Resonance pattern of low mass muon pairs produced in 38-GeV/c π-C interactions

We report the first results of experiment SERP-E-151 which examined the production of muon pairs at low mass (Mμμ≤MJ/φ) and low transverse momentum (Ptr<1.5 GeV/c) in the reaction π-C→μ+μ-+X. The measurements were carried out with a 5-meter streamer chamber with carbon targets and lead glass converters allowing the observation of the entire events. An analysed sample, consisting of ∼5000 reconstructed interactions with ∼3300 detected γ's, exceeds by an order of magnitude the statistics of other experiments detecting lepton pairs together with accompanying γ's. The direct observation of decays of resonances η→μμ γ and η'→μμγ in the μμγ-mass spectrum and ρ→μμ and η→μμ in the μμ-mass spectrum allows the conclusion that the known resonances represent the dominant source (98±7±10)% of low mass dimuons (0.2<Mμμ<0.6 GeV/c2) produced in the non-central region at XF≥0.4. This result seems to be in disagreement with the soft quark-antiquark annihilation model. 36 refs.; 7 figs.; 2 tabs