Longitudinal eccentricity decorrelations in heavy-ion collisions

In heavy-ion collisions, the harmonic flow Vn of final-state particles is driven by the eccentricity vector En that describes the shape of the initial fireball projected in the transverse plane. It was realized recently that the structure and shape of the fireball, and consequently the En, fluctuate along pseudorapidity η in a single event, En(η). This leads to eccentricity decorrelation between different η, driving the longitudinal flow decorrelations observed in the experiments. Using a Glauber model with a parametrized longitudinal structure, we have estimated the eccentricity decorrelations and related them to the measured flow decorrelation coefficients for elliptic flow n=2 and triangular flow n=3. We investigated the dependence of eccentricity decorrelations on the choice of collision system in terms of the size, asymmetry, and deformation of the nuclei. We found that these nuclear geometry effects lead to significant and characteristic patterns on the eccentricity decorrelations, which describe the measured ratios of the flow decorrelations between Xe+Xe and Pb+Pb collisions. These patterns can be searched for using existing experimental data at the Relativistic Heavy-Ion Collider and the large Hadron collider, and if confirmed, they will provide a means to improve our understanding of the initial state of the heavy-ion collisions.