Gluon spectrum in the glasma from JIMWLK evolution

TL;DR

This paper investigates how small-x gluon evolution affects the initial gluon content and spectrum in high-energy heavy-ion collisions by solving the JIMWLK equation with a daughter-dipole running coupling, starting from MV initial conditions. The evolved Wilson lines serve as inputs to a full CYM-based calculation of glasma gluon production, revealing a geometrical scaling region and a harder gluon spectrum as energy increases. The mean transverse momentum relative to the adjoint saturation scale grows (⟨p_T⟩/𝑄̃ₛ ≈ 1.5), and the total multiplicity increases slightly faster than 𝑄̃ₛ², with running coupling moderating evolution speed and UV sensitivity. The results highlight the importance of nonlinear evolution in shaping the initial glasma state and point toward future refinements, including continuum extrapolations and higher-order corrections.

Abstract

The JIMWLK equation with a "daughter dipole" running coupling is solved numerically starting from an initial condition given by the McLerran-Venugopalan model. The resulting Wilson line configurations are then used to compute the spectrum of gluons comprising the glasma inital state of a high energy heavy ion collision. The development of a geometrical scaling region makes the spectrum of produced gluons harder. Thus the ratio of the mean gluon transverse momentum to the saturation scale grows with energy. Also the total gluon multiplicity increases with energy slightly faster than the saturation scale squared.

Figures (8)

• Figure 1: Wilson line correlator (\ref{['eq:defcorr']}) in coordinate space; in lattice units (above) and as a function of the scaling variable $r Q_\mathrm{s}$ (below).
• Figure 2: Wilson line correlator (\ref{['eq:defcorr']}) (dipole cross section) in momentum space; as s function of $k_T L$ (above) and of the scaling variable $k/Q_\mathrm{s}$ (below).
• Figure 3: The evolution speed $\lambda=\, \mathrm{d} \ln Q_\mathrm{s}^2(x)/\, \mathrm{d} \ln 1/x$ as a function of the saturation scale $Q_\mathrm{s}$. The parameter values corresponding to the labels are detailed in Table \ref{['tab:params']}.
• Figure 4: Gluon spectrum at different energies, labeled by the rapidity interval of evolution starting from the MV initial condition at $y=0$. The momentum is scaled by the saturation scale $Q_\mathrm{s}$ corresponding to the rapidity in question.
• Figure 5: Gluon liberation coefficient as a function of collision energy, parametrized by the rapidity interval of evolution from the initial scale.