The production of prompt D⁰, D⁺, and D^*+ mesons was measured at midrapidity (|y| < 0.5) in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair $$ \sqrt{s_{\mathrm{NN}}} $$s NN = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0–10%) and semicentral (30–50%) collisions. The measurement was performed up to a transverse momentum (p_T) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb–Pb collisions at the LHC, the yield of D⁰ mesons was measured down to p_T = 0, which allowed a model-independent determination of the p_T-integrated yield per unit of rapidity (dN/dy). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor (R_AA) of prompt D mesons at p_T = 6–8 GeV/c for the 0–10% and 30–50% centrality classes, respectively. The D-meson RAA is compared with that of charged pions, charged hadrons, and J/ψ mesons as well as with theoretical predictions. The analysis of the agreement between the measured R_AA, elliptic (v₂) and triangular (v₃) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient D_s. Furthermore the comparison of R_AA and v₂ with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms.

A Large Ion Collider Experiment (ALICE) has been conceived and constructed as a heavy-ion experiment at the LHC. During LHC Runs 1 and 2, it has produced a wide range of physics results using all collision systems available at the LHC. In order to best exploit new physics opportunities opening up with the upgraded LHC and new detector technologies, the experiment has undergone a major upgrade during the LHC Long Shutdown 2 (2019–2022). This comprises the move to continuous readout, the complete overhaul of core detectors, as well as a new online event processing farm with a redesigned online-offline software framework. These improvements will allow to record Pb-Pb collisions at rates up to 50 kHz, while ensuring sensitivity for signals without a triggerable signature.

The first measurement of the production of pions, kaons, (anti-)protons and $$\phi $$ϕ mesons at midrapidity in Xe–Xe collisions at $$\sqrt{s_{\mathrm{NN}}} = 5.44~\text {TeV}$$sNN=5.44TeV is presented. Transverse momentum ( $$p_{\mathrm{T}}$$pT ) spectra and $$p_{\mathrm{T}}$$pT -integrated yields are extracted in several centrality intervals bridging from p–Pb to mid-central Pb–Pb collisions in terms of final-state multiplicity. The study of Xe–Xe and Pb–Pb collisions allows systems at similar charged-particle multiplicities but with different initial geometrical eccentricities to be investigated. A detailed comparison of the spectral shapes in the two systems reveals an opposite behaviour for radial and elliptic flow. In particular, this study shows that the radial flow does not depend on the colliding system when compared at similar charged-particle multiplicity. In terms of hadron chemistry, the previously observed smooth evolution of particle ratios with multiplicity from small to large collision systems is also found to hold in Xe–Xe. In addition, our results confirm that two remarkable features of particle production at LHC energies are also valid in the collision of medium-sized nuclei: the lower proton-to-pion ratio with respect to the thermal model expectations and the increase of the $$\phi $$ϕ -to-pion ratio with increasing final-state multiplicity.

This paper is a write-up of the ideas that were presented, developed and discussed at the third International Workshop on QCD Challenges from pp to A–A, which took place in August 2019 in Lund, Sweden (Workshop link:https://indico.lucas.lu.se/event/1214/). The goal of the workshop was to focus on some of the open questions in the field and try to come up with concrete suggestions for how to make progress on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.

These proceedings summarise the first measurements of anisotropic flow coefficients $v_{\mathrm{n}}$ , $2\le \mathrm{n}\le 4$ , for inclusive charged particles at mid-rapidity in Xe–Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44\mathrm{TeV}$ . The results are compared with those from Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ , in order to test the initial state (IS) models and transport properties. The resulting differences in $v_2$ and $v_3$ between the two systems are consistent with two different hydrodynamical models. Moreover, it is expected that the ratios between $v_{\mathrm{n}}$ and their corresponding eccentricities for $\mathrm{n}=2,3$ scale with transverse density. This is observed for some IS models, except for some deviations in central collisions. These results assist in constraining the initial state as well as the hydrodynamical propagation of the system.