Collectivity and electromagnetic radiation in small systems

TL;DR

This paper investigates collective behavior and electromagnetic radiation in small collision systems at high energies, using hydrodynamics to connect experimental observations with theoretical models and predict new phenomena.

Contribution

It provides a systematic hydrodynamic analysis of soft hadronic and electromagnetic observables in small systems, validating the fluid dynamic approach and making new predictions.

Findings

Quantitative agreement with existing hadronic data

Predicted sizeable thermal photon yields at low transverse momentum

Proposed higher order flow coefficients and photon enhancement predictions

Abstract

Collective behaviour has been observed in hadronic measurements of high multiplicity proton+lead collisions at the Large Hadron Collider (LHC), as well as in (proton, deuteron, helium-3)+gold collisions at the Relativistic Heavy-Ion Collider (RHIC). To better understand the evolution dynamics and the properties of the matter created in these small systems, a systematic study of the soft hadronic observables together with electromagnetic radiation from these collisions is performed, using a hydrodynamic framework. Quantitative agreement is found between theoretical calculations and existing experimental hadronic observables. The validity of the fluid dynamical description is estimated by calculating Knudsen and inverse Reynolds numbers. Sizeable thermal yields are predicted for low $p_T$ photons. Further predictions of higher order charged hadron anisotropic flow coefficients and of thermal photon enhancement are proposed.