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

Abstract

We present new results on 2-particle azimuthal ($\Delta\phi$) correlation relative to event plane and 3-particle pseudorapidity ($\Delta\eta$) 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 $\Delta\eta$-$\Delta\eta$ correlations. We found that like-sign triplets are dominated by ridge. The separated ridge, while narrow in $\Delta\phi$, is extremely broad in $\Delta\eta$. The ridge particles are not only uncorrelated to the trigger particle in $\Delta\eta$, but also uncorrelated between themselves.