Study of the dependence of direct soft photon production on the jet characteristics in hadronic Z 0 decays

An analysis of the direct soft photonproduction rate as a function of the parent jet characteristics is presented, based on hadronicevents collected by the DELPHI experiment at LEP1. The dependences of the photonrates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadronmultiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photonexcess, a similarity of the observed soft photonbehavior to that of the inner hadronic bremsstrahlungpredictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photonrate on the jet neutral and total hadronmultiplicities a prominent difference is found for the observed soft photonsignal as compared to the expected bremsstrahlungfrom final state hadrons. The observed linear increase of the soft photonproduction rate with the jet total hadronmultiplicity 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.