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

Understand

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.