Observation of anomalous scaling violation in muon pair production by 194 GeV/c π--tungsten interactions

The differential cross-section for dimuon production by 194 GeV/c π^- on W, as measured by the NA10 Collaboration, is compared with theoretical models. The wide kinematical range of the data, extending well above the ϒ resonances, provides the opportunity of a comparison with ‘realistic’ Drell-Yan models, i.e. with those allowing for scaling violation in the hadronic structure functions. The data in fact clearly indicate the failure of the ‘naïve’ Drell-Yan model, while the available ‘realistic’ versions (leading logarithm approximation and next-to-leading logarithm approximation in first order QCD), although giving a better description of the data, still disagree in the x _F andsqrt tau dependences of the cross-section at high dimuon masses. This disagreement is referred to here as ‘anomalous’ scaling violation. The dependence of the results on external inputs (nucleon and pion-sea structure functions) is analysed; it is shown that in the next-to-leading logarithm approximation the value < K>=1.03±0.03 (stat.) can be obtained for the ratio experimental/theoretical cross-section.