STAR experiment results from the beam energy scan program at RHIC

TL;DR

This paper reports initial findings from the STAR detector during RHIC's Beam Energy Scan, exploring the QCD phase diagram at various baryonic chemical potentials through Au ion collisions at different energies.

Contribution

It presents new experimental results on particle flow, correlations, and fluctuations at different energies, extending understanding of the QCD phase diagram.

Findings

Observation of differences in elliptic flow between particles and antiparticles.

Disappearance of azimuthal correlation differences at certain energies.

Deviation of fluctuation measurements from theoretical expectations.

Abstract

We present the first results using the STAR detector from the Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC). In this program, Au ion collisions at center of mass energies (\sqrt{s_{NN}}) of 7.7, 11.5 and 39 GeV allowed RHIC to extend the study of the QCD phase diagram from baryonic chemical potential values of 20 MeV to about 400 MeV. For the high net-baryon density matter at midrapidity, formed in these collisions, we report several interesting measurements. These include the observation of difference between anti-particle and particle elliptic flow, disappearance of the difference in dynamical azimuthal correlations with respect to event plane between same and opposite signed charged particles, change in slope of eccentricity at freeze-out and directed flow of protons as a function of \sqrt{s_{NN}} and the deviation of higher order fluctuations from hadron resonance gas and Poissonian expectations. Possible interpretations of these observations are also discussed.