QCD challenges from pp to AA collisions: 4th edition

Abstract

This paper is a write-up of the ideas that were presented, developed and discussed at the fourth International Workshop on QCD Challenges from pp to AA, which took place in February 2023 in Padua, Italy. The goal of the workshop was to focus on some of the open questions in the field of high-energy heavy-ion physics and to stimulate the formulation of concrete suggestions for making progresses on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.

Fig. 1

Comparisons of current- and previous-generation gluon nuclear modification factors $R_{\text{pPb}}^g$ in ${}^{208}\text{Pb}$ from the (left) EPPS [20, 25] and (right) nNNPDF [19, 26] collaborations. The shaded regions show the $68\%\text{CL}$ uncertainties. In both cases, the fit results shown in gray do not use LHCb ${\mathrm{D}}^0$ data [24], while the results shown in color do use these data

Fig. 2

$\text{J}/\psi$ photoproduction cross sections for $\gamma$Pb (left) from ALICE [49] and CMS [50] measured using nuclear breakup categories to unambiguously determine the energy dependence and for $\gamma$p (right) from HERA and LHCb measurements versus NLO predictions [57]

Fig. 3

A compilation of recent jet substructure measurements indicating a narrowing of the jet core in heavy-ion collisions. From left-to-right the figures have been extracted from Refs. [–83]

Fig. 4

Left: fraction of core and corona as a function of multiplicity from pp to PbPb. Middle and right: integrated $\mathrm{\Omega }/\pi$ yield ratio (middle) and average transverse momentum (right) as a function of charged-particle multiplicity density at midrapidity from pp to PbPb compared to EPOS 4.0.0 predictions from various parts of the collision system [121]. The “co+co” (core+corona) curve is an interpolation between the core and corona cases, with the core weight increasing continuously with multiplicity. The “full” case is equal to the co+co case with in addition the inclusion of hadron rescattering

Fig. 5

Pearson correlation coefficient $\rho (c_2\{2\},[p_T])$ as a function of multiplicity as measured by ATLAS [134] and CMS [135]

Fig. 6

Left: Baryon-to-meson ratios measured by ALICE [108], with approximate indications of the values measured at LEP (DELPHI [177]) superimposed. Right: proton-$\mathrm{\Lambda }$ correlations in azimuth [178]

Fig. 7

Charm hadron ratios vs. $p_{\text{T}}$, data in comparison to models [14] (Catania: coalescence; TAMU, GSI-Hd: SHM, TAMU with enhanced charm-baryon spectrum, GSI-Hd with the PDG one)

Fig. 8

Left: charm hadron yields in central $\text{PbPb}$ collisions, data [14] and SHM predictions. Note the scale factor of 30 for the $\text{J}/\psi$ meson. Right: The difference between the $v_2$ values of ${\mathrm{\Lambda }}_{\text{c}}^{+}$ and ${\mathrm{D}}^0$ as a function of $p_{\text{T}}$as predicted by the Catania coalescence model and POWLANG

Fig. 9

Multiplicity dependence of the cross section ratio of ${\chi }_{c1}(3872)$ and $\psi$ meson (left) and of the $T_{\mathit{cc}}^{+}$ measured yield [192] (right)

Fig. 10

Compilation of muon measurements converted to the abstract z-scale for the hadronic interaction model EPOS-LHC. The z-scale is given by $z=(ln(N_{\mu })-ln(N_{\mu ,p}))/(ln(N_{\mu },\text{Fe})-ln(N_{\mu ,p}))$, where $N_{\mu }$ is the measured muon content and $N_{\mu ,p}$ and $N_{\mu ,\text{Fe}}$ are predictions for proton and iron CRs, respectively. The energy scales of the experimental datasets shown here have been cross-calibrated as described in Ref. [256]. The expected value $z_{\text{mass}}$ based on the cosmic-ray mass composition estimated by the GSF model [257] is subtracted. Also shown are $z_{\text{mass}}$ values computed from $X_{\text{max}}$ measurements by the Pierre Auger Observatory (grey band)