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.

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PYTHIA 6.4 Physics and Manual

This paper describes the ATLAS experiment as installed in i ts experimental cavern at point 1 at CERN. It also presents a brief overview of the expec ted performance of the detector.

http://absimage.aps.org/image/SES08/MWS_SES08-2008-020010.pdf

The ATLAS Experiment at the CERN Large Hadron Collider

Theory and phenomenology of two-Higgs-doublet models

We review the present status of QCD corrections to weak decays beyond the leading-logarithmic approximation, including particle-antiparticle mixing and rare and $\mathrm{CP}$-violating decays. After presenting the basic formalism for these calculations we discuss in detail the effective Hamiltonians of all decays for which the next-to-leading-order corrections are known. Subsequently, we present the phenomenological implications of these calculations. The values of various parameters are updated, in particular the mass of the newly discovered top quark. One of the central issues in this review are the theoretical uncertainties related to renormalization-scale ambiguities, which are substantially reduced by including next-to-leading-order corrections. The impact of this theoretical improvement on the determination of the Cabibbo-Kobayashi-Maskawa matrix is then illustrated. [S0034-6861(96)00304-2]

Weak decays beyond leading logarithms

In this paper the current release of the Monte Carlo event generator Sherpa, version 1.1, is presented. Sherpa is a general-purpose tool for the simulation of particle collisions at high-energy colliders. It contains a very flexible tree-level matrix-element generator for the calculation of hard scattering processes within the Standard Model and various new physics models. The emission of additional QCD partons off the initial and final states is described through a parton-shower model. To consistently combine multi-parton matrix elements with the QCD parton cascades the approach of Catani, Krauss, Kuhn and Webber is employed. A simple model of multiple interactions is used to account for underlying events in hadron-hadron collisions. The fragmentation of partons into primary hadrons is described using a phenomenological cluster-hadronisation model. A comprehensive library for simulating tau-lepton and hadron decays is provided. Where available form-factor models and matrix elements are used, allowing for the inclusion of spin correlations; effects of virtual and real QED corrections are included using the approach of Yennie, Frautschi and Suura.

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Event generation with SHERPA 1.1

We present predictions for $t \bar t$ and $t \bar t H$ production and decay at future lepton colliders including non-resonant and interference contributions up to next-to-leading order (NLO) in perturbative QCD. The obtained precision predictions are necessary for a future precise determination of the top-quark Yukawa coupling, and allow for top-quark phenomenology in the continuum at an unprecedented level of accuracy. Simulations are performed with the automated NLO Monte-Carlo framework WHIZARD interfaced to the OpenLoops matrix element generator.

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NLO QCD Predictions for off-shell $t \bar t$ and $t \bar t H$ Production and Decay at a Linear Collider

The angular distributions in the Higgs-strahlung process $e^+e^-\to HZ\to H\bar f f$ are uniquely determined in the Standard Model. We study how these predictions are modified if non-standard couplings are present in the $ZZH$ vertex, as well as lepton-boson contact terms. We restrict ourselves to the set of operators which are singlets under standard $SU_3\times SU_2\times U_1$ transformations, CP conserving, dimension 6, helicity conserving, and custodial $SU_2$ conserving.

Anomalous couplings in the Higgs-strahlung process

A crucial test of the Standard Model is the measurement of electroweak gauge-boson scattering. In this paper, we describe a generic parameterization aimed at a realistic simulation of weak-boson scattering at the LHC. The parameterization implements resonances of all possible spin and isospin combinations, properly matched to the low-energy effective (chiral) Lagrangian, includes leading higher-order effects and contains a minimal unitarization scheme. We implement the parameterization in the Monte-Carlo event generator WHIZARD and present results for complete partonic cross-section integration and event generation. We provide a comparison with the effective W approximation that previously has been used for most WW scattering studies at hadron colliders.

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Resonances and Unitarity in Weak Boson Scattering at the LHC

We present a new method for the numerical evaluation of loop integrals which is based on the Feynman Tree Theorem. The loop integrals are replaced by phase-space integration over fictitious extra on-shell particles. This integration can be performed alongside with the Monte-Carlo integration of ordinary phase space, avoiding the time-consuming nesting of loop evaluation inside the integrand, and directly leading to NLO event generation. We systematically construct subtractions, necessary to cancel both ultraviolet divergences and the extra threshold singularities in phase-space which arise in the numerical evaluation. Infrared singularities can be dealt with by standard methods. As a proof of concept, we apply the method to NLO Bhabha scattering in QED and construct the corresponding NLO Monte Carlo event generator.

Wolfgang Kilian

Papers

NLO QCD Predictions for off-shell $t \bar t$ and $t \bar t H$ Production and Decay at a Linear Collider

Anomalous couplings in the Higgs-strahlung process

Resonances and Unitarity in Weak Boson Scattering at the LHC

Numerical evaluation of Feynman loop integrals by reduction to tree graphs

Testing Higgs self-couplings at