13 Elliptic Flow in Relativistic Heavy-Ion Collisions
183
Fig. 13.23 v 2 (EP) as a
function of η for Au +Au
collisions at different
energies as indicated in the
figure
Back et al., [68] observed that the integrated elliptic flow varies linearly with the
pseudo-rapidity η
(η − y beam ), measured with respect to the beam rapidity y beam ,
in Au+Au collisions at
√ s NN = 200, 130, 62.4 and 19.6 GeV. Figure 13.23 displays
the plot of v 2 (EP) versus η
. It is seen that elliptic flow exhibits longitudinal scaling
behaviour over the whole range of η
. The extrapolation of this to LHC energy,
predicted an increase in v 2 of 50%. However, the ALICE collaboration reported an
increase in v 2 ∼ 30% from
√
s NN = 200 GeV to
√
s NN = 2.76 TeV [28].
13.2.6 Energy Dependence
Figure 13.24 (top) displays the STAR v 2 {4} obtained using the 4-particle cumulant method versus p T for 20–30% centrality for Au+Au at
√
s NN = 39, 62.4. and
200 GeV [69]. The ALICE Pb-Pb data points for similar centrality at
√ s NN =
2.76 TeV are also shown. All data points fall on the same curve. Ratio of v 2 to
the polynomial fitted value at 200 GeV versus p T is plotted in Fig. 13.24 (bottom).
The good agreement within ∼10% is seen for wide range of energies for p T > 500
MeV/c. It is seen that v 2 {4} gets saturated for p T > 1GeV but for p T < 1 GeV/c
increase in v 2 is seen for higher energy Pb-Pb collisions at 2.76 TeV and decrease
in v 2 for lower energy taking 200 GeV as a reference which is consistent with the
overall increase in the v 2 with increase in beam energy.
The comparison of p T integrated v 2 for 20–30% centrality at different energies is
shown in Fig. 13.25 [28]. There is continuous increase in the magnitude of elliptic
flow for this centrality from RHIC to LHC. About 30% increase in v 2 is observed
with increasing energy from
√ s NN = 200 GeV to
√ s NN = 2.76 TeV. The larger v 2 is
due to the increase in the mean p T . Hydrodynamical [70] and hybrid [71–73] models
infact predicted an increase of ∼10–30% in v 2 at the LHC energy.
183
Fig. 13.23 v 2 (EP) as a
function of η for Au +Au
collisions at different
energies as indicated in the
figure
Back et al., [68] observed that the integrated elliptic flow varies linearly with the
pseudo-rapidity η
(η − y beam ), measured with respect to the beam rapidity y beam ,
in Au+Au collisions at
√ s NN = 200, 130, 62.4 and 19.6 GeV. Figure 13.23 displays
the plot of v 2 (EP) versus η
. It is seen that elliptic flow exhibits longitudinal scaling
behaviour over the whole range of η
. The extrapolation of this to LHC energy,
predicted an increase in v 2 of 50%. However, the ALICE collaboration reported an
increase in v 2 ∼ 30% from
√
s NN = 200 GeV to
√
s NN = 2.76 TeV [28].
13.2.6 Energy Dependence
Figure 13.24 (top) displays the STAR v 2 {4} obtained using the 4-particle cumulant method versus p T for 20–30% centrality for Au+Au at
√
s NN = 39, 62.4. and
200 GeV [69]. The ALICE Pb-Pb data points for similar centrality at
√ s NN =
2.76 TeV are also shown. All data points fall on the same curve. Ratio of v 2 to
the polynomial fitted value at 200 GeV versus p T is plotted in Fig. 13.24 (bottom).
The good agreement within ∼10% is seen for wide range of energies for p T > 500
MeV/c. It is seen that v 2 {4} gets saturated for p T > 1GeV but for p T < 1 GeV/c
increase in v 2 is seen for higher energy Pb-Pb collisions at 2.76 TeV and decrease
in v 2 for lower energy taking 200 GeV as a reference which is consistent with the
overall increase in the v 2 with increase in beam energy.
The comparison of p T integrated v 2 for 20–30% centrality at different energies is
shown in Fig. 13.25 [28]. There is continuous increase in the magnitude of elliptic
flow for this centrality from RHIC to LHC. About 30% increase in v 2 is observed
with increasing energy from
√ s NN = 200 GeV to
√ s NN = 2.76 TeV. The larger v 2 is
due to the increase in the mean p T . Hydrodynamical [70] and hybrid [71–73] models
infact predicted an increase of ∼10–30% in v 2 at the LHC energy.