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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
Congratulations Hongwei Ke, from Central China Normal University, who has
completed his PhD thesis on: "Beam Energy, Collision Centrality and Charge
Asymmetry Dependence of the Pion Event Anisotropy in Au+Au Collisions at
RHIC".
May 6, 2013
Congratulations Yuhui Zhu, from Shanghai Institute of Applied Physics,
Chinese Academy of Sciences, who has completed her PhD thesis on: "Study
on Hypertriton Production and Lifetime Measurement at RHIC STAR".
May 6, 2013
Congratulations Lizhu Chen, from Central China Normal University, who has
completed her PhD thesis on: "The Sixth Order Cumulant of Net-proton
Multiplicity Distributions at STAR".
May 2, 2013
Congratulations Elizabeth Oldag, from University of Texas at Austin, who
has completed her PhD thesis on: "Two-Particle Correlations in Angular and
Momentum Space in Heavy Ion Collisions at STAR".
May 2, 2013
Congratulations Alan Davila, from University of Texas at Austin, who has
completed his PhD thesis on: "Jet characterization in Au + Au collisions
at STAR".
May 2, 2013
Congratulations Masayuki Wada, from University of Texas at Austin, who has
completed his PhD thesis on: "Resonance Particles in Heavy-Ion
Collisions".
April 18, 2013
Congratulations Xin Li, from Purdue University, who has completed his PhD
thesis on: "Non-photonic electron production in p + p and Au
+ Au collisions".
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