NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC: 1. quark-antiquark annihilation

The process pp -> top anti-top bottom anti-bottom + X represents a very important background reaction to searches at the LHC, in particular to top anti-top H production where the Higgs boson decays into a bottom anti-bottom pair. A successful analysis of top anti-top H at the LHC requires the knowledge of direct top anti-top bottom anti-bottom production at next-to-leading order in QCD. We take the first step in this direction upon calculating the next-to-leading-order QCD corrections to the subprocess initiated by quark anti-quark annihilation. We devote an appendix to the general issue of rational terms resulting from ultraviolet or infrared (soft or collinear) singularities within dimensional regularization. There we show that, for arbitrary processes, in the Feynman gauge, rational terms of infrared origin cancel in truncated one-loop diagrams and result only from trivial self-energy corrections.Comment: 30 pages, LaTeX, 12 postscript figure

Four-fermion production at gamma gamma colliders: 2. Radiative corrections in double-pole approximation

The O(alpha) electroweak radiative corrections to gamma gamma --> WW --> 4f within the electroweak Standard Model are calculated in double-pole approximation (DPA). Virtual corrections are treated in DPA, leading to a classification into factorizable and non-factorizable contributions, and real-photonic corrections are based on complete lowest-order matrix elements for gamma gamma --> 4f + gamma. Soft and collinear singularities appearing in the virtual and real corrections are combined alternatively in two different ways, namely by using the dipole subtraction method or by applying phase-space slicing. The radiative corrections are implemented in a Monte Carlo generator called COFFERgammagamma, which optionally includes anomalous triple and quartic gauge-boson couplings in addition and performs a convolution over realistic spectra of the photon beams. A detailed survey of numerical results comprises O(alpha) corrections to integrated cross sections as well as to angular, energy, and invariant-mass distributions. Particular attention is paid to the issue of collinear-safety in the observables.Comment: 42 pages, latex, 34 postscript figure

t \bar{t} W production and decay at NLO

We present results for the production of a top pair in association with a W-boson at next-to-leading order. We have implemented this process into the parton-level integrator MCFM including the decays of both the top quarks and the W-bosons with full spin correlations. Although the cross section for this process is small, it is a Standard Model source of same-sign lepton events that must be accounted for in many new physics searches. For a particular analysis of same-sign lepton events in which b-quarks are also present, we investigate the effect of the NLO corrections as a function of the signal region cuts.Comment: 10 pages, 7 figure

NLO QCD corrections to pp -> ttbb+X via quark anti-quark annihilation

The process pp -> top anti-top bottom anti-bottom + X represents a very important background reaction to searches at the LHC, in particular to top anti-top H production where the Higgs boson decays into a bottom anti-bottom pair. A successful analysis of top anti-top H at the LHC requires the knowledge of direct top anti-top bottom anti-bottom production at NLO in QCD. We take the first step in this direction upon calculating the NLO QCD corrections to the subprocess initiated by quark anti-quark annihilation.Comment: 6 pages, LaTeX, 4 postscript figures, to appear in proceedings of the "9th Workshop on Elementary Particle Theory: Loops and Legs in Quantum Field Theory", Sondershausen, Germany, April 20-25, 200

Recent Progress in the Golem Project

We report on the current status of the Golem project which aims at the construction of a general one-loop evaluator for matrix elements. We construct the one-loop matrix elements from Feynman diagrams in a highly automated way and provide a library for the reduction and numerically stable evaluation of the tensor integrals involved in this approach. Furthermore, we present applications to physics processes relevant for the LHC.Comment: 7 pages, 4 figures, contrib. to proceedings of "Loops and Legs in Quantum Field Theory", 10th DESY Workshop on Elementary Particle Theory, 25-30 April 2010, Woerlitz, German

The complex-mass scheme for perturbative calculations with unstable particles

Perturbative calculations with unstable particles require the inclusion of their finite decay widths. A convenient, universal scheme for this purpose is the complex-mass scheme. It fully respects gauge-invariance, is straight-forward to apply, and has been successfully used for the calculation of various tree-level processes and of the electroweak radiative corrections to e+ e- -> 4f and H -> 4f.Comment: 5 pages, LaTeX, to appear in the proceedings of the "8th DESY Workshop on Elementary Particle Theory, Loops and Legs in Quantum Field Theory", Eisenach, 200

NLO QCD calculations with HELAC-NLO

Achieving a precise description of multi-parton final states is crucial for many analyses at LHC. In this contribution we review the main features of the HELAC-NLO system for NLO QCD calculations. As a case study, NLO QCD corrections for tt + 2 jet production at LHC are illustrated and discussed.Comment: 7 pages, 4 figures. Presented at 10th DESY Workshop on Elementary Particle Theory: Loops and Legs in Quantum Field Theory, Worlitz, Germany, April 25-30, 201

NLO QED Corrections to Hard-Bremsstrahlung Emission in Bhabha Scattering

In this paper we present a numerical implementation of the one-loop QED corrections to the hard-bremsstrahlung process e- e+ \to e- e+ gamma. These corrections can be included in the Monte Carlo event generators employed for simulating Bhabha scattering events at low-energy high-luminosity electron-positron colliders. The calculation is performed by employing the reduction method developed by Ossola, Papadopoulos and Pittau. Our results are implemented in a modular code for the numerical evaluation of the scattering amplitudes for any given phase-space point. In a similar way, we also evaluate the one-loop QED corrections to e- e+ \to mu- mu+ gamma, which represents an interesting application of the method in the presence of two different mass scales in the loops.Comment: 8 pages, 5 figures, v2 minor changes: comments and references added, matches PLB versio