609 research outputs found
Helac-nlo
Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program
that is capable of numerically evaluating QCD virtual corrections to scattering
amplitudes. A detailed presentation of the algorithm is given, along with
instructions to run the code and benchmark results. The program is part of the
HELAC-NLO framework that allows for a complete evaluation of QCD NLO
corrections.Comment: minor text revisions, version to appear in Comput.Phys.Commu
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
Matching the Nagy-Soper parton shower at next-to-leading order
We present an MC@NLO-like matching of next-to-leading order QCD calculations
with the Nagy-Soper parton shower. An implementation of the algorithm within
the HELAC-DIPOLES Monte Carlo generator is used to address the uncertainties
and ambiguities of the matching scheme. First results obtained using the
Nagy-Soper parton shower implementation in DEDUCTOR in conjunction with the
HELAC-NLO framework are given for the pp -> top anti-top j + X process at the
LHC with sqrt(s)=8 TeV. Effects of resummation are discussed for various
observables.Comment: 53 pages, 18 figures, 3 tables. References and a few typos corrected,
acknowledgments added, dependence on the variation of the starting shower
time corrected, version to appear in JHE
Standard Model Higgs boson production in association with a top anti-top pair at NLO with parton showering
We present predictions for the production cross section of a Standard Model
Higgs boson in association with a top-antitop pair at next-to-leading order
accuracy using matrix elements obtained from the HELAC-Oneloop package. The NLO
prediction was interfaced to the PYTHIA and HERWIG shower Monte Carlo programs
with the help of POWHEG-Box, allowing for decays of massive particles,
showering and hadronization, thus leading to final results at the hadron level.Comment: 14 pages, 9 figure
On the ratio of ttbb and ttjj cross sections at the CERN Large Hadron Collider
Triggered by ongoing experimental analyses, we report on a study of the cross
section ratio sigma(pp -> ttbb)/sigma(pp -> ttjj) at the next-to-leading order
in QCD, focusing on both present and future collider energies: sqrt{s}= 7, 8,
13 TeV. In particular, we provide a comparison between our predictions and the
currently available CMS data for the 8 TeV run. We further analyse the
kinematics and scale uncertainties of the two processes for a single set of
parton distribution functions, with the goal of assessing possible correlations
that might help to reduce the theoretical error of the ratio and thus enhance
the predictive power of this observable. We argue that the different jet
kinematics makes the ttbb and ttjj processes uncorrelated in several
observables, and show that the scale uncertainty is not significantly reduced
when taking the ratio of the cross sections.Comment: 23 pages, 10 figures, 3 tables, some issues clarified,
acknowledgement and references added, version to appear in JHE
Top quark pair production in association with a jet with NLO parton showering
We compute the production cross section of a top-antitop pair in association
with a jet at hadron colliders at next-to-leading order accuracy matched with
parton shower algorithms to make predictions at the hadron level. The parton
shower allows for including the decay of the top quarks at the leading order
accuracy. We use a framework based on three well established numerical codes,
the POWHEG-BOX, used for the calculation of the cross section, HELAC, which
generates the matrix elements for the Born-level, real emission and the virtual
part, and finally a parton shower program, such as PYTHIA or HERWIG, which
generate the parton-shower and hadronization.Comment: changed title, bug in HERWIG analysis corrected, published version
with restructured discussion, but main conclusions unchange
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