Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb-Pb and Xe-Xe collisions

Understand

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons $v_2${$\Psi_{\rm SP}$} in collisions of Pb ions at center-of-mass energy per nucleon-nucleon pair $\sqrt{s_{\rm NN}}$=2.76 TeV and Xe ions at $\sqrt{s_{\rm NN}}$=5.44 TeV are reported.

• The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum.
• The ratio between $v_2${$\Psi_{\rm SP}$} and the elliptic flow coefficient relative to the participant plane $v_2${4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality.
• This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies.