Hanbury Brown--Twiss Interferometry and Collectivity in Small Systems

TL;DR

This paper explores how Hanbury Brown--Twiss interferometry reveals the space-time and momentum evolution in small nuclear collision systems, suggesting hydrodynamic behavior at high multiplicity as a key factor.

Contribution

It establishes a hydrodynamic baseline for HBT radii dependence on multiplicity in small systems, aiding in distinguishing models of collectivity in nuclear collisions.

Findings

HBT radii depend on multiplicity and transverse momentum in small systems.

Hydrodynamics can naturally explain features of HBT radii in high-multiplicity events.

Provides a baseline for future comparisons in collectivity studies.

Abstract

Hanbury Brown--Twiss interferometry (HBT) provides crucial insights into both the space-time structure and the momentum-space evolution of ultrarelativistic nuclear collisions at freeze-out. In particular, the dependence of the HBT radii on the transverse pair momentum $K_T$ and the system charged multiplicity $dN_{\mathrm{ch}}/d\eta$ may reflect the mechanisms driving collective behavior in small systems. This paper argues that certain features observed in the multiplicity dependence of the HBT radii can be naturally understood if small systems evolve hydrodynamically at high-multiplicity. This study thus establishes a baseline for the multiplicity dependence of HBT in hydrodynamics which may prove useful in discriminating between competing models of collectivity in nuclear collisions.