SN0493 : Experimental Study of the QCD Phase Diagram & Search for the Critical Point: Selected Arguments for the Run-10 Beam Energy Scan

Author(s):The STAR Collaboration (B. I. Abelev et al.)
Date:Jun. 4, 2009
File(s): BES-Short-v8.3.pdf

We present an overview of the main ideas that have emerged from discussions within STAR for the Beam Energy Scan (BES).  The formulation of this concise and abridged document is facilitated by the existence of a much longer and more comprehensive companion document (Ref [1]).  The compelling arguments and motivations for the physics of our proposed Beam Energy Scan program, which have a particular role in guiding the run plan as set out in our discussion of Tables 1 and 2 of this document, are (not in order of priority):

A.   A search for turn-off of new phenomena already established at higher RHIC energies; QGP signatures are the most obvious example, but we define this category more broadly.  If our current understanding of RHIC physics and these signatures is correct, a turn-off must be observed in several signatures, and such corroboration is an essential part of the “unfinished business” of QGP discovery.  The particular observables that STAR has identified as the essential drivers of our run plan are:         

(A-1)  Constituent-quark-number scaling of v2 , indicating partonic degrees of freedom;

(A-2)  Hadron suppression in central collisions as characterized by the ratio RCP ;

(A-3)  Untriggered pair correlations in the space of pair separation in azimuth and pseudorapidity, which elucidate the ridge phenomenon;

(A-4)  Local parity violation in strong interactions, an emerging and important RHIC discovery in its own right, is generally believed to require deconfinement, and thus also is expected to turn-off at lower energies. 

B.   A search for signatures of a phase transition and a critical point.  The particular observables that we have identified as the essential drivers of our run plan are:

(B-1)  Elliptic & directed flow for charged particles and for identified protons and pions, which have been identified by many theorists as highly promising indicators of a “softest point” in the nuclear equation of state;

(B-2)  Azimuthally-sensitive femtoscopy, which adds to the standard HBT observables by allowing the tilt angle of the ellipsoid-like particle source in coordinate space to be measured; these measurements hold promise for identifying a softest point, and complements the momentum-space information revealed by flow measurements, and

(B-3) Fluctuation measures, indicated by large jumps in the baryon, charge and strangeness susceptibilities, as a function of system temperature – the most obvious expected manifestation of critical phenomena.

Keywords:Beam Energy Scan, Critical Point, QCD Phase Diagram, QGP Onset