Medium-induced radiation with vacuum propagation in the pre-hydrodynamics phase

TL;DR

This paper extends the BDMPS-Z framework to include medium-induced radiation during the pre-hydrodynamics phase, revealing significant effects on jet quenching observables by modeling vacuum propagation before QGP formation.

Contribution

It introduces a generalized model for medium-induced radiation that accounts for vacuum propagation in the early stages of heavy-ion collisions, highlighting its impact on jet quenching observables.

Findings

Replacing initial medium with vacuum reduces low-energy gluon emission.

High-energy gluon emission remains largely unaffected by initial medium replacement.

Considering vacuum propagation increases the azimuthal asymmetry $v_2$ in high-$p_T$ particles.

Abstract

The recent discovery of the potential of jet quenching observables to constrain the initial stages after a heavy-ion collision makes imperative to have a better understanding of the process of medium-induced radiation before the formation of the quark-gluon plasma (QGP) and its impact on observables at high-$p_T$. In this work, we generalize the BDMPS-Z framework for medium-induced radiation to account for additional emissions occurring before the creation of the QGP. For simplicity, we assume that during the pre-hydrodynamics phase the hard parton propagates as in vacuum. This set-up, allows us to isolate the contribution from the additional initial radiation by comparing with the usual scenarios in which the emitter is created inside the medium but with different starting points. Using both a numerical implementation of the fully resummed emission spectrum and the usual analytical approximations, we find that replacing an initial slab of the medium by vacuum yields to a significant reduction of the emission spectrum for low radiated gluon energies, while the high-energy tails remain largely unmodified. Finally, we assess the effect of replacing the initial medium by vacuum propagation on the single-inclusive particle suppression $R_{AA}$ and high-$p_T$ azimuthal asymmetry $v_2$. Our findings indicate that considering vacuum propagation prior to hydrodynamization leads to an increase in the $v_2$, thus corroborating the importance of the treatment of jet quenching in the initial stages for the correct description of both observables.