Initial-State Bremsstrahlung versus Final-State Hydrodynamic Sources of Azimuthal Harmonics in p+A at RHIC and LHC

TL;DR

This paper compares initial-state gluon bremsstrahlung effects and final-state hydrodynamic models in explaining azimuthal harmonics observed in proton-nucleus collisions at RHIC and LHC, highlighting similar predictions from both approaches.

Contribution

It introduces a perturbative QCD-based initial-state model involving Color Scintillation Arrays that can produce azimuthal harmonics comparable to hydrodynamic models.

Findings

Initial-state CSA can generate azimuthal harmonics similar to final-state hydrodynamics.

The similarity challenges the exclusive interpretation of flow as a final-state effect.

The dominant mechanism in p+A and A+A collisions remains an open question.

Abstract

Recent pT<2~GeV azimuthal correlation data from the Beam Energy Scan (BES) and D+Au runs at RHIC/BNL and, especially, the surprising similarity of azimuthal $v_n\{2m\}(p_T)$ ``transeverse flow'' harmonics in $p+Pb$ and $Pb+Pb$ at LHC have challenged the uniqueness of local equilibrium ``perfect fluid'' interpretations of those data. We report results at QM14 on azimuthal harmonics associated with initial-state non-abelian ``wave interference'' effects predicted by perturbative QCD gluon bremsstrahlung and sourced by Color Scintillation Arrays (CSA) of color antennas. CSA are naturally identified with multiple projectile and target beam jets produced in inelastic p+A reactions. We find a remarkable similarity between azimuthal harmonics sourced by initial state CSA and those predicted with final state perfect fluid models of high energy p+A reactions. The question of which mechanism dominates in $p+A$ and $A+A$ remains open at this time.