Pion and nucleon structure as probed in the reaction π±N → μ+μ-X at 253 GeV

New results are presented from Fermilab experiment E615, in which hadroproduction of muon pairs allows a determination of the quark structure of the initial-state hadrons in the context of a Drell-Yan model. Comparison of muon-pair production by π+ and π- beams shows the cross-section ratio follows Drell-Yan model expectations, except for a dip in σ(π+)/σ(π-) near xπ near 1. The same data are also used to extract the ratio of the sea to valence quark distributions in the nucleon, with improved accuracy for xN<0.1 compared to deep-inelastic neutrino scattering. The Drell-Yan analysis of continuum muon-pair production has been extended down to a mass of 3.2 GeV/c2 for the π- data sample. This provides stronger evidence of a rise in the nucleon structure function for xN<0.06 compared to that extracted in deep-inelastic lepton scattering. The issue is raised of the nonuniqueness of the definition of xπ and xN used in the Drell-Yan analysis. A definition proposed by Soper has superior invariance properties to that commonly used. An analysis based on this definition yields generally similar results for the pion and nucleon structure function, compared to use of the common definition. However, the pion structure function shows a larger intercept at xπ=1 when the definition of Soper is used. (orig.)