Calculations with off-shell matrix elements, TMD parton densities and TMD parton showers

TL;DR

This paper introduces a novel calculation framework combining off-shell matrix elements, TMD parton densities, and a newly developed TMD parton shower, enabling more accurate simulations of high-energy particle collisions.

Contribution

It develops an integrated approach using KaTie and CASCADE to incorporate TMD parton densities and showers, improving the modeling of initial state radiation in high-energy processes.

Findings

Good agreement with LHC dijet azimuthal de-correlation data

Development of a full set of TMD parton densities

Implementation of an initial state TMD parton shower

Abstract

A new calculation using off-shell matrix elements with TMD parton densities supplemented with a newly developed initial state TMD parton shower is described. The calculation is based on the KaTie package for an automated calculation of the partonic process in high-energy factorization, making use of TMD parton densities implemented in TMDlib. The partonic events are stored in an LHE file, similar to the conventional LHE files, but now containing the transverse momenta of the initial partons. The LHE files are read in by the CASCADE package for the full TMD parton shower, final state shower and hadronization from PYTHIA where events in HEPMC format are produced. We have determined a full set of TMD parton densities and developed an initial state TMD parton shower, including all flavors following the TMD distribution. As an example of application we have calculated the azimuthal de-correlation of high pt dijets as measured at the LHC and found very good agreement with the measurement when including initial state TMD parton showers together with conventional final state parton showers and hadronization.