Evidence for the Collective Nature of Radial Flow in Pb + Pb Collisions with the ATLAS Detector

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, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (pT) dependence of radial flow fluctuations (v0(pT)) over 0.5 < pT < 10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb + Pb collisions at sqrt(sNN) = 5.02 TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in pT, and centrality-independent shape in pT. The comparison with a hydrodynamic model demonstrates the sensitivity of v0(pT) 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.