Soft photon production from real-time dynamics of jet fragmentation

TL;DR

This paper investigates the longstanding puzzle of anomalous soft photon production in heavy ion collisions, proposing a nonperturbative QCD-inspired model to explain the excess observed experimentally.

Contribution

The study introduces an exactly solvable 1+1 dimensional Abelian model that captures key QCD features to analyze soft photon production in jet fragmentation.

Findings

Qualitative agreement with experimental data on soft photon excess

Model reproduces key features of nonperturbative QCD effects

Provides insights into the mechanism behind anomalous soft photon production

Abstract

Soft photons produced in heavy ion collisions are an important tool for probing the properties of the quark-gluon plasma. It is therefore crucial to understand the background - soft photons produced in elementary collisions. Low theorem states that soft photon production in hadron collisions is dominated by Bremsstrahlung off charged initial and final state hadrons. Surprisingly, almost every experiment observed an enhancement (by a factor of $2\div 5$) above Low theorem's prediction. This is the longstanding puzzle of "anomalous soft photon production." The phenomenon is not observed in processes with leptonic final states, which suggests that the mechanism is due to nonperturbative QCD evolution. We study this phenomenon using an exactly soluble, massless, Abelian model in $1+1$ dimensions which shares with QCD many important properties: confinement, chiral symmetry breaking, axial anomaly and $\theta$-vacuum. We then apply this model to the soft photon production in the fragmentation of jets produced in $Z^0$ decays and find a qualitative agreement with the data.