Forward-backward flow correlations in relativistic heavy-ion collisions

TL;DR

This paper investigates the torque effect in relativistic heavy-ion collisions, linking initial fireball fluctuations to observable flow correlations across rapidities, and proposes experimental measures to quantify this phenomenon.

Contribution

It introduces the concept of the torque effect in heavy-ion collisions and connects initial state fluctuations to measurable flow correlations across rapidities.

Findings

Average torque angle deviation is about 20 degrees in central collisions.

Hydrodynamic expansion transmits the initial torque to collective flow.

Proposed cumulant-based measures enable experimental extraction of the torque effect.

Abstract

We discuss the torque effect in the initial fireball formed in relativistic heavy-ion collisions, manifesting itself, on the event-by-event basis, in a relative angle between the principal axes of the transverse momentum distributions in the forward and backward rapidity regions. The torque follows from two natural features: 1) the sources of particles (e.g. wounded nucleons) emit predominantly in their forward hemispheres, and 2) there exist fluctuations in the transverse distribution of sources from the two colliding nuclei. On the average, the standard event-by-event deviation of the relative torque angle is about 20 degrees for the central and 10 degrees for the mid-peripheral collisions. The hydrodynamic expansion of a torqued fireball leads to a torqued collective flow of the fluid, which, in turn, yields torqued principal axes of the transverse-momentumdistributions at different rapidities. We discuss experimental measures based on cumulants involving particles in different rapidity regions, which allow for a quantitative extraction of the effect from the experimental data. We estimate the non-flow contributions from resonance decays with the help of THERMINATOR.