Particle-type dependence of azimuthal anisotropy and nuclearmodification of particle production in Au+Au collisions at sNN = 200GeV Page: 4 of 6
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" A+A
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of Nbin [15].
c13
0
0
W5
CE0.3-
0.2
0.10
0
A Ks
s2 h+h
Hydro calculations . -
- --- .
- P
A... _ . . . _ .... . _ ._ . _ .. . _ ' 12
4
Transverse Momentum PT (GeV/c)
FIG. 1: (color online). The minimum-bias (0 80% of the col-
lision cross section) v2 (PT) for Ks, A + A and h'. The error
bars shown include statistical and point-to-point systematic
uncertainties from the background. The additional non-flow
systematic uncertainties are approximately -20%. Hydrody-
namical calculations of v2 for pions, kaons, protons and lamb-
das are also plotted [10].
Fig. 1 shows minimum-bias v2 for K3, A + A and
charged hadrons (h ). The analysis method used to ob-
tain the charged hadron v2 is described in Ref. [7]. Fig. 1
also shows hydrodynamic model calculations of v2 for pi-
ons, kaons, protons, and lambdas [10]. At low PT, v2
is consistent with hydrodynamical calculations, in agree-
ment with the previous results at sNN 130 GeV [9].
This Letter establishes the particle-type dependence of
the v2 saturation at intermediate PT. In contrast to hy-
drodynamical calculations, where at a given PT, heav-
ier particles have smaller v2 values, at intermediate PT,
VA > vz'. The PT scale where v2 deviates from the hy-
drodynamical prediction is ~ 2.5 GeV/c for A + A and
~ 1 GeV/c for K3.0.3
0 0.2
0.1
00 2 4 0 2 4 0 2 4
Transverse Momentum PT (GeV/c)
FIG. 2: (color online). The V2 of Ks and A + A as a func-
tion of PT for 30 70%, 5 30% and 0 5% of the collision cross
section. The error bars represent statistical errors only. The
non-flow systematic errors for the 30 70%, 5 30% and 0 5%
centralities are -25%, -20% and -80% respectively.
Fig. 2 shows v2 of KS and A + X for three centrality
intervals: 30 70%, 5 30%, and 0 5% of the geometricalcross section. In each centrality bin, v2(PT) rises at low
PT and saturates at intermediate PT. The values of v2 at
saturation are particle-type and centrality dependent.
If partons that fragment into (anti-)lambdas lose more
energy than those that fragment into kaons, a particle-
type dependence for v2 may develop at high PT with
vz > o%. In this case, A + A yields should be more sup-
pressed than kaon yields. Fig. 3 shows RCP for KS, K ,
and A + A using the 5% most central collisions, normal-
ized by peripheral collisions (40-60% and 60-80%). For
charged hadrons, these peripheral bins approximately fol-
low Nbi~ scaling without medium modification [15]. The
bands in Fig. 3 show the expected values of RCP for
binary and participant (Npart) scaling including system-
atic variations from the calculation [15]. For most of the
intermediate PT region, RCP for A + X is similar to ex-
pectations of Nbi~ scaling and RP < RCP. The PT
scales associated with the saturation and reduction of
RCP also depend on the particle type. For both species,
the PT where RCP begins to decrease approximately co-
incides to the PT where v2 in Fig. 1 saturates. At high PT
(PT > 5.0 GeV/c), RCP values for Ks and A+A are con-
sistent with the value for charged hadron RCP, indicating
that the baryon enhancement observed at intermediate
PT in central Au+Au collisions ends at PT ~ 5 GeV/c.
The particle-type dependence of v2 and RCP at interme-
diate PT are in contradiction to expectations from energy
loss followed by fragmentation in vacuum.
Nuclear modifications such as shadowing and initial-
state rescattering [22, 23] may affect RCP but they are
not expected to give rise to such a large variation with
particle-type (e.g. [24]). At lower beam energy, the en-
hancement of yields in p+A collisions at intermediate
PT (i.e. the Cronin effect [25]) is larger for baryons
than mesons [22]. The Cronin effect has been attributed
to initial-state rescattering, and is expected to decrease
with increasing beam energy [23]. Alternatively, a strong
particle-type dependence of the Cronin effect may indi-
cate a nuclear modification to the parton fragmentation.
Although the effects of shadowing, initial-state rescat-
tering and non-flow deserve further investigation, the
particle-type and PT dependence of v2 and RCP may re-
veal a cross-over from a PT region dominated by bulk
partonic matter hadronization to one dominated by sin-
gle parton fragmentation. Our measurements indicate
that the cross-over would occur at PT ~ 4 5 GeV/c.
The larger A + A RCP at intermediate PT shows that
the A + A yield increases with parton density faster than
the kaon yield. Multi-parton mechanisms such as gluon
junctions [26], quark coalescence [2], or recombination [3]
can naturally lead to a stronger dependence on parton
density for baryon production than meson production.
Models using coalescence or recombination mechanisms
in particle production predict that at intermediate PT
v2 will follow a number-of-constituent-quark scaling [2].
Fig. 4 shows v2 of KS and A+X as a function of PT, whereAKs *A+A
-A A
----------------
(a) 30 -70% (b) 5-30 % (c0-5 .
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Adams, J.; Adler, C.; Aggarwal, M. M.; Ahammed, Z.; Amonett, J.; Anderson, B. D. et al. Particle-type dependence of azimuthal anisotropy and nuclearmodification of particle production in Au+Au collisions at sNN = 200GeV, article, June 18, 2003; United States. (https://digital.library.unt.edu/ark:/67531/metadc883843/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.