52 research outputs found
Two-Loop Threshold Singularities, Unstable Particles and Complex Masses
The effect of threshold singularities induced by unstable particles on
two-loop observables is investigated and it is shown how to cure them working
in the complex-mass scheme. The impact on radiative corrections around
thresholds is thoroughly analyzed and shown to be relevant for two selected LHC
and ILC applications: Higgs production via gluon fusion and decay into two
photons at two loops in the Standard Model. Concerning Higgs production, it is
essential to understand possible sources of large corrections in addition to
the well-known QCD effects. It is shown that NLO electroweak corrections can
incongruently reach a 10 % level around the WW vector-boson threshold without a
complete implementation of the complex-mass scheme in the two-loop calculation.Comment: LaTeX, 12 pages, 7 figure
NNLO Computational Techniques: the Cases H -> gamma gamma and H -> g g
A large set of techniques needed to compute decay rates at the two-loop level
are derived and systematized. The main emphasis of the paper is on the two
Standard Model decays H -> gamma gamma and H -> g g. The techniques, however,
have a much wider range of application: they give practical examples of general
rules for two-loop renormalization; they introduce simple recipes for handling
internal unstable particles in two-loop processes; they illustrate simple
procedures for the extraction of collinear logarithms from the amplitude. The
latter is particularly relevant to show cancellations, e.g. cancellation of
collinear divergencies. Furthermore, the paper deals with the proper treatment
of non-enhanced two-loop QCD and electroweak contributions to different
physical (pseudo-)observables, showing how they can be transformed in a way
that allows for a stable numerical integration. Numerical results for the
two-loop percentage corrections to H -> gamma gamma, g g are presented and
discussed. When applied to the process pp -> gg + X -> H + X, the results show
that the electroweak scaling factor for the cross section is between -4 % and +
6 % in the range 100 GeV < Mh < 500 GeV, without incongruent large effects
around the physical electroweak thresholds, thereby showing that only a
complete implementation of the computational scheme keeps two-loop corrections
under control.Comment: LaTeX, 70 pages, 8 eps figure
Next-to-Next-to-Leading Electroweak Logarithms for W-Pair Production at LHC
We derive the high energy asymptotic of one- and two-loop corrections in the
next-to-next-to-leading logarithmic approximation to the differential cross
section of -pair production at the LHC. For large invariant mass of the
W-pair the (negative) one-loop terms can reach more than 40%, which are
partially compensated by the (positive) two-loop terms of up to 10%.Comment: 23 pages, 9 figures, added explanations in section 3, corrected typos
and figures 7, 8,
Recursive generation of one-loop amplitudes in the Standard Model
We introduce the computer code Recola for the recursive generation of tree-level and one-loop amplitudes in the Standard Model. Tree-level amplitudes are constructed using off-shell currents instead of Feynman diagrams as basic building blocks. One-loop amplitudes are represented as linear combinations of tensor integrals whose coefficients are calculated similarly to the tree-level amplitudes by recursive construction of loop off-shell currents. We introduce a novel algorithm for the treatment of colour, assigning a colour structure to each off-shell current which enables us to recursively construct the colour structure of the amplitude efficiently. Recola is interfaced with a tensor-integral library and provides complete one-loop Standard Model amplitudes including rational terms and counterterms. As a first application we consider Z + 2 jets production at the LHC and calculate with Recola the next-to-leading-order electroweak corrections to the dominant partonic channels.ISSN:1126-6708ISSN:1029-847
Strongly-ordered infrared limits for subtraction counterterms from factorisation
After a brief introduction to the problem of subtraction of infrared divergences for high-order collider observables, we present a preliminary study of strongly-ordered soft and collinear multiple radiation from the point of view of factorisation. We show that the matrix elements of fields and Wilson lines that describe soft and collinear radiation in factorised scattering amplitudes can be re-factorised in strongly-ordered limits, providing a systematic method to compute them, to characterise their singularity structure, and to build local subtraction counterterms for strongly-ordered configurations. Our results provide tools for a detailed organisation of subtraction algorithms, in principle to all orders in perturbation theory
Strongly-ordered infrared limits for subtraction counterterms from factorisation
After a brief introduction to the problem of subtraction of infrared divergences for high-order collider observables, we present a preliminary study of strongly-ordered soft and collinear multiple radiation from the point of view of factorisation. We show that the matrix elements of fields and Wilson lines that describe soft and collinear radiation in factorised scattering amplitudes can be re-factorised in strongly-ordered limits, providing a systematic method to compute them, to characterise their singularity structure, and to build local subtraction counterterms for strongly-ordered configurations. Our results provide tools for a detailed organisation of subtraction algorithms, in principle to all orders in perturbation theory
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