Muon pair production in heavy ion interactions at 200 GeV per nucleon

The NA38 experiment measures muon pair production in 200 GeV per nucleon nucleus-nucleus interactions at the CERN SPS. Dimuon production is studied as a function of the neutral transverse energy produced in the collisions, which reflects the energy density reached in the interaction. The J/ψ yield relative to muon pairs in the mass continuum decreases with increasing energy density. The atomic number dependence of the cross sections has been studied using the parametrization σ ∞ (A_proj.A_targ.)^α which leads to α = 0.91 ± 0.04 for the J/ψ, and α = 1.01 ± 0.04 for muon pairs in the mass range [1.7, 2.7] GeV/c². The study of dimuon transverse momentum distributions shows an enhancement of the J/ψ suppression for low P_T values. The average values 〈P_T〉 and 〈P_T²〉 increase with increasing transverse energy in case of the J/ψ, and are rather flat for muon pairs in the mass continuum. Such a behaviour is expected in case of quark-gluon plasma formation. However, a model based on parton multiple scattering in the initial state leads to a variation of 〈P_T²〉 as a function of the mean thickness of nuclear matter encountered by the partons, which seems also to account for the observed effect. In addition, low mass dimuons are studied. An enhancement of the φ/(ρ{variant} + ω) ratio is observed with increasing energy density.