Quark Gluon Plasma an Color Glass Condensate at RHIC? The perspective from the BRAHMS experiment

TL;DR

This paper surveys BRAHMS results from RHIC to evaluate evidence for a quark-gluon plasma and a Color Glass Condensate initial state. It shows the system achieves high energy density and exhibits chemical and, importantly, partonic-level thermal-like equilibration with strong collective flow, consistent with a deconfined medium. It also highlights jet-quenching-like suppression of high-pT hadrons in central Au+Au and a contrasting lack of suppression in d+Au at midrapidity, while forward-rapidity data suggest CGC-like initial-state effects. Taken together, the findings strongly favor a dense, partonic medium with potential CGC initial conditions, though not all classic QGP signatures are unambiguously observed, underscoring the need for further falsifiable tests and theory.

Abstract

We review the main results obtained by the BRAHMS collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so called quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e. the proposed Color Glass Condensate.

Figures (18)

• Figure 1: Rapidity density of net protons (i.e. number of protons minus number of antiprotons) measured at AGS, SPS, and RHIC (BRAHMS) for central collisions. At RHIC, where the beam rapidity is $y=5.4$, the full distribution cannot be measured with current experiments, but BRAHMS will be able to extend its unique results to y=3.5 from the most recent high statistics Au+Au run, corresponding to measurements extending to 2.3 degrees with respect to the beam direction.
• Figure 2: Insert: two possible net-baryon distributions (Gaussian in $p_T$ and 6'th order polynomial) respecting baryon number conservation. In going from net-proton to net-baryon distributions we have assumed that $N(n)\approx N(p)$ and have scaled hyperon yields known at midrapidity to forward rapidity using HIJING. Even assuming that all missing baryons are located just beyond the acceptance edge or at the beam rapidity, quite tight limits on the rapidity loss of colliding Au ions at RHIC can be set (main panel).
• Figure 3: Pseudorapidity densities (multiplicities) of charged particles measured by BRAHMS for $\sqrt{s_{NN}}=\unit[200] GeV$ Au+Au collisions for various centralities. The integral of the most central distribution $0-5\%$ corresponds to about 4600 charged particles BRAHMSmult.
• Figure 4: Multiplicity of charged particles per participant pair around midrapidity, as a function of $\sqrt{s_{NN}}$. The figure shows that the particle production in Au+Au collisions at the RHIC top energy , around $\eta=0$, exceeds that seen in p+p collisions by 40-50%.
• Figure 5: Top panel: rapidity density distribution for positive and negative pions, kaons and protons measured by Brahms. The shown data have not been corrected for feed down. The lines show Gaussian fits to the measured distributions. Bottom panel: average $m_T$ distributions as a function of rapidity. From DjamelPhD.