752 research outputs found
in FDH
We investigate the regularization-scheme dependent treatment of
in the framework of dimensional regularization, mainly focusing on the
four-dimensional helicity scheme (FDH). Evaluating distinctive examples, we
find that for one-loop calculations, the recently proposed four-dimensional
formulation (FDF) of the FDH scheme constitutes a viable and efficient
alternative compared to more traditional approaches. In addition, we extend the
considerations to the two-loop level and compute the pseudo-scalar form factors
of quarks and gluons in FDH. We provide the necessary operator renormalization
and discuss at a practical level how the complexity of intermediate
calculational steps can be reduced in an efficient way.Comment: 28 pages, 7 figure
Two-loop Correction to the Leptonic Decay of Quarkonium
Applying asymptotic expansions at threshold, we compute the two-loop QCD
correction to the short-distance coefficient that governs the leptonic decay
of a S-wave quarkonium state and discuss its impact on the
relation between the quarkonium non-relativistic wave function at the origin
and the quarkonium decay constant in full QCD.Comment: 7 pages, LaTeX, 1 figure included via epsf.st
Renormalization-group improved fully differential cross sections for top pair production
We extend approximate next-to-next-to-leading order results for top-pair
production to include the semi-leptonic decays of top quarks in the
narrow-width approximation. The new hard-scattering kernels are implemented in
a fully differential parton-level Monte Carlo that allows for the study of any
IR-safe observable constructed from the momenta of the decay products of the
top. Our best predictions are given by approximate NNLO corrections in the
production matched to a fixed order calculation with NLO corrections in both
the production and decay subprocesses. Being fully differential enables us to
make comparisons between approximate results derived via different (PIM and
1PI) kinematics for arbitrary distributions. These comparisons reveal that the
renormalization-group framework, from which the approximate results are
derived, is rather robust in the sense that applying a realistic error estimate
allows us to obtain a reliable prediction with a reduced theoretical error for
generic observables and analysis cuts
Fully differential NLO predictions for the radiative decay of muons and taus
We present a general purpose Monte Carlo program for the calculation of the
radiative muon decay and the radiative decays
and at
next-to-leading order in the Fermi theory. The full dependence on the lepton
masses and polarization of the initial-sate lepton are kept. We study the
branching ratios for these processes and show that fully-differential
next-to-leading order corrections are important for addressing a tension
between BaBar's recent measurement of the branching ratio and the Standard Model prediction. In addition, we
study various distributions of the process and
obtain precise predictions for the irreducible background to
searches, tailored to the geometry of the MEG detector.Comment: 14 pages, 5 figures, published versio
Computation of in FDH and DRED: renormalization, operator mixing, and explicit two-loop results
The amplitude relevant for Higgs production via gluon fusion is
computed in the four-dimensional helicity scheme (FDH) and in dimensional
reduction (DRED) at the two-loop level. The required renormalization is
developed and described in detail, including the treatment of evanescent
-scalar contributions. In FDH and DRED there are additional
dimension-5 operators generating the vertices, where can either be
a gluon or an -scalar. An appropriate operator basis is given and the
operator mixing through renormalization is described. The results of the
present paper provide building blocks for further computations, and they allow
to complete the study of the infrared divergence structure of two-loop
amplitudes in FDH and DRED
SCET approach to regularization-scheme dependence of QCD amplitudes
We investigate the regularization-scheme dependence of scattering amplitudes
in massless QCD and find that the four-dimensional helicity scheme (FDH) and
dimensional reduction (DRED) are consistent at least up to NNLO in the
perturbative expansion if renormalization is done appropriately. Scheme
dependence is shown to be deeply linked to the structure of UV and IR
singularities. We use jet and soft functions defined in soft-collinear
effective theory (SCET) to efficiently extract the relevant anomalous
dimensions in the different schemes. This result allows us to construct
transition rules for scattering amplitudes between different schemes (CDR, HV,
FDH, DRED) up to NNLO in massless QCD. We also show by explicit calculation
that the hard, soft and jet functions in SCET are regularization-scheme
independent.Comment: 46 pages, 6 figure
Top physics: An overview
This is a very brief overview of the current status of top-quark physics at hadron colliders, a selection of some mainly theoretical aspects without any claim of completeness
Small-mass effects in heavy-to-light form factors
We present the heavy-to-light form factors with two different non-vanishing
masses at next-to-next-to-leading order and study its expansion in the small
mass. The leading term of this small-mass expansion leads to a factorized
expression for the form factor. The presence of a second mass results in a new
feature, in that the soft contribution develops a factorization anomaly. This
cancels with the corresponding anomaly in the collinear contribution. With the
generalized factorization presented here, it is possible to obtain the leading
small-mass terms for processes with large masses, such as muon-electron
scattering, from the corresponding massless amplitude and the soft
contribution.Comment: 20 pages, 4 figures, 1 ancillary file, published versio
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