Future physics opportunities for high-density QCD at the LHC with heavy-ion and proton beams

TL;DR

The report outlines a comprehensive HL/HE-LHC blueprint to study high-density QCD with ions and protons, centering on four goals: macroscopic QGP properties, microscopic parton dynamics, a unified picture of particle production across system sizes, and nuclear parton densities with saturation searches. It details experimental strategies across Pb–Pb, p–Pb, and pp, including high-luminosity runs, high-multiplicity pp studies, and lighter ion runs, with emphasis on hard and soft probes, heavy flavor, quarkonia, light nuclei and anti-nuclei, as well as fluctuations of conserved charges. It highlights the potential to constrain transport coefficients, test non-linear QCD at small x, and connect collider observables to neutron-star physics via hyperon interactions and hypernuclei. The document also provides performance projections for detector upgrades and data-taking schedules, and discusses implications for astrophysics and dark matter, as well as cosmic-ray physics. Collectively, the work aims to deepen our microscopic and macroscopic understanding of QCD matter under extreme conditions and to inform future facility designs and theoretical frameworks.

Abstract

The future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented. Four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucleus) systems, the exploration of parton densities in nuclei in a broad ($x$, $Q^2$) kinematic range and the search for the possible onset of parton saturation. In order to address these scientific goals, high-luminosity Pb-Pb and p-Pb programmes are considered as priorities for Runs 3 and 4, complemented by high-multiplicity studies in pp collisions and a short run with oxygen ions. High-luminosity runs with intermediate-mass nuclei, for example Ar or Kr, are considered as an appealing case for extending the heavy-ion programme at the LHC beyond Run 4. The potential of the High-Energy LHC to probe QCD matter with newly-available observables, at twice larger center-of-mass energies than the LHC, is investigated.

Figures (72)

• Figure 1: Top: comparison of predictions for the coalescence parameters for (hyper-)nuclei with $\mathrm{A}$ = 3, 4 from the Blast-Wave + GSI-Heidelberg thermal statistical model and nucleon coalescence as a function of the radius ($R$) of the particle emitting source. For each (hyper-)nucleus, the radius $r$ considered by the coalescence model is reported in the legend. For $^{3}_{\Lambda}\mathrm{H}$ ($^{4}_{\Lambda}\mathrm{H}$), two values of the radius are considered: the lower value represents the average separation of the three (four) constituents, whereas the larger $r$ corresponds to the average separation between the $\Lambda$ and the deuteron (triton) core. See Bellini:2018epz for full details on the models. Middle: projection of the relative statistical uncertainty achievable with a minimum bias $\mathrm{Pb}$--$\mathrm{Pb}$ integrated luminosity of $L_{\text{int}}$ = 10 $\mathrm{nb}^{-1}$ and the upgraded ALICE detector (in red) compared to the relative statistical uncertainty of the Run 2 measurements (in black). Bottom: significance in the discrimination between the two models, assuming 10$\%$ and 20$\%$ systematic uncertainty in addition to the statistical uncertainty expected with $L_{\text{int}}$ = 10 $\mathrm{nb}^{-1}$. For $^{3}_{\Lambda}\mathrm{H}$, the coalescence prediction considered is for $r = 6.8$ fm (corresponding to the black continuous lines in the top panel). Figure from Ref. ALICE-PUBLIC-2019-001.
• Figure 2: Left: (raw) yield of anti-nuclei in the $2 <p\sb{\mathrm{T}}\Xspace< 10~\mathrm{GeV}/ c$ interval, detectable in 0--10$\%$ central $\mathrm{Pb}$--$\mathrm{Pb}$ collisions with the ALICE detector as a function of the minimum bias luminosity. Right: Projected significance of anti-hyper-nuclei measurements in central $\mathrm{Pb}$--$\mathrm{Pb}$ collisions in Runs 3 and 4 with ALICE as a function of the integrated minimum-bias luminosity. In both panels, the arrow represents the minimum bias $\mathrm{Pb}$--$\mathrm{Pb}$ luminosity anticipated for the end of Run 2. The dashed vertical line marks the projections with $L_{\text{int}}$ = 10 $\mathrm{nb}^{-1}$. The bands represents the uncertainty on model prediction for the yield (see text for details). Figures from Ref. ALICE-PUBLIC-2019-001.
• Figure 3: Expected p$\Xi^-$ + $\bar{\mathrm{p}}\Xi^+$ correlation for $\mathrm{pp}$ collisions at $\sqrt{s} = 5.5$ TeV and $4\times 10^{11}$ minimum bias events, corresponding to $L_{\text{int}}$ = 6 $\mathrm{pb}^{-1}$. Only statistical errors have been estimated. Figure from Ref. ALICE-PUBLIC-2019-001.
• Figure 4: Ratio of sixth to second-order baryon number susceptibilities from lQCD. The left-hand figure is from Bazavov:2017dus. The right-hand figure is calculated from recent lQCD data on sixth and second order susceptibilities from Borsanyi:2018grb.
• Figure 5: $\kappa_{4}/\kappa_{2}$ and $\kappa_{6}/\kappa_{2}$ as calculated within PQM Almasi:2017bhq model (open symbols). After taking into account contributions from participant nucleon fluctuations and global baryon number conservation Braun-Munzinger:2016yjzBraun-Munzinger:2018yru, the deviations from unity decrease (closed symbols).