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

TL;DR

This paper investigates the ridge phenomenon in heavy-ion collisions by analyzing 2- and 3-particle correlations, revealing that the ridge is asymmetric, charge-dependent, and uncorrelated in pseudorapidity, providing new insights into its physics.

Contribution

It introduces a novel analysis of charge ordering and correlation properties to distinguish jet-like and ridge components in heavy-ion collisions.

Findings

Ridge is asymmetric relative to the event plane.

Like-sign triplets are dominated by ridge.

Ridge particles are uncorrelated in pseudorapidity and among themselves.

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.