Glasma flux tubes and the near side ridge phenomenon at RHIC

TL;DR

The paper addresses the origin of the near-side ridge observed at RHIC by proposing that boost-invariant Glasma flux tubes, formed in the initial Color Glass Condensate interaction, generate long-range rapidity correlations that survive into the later quark-gluon plasma. By computing the leading classical two-particle correlation in the CGC/MV framework, the authors show a boost-invariant, flux-tube–driven contribution to $C(p_\perp,q_\perp)$ that scales as $\sim \frac{1}{S_\perp Q_s^2}$ and relates directly to the inclusive spectra via $C \approx \frac{\kappa}{S_\perp Q_s^2}\left\langle \frac{dN}{dy_p d^2{\boldsymbol p}_\perp}\right\rangle\left\langle \frac{dN}{dy_q d^2{\boldsymbol q}_\perp}\right\rangle$, with $\kappa$ order unity. When coupled to strong radial flow, these correlations become azimuthally collimated, producing a ridge whose amplitude scales as $A \sim \frac{1}{\alpha_s(Q_s)}(\gamma_B-\gamma_B^{-1})$, and whose centrality dependence follows from the growth of the saturation scale $Q_s$ with system size. The results provide a coherent link between early-time Glasma dynamics and final-state collective expansion, offering a natural explanation for the STAR ridge and guiding quantitative comparisons with data.

Abstract

We investigate the consequences of long range rapidity correlations in the Glasma. Particles produced locally in the transverse plane are correlated by approximately boost invariant flux tubes of longitudinal color electric and magnetic fields that are formed when two sheets of Colored Glass Condensate pass through one another, each acquiring a modified color charge density in the collision. We argue that such long range rapidity correlations persist during the evolution of the Quark Gluon Plasma formed later in the collision. When combined with transverse flow, these correlations reproduce many of the features of the recently observed ridge events in heavy ion collisions at RHIC.

Figures (10)

• Figure 1: The red and green cones are the location of the events in causal relationship with the particles $A$ and $B$ respectively. Their intersection is the location in space-time of the events that may correlate the particles $A$ and $B$.
• Figure 2: (a) Top Figure: The ridge as seen in the measurement of two particle correlations with minimal cut on particle momenta. Mixed events have been subtracted, as have been the effects of azimuthally asymmetric flow. The centrality bin here is 19-28% b) Bottom Figure: The height of the ridge as a function of the number of binary collisions per participant. Both figures are preliminary STAR figures from Ref. Daugherity-QM2008
• Figure 3: The width in rapidity and azimuthal angle of the ridge as a function of the number of binary collisions per participant.
• Figure 4: Glasma flux tubes. The transverse size of the flux tubes is of order $1/Q_s$.
• Figure 5: Top Figure: A classical diagram which yields a non-vanishing two particle correlation after averaging over the color sources. Bottom Figure: A contribution to the correlation function associated with a quantum correction to the classical field.