Torbjörn Sjöstrand
Given the current landscape in experimental high-energy physics, these lectures are focused on applications of event generators for hadron colliders like the Tevatron and LHC. Section 2 contains a first overview of the physics picture and the generator landscape. Thereafter section 3 describes the usage of matrix elements, section 4 the important topics of initial- and final-state showers, and section 5 how showers can be matched to different hard processes. The issue of multiparton interactions and their role in mimimum-bias and underlying-event physics is introduced in section 6, followed by some comments on hadronization in section 7. The article concludes with an outlook on the ongoing generator-development work in section 8.
Torbjörn Sjöstrand, Marius Utheim
In this article, a framework for hadronic rescattering in the general-purpose Pythia event generator is introduced. The starting point is the recently presented space-time picture of the hadronization process. It is now extended with a tracing of the subsequent motion of the primary hadrons, including both subsequent scattering processes among them and decays of them. The major new component is cross-section parameterizations for a range of possible hadron-hadron combinations, applicable from threshold energies upwards. The production dynamics in these collisions has also been extended to cope with different kinds of low-energy processes. The properties of the model are studied, and some first comparisons with LHC pp data are presented. Whereas it turns out that approximately half of all final particles participated in rescatterings, the net effects in pp events are still rather limited, and only striking in a few distributions. The new code opens up for several future studies, however, such as effects in pA and AA collisions.
Torbjörn Sjöstrand
The evolution of the widely-used PYTHIA particle physics event generator is outlined, from the early days to the current status and plans. The key decisions and the development of the major physics components are put in context.
Guillaume Albouy, Jared Barron, Hugues Beauchesne, Elias Bernreuther, Marcella Bona, Cesare Cazzaniga, Cari Cesarotti, Timothy Cohen, Annapaola de Cosa, David Curtin, Zeynep Demiragli, Caterina Doglioni, Alison Elliot, Karri Folan DiPetrillo, Florian Eble, Carlos Erice, Chad Freer, Aran Garcia-Bellido, Caleb Gemmell, Marie-Hélène Genest, Giovanni Grilli di Cortona, Giuliano Gustavino, Nicoline Hemme, Tova Holmes, Deepak Kar, Simon Knapen, Suchita Kulkarni, Luca Lavezzo, Steven Lowette, Benedikt Maier, Seán Mee, Stephen Mrenna, Harikrishnan Nair, Jeremi Niedziela, Christos Papageorgakis, Nukulsinh Parmar, Christoph Paus, Kevin Pedro, Ana Peixoto, Alexx Perloff, Tilman Plehn, Christiane Scherb, Pedro Schwaller, Jessie Shelton, Akanksha Singh, Sukanya Sinha, Torbjörn Sjöstrand, Aris G. B. Spourdalakis, Daniel Stolarski, Matthew J. Strassler, Andrii Usachov, Carlos Vázquez Sierra, Christopher B. Verhaaren, Long Wang
In this work, we consider the case of a strongly coupled dark/hidden sector, which extends the Standard Model (SM) by adding an additional non-Abelian gauge group. These extensions generally contain matter fields, much like the SM quarks, and gauge fields similar to the SM gluons. We focus on the exploration of such sectors where the dark particles are produced at the LHC through a portal and undergo rapid hadronization within the dark sector before decaying back, at least in part and potentially with sizeable lifetimes, to SM particles, giving a range of possibly spectacular signatures such as emerging or semi-visible jets. Other, non-QCD-like scenarios leading to soft unclustered energy patterns or glueballs are also discussed. After a review of the theory, existing benchmarks and constraints, this work addresses how to build consistent benchmarks from the underlying physical parameters and present new developments for the PYTHIA Hidden Valley module, along with jet substructure studies. Finally, a series of improved search strategies is presented in order to pave the way for a better exploration of the dark showers at the LHC.
Torbjörn Sjöstrand
PYTHIA is a general-purpose event generator for multiparticle production in high-energy physics. After a general introduction and program news survey, some areas of recent physics progress are considered: the matching to matrix elements, especially for W/Z production; charm and bottom hadronization; multiple interactions; and interconnection effects. The report concludes with some words on the future, specifically the ongoing transition to C++.
Torbjorn Sjostrand
Several topics of interest in soft photoproduction physics are discussed. These include jet universality issues (particle flavour composition), the subdivision into event classes, the buildup of the total photoproduction cross section and the effects of multiple interactions.
Torbjorn Sjostrand
This is the latest edition of the physics description and manual of the PYTHIA and JETSET programs for event generation in High Energy Physics.
Torbjörn Sjöstrand
There are experimental evidence for the occurrence of colour reconnection, but the mechanisms involved are far from understood. Previous reconnection studies are briefly summarized, and some potential implications for LHC physics are outlined.
Christian Bierlich, Smita Chakraborty, Nishita Desai, Leif Gellersen, Ilkka Helenius, Philip Ilten, Leif Lönnblad, Stephen Mrenna, Stefan Prestel, Christian T. Preuss, Torbjörn Sjöstrand, Peter Skands, Marius Utheim, Rob Verheyen
This manual describes the PYTHIA 8.3 event generator, the most recent version of an evolving physics tool used to answer fundamental questions in particle physics. The program is most often used to generate high-energy-physics collision "events", i.e. sets of particles produced in association with the collision of two incoming high-energy particles, but has several uses beyond that. The guiding philosophy is to produce and reproduce properties of experimentally obtained collisions as accurately as possible. The program includes a wide ranges of reactions within and beyond the Standard Model, and extending to heavy ion physics. Emphasis is put on phenomena where strong interactions play a major role. The manual contains both pedagogical and practical components. All included physics models are described in enough detail to allow the user to obtain a cursory overview of used assumptions and approximations, enabling an informed evaluation of the program output. A number of the most central algorithms are described in enough detail that the main results of the program can be reproduced independently, allowing further development of existing models or the addition of new ones. Finally, a chapter dedicated fully to the user is included towards the end, providing pedagogical examples of standard use cases, and a detailed description of a number of external interfaces. The program code, the online manual, and the latest version of this print manual can be found on the PYTHIA web page: https://www.pythia.org/
Stefano Moretti, Leif Lönnblad, Torbjörn Sjöstrand
Over the years, many jet clustering algorithms have been proposed for the analysis of hadronic final states in $e^+e^-$ annihilations. These have somewhat different emphasis and are therefore more or less suited for various applications. We here review some of the most used and compare them from a theoretical and experimental point of view.
Jesper R. Christiansen, Torbjörn Sjöstrand
The effects of colour reconnection (CR) at $\mathrm{e}^+\mathrm{e}^-$ colliders are revisited, with focus on recently developed CR models. The new models are compared with the LEP2 measurements for $\mathrm{e}^+\mathrm{e}^- \to \mathrm{W}^+\mathrm{W}^- \to\mathrm{q}_1 \overline{\mathrm{q}}_2 \mathrm{q}_3 \overline{\mathrm{q}}_4$ and found to lie within their limits. Prospects for constraints from new high-luminosity $\mathrm{e}^+\mathrm{e}^-$ colliders are discussed. The novel arena of CR in Higgs decays is introduced, and illustrated by shifts in angular correlations that would be used to set limits on a potential $CP$-odd admixture of the 125 GeV Higgs state.
Torbjorn Sjostrand, Stephen Mrenna, Peter Skands
The PYTHIA program can be used to generate high-energy-physics `events', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, within and beyond the Standard Model, with emphasis on those where strong interactions play a role, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics input is summarized here, for areas such as hard subprocesses, initial- and final-state parton showers, underlying events and beam remnants, fragmentation and decays, and much more. Furthermore, extensive information is provided on all program elements: subroutines and functions, switches and parameters, and particle and process data. This should allow the user to tailor the generation task to the topics of interest.
Torbjörn Sjöstrand
The structure of events in high-energy collisions is complex and not predictable from first principles. Event generators allow the problem to be subdivided into more manageable pieces, some of which can be described from first principles, while others need to be based on appropriate models with parameters tuned to data. In these lectures we provide an overview, discuss how matrix elements are used, introduce the machinery for initial- and final-state parton showers, explain how matrix elements and parton showers can be combined for optimal accuracy, introduce the concept of multiple parton--parton interactions, comment briefly on the hadronization issue, and provide an outlook for the future.
Torbjörn Sjöstrand, Leif Lönnblad, Stephen Mrenna
The PYTHIA program can be used to generate high-energy-physics `events', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, with emphasis on those where strong interactions play a role, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics input is summarized here, for areas such as hard subprocesses, initial- and final-state parton showers, beam remnants and underlying events, fragmentation and decays, and much more. Furthermore, extensive information is provided on all program elements: subroutines and functions, switches and parameters, and particle and process data. This should allow the user to tailor the generation task to the topics of interest. The information in this edition of the manual refers to PYTHIA version 6.200, of 31 August 2001. The official reference to the latest published version is T. Sjöstrand, P. Edén, C. Friberg, L. Lönnblad, G. Miu, S. Mrenna and E. Norrbin, Computer Physics Commun. 135 (2001) 238.
Torbjörn Sjöstrand
Recent improvements in the PYTHIA event generator are summarized: new hard subprocesses, gamma*-gamma* physics, QCD final-state showers, and more.
Valery A. Khoze, Torbjorn Sjostrand
The process $\ee \to \W^+\W^- \to \f_1 \fbar_2 \, \f_3 \fbar_4$ contains charges in the initial, intermediate and final stages. This gives a rich selection of possible QED interconnection effects. Coulomb interaction is the simplest of these, and can thus be used to explore consequences. We study a number of experimental observables, with emphasis on those related to the W momentum distribution. Second-order Coulomb effects are shown to be practically negligible. The limited LEP~2 statistics will not allow detailed tests, so any theory uncertainty will be reflected in the systematic error on the W mass. Currently the uncertainty from this source may be as high as 20~MeV.
Andy Buckley, Jonathan Butterworth, Stefan Gieseke, David Grellscheid, Stefan Hoche, Hendrik Hoeth, Frank Krauss, Leif Lonnblad, Emily Nurse, Peter Richardson, Steffen Schumann, Michael H. Seymour, Torbjorn Sjostrand, Peter Skands, Bryan Webber
We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hard-scattering matrix elements for processes of interest, at both leading and next-to-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diffractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the ARIADNE, Herwig++, PYTHIA 8 and SHERPA generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.
Torbjörn Sjöstrand
Pythia 8, the C++ rewrite of the commonly-used Pythia event generator, is now available in a first full-fledged version 8.1. The older Pythia 6.4 generator in Fortran 77 is still maintained, for now, but users are strongly recommended to try out and move to the new version as soon as feasible.
Torbjörn Sjöstrand
Event generators play a crucial role in the exploration of LHC physics. This presentation summarizes news and plans for the three general-purpose pp generators HERWIG, PYTHIA and SHERPA, as well as briefer notes on a few other generators. Common themes, such as the matching and merging between matrix elements and parton showers, are highlighted. Other topics include a historical introduction, from the Lund perspective, and comments on the role of MCnet.
Torbjörn Sjöstrand
An algorithm for an improved description of final-state QCD radiation is introduced. It is matched to the first-order matrix elements for gluon emission in a host of decays, for processes within the Standard Model and the Minimal Supersymmetric extension thereof.