Goldstone theory of gluon saturation in high energy QCD: From color glass to color superfluid condensate

Abstract

In the high energy Regge asymptotics of QCD, the rapid overpopulation of gluons leads to gluon saturation, characterized by nonperturbative semi-classical lumpy configurations of size $1/Q_S$, where $Q_S\gg Λ_{\rm QCD}$ is a dynamical semi-hard saturation scale. We discuss here a generalization of the Color Glass Condensate (CGC) framework describing this phenomenon that is obtained by carefully treating the breaking of global symmetries by the condensate. In particular, we show that the breaking of a global subgroup of color results in a Goldstone effective field theory (EFT) whose dynamics is characterized by a Goldstone decay constant $f_G = \sqrt{N} Q_S$, where $N=O(1/α_S)$ is the phase space occupancy of the condensate, which is large for $α_S(Q_S)\ll 1$. Further, the coupling of these colored Goldstones to collinear gluon modes leads to a depletion of the condensate, thereby modifying the static shockwave scattering framework of the CGC on timescales controlled by $f_G$. The CGC EFT on these longer time scales is a novel color superfluid. Low frequency modes with $ω\leq α_S Q_S$ in an external probe of the CGC are sensitive to superfluid dynamics, with interesting physical consequences for deeply inelastic scattering and ultrarelativistic nuclear collisions.