Monte Carlo Generators

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.Comment: 23 pages, lectures presented at the 2006 European School of High-Energy Physics, Aronsborg, Sweden, 18 June -- 1 July 200

PYTHIA 8 Status Report

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.Comment: 5 pages, to appear in the proceedings of the HERA and the LHC workshop, 26--30 May 2008, CER

Status and developments of event generators

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.Comment: 22 pages, 9 figures, presented at the Fourth Annual Large Hadron Collider Physics Conference (LHCP2016), 13--18 June 2016, Lund, Swede

Collective Effects: the viewpoint of HEP MC codes

Collective effects are observed in high-multiplicity pp events, similar to the signals traditionally attributed to the formation of a Quark Gluon Plasma in heavy ion collisions. In core--corona models it is assumed that a partial plasma formation is indeed possible also in pp, but here the focus is on several recent models that attempt to explain pp data without invoking plasma formation. These attempts are partly successful, but there is still not a unified framework.Comment: 8 pages, presented at XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018

QCD Interconnection Effects

Heavy objects like the W, Z and t are short-lived compared with typical hadronization times. When pairs of such particles are produced, the subsequent hadronic decay systems may therefore become interconnected. We study such potential effects at Linear Collider energies.Comment: 1+7 pages, 6 eps figures, to appear in the Proceedings of the Workshop on the development of future linear electron-positron colliders for particle physics studies and for research using free electon lasers, Lund, Sweden, 23-26 September 199

Colour reconnection and its effects on precise measurements at the LHC

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.Comment: 8 pages, 1 figure, to appear in the proceedings of the XLIII International Symposium on Multiparticle Dynamics (ISMD13), Chicago, USA, 15 -- 20 September 201

Monte Carlo Tools

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.Comment: 27 pages, to appear in the proceedings of the 65th Scottish Universities Summer School in Physics: LHC Physics, St Andrews, 16 - 29 August 200

Jet Production by Virtual Photons

The production of jets is studied in collisions of virtual photons, gamma*-p and gamma*-gamma*, specifically for applications at HERA and LEP2. Photon flux factors are convoluted with matrix elements involving either direct or resolved photons and, for the latter, with parton distributions of the photon. Special emphasis is put on the range of uncertainty in the modeling of the resolved component. The resulting model is compared with existing data.Comment: 1+36 pages, LaTeX2e, 63 eps figures, submitted to Eur. Phys. J.

Multiparton Interactions with an x-dependent Proton Size

Theoretical arguments, supported by other indirect evidence, suggest that the wave function of high-x partons should be narrower than that of low-x ones. In this article, we present a modification to the variable impact parameter framework of Pythia 8 to model this effect. In particular, a Gaussian hadronic matter profile is introduced, with a width dependent on the x value of the constituent being probed. Results are compared against the default single- and double-Gaussian profiles, as well as an intermediate overlap function