad chambers that constitute the PHENIX central arm tracking systems (CNT) [11]. Both of these detectors cover azimuthal angle π/2 and the pseudorapidity range |η| < 0.35. The azimuthal anisotropies of the particle produced at forward/backward rapidity are measured with the event plane technique [12]. The event plane angle Ψobs n is determined on an event-by-event basis for each harmonic n. The magnitudes of the azimuthal anisotropies are evaluated as the coefficients vn in the Fourier expansion of the produced particle distributions with respect to the event plane, dN/dφ ∝ 1 + 2vn cos(n[φ − Ψn ]) (1) n=1
vn
= cos([φ − Ψn ]) obs = cos([φ − Ψobs n ])/Res{Ψn },
(2) (3)
where φ is the azimuthal angle of produced particle and Res{Ψn } = cos(n[Ψobs n − Ψn ]) is the event plane resolution. In Cu+Au collisions, the second order and third order event planes Ψ2 and Ψ3 are determined by using the CNT. The CNT event plane is determined with the tracks restricted to low pT ( less than 2 GeV/c ) in order to reduce jet contributions. To estimate the event plane resolutions of the CNT for second order and third order harmonics, we use a three sub-event method [12] with the combination of the event-plane correlations among BBCS (−3.9 < η < −3), CNTs and BBCN (3 < η < 3.9). The numerical formula for the estimation of the CNT event plane resolution with the three sub-event method is given as follows, obs obs obs cos(n[Ψobs n,CNT − Ψn,BBCS ])cos(n[Ψn,CNT − Ψn,BBCN ]) obs Res{Ψn,CNT } = (4) obs cos(n[Ψobs n,BBCS − Ψn,BBCN ]) In this study, the forward/backward vn is measured using the particles detected in the BBC. We first measure the vn using the azimuthal position of each of the PMTs. Then, we convert the measured vmes into n the true vtrue n . Since the BBC can not reconstruct charged particle tracks, the azimuthal anisotropy measured using the PMTs includes background contributions, such as secondary particles. To devise a correction that accounts for these background contributions, we rely on a GEANT simulation [13]. In this study, the systematic uncertainties from the measurements of the event plane and the input particle distributions in the simulation are estimated. The systematic uncertainties from the different choice of detectors used to determine the event plane are estimated by comparing the vn measured with respect to BBCS and BBCN. In the simulation study, we evaluate the systematic uncertainties by varying the input shapes of the particles’ pT , η, and vn (pT ) distributions.
H. Nakagomi / Nuclear Physics A 967 (2017) 912–915
914
0.06
Cu+Au 200 GeV Au-going
Cu+Au 200 GeV Au-going
0.01
Cu+Au 200 GeV Cu-going
sNN=200 GeV
0.02
Cu+Au 200 GeV Cu-going Charged hadron 3<|η|<3.9 sNN=200 GeV
v3
v2
0.04
Charged hadron 3<|η|<3.9
0 0 100 200 300 400 500 dNlocal /dη ch
0.005
0 0 100 200 300 400 500 dNlocal /dη ch
Fig. 1. Measurements of v2 and v3 for charged hadrons at forward/backward η (3 < |η| < 3.9) in Cu+Au √ collisions at sNN = 200 GeV as a function of the local charged particle multiplicity. The Au-going side (−3.9 < η < −3) and the Cu-going side (3 < η < 3.9) results are shown in blue and red markers, respectively.
3. Results and discussions Figure 1 shows the v2 and v3 values measured in the forward/backward direction (3 < |η| < 3.9) for charged hadrons as a function of local dNch /η in the corresponding forward/backward pseudorapidity (3 < |η| < 3.9) in Cu+Au collisions. The vn data on the Au-going side (−3.9 < η < −3) and Cu-going side (3 < η < 3.9) are shown with blue and red markers respectively, and the systematic uncertainties are shown with shaded boxes. The v2 values at both of Au-going and Cu-going directions decrease with increasing dNch /dη. On the other hand, the v3 values at both of Au-going and Cu-going directions decrease at low dNch /dη. In order to study how the participant eccentricity of the initial geometry, εn , influences the measured vn values at forward/backward η, we study the empirical relation among vn , εn and dNch /dη, vn /εn ∝ f (dNch /dη) used for the study of εn [14]. In addition to the standard definition of participant eccentricity that uses the location of all participant nucleons from both nuclei [14] and is symmetric in η, we define a η-dependent εn , which is based on the participants from the Cu nucleus in the forward direction (positive η) and the participants from the Au nucleus in the backward direction (negative η). We study ηasymmetric/symmetric εn . These eccentricities are estimated from a Monte Carlo Glauber model as follows: In η-asymmetric εn , Au-going side is given by participants in Au nuclei εn,Au , and Cu-going side is given by participants in Cu nuclei εn,Cu : εn,Au(Cu) =
rn cos(n[φAu(Cu) − Φn,Cu+Au ]) , rn
(5)
where Φn,Cu+Au is participant plane determined by all participants in both Au and Cu nuclei, φAu(Cu) are the azimuthal coordinates of the participants in the Au (Cu) nucleus, and r is their radial coordinate. Since the vn values are measured with respect to the single midrapidity event plane, the participant plane angle Φn,Cu+Au given by participants in both Au and Cu nuclei is used. In η-symmetric εn , general εn,Cu+Au given by participants in both Au and Cu nuclei is assumed at both of Au-going and Cu-going directions. Figure 2 shows the eccentricity-scaled vn at forward/backward η by the η-asymmetry of εn in Cu+Au collisions : the vn values measured on the Cu-going side and the Au-going side vn are scaled with the εn,Cu and the εn,Au respectively. For both of the second and third harmonic v2 and v3 results, the eccetricity-scaled values vn /εn,Au(Cu) are higher on the Cu-going side than on the Au-going side. Figure 3 shows the eccentricity-scaled vn at forward/backward η where the standard η-symmetric definition of εn,Cu+Au is used both on the Cu-going side, and on the Au-going side. The values of vn /εn , Cu+Au converge to a universal curve as a function of the local dNch /dη. This indicates that the εn,Cu+Au at forward/backward η is favored over a η-dependent definition and the difference of vn at forward/backward η is likely caused by the difference in the forward/backward dNch /dη.
H. Nakagomi / Nuclear Physics A 967 (2017) 912–915
0.2
Cu+Au Au-going ε2,Au-going = ε2,Au
Cu+Au Au-going ε3,Au-going = ε3,Au
Cu+Au Cu-going ε2,Cu-going = ε2,Cu
Cu+Au Cu-going ε3,Cu-going = ε3,Cu
0.15
0.05
v3/ε3
v2/ε2
0.1
0.05
Charged hadron 3<|η|<3.9
Charged hadron 3<|η|<3.9
sNN=200 GeV
sNN=200 GeV
0
0
0.2
200
dNlocal ch /dη
0
400
0
200
dNlocal ch /dη
Cu+Au Au-going ε2,Au-going = ε2,Cu+Au
Cu+Au Au-going ε3,Au-going = ε3,Cu+Au
Cu+Au Cu-going ε2,Cu-going = ε2,Cu+Au
Cu+Au Cu-going ε3,Cu-going = ε3,Cu+Au
0.15
0.05
v3/ε3
v2/ε2
0.1
0.05
Charged hadron 3<|η|<3.9
Charged hadron 3<|η|<3.9
0
200
dNlocal ch /dη
400
Fig. 2. Forward/backward (3 < |η| < 3.9) vn divided with η-asymmetric ε in Cu+Au collisions at √n sNN = 200 GeV. The vn measured on the Au-going side (−3.9 < η < −3) and the Cu-going side (3 < η < 3.9) are divided by the εn,Au given by the participants in the Au nucleus and the εn,Cu given by the participants in the Cu nucleus, respectively.
Fig. 3. Forward/backward (3 < |η| < 3.9) vn divided by η-symmetric εn in Cu+Au √ collisions at sNN = 200 GeV. The vn measured on both the Au-going side (−3.9 < η < −3) and the Cu-going side (3 < η < 3.9) are divided by the εn,Cu+Au given by the participants in both Cu and Au nuclei.
sNN=200 GeV
sNN=200 GeV
0
400
915
0
0
200
dNlocal ch /dη
400
4. Summary We have presented measurements of the azimuthal anisotropy strength vn for charged hadrons measured √ at forward/backward pseudorapidity in Cu+Au collisions at sNN = 200 GeV. Larger vn is observed on the Au-going side than on the Cu-going side. To understand the role of the initial geometry in the longitudinal direction in the observed η dependence of vn , we utilize two definitions of participant eccentricity: η-symmetric and η-asymmetric εn to scale the vn values. We find that vn /εn follows a universal trend as a function of the local charged-particle density when the scaled with η-independent eccentricity. The difference in the magnitude of vn between the Au-going side and the Cu-going side may be related to the different local charged-particle density at forward/backward pseudorapidity. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A. Adare et al. (PHENIX Collaboration) Phys. Rev. Lett. 107, 252301 (2011) A. Adare et al. (PHENIX Collaboration) Phys. Rev. C 94, 054910 (2016) M. L. Miller, K. Reygers, S. J. Sanders, and P. Steinberg, DOI: 10.1146/annurev.nucl.57.090506.123020 (2007) D. Kharzeev, M. Nardi, Phys. Lett. B 507, 121 (2001) B. Schenke, P. Tribedy and R. Venugopalan, Phys. Rev. Lett. 108, 252301 (2012) V. Khachatryan et al. (CMS Collaboration), Phys. Rev. C 92, 034911 (2015) L.-G. Pang, H. Petersen, G.-Y. Qin, V. Roy, and X.-N. Wang, Eur. Phys. J. A 52, 97 (2016) P. Bozek and W. Broniowski, Physics Lett. B 752, 206 (2016) B. Schenke and S. Schlichting, Physics Rev. C 94, 044907 (2016) H. Nakagomi for the PHENIX Collaboration, https://indico.cern.ch/event/433345/contributions/2358190/ poster presentation, Quark Matter 2017 K. Adcox, et al., PHENIX detector overview, Nucl. Instrum. Meth. A 499 469 (2003) A. M. Poskanzer and S. A. Voloshin, Phys. Rev. C 58, 1671 (1998) GEANT 3.2.1 manual (CERN, Geneva, 1993) B. Alver et al. (PHOBOS Collaboration) Phys. Rev. Lett. 98, 242302 (2007)