Backwards evolved initial state parton showers

A Monte Carlo model for initial state parton showers is presented. Showers are generated by working backwards from the hard scattering to unevolved structure functions at a reference scale Q02. This backward evolution formalism allows the incorporation of exact kinematic constraints at all stages of shower generation. The physical regions for deep-inelastic showers and hard scattering showers (with an additional fixed time-like scale s) are different, leading to kinematic differences in scale-dependent evolution. Shower estimates of these kinematic differences are shown to be compatible with the O(αs) corrections for the Drell-Yan process. Transverse momentum distributions are discussed, with attention given to the merging of shower model results and exact, low-order QCD cross sections. (orig.)