Speaker
Prashanth Shanmuganathan
(Kent State University, USA)
Description
The Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider aims to study the QCD phase diagram in regions where net baryon density is large, a region where a critical point may exist. Possible signatures of a softening of the QCD equation of state have been reported at BES energies, and directed flow (rapidity-odd $v_1(y)$) is one of the more striking examples in this category. This talk will focus on $v_1$, and its slope $dv_1/dy$, near midrapidity. Ten identified particle types will be presented: $\Lambda$, $\bar\Lambda$, $\phi$, $K^0_s$, $K^\pm$, $p$, $\bar{p}$, and $\pi^\pm$ produced in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27 and 39 GeV as a function of rapidity. For $\Lambda$, $K^0_s$, $K^\pm$, $p$ and $\pi^\pm$, all results are presented in the centrality bins $0-5\%$, $5-10\%$, $10-20\%$, $20-30\%$, $30-40\%$, $40-50\%$, $50-60\%$, $60-70\%$ and $70-80\%$.
At intermediate centrality (10-40\%), $dv_1/dy$ shows a minimum near 14.5 GeV for several particle species while $p$ and $\Lambda$ also show a sign change. The $\phi$ meson appears to follow $\bar{p}$ at the higher BES energies, while protons, kaons and pions have a lower magnitude than $\bar{p}$ at 14.5 GeV and above. $\Lambda$ and $p$ are consistent within errors at all energies. $dv_1/dy$ shows a strong centrality dependence, especially for $p$ and $\Lambda$ at the lower beam energies. Results for net kaons and net protons will also be discussed.
The UrQMD model shows a qualitatively similar trend for only a subset of the ten particle types under investigation. This comprehensive set of directed flow measurements for several identified baryons, antibaryons and mesons, spanning BES energies and all centralities, offers a powerful constraint on model calculations.
| On behalf of collaboration: | STAR |
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Primary author
Prashanth Shanmuganathan
(Kent State University, USA)
