The Pythia8 event generator implements many aspects of a detailed calculation of high energetic hadronic collisions. It is mainly focussed on simulating proton-proton collision processes as in the LHC, however, it is possible to generate events with other beam types such as electron beams for DIS events. Using “vanilla-”Pythia, one has many hard scattering calculation possibilities, the standard parton showers for initial- (ISR) and final- (FSR) state radiation, multi-parton interactions and hadronization models to its dispose. The parton showers simulate the emissions that a proton undergoes before and after the hard process. The showers and MPI are interleaved, which means that they are performed simultaneously when evolving backwards in the transverse momentum scale.

With recently new developed ideas to do event generation using transverse momentum dependent parton densities (TMDs), the aim is to understand more about the role of the current ISR and FSR before starting to implement a new shower in the Pythia framework. Subject of this study are the event kinematics and generated pT scales of emissions. Checks with a single parton emission in DIS have been performed in Pythia to reveal some information about the first parton splitting and the event kinematics. Scatter plots of the first emission in DIS gave a hint that the method of dealing with recoil effects influence the distribution of parton emissions in the phase space. Larger transverse momenta are reached when applying a global recoil instead of a recoil only within a dipole. With checks that involve multiple splittings in one event and looking in different frames of reference, we observed that from the DIS studies we cannot draw strong conclusions on the recoil prescriptions. However, with more skills and knowledge on the procedures from Pythia, a starting point is made to develop a modified version of the shower to include TMDs. The SimpleSpace shower will be used as a template for the TMD shower. Some of the elements can be adapted according to the PB method. The evolution scale will not anymore be related to the transverse momentum, but to the rescaled transverse momentum. The picking of possible scales - done with the veto algorithm - needs to be done according to a Sudakov that is calculated using TMDs. Eventually, the TMD initial shower needs to be interfaced with the final state shower appropriately.

Mees van Kampen
November, 2020 to January, 2021