Systematics of parton-medium interaction from RHIC to LHC

TL;DR

This paper discusses how the flatter parton spectra at LHC energies enable better discrimination between different parton-medium interaction models, compared to RHIC energies where spectra are too steep.

Contribution

It highlights the importance of combined RHIC and LHC data to distinguish between competing models of parton energy loss in heavy-ion collisions.

Findings

LHC data probes deeper into parton-medium interaction models.

Spectral shape differences at LHC improve model discrimination.

Energy loss models are mainly constrained by pathlength dependence at RHIC.

Abstract

Despite a wealth of experimental data for high-P_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. A key reason is that the pQCD parton spectrum at RHIC is falling so steeply that distinguishing even a moderate shift in parton energy from complete parton absorption is essentially impossible. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and, as a result, at RHIC energies only the pathlength dependence of energy loss offers some discriminating power. At LHC however, this is no longer the case: Due to the much flatter shape of the parton p_T spectra originating from 2.76 AGeV collisions, the available data probe much deeper into the model dynamics. A simultaneous fit of the nuclear suppression at both RHIC and LHC energies thus has great potential for discriminating between various models that yield equally good descriptions of RHIC data alone.