Dynamical correlations across momentum scales in the Quark-Gluon Plasma

TL;DR

The work addresses how Quark-Gluon-Plasma dynamics couple across momentum scales, connecting soft collective flow and hard jet quenching through the $p_T$-differential radial-flow observable $v_{0}(p_T)$. It introduces a spectrum-decomposition framework with soft, mid, and hard components and analyzes event-by-event fluctuations via the reference-aligned covariance $V_{0}$, using the factorization ratio $r(p_{T1},p_{T2})$ and the eigenvalue ratio $\lambda_{2}/\lambda_{1}$ to quantify the number and coherence of active fluctuation modes. A three-component model reproduces the observed rise–plateau–downturn of $v_{0}(p_T)$ and reveals how soft coherence gives way to multi-mode dynamics through mid and hard sector fluctuations, with kaon-specific decorrelations highlighted. This mode-based approach provides a practical, experimentally accessible way to map soft–mid–hard correlations and to constrain the collective-to-partonic transition in heavy-ion collisions, with potential cross-system and energy-system insights.

Abstract

Experimental probes of the Quark-Gluon Plasma (QGP) generated in heavy-ion collisions span a broad range in momentum scale: low transverse momentum (low $p_T$) measurements probe collective dynamics, while high $p_T$ measurements probe the response to QGP excitation by jets (jet quenching). However, the dynamical interplay between QGP collective dynamics and jet quenching is currently poorly understood. We present a new framework for exploring dynamical correlations across momentum scales in heavy-ion collisions, based on the $p_T$-differential radial-flow observable $v_0(p_T)$. Measured $v_0(p_T)$ phenomenology is traced to the evolution in strength and coherence of distinct underlying fluctuation modes. We then propose new experimental observables to quantify this evolution. The eigenvalue ratio $λ_2/λ_1$ of the reference-aligned covariance matrix $V_0$ is shown to measure the effective fluctuation rank, while the $p_T$-dependence of the corresponding eigenvectors maps the evolution from a single coherent soft mode to multi-mode dynamics including coalescence and jet quenching. These observables map the soft-mid-hard correlation structure and provide a unified description of the collective-to-partonic transition in the QGP.

Figures (4)

• Figure 1: Three-stage construction of $v_{0}$($p_{\rm T}$) for $\pi^{\pm}$, $K^{\pm}$, and $p/\bar{p}$ in 30--40% Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. (a) Soft-only fluctuations (solid) and soft fluctuations with a static mid-$p_{\rm T}$ component (dashed). (b) Soft+mid fluctuations (solid), augmented by a static hard tail (dashed). (c) Soft+mid+hard fluctuations with slope modulation (solid) or yield modulation (dashed) of the hard component. Shaded bands denote statistical uncertainties from bootstrap resampling. ALICE data from ALICE:2025iud.
• Figure 1: Inclusive transverse-momentum spectra of $\pi^{\pm}$, $K^{\pm}$, and $p/\bar{p}$ in 30--40% Pb--Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV compared with the three-component model. Solid curves show the total spectra, while others indicate the soft, mid-$p_T$ (coalescence-like), and hard (fragmentation-like) components.
• Figure 2: Diagnostics of the reference-aligned covariance matrix $V_0(p_{\rm T1},p_{\rm T2})$ for pions. Left panels: two-bin factorization ratio $r(p_{\rm T1},p_{\rm T2})$ for (a) soft-only, (b) soft+mid, and (c) soft+mid+hard (slope-modulated) configurations. Right panels: corresponding normalized leading and subleading eigenvectors $u_{1}(p_{\rm T})$ and $u_{2}(p_{\rm T})$, together with the subleading-to-leading eigenvalue ratio $\lambda_2/\lambda_1$.
• Figure 3: Subleading-to-leading eigenvalue ratio $\lambda_2/\lambda_1$ for $\pi^\pm$, $K^\pm$, and $p/\bar{p}$ for different sets of fluctuation modes.