Evidence for the collective nature of radial flow in Pb+Pb collisions with the ATLAS detector

Abstract

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system's radial expansion, has received less attention. Notably, experimental evidence for the global and collective nature of radial flow has been lacking. This Letter presents the first measurement of transverse momentum ($p_{\mathrm{T}}$) dependence of radial flow fluctuations ($v_0(p_{\mathrm{T}})$) over $0.5<p_{\mathrm{T}}<10$ GeV, using a two-particle correlation method in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in $p_{\mathrm{T}}$, and centrality-independent shape in $p_{\mathrm{T}}$. The comparison with a hydrodynamic model demonstrates the sensitivity of $v_0(p_{\mathrm{T}})$ to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.

Figures (11)

• Figure 1: Schematic illustration of how radial flow fluctuations lead to correlations between the EbE $\pT$-differential yield $n(\pT)$ and the EbE average transverse momentum $[\pT]$. The blue curve represents an event with larger-than-average radial flow, the red curve an event with smaller-than-average radial flow, and the black curve the ensemble-averaged spectrum. Events with larger radial flow have flatter spectra, resulting in a positive covariance ($\left\langle \delta n(\pT)\delta[\pT]\right\rangle>0$) at $\pT\gtrsim \left\langle [\pT]\right\rangle$ and a negative covariance ($\left\langle \delta n(\pT)\delta[\pT]\right\rangle<0$) at $\pT\lesssim \left\langle [\pT]\right\rangle$Parida:2024ckk. The zero-crossing point of $v_0(\pT)$, $\pT \approx \langle[\pT] \rangle$, depends on the $\pT$ range used for measurement, which is 0.5--10 GeV in this analysis, but does not depend on $p_{\mathrm{T}}^{\mathrm{ref}}$ (see Ref. Bhatta:2025oyp).
• Figure 2: Centrality dependence of $v_0$ (a) and $v_0/v_0^{0-5\%}$ (b) for three $p_{\mathrm{T}}^{\mathrm{ref}}$ ranges, and normalized covariance from Eq. \ref{['eq:1']} (c) and $v_0(\pT)$ (d) obtained in 0--5% centrality (top) and 50--60% centrality (bottom) in three $p_{\mathrm{T}}^{\mathrm{ref}}$ ranges. The color lines indicate the predictions from the $\textsc{Hijing}$ model containing only non-flow correlations. Bars and shaded areas indicate statistical and total uncertainties, respectively.
• Figure 3: $v_0(\pT)$ for $\eta_{\mathrm{gap}}$=0, 1, 2, and 3 in 0--5% (a) and 60--70% (b) centrality. Bars and shaded areas indicate statistical and total uncertainties, respectively.
• Figure 4: The $v_0(\pT)$ (a) and $v_0(\pT)/v_0$ (b) for different centrality ranges. Bars and shaded areas represent statistical and total uncertainties, respectively.
• Figure 5: The $v_0(\pT)/v_0$ as a function of $\pT$ (a) and $\pT/\bar{p}_{\mathrm{T}}$ (b) in 0--5% centrality range, where $\bar{p}_{\mathrm{T}}$ is the mean $\pT$ in 0.5--10 GeV. Bars and shaded areas represent statistical and total uncertainties, respectively. They are compared with hydrodynamic model predictions without viscosity (blue), with a constant shear viscosity (in terms of shear viscosity to entropy density ratio of $\eta/s = \frac{1}{4\pi}$, magenta), and with a temperature-dependent bulk viscosity (grey) Ryu:2015vwaRyu:2017qzn. The shaded bands on the model curves represent the statistical uncertainties.