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STAR focus: Hunting for the QCD Critical Point

Highlights from the STAR paper: Energy Dependence of Moments of Net-proton Multiplicity Distributions at RHIC.


In the phase diagram of Quantum Chromodynamics (QCD–theory of strong interactions), it is conjectured on the basis of theoretical calculations that there will be a critical point (CP) at high temperature and non-zero baryonic chemical potential region. Experimental confirmation of the QCD Critical Point will be an excellent test of QCD theory in the non-perturbative region and the milestone of exploring the QCD phase diagram. This is one of the main goals of the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Due to the high sensitivity to the correlation length of dynamical systems and directly connected to the susceptibility in the Lattice QCD, higher moments of net-proton distributions have been applied to search for the QCD Critical Point in the heavy-ion collision experiment at STAR. The results were recently published in
Phys. Rev. Lett. 112, 032302 (2014).

In this article, we report the beam energy (√s = 7.7 - 200 GeV) and collision centrality dependence of the mean (M), standard deviation (σ), skewness (S), and kurtosis (κ) of the net-proton multiplicity distributions in Au + Au collisions. The measurements are carried out by the STAR experiment at mid-rapidity (|y|< 0.5) in the first phase of the BES at RHIC.


Left Fig. The products of moments (Sσ and κσ2) in 0-5% Au+Au collisions are found to have values significantly below the Skellam expectation and have largest significance of deviations (the difference between data and Skellam baseline divided by the errors) for 19.6 and 27 GeV, with values of 3.2 and 3.4 for κσ2 and 4.5 and 5.6 for Sσ, respectively. These observables are not reproduced by either a non-CP transport model or by a hadron resonance model. The measurements are reasonably described by assuming independent production of proton and anti-protons, indicating that there are no apparent correlations between the protons and anti-protons for the observable presented. However at the lower beam energies, the net-proton measurements are dominated by the shape of the proton distributions only.


In summary, measurements of the higher moments of the net-proton distributions in Au+Au collisions over a wide range of μB have been presented to search for a possible CP. For √s > 39 GeV, Sσ and κσ2 values are similar for central, peripheral Au+Au collisions and p+p collisions. Deviations for both Sσ and κσ2 from HRG and Skellam expectations are observed for √s < 27 GeV. Higher statistics data for √s < 19.6 GeV will help to quantitatively understand the energy dependence of Sσ and κσ2. The conclusions on the existence of CP can be made only after comparison to QCD calculations with CP behavior which include the dynamics associated with heavy-ion collisions.

Posted Jan. 25, 2014

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January 22, 2014
Congratulations to Witek Borowski, from the University of Nantes, who has received the university approval for his Ph.D thesis on: "Open Charm in heavy ion and p+p collisions with STAR at RHIC”

December 18, 2013
Congratulations to Brian Page, from Indiana University, who has defended his Ph.D thesis on: "The Di-jet Cross Section and Double-Spin Asymmetry at Mid-Rapidity from Polarized p+p Collisions at √s = 200 GeV at RHIC".

December 17, 2013
Congratulations to John Novak, from Michigan State University, who has defended his Ph.D thesis on: "Energy Dependence of Fluctuation and Correlation Observables of Transverse Momentum in Heavy-Ion Collisions".

December 16, 2013
Congratulations to Chris Anson, from Ohio State University, who has defended his Ph.D thesis on: "Energy-dependent Hanbury Brown Twiss interferometry and the freeze-out eccentricity of heavy ion collisions at STAR".

December 9, 2013
Congratulations to Nihar Sahoo, from VECC at Kolkata, who has defended his Ph.D thesis on: "Probing the QCD Critical Point by higher moments of net-charge distributions at RHIC energies".


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