Transport of hard probes in the glasma

Abstract

The quark-gluon plasma (QGP) formed in relativistic heavy-ion collisions at RHIC and the LHC thermalizes from an out-of-equilibrium state of strong classical gluon fields known as the glasma. Jets and heavy quarks, produced in the initial scatterings, traverse these pre-equilibrium fields, but the glasma stage is neglected in phenomenology, which may bias the extracted QGP transport coefficients. This thesis studies its impact on jet and heavy quark transport. The glasma is described in the Color Glass Condensate framework by solving the classical Yang-Mills equations on a real-time lattice. The fields initially form longitudinal electric and magnetic flux tubes whose transverse correlation length is set by the saturation scale $Q_s$. Probes are studied with both classical transport equations and quantum evolution in the light-front Hamiltonian approach. The transport coefficients for jets and heavy quarks are much larger than in the QGP and anisotropic between transverse and longitudinal directions. The azimuthal decorrelation of back-to-back $c\bar{c}$ pairs in the glasma is comparable to that from the QGP, making $D\bar{D}$ angular correlations a potentially measurable glasma signature. The glasma also modifies the nuclear modification factor $R_{AA}$, though the effect is moderate compared to the nuclear parton distribution contribution. A systematic gauge-invariance analysis shows that the distinction between kinetic and canonical momenta is essential when comparing classical and quantum results, and that Coulomb gauge fixing largely reduces gauge artifacts. Finally, the eikonal quantum evolution of a high-energy quark is studied with time-dependent Basis Light-Front Quantization. The resulting momentum broadening agrees with the classical estimates and lays the groundwork for including quantum radiation.