The torque effect and fluctuations of entropy deposition in rapidity in ultra-relativistic nuclear collisions

Abstract

The decorrelation of the orientation of the event-plane angles in the initial state of relativistic Pb-Pb and p-Pb collisions, the "torque effect," is studied in a model of entropy deposition in the longitudinal direction involving fluctuations of the longitudinal source profile on large scales. The radiation from a single wounded nucleon is asymmetric in space-time rapidity. It is assumed that the extent in rapidity of the region of deposited entropy is random. Fluctuations in the deposition of entropy from each source increase the event-plane decorrelation: for Pb-Pb collisions they improve the description of the data, while for p-Pb collisions the mechanism is absolutely essential to generate any sizable decorrelation. We also show that the experimental data for rank-four flow may be explained via folding of the elliptic flow. The results suggest the existence of long range fluctuations in the space-time distribution of entropy in the initial stages of relativistic nuclear collisions.