259 research outputs found
t tbar W and t tbar Z Hadroproduction at NLO accuracy in QCD with Parton Shower and Hadronization effects
We present theoretical predictions for the hadroproduction of t tbar W+, t
tbar W- and t tbar Z at LHC as obtained by matching numerical computations at
NLO accuracy in QCD with Shower Monte Carlo programs. The calculation is
performed by PowHel, relying on the POWHEG-BOX framework, that allows for the
matching between the fixed order computation, with input of matrix elements
produced by the HELAC-NLO collection of event generators, and the Parton Shower
evolution, followed by hadronization and hadron decays as described by PYTHIA
and HERWIG. We focus on the dilepton and trilepton decay channels, studied
recently by the CMS Collaboration.Comment: 21 pages 12 figure
Z0 - boson production in association with a top anti-top pair at NLO accuracy with parton shower effects
We present predictions for the production cross section of a Standard Model
Z0-boson in association with a top-antitop pair at the next-to-leading order
accuracy in QCD, matched with shower Monte Carlo programs to evolve the system
down to the hadronization energy scale. We adopt a framework based on three
well established numerical codes, namely the POWHEG-BOX, used for computing the
cross section, HELAC-NLO, which generates all necessary input matrix elements,
and finally a parton shower program, such as PYTHIA or HERWIG, which allows for
including t-quark and Z0-boson decays at the leading order accuracy and
generates shower emissions, hadronization and hadron decays.Comment: 10 pages, 5 figures; found and corrected a bug in the
phenomenological analysis, just affecting Fig.4 - 5 that turn out to change
slightly with respect to our previous version and the cross-section values
after all cuts. Conclusions qualitatively unchange
Separation of soft and collinear infrared limits of QCD squared matrix elements
We present a simple way of separating the overlap between the soft and collinear factorization formulae of QCD squared matrix elements. We check its validity explicitly for single and double unresolved emissions of tree-level processes. The new method makes possible the definition of helicity-dependent subtraction terms for regularizing the real contributions in computing radiative corrections to QCD jet cross sections. This implies application of Monte Carlo helicity summation in computing higher order corrections
One-loop Singular Behaviour of QCD and SUSY QCD Amplitudes with Massive Partons
We discuss the structure of infrared and ultraviolet singularities in on-shell QCD and supersymmetric QCD amplitudes at one-loop order. Previous results, valid for massless partons, are extended to the case of massive partons. Using dimensional regularization, we present a general factorization formula that controls both the singular epsilon-poles and the logarithmic contributions that become singular for vanishing masses. We introduce generalized Altarelli--Parisi splitting functions and discuss their relations with the singular terms in the amplitudes. The dependence on the regularization scheme is also considered
Precision studies for Drell-Yan processes at NNLO
We present a detailed comparison of the fixed-order predictions computed by
four publicly available computer codes for Drell-Yan processes at the LHC and
Tevatron colliders. We point out that while there is agreement among the
predictions at the next-to-leading order accuracy, the predictions at the
next-to-next-to-leading order (NNLO) differ, whose extent depends on the
observable. The sizes of the differences in general are at least similar,
sometimes larger than the sizes of the NNLO corrections themselves. We
demonstrate that the neglected power corrections by the codes that use global
slicing methods for the regularization of double real emissions can be the
source of the differences. Depending on the fiducial cuts, those power
corrections become linear, hence enhanced as compared to quadratic ones that
are considered standard.Comment: 24 pages LaTeX, 17 figures, (journal version
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