Summing threshold logs in a parton shower

Abstract

When parton distributions are falling steeply as the momentum fractions of the partons increases, there are effects that occur at each order in ${\alpha }_s$ that combine to affect hard scattering cross sections and need to be summed. We show how to accomplish this in a leading approximation in the context of a parton shower Monte Carlo event generator.

Comments: 83 pages, 8 figures