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(to be submitted to Zeit. Phys. C} for muons, and ~ 40% for electrons.level, on any new source of lepton pairs is ~ 20% of the hadronic decay contribution of the major hadronic sources is set by the data. The upper limit, at 90% confidence decays, and there is no need to invoke any "unconventional" source. The normalisation the low-mass spectrum can be explained satisfactorily by lepton pairs from hadronic p-Be collisions at 450 GeV/ c at the CERN SPS. For both electron and muon pairsWe report on the production of low-mass electron pairs and muon pa.irs in Area. An overview of the apparatus is shown in Figure la. The main components are OCR OutputThe HELIOS spectrometer is situated in the H8 beam line of the CERN SPS North 2.2 Apparatus reliably and stably throughout the experiment.The intensity was ~ 106 per burst. The targeting of the beam on to the wire worked 0.1%), a transverse diameter less than 50 pm and divergence ~ 0.2 mrad at the target.eriment to match the wire target. This beam has excellent momentum resolution (6p/ p A special 450 GeV/ c proton f'micro"-bea.m was developed for the HELIOS exp from the decay of hadrons produced in the interaction.of only 125 pm diameter, in order to minimize the radiation length traversed by photons the design. Accordingly, we have used a 4 cm long (10% interaction length) Be wire target experiment, and so the suppression of e'*'c" pairs from conversions was a key feature ofThe study of low-mass lepton pairs was one of the prime motivations of the HELIOS analysis is presented in section 4, and in section 5 results are summarised and conclusions ing, and data-taking, followed by the event reconstruction and selection in section 3. TheThe plan of this paper is as follows. In section 2 we describe the apparatus, trigger any "unconventional" source. Upper limits on any new source are presented.can be accounted for by lepton pairs from the decay of hadrons, and there is no need forThe main result is that low-mass lepton pairs, produced centrally at `/E 2 29 GeV, a measurement of the total charged multiplicity of the event.electron identincation by both transition radiation and calorimetry; a double measurement of the momentum (or energy) of both muons and electrons;Other noteworthy points are:of certain Dalitz decay modes;the measurement of photons as well as charged leptons, affording direct measurement the detector, producing two essentially independent measurements of lepton pairs; the analysis of both electron pairs and muon pairs, emphasising different aspects of from conventional sources. The most important features of the experimental approach are:Be collisions (\/Z cx 29 GeV) in the central rapidity region, is compared to the expectationIn this paper the production of electron and muon pairs, produced in 450 GeV/ c p their production level in ordinary hadronic collisions.plasma formation in relativistic heavy-ion collisions [3]: it is essential then to understand process Furthermore, lepton pairs have been suggested as a signature for quark-gluon implied by conven...

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