Tilted emitting source in the heavy-ion collisions

Physics Department, The Ohio State University, Columbus, Ohio 43210, USA

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Published online: 17 April 2026

Abstract

The three-dimensional structure of the initial state in heavy-ion collisions has recently drawn a lot of attention. One of its key features is the tilt of the particle-emitting source away from the beam direction, which becomes more pronounced at lower collision energies. This tilt, set by the collision geometry, plays an important role in understanding observables like directed flow and particle polarization. Still, measuring the tilt directly in experiments is far from simple.

The azimuthally sensitive femtoscopy method makes it possible to study tilt by analyzing momentum correlations of pion pairs. In this work we apply it to Au+Au collisions at √s_NN = 7.7 GeV. While one might expect the tilt to change mainly with centrality, our results instead show a much stronger dependence on the pair momentum.

This provides a framework that STAR can use with the ongoing high-statistics BES-II data to extract tilt more precisely and to build a clearer picture of how the three-dimensional initial state shapes final observables.