Triple collinear emissions in parton showers

Abstract

A framework to include triple collinear splitting functions into parton showers is presented, and the implementation of flavor-changing NLO splitting kernels is discussed as a first application. The correspondence between the Monte-Carlo integration and the analytic computation of NLO DGLAP evolution kernels is made explicit for both timelike and spacelike parton evolution. Numerical simulation results are obtained with two independent implementations of the new algorithm, using the two independent event generation frameworks PYTHIA and SHERPA.

Combining states without scale hierarchies with ordered parton showers

“…The effect of including ME corrections for ordered partonshower splittings is minor compared to the uncorrected shower. This means that the method does not deteriorate the shower resummation, and gives us confidence that the improvement does not interfere with other improvement strategies [57][58][59]. The main improvements stem from a careful treatment of contributions from phase-space regions that are not accessible by ordered parton showers.…”

Implementing NLO DGLAP evolution in parton showers

“…At next-to-leading order the solution will be different, as the splitting functions include contributions from threeparticle final states that have been integrated out. We will show in the following how this problem can be approached [55].…”

“…We will thereby focus on the refinements needed in order to realize NLO accurate parton evolution. This includes the implementation of the complete set of splitting kernels at O(α 2 s ), a subset of which are the flavor-changing kernels discussed in [55]. Another significant change concerns the implementation of symmetry factors.…”

“…In order to account for their more involved flavor structure, we must simulate them fully differentially in the 1 → 3 phase space. To this end, we use the method presented in [55]. All other splitting functions have an analogous 1 → 2 topology and are implemented using this topology.…”

“…We employ the non-flavor changing NLO splitting functions in the MS scheme in their integrated form [49][50][51][52][53][54], and we include the flavor-changing NLO splitting kernels fully differentially using the method presented in [55]. We identify the contribution to the NLO splitting functions which is already included in the leading-order (LO) realization of the parton-shower, and correspondingly subtract it from the NLO splitting function.…”