Speaker
Description
Radial and anisotropic flow are key observables for probing the expansion dynamics of the quark-gluon plasma (QGP). The first measurement of transverse momentum–dependent radial flow fluctuations, v0(pT), in Pb+Pb collisions is presented. The $v_0(p_T)$ observable exhibits three key features: (i) long-range correlations in pseudorapidity, (ii) factorization in $p_T$, and (iii) a centrality-independent shape. Observations of these features provide compelling evidence that radial flow originates from collective expansion. Comparisons with hydrodynamic models show strong sensitivity to bulk viscosity, highlighting $v_0(p_T)$ as a powerful new probe of QGP transport properties. The first measurement of directed flow, $v_1$, relative to the spectator plane with the ATLAS detector at the LHC is also reported. The measurement leverages experimental capabilities enabled by the installation of a new Reaction Plane Detector within the ATLAS Zero Degree Calorimeter. The results will inform modeling of the initial-state conditions and the electromagnetic fields generated by spectator nuclei in heavy-ion collisions.
