HIGHER-TWIST EFFECTS IN THE REACTION PI-N-]MU+MU-X AT 253GEV/C

The distributions of quarks in the pion and nucleon are extracted from measurements of the reaction pi- N --> mu+mu- X at 253 GeV/c in a naive Drell-Yan analysis, as well as QCD-corrected analyses at leading-log and next-to-leading-log order. As x-pi --> 1 the pion structure function shows a term that varies as 1/m-mu-mu-4, which we interpret as a higher-twist effect. Additionally, the angular distribution of the mu+ in the muon-pair rest frame tends towards sin2-theta as x-pi --> 1 and as m-mu-mu --> 0 in a manner consistent with higher-twist effects. When the strongly mass-dependent higher-twist effects are included as part of the pion structure function, the nucleon structure function agrees well with leading-twist results from deeply inelastic lepton-hadron scattering. A significant advance of the present work is the extension of the analysis to low masses by the subtraction of the J/psi and psi' resonances from the continuum. Our analysis covers the kinematic range 0.4 < x-pi < 1.0 and 0.02 < x(N) < 0.33 with 3.0 < m-mu-mu < 8.55 GeV/c2. Cross sections for psi' production are presented in an appendix.