Parton energy loss in cold QCD matter – COLDLOSS

Quarks and gluons (named partons) traversing a QCD medium – either quark-gluon plasma but also confined, cold nuclear matter – are expected to lose energy dominantly through gluon radiation, leading to the phenomenon of jet quenching observed in high-energy heavy ion collisions. Understanding quantitatively parton energy loss is thus a central question of this field, which the present proposal addresses. More specifically, the objective of the project is a systematic investigation of parton energy loss processes in cold nuclear matter. This ambitious program will be carried out through detailed and complementary theoretical, phenomenological, and experimental studies, with measurements performed in hadron-nucleus collisions from fixed-target facilities (COMPASS at the CERN Super Proton Synchrotron, SPS) to colliders (CMS at the Large Hadron Collider, LHC). The synergy between theory, phenomenology and experiment is at the heart of this proposal.

The effects of energy loss in the fully coherent regime on various QCD processes will be investigated, such as forward production of light hadrons, heavy hadrons (D and B mesons) and Drell-Yan lepton pairs, in pi-A and p-A collisions at different center-of-mass energies. Another important task is the development of a unified framework enabling the rigorous treatment of parton energy loss in the two most relevant dynamical regimes, namely the Landau-Pomeranchuk-Migdal (partially coherent) regime and the fully coherent regime. In addition, both radiative energy loss processes and parton saturation – expected in nuclei at small values of Bjorken-x and an important topic in the phenomenology of high-energy nuclear collisions – will also be implemented in a unified approach based on first principles. Resulting from this theoretical work, detailed phenomenology of QCD processes in hadron-nucleus collisions will allow for the interpretation of the present data and for providing reliable predictions on future measurements. Closely related to radiative energy loss, the transverse momentum broadening of partons propagating through cold QCD matter will also be investigated, at both theoretical and phenomenological levels. On the experimental side, the measurements of Drell-Yan lepton pairs and quarkonium production will be performed in pion-nucleus collisions at SPS energy by the COMPASS experiment. At the LHC, Drell-Yan, quarkonia and heavy mesons will be measured by the CMS collaboration, as well as forward dijet production. The measurements performed at both SPS and LHC, together with past measurements at Fermilab (FNAL) and Brookhaven (RHIC), will allow for the extraction of the cold nuclear matter transport coefficient at different energies and using different processes. In fine, the phenomenological and experimental results will clarify the contributions of radiative energy loss (in both LPM and fully coherent regimes) on the production of hard processes in nuclear collisions.

On top of these research studies, an important aspect of the proposal is that of education of young researchers. This project will allow for the organization of two international high-level schools on QCD, in 2019 and 2021, gathering PhD students and young researchers together with world-class lecturers. The proposal will also lead to outreach activities through the publication of brochures on particle physics and astrophysics, intended to a general audience, as well as for the organization of conferences in high-schools and during public events.