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.