Method for studying the rapidity fluctuation and de-correlation of harmonic flow in heavy-ion collisions

An "event-shape-twist" technique is proposed to study the longitudinal dynamics of harmonic flow, in particular the effects of rapidity fluctuation and event-plane de-correlation. This technique can distinguish between two types of rapidity de-correlation effects: a systematic rotation versus a random fluctuation of flow angles along the rapidity direction. The technique is demonstrated and the magnitude of the two de-correlation effects is predicted by using a multiphase transport model via a single-particle analysis and a two-particle correlation analysis. An observed de-correlation can be attributed to a systematic rotation of event-plane angle along the pseudorapidity, consistent with a collective response to an initial state twist of the fireball proposed by Bozek et al. This rotation is also observed for several higher-order harmonics with the same sign and similar magnitudes.