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

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 the change is moderate, 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.