An introduction to PYTHIA 8.2

Abstract

The Pythia program is a standard tool for the generation of events in high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multiparticle final state. It contains a library of hard processes, models for initial-and final-state parton showers, matching and merging methods between hard processes and parton showers, multiparton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and several interfaces to external programs. Pythia 8.2 is the second main release after the complete rewrite from Fortran to C++, and now has reached such a maturity that it offers a complete replacement for most applications, notably for LHC physics studies. The many new features should allow an improved description of data. Catalogue identifier of previous version: ACTU v3 0 Journal reference of previous version: T. Sjöstrand, S. Mrenna and P. Skands, Computer Physics Commun. 178 (2008) 852 Does the new version supersede the previous version?: yes Nature of problem: high-energy collisions between elementary particles normally give rise to complex final states, with large multiplicities of hadrons, leptons, photons and neutrinos. The relation between these final states and the underlying physics description is not a simple one, for two main reasons. Firstly, we do not even in principle have a complete understanding of the physics. Secondly, any analytical approach is made intractable by the large multiplicities. Solution method: complete events are generated by Monte Carlo methods. The complexity is mastered by a subdivision of the full problem into a set of simpler separate tasks. All main aspects of the events are simulated, such as hard-process selection, initial-and final-state radiation, beam remnants, fragmentation, decays, and so on. Therefore events should be directly comparable with experimentally observable ones. The programs can be used to extract physics from comparisons with existing data, or to study physics at future experiments. Reasons for the new version: improved and expanded physics models Summary of revisions: hundreds of new features and bug fixes, allowing an improved modeling Restrictions: depends on the problem studied Unusual features: none Running time: 10-1000 events per second, depending on process studied

Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD

“…We compute the spectrum dσ/dE b-tag according to the same definition described earlier, preparing LO partonlevel events for the process in (2) with MadGraph5 _aMC@NLO [52] and subsequently feeding them to Pythia 8.210 for parton showering [53]. We remark that the latter Monte Carlo package has been tuned to data to give a better description on heavy quark fragmentation [54] and has been investigated, by the ATLAS Collaboration, for the sensitivity to its parton shower parameters in the context of tt production at the LHC [55].…”

Probing triple-Higgs productions via4b2γdecay channel at a 100 TeV hadron collider

“…The parton-level background events are generated by MadGraph5/aMC@NLO [40] directly and showered through PYTHIA 8 [41]. In this paper, we only consider events with at least two tagged b-jets, i.e.…”

Deep-learning top taggers or the end of QCD?

“…The lepton kinematics can then be used to estimate the transverse momentum of the hadronically decaying top quark. We simulate a 14 TeV LHC hadronic tt sample and a QCD dijet sample with Pythia8 [42], ignoring multiple interactions. While one could clearly include pile-up in the simulations, understanding and removing it requires information beyond the calorimeter images, for examples from tracks.…”