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

Remarks on the particle multiplicities at LHC energies

Recent ALICE data for the multiplicity distributions in the central rapidity bins at LHC energies are compared with the results from two default versions of the PYTHIA 8 generator. We find that, contrary to the earlier versions of PYTHIA, the model overestimates the increase of average multiplicity with energy. Tuning two of the model parameters one obtains reasonable agreement with data. The dependence of the normalized moments of the distribution on the rapidity bin width and on energy is also qualitatively correct.Comment: 6 pages, 1 figur

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

Status of ATLAS and Preparation for the Pb-Pb Run

The ATLAS experiment took its first beam data in September 2008 and is actively preparing for the planned start of LHC collision data-taking in 2009. This preparation includes hardware and software commissioning, as well as calibration and cosmic data analysis. The status and performance of the ATLAS detector will be discussed, with a view towards the Pb+Pb run expected in 2010.Comment: 8 pages, 14 figures, corrected typos, added references, figures corrected for better legibility in B&W - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

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

Semiclassical Nonconcentration near Hyperbolic Orbits

For a large class of semiclassical pseudodifferential operators, including Schr\"odinger operators, $P (h) = -h^2 \Delta_g + V (x)$, on compact Riemannian manifolds, we give logarithmic lower bounds on the mass of eigenfunctions outside neighbourhoods of generic closed hyperbolic orbits. More precisely we show that if $A$ is a pseudodifferential operator which is microlocally equal to the identity near the hyperbolic orbit and microlocally zero away from the orbit, then $$\| u \| \leq C (\sqrt{\log(1/h)}/ h) \| P (h)u \| + C \sqrt {\log(1/h)} \| (I - A) u \| .$$ This generalizes earlier estimates of Colin de Verdi\`ere-Parisse \cite{CVP} obtained for a special case, and of Burq-Zworski \cite{BZ} for real hyperbolic orbits.Comment: 45 pages, 5 figure

On the sign-imbalance of partition shapes

Let the sign of a standard Young tableau be the sign of the permutation you get by reading it row by row from left to right, like a book. A conjecture by Richard Stanley says that the sum of the signs of all SYTs with n squares is 2^[n/2]. We present a stronger theorem with a purely combinatorial proof using the Robinson-Schensted correspondence and a new concept called chess tableaux. We also prove a sharpening of another conjecture by Stanley concerning weighted sums of squares of sign-imbalances. The proof is built on a remarkably simple relation between the sign of a permutation and the signs of its RS-corresponding tableaux.Comment: 12 pages. Better presentatio

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