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

Abstract

We present new results on 2-particle azimuthal (Δ ϕ) correlation relative to event plane and 3-particle pseudorapidity (Δ η) correlation at mid-rapidity in Au+Au collisions at s N N = 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 Δ η-Δ η correlations. We found that like-sign triplets are dominated by ridge. The separated ridge, while narrow in Δ ϕ, is extremely broad in Δ η. The ridge particles are not only uncorrelated to the trigger particle in Δ η, but also uncorrelated between themselves.