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STAR focus: Observation of an
Energy-Dependent Difference in Elliptic Flow between Particles and Antiparticles in
Relativistic Heavy Ion Collisions
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We present elliptic flow (v2) measurements for identified
particles at mid-rapidity in Au+Au collisions, measured by the STAR
experiment in the Beam Energy Scan at RHIC at √sNN= 7.7-62.4
GeV. A beam-energy dependent difference of the values of v2
between particles and corresponding anti-particles was observed. The
difference increases with decreasing beam energy and is larger for baryons
compared to mesons. This implies that, at lower energies, particles and
anti-particles are not consistent with the universal
number-of-constituent-quark (NCQ) scaling of v2 - one of the
evidences in support of the sQGP formation at √sNN = 200
GeV. The results were recently published
in Phys.
Rev. Lett. 110, 142301 (2013).
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Left figure: The difference in v2
between particles X and their corresponding
anti-particles X
(see legend) as a function of √sNN for 0-80% central
Au+Au collisions. The dashed lines in the plot are fits with a
power-law function. The error bars depict the combined statistical
and systematic errors. The difference in the v2 values
for particles and anti-particles increases with decreasing beam
energy. The energy dependence in shape and magnitude is similar
for all baryons. Baryons show a larger difference compared to
mesons. |
The
v2(mT-m0) and possible NCQ scaling
were investigated for particles and anti-particles separately, as
shown in the lower figure. The baryons and mesons are clearly
separated for √sNN = 62.4 GeV at
(mT-m0) > 1 GeV/c2. While the
effect is present for particles at √sNN = 11.5 GeV, no
such separation is observed for the anti-particles at this energy in
the measured (mT-m0) range up to 2 GeV/c2
. The lower panels of the figure depict the difference of the
baryon v2 relative to a fit to the meson v2
data with the pions excluded from the fit. The anti-particles at
√sNN = 11.5 GeV show a smaller difference compared to the
particles. |
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In summary, the first observation of a
beam-energy dependent difference in v2(pT) between particles and
corresponding anti-particles for minimum bias √sNN = 7.7-62.4 GeV Au+Au
collisions at mid-rapidity is reported. The difference increases with decreasing beam
energy. It is apparent that, at the lower energies, particles and anti-particles are no
longer consistent with the single NCQ scaling that was observed for √sNN = 200
GeV. However, for the group of particles the NCQ scaling holds within
±10% while for the group of anti-particles the
difference between baryon and meson v2 continues to decrease to lower
energies.
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Posted Apr. 16, 2013
Previous STAR Focus Features
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community links
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collaboration links
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STAR Analysis Meeting, July 16 - 19, 2013, Purdue
STAR Regional Meeting (Heavy Quark & Jets) June 13 - 15th, 2013, Prague
STAR Collaboration Meeting, Feb. 24 - Mar. 1, 2013, BNL
STAR Analysis and Upgrade Meeting, Nov. 28 - Dec. 2, 2012, UCLA
STAR Regional Meeting: Heavy Quark Production and Correlations Nov. 5 - 7, 2012, Warsaw
STAR Collaboration Meeting, August 5-10, 2012, BNL
STAR Upgrade Workshop, June 11-12, 2012, BNL
STAR Topical Workshop, May 23-25, 2012, Rice U.
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recent news
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May 6, 2013
Congratulations Jie Zhao, from Shanghai Institute of Applied Physics,
Chinese Academy of Sciences, who has completed his PhD thesis on:
"Di-electron Production from 200GeV Au+Au Collisions at RHIC".
May 6, 2013
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