Results of the HELIOS Collaboration on low mass dilepton and soft photon production in pBe, pW and SW collisions

Abstract

We report results of the HELIOS Collaboration on low-mass dilepton and soft photon production in 450 GeV/ c proton-beryllium interactions and preliminary results on sulphur tungsten collisions. Both electron and muon pairs were measured in a double spectrometer with similar kinematic acceptance for both parts. The observed dilepton yield is found to be in good agreement with the expected cumulative lepton pair production by meson decays. Based on the simultaneous direct measurements of the η, via the Dalitz decay channel, which represents the main component of the decay background, the remaining systematic errors of this experiment are as small as 20–30%. With a specially modified experimental set-up the HELIOS-3 collaboration measured low-mass muon pairs in 200 GeV/ c proton-tungsten and sulfur-tungsten collisions. We show first preliminary results. For this analysis the mesonic Dalitz decay contributions are evaluated using cross-section parametrization leading to larger systematic errors. No significant additional dilepton source is detected within the framework of this preliminary analysis. Soft ( p T ≤ 50 MeV/ c) photon production was measured with two special independent set-ups in 450 GeV/ c proton beryllium collisions. The observed production rates and spectra are well reproduced by hadronic bremsstrahlung calculations.