Exponentiating virtual imaginary contributions in a parton shower

Abstract

The operator in a parton shower algorithm that represents the imaginary part of virtual Feynman graphs has a nontrivial color structure and is large because it is proportional to a factor of 4π. In order to improve the treatment of color in a parton shower, it may help to exponentiate this phase operator. We show that it is possible to do so by exponentiating matrices that are no larger than 14×14. Using the example of the probability to have a gap in the rapidity interval between two high transverse momentum jets, we test this exponentiation algorithm by comparing to the result of treating the phase operator perturbatively. We find that the exponentiation works, but that the net effect of the exponentiated phase operator is quite small for this problem, so that one can as well use the perturbative approach.