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Abstract 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. 

The production of the $\psi \left(2S\right)$charmonium state was measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity ( $2.5<y<4$). The measurement of the ratio of the inclusive production cross sections of the $\psi \left(2S\right)$and $\mathrm{J}/\psi$resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region $p_T<12\mathrm{GeV}/c$. The results are compared with the corresponding measurements in $pp$collisions, by forming the double ratio $[{\sigma }^{\psi \left(2S\right)}/{\sigma }^{J/\psi }{]}_{\mathrm{Pb}\text{−}\mathrm{Pb}}/[{\sigma }^{\psi \left(2S\right)}/{\sigma }^{J/\psi }{]}_{pp}$. It is found that in Pb-Pb collisions the $\psi \left(2S\right)$is suppressed by a factor of $\sim 2$with respect to the $J/\psi$. The $\psi \left(2S\right)$nuclear modification factor $R_{\mathrm{AA}}$was also obtained as a function of both centrality and $p_T$. The results show that the $\psi \left(2S\right)$resonance yield is strongly suppressed in Pb-Pb collisions, by a factor of up to $\sim 3$with respect to $pp$. Comparisons of cross section ratios with previous Super Proton Synchrotron findings by the NA50 experiment and of $R_{\mathrm{AA}}$with higher- $p_T$results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC. © 2024 CERN, for the ALICE Collaboration2024CERN 

Abstract The interactions of kaons (K) and antikaons ($$\mathrm {\overline{K}}$$ $\overline{K}$) with few nucleons (N) were studied so far using kaonic atom data and measurements of kaon production and interaction yields in nuclei. Some details of the three-body KNN and$$\mathrm {\overline{K}}$$ $\overline{K}$NN dynamics are still not well understood, mainly due to the overlap with multi-nucleon interactions in nuclei. An alternative method to probe the dynamics of three-body systems with kaons is to study the final state interaction within triplet of particles emitted in pp collisions at the Large Hadron Collider, which are free from effects due to the presence of bound nucleons. This Letter reports the first femtoscopic study of p–p–K$$^+$$ ${}^{+}$and p–p–K$$^-$$ ${}^{-}$correlations measured in high-multiplicity pp collisions at$$\sqrt{s}$$ $\sqrt{s}$= 13 TeV by the ALICE Collaboration. The analysis shows that the measured p–p–K$$^+$$ ${}^{+}$and p–p–K$$^-$$ ${}^{-}$correlation functions can be interpreted in terms of pairwise interactions in the triplets, indicating that the dynamics of such systems is dominated by the two-body interactions without significant contributions from three-body effects or bound states. 

Abstract The elliptic flow$$(v_2)$$ $\left(v_2\right)$of$${\textrm{D}}^{0}$$ ${\text{D}}^0$mesons from beauty-hadron decays (non-prompt$${\textrm{D}}^{0})$$ ${\text{D}}^0)$was measured in midcentral (30–50%) Pb–Pb collisions at a centre-of-mass energy per nucleon pair$$\sqrt{s_{\textrm{NN}}} = 5.02$$ $\sqrt{s_{\text{NN}}}=5.02$ TeV with the ALICE detector at the LHC. The$${\textrm{D}}^{0}$$ ${\text{D}}^0$mesons were reconstructed at midrapidity$$(|y|<0.8)$$ $\left(\right|y|<0.8)$from their hadronic decay$$\mathrm {D^0 \rightarrow K^-\uppi ^+}$$ $D^0\to K^{-}{\pi }^{+}$, in the transverse momentum interval$$2< p_{\textrm{T}} < 12$$ $2<p_{\text{T}}<12$ GeV/c. The result indicates a positive$$v_2$$ $v_2$for non-prompt$${{\textrm{D}}^{0}}$$ ${\text{D}}^0$mesons with a significance of 2.7$$\sigma $$ $\sigma$. The non-prompt$${{\textrm{D}}^{0}}$$ ${\text{D}}^0$-meson$$v_2$$ $v_2$is lower than that of prompt non-strange D mesons with 3.2$$\sigma $$ $\sigma$significance in$$2< p_\textrm{T} < 8~\textrm{GeV}/c$$ $2<p_{\text{T}}<8\text{GeV}/c$, and compatible with the$$v_2$$ $v_2$of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties. 

A<sc>bstract</sc> The cross section for coherent photonuclear production of J/ψis presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of$$ \sqrt{{\textrm{s}}_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV. Cross sections are presented in five different J/ψrapidity ranges within |y| < 4, with the J/ψreconstructed via its dilepton decay channels. In some events the J/ψis not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 <W_γPb,n< 920 GeV, whereW_γPb,nis the centre-of-mass energy per nucleon of theγPb system. This range corresponds to a Bjorken-xinterval spanning about three orders of magnitude: 1.1 × 10⁻⁵<x< 3.3 × 10⁻². In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.