Quarkonium production in parton showers

Naomi Cooke
June, 2021 to January, 2022

Quarkonia production has been a long-standing puzzle in particle
physics. These are mesons which contain quarks of the same flavour,
for example J/ψ. The polarisation measurement of J/ψ is expected to
have significant transverse polarisation at large pT, but has been
experimentally observed to be consistent with zero. Hard production of
onia processes using NRQCD formalism are available in the Pythia8
framework. However, these processes alone cannot fully describe the
data; LHCb and CMS have shown with normalised cross section

TMD shower in Pythia

Mees van Kampen
November, 2020 to January, 2021

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.

Azimuthal di-jet correlations

Armando Bermudez Martinez
April, 2018 to October, 2018

Recently in CMS, we have measured the azimuthal separation between the two leading jets in multi-jet events when these are approximately back-to-back. We have also measured the more exclusive and complementary azimuthal separation between the leading jets when the presence of at least an extra much softer jet is required in the event. We have observed a tension between the data and the theory predictions from parton shower event generators as well as from improved predictions using higher order matrix elements.

Hadronization in Pythia8

Nadine Fischer
June, 2016 to December, 2016

The MCnet short-term project aimed at implementing modifications to the conventional string fragmentation framework in the Pythia event generator. In the current form of the fragmentation, strings are stretched between quark-antiquark pairs. As a pair moves apart the string may break up by producing a new pair of a quark and antiquark which receive opposite and compensating kicks in transverse momentum according to a Gaussian distribution, independently of their flavour.

CKKW Matching and Merging with HEJ

Helen Brooks
March, 2016 to June, 2016

HEJ is a parton-level Monte Carlo generator designed to calculate cross sections for multi-jet processes at the LHC in the limit where the jets are widely separated in rapidity. The HEJ formalism provides an all-order description of wide angle radiation, which becomes logarithmically enhanced in this limit. In the HEJ treatment, the jets which arise will only contain a small number partons, that is, they will be largely unpopulated.

Color reconnection effects in top physics

Spyridon Argyropoulos
April, 2014 to July, 2014

Colour reconnection (CR) is expected to occur at a significant rate at the LHC, and to have a non-negligible impact on event properties. The reason is that the high density of particle production points to a high rate of multiparton interactions which, in combination with parton showers, gives a large number of perturbatively defined partons emerging from typical proton-proton collisions. These partons give many force fields - strings or clusters - that are stretched between matching colour and anticolour charges.

Incorporating Spin Effects into Pythia 8

Philip Ilten
September, 2010 to January, 2011

The objectives of the LHC physics program include tests of the Standard Model, searches/studies of the Higgs boson(s), and searches for physics beyond the Standard Model. In all three areas polarization of gauge bosons and heavy fermions such as the tau and top quark will play an important role, and yet currently Pythia does not include spin correlation effects except in a few cases. Because of the importance of Pythia and the need for spin handling, it is critical that spin correlation effects are incorporated into Pythia as soon as possible.

A C++ modified implementation of the routine Q-PYTHIA

Irais Bautista Guzman
January, 2010 to April, 2010

For the older Pythia 6.4 code the Q-PYTHIA routines have added modifications in the context of jet production in heavy-ion collisions. Medium effects are introduced through an additive term in the splitting functions, computed in the multiple-soft scattering approximation. The observable effects of this modification are studied for different quantities such as fragmentation functions and the hump-backed plateau, in terms of evolution variables: virtuality, transverse momentum and angle.

Development of diffraction in PYTHIA

Sparsh Navin
May, 2009 to September, 2099

Up until this point the modelling of diffraction in Pythia has been primitive. This is a problem for the ALICE collaboration, since typically efficiencies and uncertainties are evaluated by comparing Pythia and Phojet, and these two give appreciably different predictions for diffractive topologies. An improved diffractive description in Pythia would give the ALICE experiment a reduction of the systematic uncertainty on trigger efficiencies and an improved understanding of the underlying event.