Mass, quark-number, and s NN − − − − √ dependence of the second and fourth flow harmonics in ultra-relativistic nucleus-nucleus collisions
Abstract
We present STAR measurements of the azimuthal anisotropy parameter
v
2
for pions, kaons, protons,
Λ
,
Λ
¯
,
Ξ+
Ξ
¯
, and
Ω+
Ω
¯
, along with
v
4
for pions, kaons, protons, and
Λ+
Λ
¯
at mid-rapidity for Au+Au collisions at
s
NN
−
−
−
√
=62.4
and 200 GeV. The
v
2
(
p
T
)
values for all hadron species at 62.4 GeV are similar to those observed in 130 and 200 GeV collisions. For observed kinematic ranges,
v
2
values at 62.4, 130, and 200 GeV are as little as 10%--15% larger than those in Pb+Pb collisions at
s
NN
−
−
−
√
=17.3
GeV. At intermediate transverse momentum (
p
T
from 1.5--5 GeV/c), the 62.4 GeV
v
2
(
p
T
)
and
v
4
(
p
T
)
values are consistent with the quark-number scaling first observed at 200 GeV. A four-particle cumulant analysis is used to assess the non-flow contributions to pions and protons and some indications are found for a smaller non-flow contribution to protons than pions. Baryon
v
2
is larger than anti-baryon
v
2
at 62.4 and 200 GeV perhaps indicating either that the initial spatial net-baryon distribution is anisotropic, that the mechanism leading to transport of baryon number from beam- to mid-rapidity enhances
v
2
, or that anti-baryon and baryon annihilation is larger in the in-plane direction.