Initial State Parton Showers Beyond Leading Order

We derive a new method for initial-state collinear showering in Monte-Carloevent generators which is based on the use of unintegrated parton correlationfunctions. Combined with a previously derived method for final-state showering,the method solves the problem of treating both the hard scattering and theevolution kernels to be used in arbitrarily non-leading order. Although we onlytreat collinear showering, so that further extensions are needed for QCD, wehave discovered several new results: (1) It is better to generate exact partonkinematics in the hard scattering rather than with the subsequent partonshowering, and similarly at each step of the showering. (2) Parton showering isthen done conditionally on the exact energy-momentum of the initiating parton.(3) We obtain a factorization for structure functions in terms of partoncorrelation functions so that parton kinematics can be treated exactly from thebeginning. (4) We obtain two factorization properties for parton correlationfunctions, one in terms of ordinary parton densities and one, suitable forevent generation, in terms of parton correlation functions themselves.Comment: 45 page