NA7-Hf-QGP: Quark-Gluon Plasma characterisation with heavy flavour probes

Heavy flavor (HF) quarks (charm and beauty) have been identified in the last years as a reliable mean to study the properties of the Quark Gluon Plasma (QGP), which is created in ultra-relativistic heavy ion collisions at the LHC. The reasons are the following: heavy quarks are produced in hard processes with very short formation time. They hence experience the early phase of the QGP and the full evolution of the system. Additionally, the elementary production processes can be calculated reliably. The comparison of experimental measurements with the results of transport calculations provides information on the transport coefficients of the QGP and on the degree of thermalization of heavy quarks in the system. Heavy quarks with large transverse momentum, which do not come to equilibrium, are unique probes to study the parton energy loss mechanism in the QGP, and its dependence on the parton type and ass (gluon vs. quark and light vs. heavy quarks, respectively). The perspective to characterize with high precision the QGP properties with heavy flavor observables has motivated the High Luminosity Heavy Ion Phase (HL-HI) of the LHC (starting with run-3 in 2021-2023, first ion run in November 2021), after the 2nd long shutdown of the LHC facility (LS2 in 2019-2020), and the detector upgrade of the dedicated heavy-ion experiment (ALICE). The other major LHC experiments (ATLAS, CMS and LHCb) will also provide excellent and complementary measurements (in term of kinematic domains), thus providing an exhaustive set of precise new measurements. Before the HL-HI phase, still a major advance in this field is expected thanks to the harvest of data from the 2nd and last Pb-Pb data-taking of the LHC run-2 (November 2018). The joined effort of experimentalists and theorists is indispensable to assess our knowledge, approve or refine chosen direction and define experimental priorities and the best analyzing strategies for the upcoming high precision data.

On the theoretical side several approaches have been proposed which describe the few presently available open HF data within the still large experimental uncertainties. They are based on quite different assumptions on the non-perturbative interaction of heavy quarks with the QGP. The challenge for theory is to provide predictions for more differential observables, including various kinds of heavy-flavour correlations, and to address the issue of possible collective effects for heavy quarks in high-multiplicity collisions of small systems, like proton–proton and proton–nucleus. High precision data, additional observables as well as a careful analysis of the open heavy flavor production in pp and pA reactions are necessary to derive the fundamental properties (transport coefficients) of the QGP. Objective of the present proposal is a systematic comparison of these different approaches in order to discriminate between them and to eliminate uncertainties like details of the expansion of the QGP, initial state fluctuations or the influence of nuclear shadowing, which influence the light hadron observables as well and can therefore be constrained independently. To this aim we will develop a new interface between the experimental data and the results of the theoretical models. Presently in Europe five theory groups have developed event by event generators including heavy quarks: Frankfurt (Greiner), Catania (Greco), GSI (Bratkovskaya), FIAS (Bleicher), Nantes (Aichelin, Gossiaux, Werner)). This interface will allow us imposing on the theoretical simulations the same acceptance cuts and event constraints (e.g. multiplicity) as on the experimental data. Such an approach will reduce the systematic uncertainties and improve reliability when comparing theory with experiment, thus allowing for a systematic interpretation of the open heavy flavor data in particular in the prospect of the coming high precision data.

The second main objective is to advance the understanding of hidden charm and bottom mesons in ultra-relativistic heavy ion reactions. The description of hidden HF production is even more challenging and significant experimental and theoretical advances are required. The open questions include: up to which temperature can the different quarkonium states survive in a QGP and how do these color neutral objects interact with the QGP? How important is recombination, that a c(b) and a cbar (bbar) originating from different hard parton-parton interaction form later a hidden charm (bottom) meson? How this recombination can be described in an expanding medium? How do quarkonia interact with the expanding gas of hadrons? How constraining is the precise knowledge of the total charm (beauty) production crosssection? What are other experimental observables to constraint the models? One of the main difficulties in this sector is the multi-disciplinary aspect of the problems. The competences of different communities, which often speak a different language, are required. In order to understand how hidden HF mesons are formed in an expanding QGP, people from lattice gauge groups have to work together with people who have developed transport approaches and with those who are more familiar with the vacuum physics of hidden heavy mesons. In Europe, the different competences of small university groups need to be gathered within a strong and efficient collaboration to address efficiently the current challenges. These groups necessitate of dedicated funding to form interdisciplinary collaborations. Two workshops will be organized to bring the exports from the different communities together to discuss, plan and develop a common multi-disciplinary approach to the problem. The participation of the young postdocs of the network will be mandatory for carrying the knowledge from one laboratory to another, for seeding future collaborations and fostering further progress.