Baseline measures for net-proton distributions in high energy heavy-ion collisions

We report a systematic comparison of the recently measured cumulants of the net-proton distributions for 0-5% central Au+Au collisions in the first phase of the Beam Energy Scan (BES) Program at the Relativistic Heavy Collider facility to various kinds of possible baseline measures. These baseline measures correspond to assuming that the proton and anti-proton distributions, follow Poisson statistics, Binomial statistics, obtained from a transport model calculation and from a hadron resonance gas model. The higher order cumulant net-proton data corresponding to the center of mass energies ($sqrt{s_{NN}}$) of 19.6 and 27 GeV are observed to deviate from all the baseline measures studied. The deviations are predominantly due to the difference in shape of the proton distributions between data and those obtained in the baseline measures. We also present a detailed study on the relevance of the independent production approach as a baseline for comparison with the measurements at various beam energies. Our studies points to the need for a proper comparison of the experimental measurements to QCD calculations in order to extract the exact physics process that leads to deviation of the data from the baselines presented.

Three-particle coincidence of the long range pseudorapidity correlation in high energy nucleus-nucleus collisions

We report the first three-particle coincidence measurement in pseudorapidity ($Deltaeta$) between a high transverse momentum ($p_{perp}$) trigger particle and two lower $p_{perp}$ associated particles within azimuth $mid$$Deltaphi$$mid$$<$0.7 in $sqrt{{it s}_{NN}}$ = 200 GeV $d$+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jet-like component and the ridge (long-range $Deltaeta$ correlation). The results indicate that the particles from the ridge are uncorrelated in $Deltaeta$ not only with the trigger particle but also between themselves event-by-event. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jet-like component.

Addressing the physics of the ridge by 2- and 3-particle correlations at STAR

We present new results on 2-particle azimuthal ($Deltaphi$) correlation relative to event plane and 3-particle pseudorapidity ($Deltaeta$) correlation at mid-rapidity in Au+Au collisions at $sqrt{{it s}_{NN}}$ = 200 GeV, measured by the STAR experiment. While jet-like correlation is symmetric, ridge is found to be asymmetric when trigger particle azimuth is between in- and out-of-plane. The charge ordering properties between associated and trigger particles are exploited to separate jet-like and ridge contributions in 3-particle $Deltaeta$-$Deltaeta$ correlations. We found that like-sign triplets are dominated by ridge. The separated ridge, while narrow in $Deltaphi$, is extremely broad in $Deltaeta$. The ridge particles are not only uncorrelated to the trigger particle in $Deltaeta$, but also uncorrelated between themselves.

Identifying the underlying physics of the ridge via 3-particle $Deltaeta-Deltaeta$ correlations

We present the first results on 3-particle $Deltaeta$-$Deltaeta$ correlations in minimum bias $d$+Au, peripheral and central Au+Au collisions at $sqrt{{it s}_{NN}}$ = 200 GeV measured by the STAR experiment. The analysis technique is described in detail. The ridge particles, observed at large $Deltaeta$ in dihadron correlations in central Au+Au collisions, appear to be uniformly distributed over the measured $Deltaeta$-$Deltaeta$ region in 3-particle correlation. The results, together with theoretical models, should help further our understanding of the underlying physics of the ridge.