99 research outputs found
Two loop integrals and QCD scattering
We present the techniques for the calculation of one- and two-loop integrals contributing to the virtual corrections to 2→2 scattering of massless particles. First, tensor integrals are related to scalar integrals with extra powers of propagators and higher dimension using the Schwinger representation. Integration By Parts and Lorentz Invariance recurrence relations reduce the number of independent scalar integrals to a set of master integrals for which their expansion in є = 2 — D/2 is calculated using a combination of Feynman parameters, the Negative Dimension Integration Method, the Differential Equations Method, and Mellin-Barnes integral representations. The two-loop matrix-elements for light-quark scattering are calculated in Conventional Dimensional Regularisation by direct evaluation of the Feynman diagrams. The ultraviolet divergences are removed by renormalising with the MS scheme. Finally, the infrared singular behavior is shown to be in agreement with the one anticipated by the application of Catani's formalism for the infrared divergences of generic QCD two-loop amplitudes
Mixed QCD-electroweak corrections to Higgs boson production in gluon fusion
We compute the 3-loop O(\alpha \alpha_s) correction to the Higgs boson
production cross section arising from light quarks using an effective theory
approach. Our calculation probes the factorization of QCD and electroweak
perturbative corrections to this process. We combine our results with the best
current estimates for contributions from top and bottom quarks to derive an
updated theoretical prediction for the Higgs boson production cross section in
gluon fusion. With the use of the MSTW 2008 parton distribution functions that
include the newest experimental data, our study results in cross sections
approximately 4-6% lower for intermediate Higgs boson masses than those used in
recent Tevatron analyses that imposed a 95% confidence level exclusion limit of
a Standard Model Higgs boson with M_H=170 GeV.Comment: 16 pgs., 5 figs. References and discussion added. Numerical results
updated to use recent MSTW 2008 PDFs, which decrease the predicted Tevatron
cross sectio
The gluon-fusion uncertainty in Higgs coupling extractions
We point out that the QCD corrections to the gluon-fusion Higgs boson
production cross section at the LHC are very similar to the corrections to the
Higgs decay rate into two gluons. Consequently, the ratio of these two
quantities has a theoretical uncertainty smaller than the uncertainty in the
cross section alone by a factor of two. We note that since this ratio is the
theoretical input to analyses of Higgs coupling extractions at the LHC, the
reduced uncertainty should be used; in previous studies, the full cross section
uncertainty was employed.Comment: 4 pages, 3 figure
Higgs boson production at hadron colliders: differential cross sections through next-to-next-to-leading order
We present a calculation of the fully differential cross section for Higgs
boson production in the gluon fusion channel through next-to-next-to-leading
order in perturbative QCD. We apply the method introduced in
\cite{Anastasiou:2003gr} to compute double real emission corrections. Our
calculation permits arbitrary cuts on the final state in the reaction . It can be easily extended to include decays of the Higgs boson into
observable final states. In this Letter, we discuss the most important features
of the calculation, and present some examples of physical applications that
illustrate the range of observables that can be studied using our result. We
compute the NNLO rapidity distribution of the Higgs boson, and also calculate
the NNLO rapidity distribution with a veto on jet activity.Comment: 4 pgs, 2 figs; references adde
One-loop QCD contributions to differential cross-sections for Higgs production at NLO
We present one-loop contributions to the fully differential Higgs boson
gluon-fusion cross-section for Higgs production via gluon fusion. Our results
constitute a necessary ingredient of a complete NLO determination of the
cross-section. We perform our computation using a subtraction method for the
treatment of soft and collinear singularities. We identify the infrared
divergent parts in terms of universal splitting and eikonal functions, and
demonstrate how phase-space integrations yield poles (up to ) in
the dimensional regulator . We compute the coefficients of
the expansion, including the finite part numerically. As a
demonstration of our numerical implementation, we present the corrections at
NLO due to one-loop amplitudes in the rapidity and transverse momentum of
the Higgs boson.Comment: 25 pages, 2 figure
Removing infrared divergences from two-loop integrals
Feynman amplitudes at higher orders in perturbation theory generically have
complex singular structures. Notwithstanding the emergence of many powerful new
methods, the presence of infrared divergences poses significant challenges for
their evaluation. In this article, we develop a systematic method for the
removal of the infrared singularities, by adding appropriate counterterms that
approximate and cancel divergent limits point-by-point at the level of the
integrand. We provide a proof of concept for our method by applying it to
master-integrals that are found in scattering amplitudes for representative
two-to-two scattering processes of massless particles. We demonstrate that,
after the introduction of counterterms, the remainder is finite in four
dimensions. In addition, we find in these cases that the complete singular
dependence of the integrals can be obtained simply by analytically integrating
the counterterms. Finally, we observe that our subtraction method can be also
useful in order to extract in a simple way the asymptotic behavior of Feynman
amplitudes in the limit of small mass parameters.Comment: 53 pages, 13 figure
Dilepton rapidity distribution in the Drell-Yan process at NNLO in QCD
We compute the rapidity distribution of the virtual photon produced in the
Drell-Yan process through next-to-next-to-leading order in perturbative QCD. We
introduce a powerful new method for calculating differential distributions in
hard scattering processes. This method is based upon a generalization of the
optical theorem; it allows the integration-by-parts technology developed for
multi-loop diagrams to be applied to non-inclusive phase-space integrals, and
permits a high degree of automation. We apply our results to the analysis of
fixed target experiments.Comment: 5 pages, revte
Light Quark Mediated Higgs Boson Threshold Production in the Next-to-Leading Logarithmic Approximation
We study the amplitude of the Higgs boson production in gluon fusion mediated
by a light quark loop and evaluate the logarithmically enhanced radiative
corrections to the next-to-leading logarithmic approximation which sums up the
terms of the form to all orders in the strong
coupling constant. This result is used for the calculation of the process cross
section near the production threshold and gives a quantitative estimate of the
three and four-loop bottom quark contribution to the Higgs boson production at
the Large Hadron Collider.Comment: 25 pages, 6 figure
The one-loop gluon amplitude for heavy-quark production at NNLO
We compute the one-loop QCD amplitude for the process gg-->Q\bar{Q} in
dimensional regularization through order \epsilon^2 in the dimensional
regulator and for arbitrary quark mass values. This result is an ingredient of
the NNLO cross-section for heavy quark production at hadron colliders. The
calculation is performed in conventional dimensional regularization, using well
known reduction techniques as well as a method based on recent ideas for the
functional form of one-loop integrands in four dimensions.Comment: 27 pages, 3 figure
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