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