Low-energy direct photon production in p p and alpha alpha collisions at the cern intersecting storage rings

High transverse momentum (p(,T)) direct photons have given us some of our best evidence for low order perturbative QCD proces- ses in hadron collisions. It is also important to examine hadronic collisions with a weakly interacting probe in the low p(,T) region, where hadron interactions involve very complicated phenomena. A previous experiment at SQRT.(s) = 12 GeV has observed an excess of low p(,T) direct photons. The present experiment has studied direct soft photons in pp interactions at SQRT.(s) = 63 GeV and alpha-alpha interactions at SQRT.(s(,NN)) = 31.5 GeV at the CERN ISR. Comparisons of photon production with respect to track production in pp minimum bias events with that in pp events with high transverse energy (E(,T)) and with alpha-alpha minimum bias events are investigated. For alpha-alpha minimum bias data, within experimental errors there was no excess of photons with respect to tracks, compared with pp minimum bias data. But for pp high E(,T) data, we observed an interesting effect: as p(,T) increases, the ratio of photons to tracks increases, compared with pp minimum bias data.