The Glasma and the Hard Ridge

TL;DR

This paper investigates the origin of the ridge phenomenon in particle correlations observed in high-energy collisions, focusing on the role of Glasma initial conditions and their extension to transverse momentum dependence.

Contribution

It extends previous work by analyzing how the ridge structure depends on the transverse momentum of particle pairs and quantifies the soft contribution to the hard ridge.

Findings

Glasma initial conditions naturally produce long-range rapidity correlations.

The ridge structure's dependence on transverse momentum is characterized.

The soft contribution to the hard ridge is quantitatively determined.

Abstract

Correlation measurements indicate that excess two particle correlations extend over causally disconnected rapidity ranges. Although, this enhancement is broad in relative rapidity $\eta=\eta_1 - \eta_2$, it is focused in a narrow region in relative azimuthal angle $\phi=\phi_1 - \phi_2$. The resulting structure looks like a ridge centered at $\eta = \phi=0$. Similar ridge structures are observed in correlations of particles associated with a jet trigger (the hard ridge) and in correlations without a trigger (the soft ridge). The long range rapidity behavior requires that the correlation originates in the earliest stage of the collision, and probes properties of the production mechanism. Glasma initial conditions as predicted by the theory of Color Glass Condensate and provide a and early stage correlation that naturally extends far in rapidity. We have previously shown that the soft ridge is a consequence of particles forming from an initial Glasma phase that experience a later stage transverse flow. We extend this work to study the ridge dependence on the $p_t$ of the correlated pairs. We then determine the soft contribution to the hard ridge.