Long-range transverse momentum correlations and radial flow in Pb$-$Pb collisions at the LHC with ALICE

This Letter presents measurements of long-range transverse-momentum correlations using a new observable, $v_{0}(p_\mathrm{T})$, serving as a probe of event-by-event radial-flow fluctuations, the underlying radial expansion, and the medium properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low $p_\mathrm{T}$, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher $p_\mathrm{T}$ ($> 3$ GeV/$c$), protons exhibit larger $v_{0}(p_\mathrm{T})$ than pions and kaons, in agreement with expectations from quark-recombination models. Comparisons to viscous hydrodynamic calculations with varying bulk viscosity and equation of state demonstrate the sensitivity of the $v_{0}(p_\mathrm{T})$ observable to these key medium properties. The findings establish $v_{0}(p_\mathrm{T})$ as a valuable addition to the set of observables used in Bayesian analyses for extracting the transport properties and constraining the equation of state of strongly interacting matter, while also helping to systematically explore its sensitivity and impact within such global studies.

💡 Research Summary

In this Letter the ALICE Collaboration reports the first differential measurement of long‑range transverse‑momentum correlations in Pb–Pb collisions at √sNN = 5.02 TeV using a newly introduced observable, v₀(pT). The observable is defined as the normalized covariance between the event‑by‑event multiplicity fraction in a given pT bin and the mean pT measured in a pseudorapidity window separated by a gap Δη = 0.4, thereby suppressing short‑range non‑flow contributions while retaining correlations that arise from the collective expansion of the medium.

The analysis uses about 80 million minimum‑bias Pb–Pb events recorded in 2018. Charged tracks are reconstructed in the central barrel (|η| < 0.8, 0.2 < pT < 10 GeV/c) with the ITS, TPC and TOF detectors. Particle identification for pions, kaons and protons is performed with a Bayesian combination of dE/dx and time‑of‑flight information, achieving purities above 95 % over the full pT range. Systematic uncertainties are evaluated by varying event‑selection criteria, centrality definitions, track quality cuts, the size of the η‑gap, and PID thresholds; statistical uncertainties are obtained via bootstrap resampling.

The measured v₀(pT) for inclusive charged particles shows a characteristic sign change: it is negative at low pT (pT ≲ 0.8 GeV/c), rises approximately linearly up to ≈ 4 GeV/c, and then deviates from the linear trend. The slope of the linear region grows from central to peripheral collisions, reflecting larger event‑by‑event mean‑pT fluctuations in smaller systems. At high pT (> 4 GeV/c) the increase slows down in central and semi‑central events, while peripheral collisions retain a more monotonic rise, indicating an increasing contribution from hard processes.

Identified‑particle results reveal a clear mass ordering for pT < 3 GeV/c (π < K < p), consistent with hydrodynamic radial flow. Above 3 GeV/c protons exhibit a larger v₀(pT) than pions and kaons, a pattern reminiscent of the baryon‑meson splitting observed in anisotropic flow coefficients and naturally explained by quark‑recombination models. The separation between protons and mesons is strongest in central collisions and diminishes toward peripheral events.

The data are compared to two model frameworks. The IP‑Glasma + MUSIC + UrQMD calculation, which incorporates temperature‑dependent shear (η/s) and bulk (ζ/s) viscosities and a lattice‑QCD based equation of state, reproduces the measurements up to pT ≈ 2 GeV/c across all centralities. Beyond this region the model underestimates v₀(pT), reflecting the growing importance of jet‑medium interactions and non‑hydrodynamic effects that are not fully captured. The HIJING model, lacking collective flow, fails to describe the low‑pT region but qualitatively matches peripheral data at higher pT, underscoring the dominance of hard scattering in small, low‑density systems.

When v₀(pT) is scaled by its pT‑integrated value v₀ = σ