Gluon Quasi Particles and the CGC Density Matrix

TL;DR

This paper extends the calculation of the density matrix and entanglement entropy in the Color Glass Condensate framework by including saturation effects, revealing a quasi-particle basis with a Boltzmann distribution and identifying an intermediate momentum regime.

Contribution

It introduces leading saturation corrections into the density matrix calculation, showing it is diagonal in a quasi-particle basis with a Boltzmann form, and characterizes the semi-hard momentum regime in the CGC.

Findings

Density matrix is diagonal in the quasi-particle basis.

Quasi particles behave as massless 2D bosons with a specific temperature.

Identification of an intermediate momentum regime between hard and soft scales.

Abstract

We revisit and extend the calculation of the density matrix and entanglement entropy of a Color Glass Condensate by including the leading saturation corrections in the calculation. We show that the density matrix is diagonal in the quasi particle basis, where it has the Boltzmann form. The quasi particles in a wide interval of momenta behave as massless two-dimensional bosons with the temperature proportional to the typical semi-hard scale $T=Q_s/\sqrt{\alpha_sN_c}$. Thus the semi-hard momentum region $Q_s<k<Q_s/\sqrt{\alpha_sN_c}$ arises as a well-defined intermediate regime between the perturbatively hard momenta and the nonperturbative soft momenta $k<Q_s$ in the CGC description of a hadronic wave function.