Soft photons in the Lund model

Abstract

We investigate the emission of real and virtual photons with small transverse momenta close to zero cms rapidity in proton-proton collisions. We consider a model, built upon the space-time structure of the Lund string fragmentation model combined with the Fritiof model for hadronic interactions. The model contains a gauge and Lorentz invariant, albeit semiclassical, treatment of the currents from the final state hadrons as well as their valence constituents. We further investigate the effects of resonance decay, increase of the transverse momentum width of the string fluxtube, gluon emission leading to “bent” strings and finally also the occurrence of polarization charges (virtual q q -production) along the string world-sheet. We show that the spectrum for real and virtual (e +e − pairs) photon emission always will be within a factor of 2 to 3 from the long wavelength limit (the Low limit) for transverse momenta up to a few hundred MeV/ c. We conclude that if the present experimental indications for a much larger source of emission should be confirmed, then a major revision will be necessary in our present understanding of the space-time structure and/or the energy densities available in hadron fragmentation.