Study of the Dependence of Direct Soft Photon Production on the Jet Characteristics in Hadronic Z^0 Decays

TL;DR

This study investigates how the rate of direct soft photon production in hadronic Z^0 decays depends on jet characteristics, revealing similarities to bremsstrahlung predictions and highlighting differences related to neutral hadron multiplicities.

Contribution

It provides the first detailed analysis of soft photon production dependence on various jet properties in Z^0 decays, revealing new insights into the fragmentation process.

Findings

Soft photon rates correlate with jet momentum and charged multiplicity similarly to bremsstrahlung.

A significant excess of soft photons is observed compared to bremsstrahlung predictions for neutral and total hadron multiplicities.

Soft photon production increases linearly with total hadron multiplicity, especially with neutral hadron pairs.

Abstract

An analysis of the direct soft photon production rate as a function of the parent jet characteristics is presented, based on hadronic events collected by the DELPHI experiment at LEP1. The dependences of the photon rates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadron multiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photon excess, a similarity of the observed soft photon behaviour to that of the inner hadronic bremsstrahlung predictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photon rate on the jet neutral and total hadron multiplicities a prominent difference is found for the observed soft photon signal as compared to the expected bremsstrahlung from final state hadrons. The observed linear increase of the soft photon production rate with the jet total hadron multiplicity and its strong dependence on the jet neutral multiplicity suggest that the rate is proportional to the number of quark pairs produced in the fragmentation process, with the neutral pairs being more effectively radiating than the charged ones.