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star focus:
Azimuthal harmonics in small and large collision systems at RHIC top energies
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The STAR Collaboration has recently published, Azimuthal Harmonics in Small
and Large Collision Systems at RHIC Top Energies,
in Physical
Review Letter, 122, 172301(2019).
This publication reports and compares recent integral and differential
measurements (obtained with the STAR TPC), of the flow harmonics ($v_n, n=1-3$)
for charged hadrons produced in U+U collisions ($\sqrt{s_{{NN}}}$ = 193 GeV) and
Au+Au, Cu+Au, Cu+Cu, d+Au, and p+Au collisions ($\sqrt{s_{{NN}}}$ = 200 GeV).
The measurements for these disparate collision-systems, allow systematic
variations of the initial-state eccentricity and its fluctuations, as well as
the size of the produced fireball at approximately the same collision
energy. All are expected to influence the magnitude of $v_n$. The comparisons
between the measurements for these small, medium and large collision systems,
give unique insights into the role of final-state interactions as the
collision-system size is varied. Similarly, the measurements provide more
discerning constraints which can aid extraction of the temperature-dependent
specific shear viscosity of the hot and dense media created in the
collisions.
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Two-particle azimuthal correlation functions (a-f) and four-particle
cumulants (g) for pT-integrated track pairs. Results are shown for U+U (a)
collisions (193 GeV) and Au+Au (b), Cu+Au (c), Cu+Cu (d), d+Au (e) and p+Au (f)
collisions (200 GeV) for the same charged particle multiplicity $N_{ch}$. Panel
(g) shows the four-particle second-order cumulant vs. $N_{ch}$, obtained with
the three sub-events method from the same data sets.
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The measurements exploit both the two- and multi-particle correlation
techniques to extract $v_n$ as a function of the transverse momentum ($p_T$) and
mean charged particle multiplicity $N_{ch}$. The first figure shows
representative correlation functions (a-f) and four particle cumulants (g) which
accentuate the qualitative similarity between the charged particle azimuthal
distributions obtained for the full range of collision-system sizes. Further
quantitative study reveals that for a fixed value of $N_{ch}$, the
$v^{even}_{1}$ and $v_{3}$ coefficients are essentially independent of the
colliding species, indicating that for a given $N_{ch}$, the fluctuation-driven
initial-state eccentricities, $\varepsilon_{1}$ and $\varepsilon_{3}$, are
system independent. By contrast, the $v_{2}$ coefficients indicate sizeable
variations [for fixed $N_{ch}$] with the colliding species, showing the strong
collision-system dependence of the shape-driven eccentricity
$\varepsilon_{2}$.
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$v_{2}/\varepsilon_{2}$
vs. $N_{ch}^{-1/3}$ for U+U, Au+Au, Cu+Au, Cu+Cu, d+Au and p+Au collisions as
indicated. The open boxes indicate systematic uncertainties. The dotted line
represents an exponential fit to the data. The inset shows the respective ratios
of the slopes extracted for each system relative to the slope extracted from a
fit to the combined data sets (Slope = 0.82 $\pm$ 0.02).
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The second figure shows the $N_{ch}$ dependence of the eccentricity-scaled
second harmonic $v_{2}/\varepsilon_{2}$, for the respective collision
systems. Since such patterns reflect the influence of viscous attenuation in
the hot and dense media created in the collisions, they suggest a
collision-system-independent viscous attenuation coefficient. That is, the
implied specific shear viscosity coefficient is independent of the
collision-system and consequently, the initial-state eccentricity
spectrum. Note that this pattern of viscous attenuation suggests a strong
dependence on the dimensionless size of the produced media, where $N_{ch}$ is
proportional to this size.
The detailed comparisons of the reported measurements highlight the
sensitivity of vn to the magnitude of the initial-state eccentricity, system
size and the final-state interactions in the expanding matter. Thus, they lend
essential insight into the role of final-state interactions in the system
studied and provide significant constraints for model calculations with an eye
toward extraction of the temperature-dependent specific shear viscosity.
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Posted May 31, 2018
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