An unresolved problem in J/psi phenomenology is a systematic understanding ofthe differential photoproduction cross section, dsigma/dz [gamma + p -> J/psi +X], where z= E_psi/E_gamma in the proton rest frame. In the non-relativisticQCD (NRQCD) factorization formalism, fixed-order perturbative calculations ofcolor-octet mechanisms suffer from large perturbative and nonperturbativecorrections that grow rapidly in the endpoint region, z -> 1. In this paper,NRQCD and soft collinear effective theory are combined to resum these largecorrections to the color-octet photoproduction cross section. We derive afactorization theorem for the endpoint differential cross section involving theparton distribution function and the color-octet J/psi shape functions. A oneloop matching calculation explicitly confirms our factorization theorem atnext-to-leading order. Large perturbative corrections are resummed using therenormalization group. The calculation of the color-octet contribution todsigma/dz is in qualitative agreement with data. Quantitative tests of theuniversality of color-octet matrix elements require improved knowledge of shapefunctions entering these calculations as well as resummation of thecolor-singlet contribution which accounts for much of the total cross sectionand also peaks near the endpoint.
展开▼
