New proofs for the two Barnes lemmas and an additional lemma

Mellin-Barnes (MB) representations have become a widely used tool for the evaluation of Feynman loop integrals appearing in perturbative calculations of quantum field theory. Some of the MB integrals may be solved analytically in closed form with the help of the two Barnes lemmas which have been known in mathematics already for one century. The original proofs of these lemmas solve the integrals by taking infinite series of residues and summing these up via hypergeometric functions. This paper presents new, elegant proofs for the Barnes lemmas which only rely on the well-known basic identity of MB representations, avoiding any series summations. They are particularly useful for presenting and proving the Barnes lemmas to students of quantum field theory without requiring knowledge on hypergeometric functions. The paper also introduces and proves an additional lemma for a MB integral \int dz involving a phase factor exp(+-i pi z).Comment: 6 page

NNLO non-resonant corrections to threshold top-pair production from e+ e- collisions: Endpoint-singular terms

We analyse the subleading non-resonant contributions to the e+ e- -> W+ W- b bbar cross section at energies near the top-antitop threshold. These correspond to next-to-next-to-leading-order (NNLO) corrections with respect to the leading-order resonant result. We show that these corrections produce 1/epsilon endpoint singularities which precisely cancel the finite-width divergences arising in the resonant production of the W+ W- b bbar final state from on-shell decays of the top and antitop quarks at the same order. We also provide analytic results for the (m_t/Lambda)^2, (m_t/Lambda) and (m_t/Lambda)^0 log(Lambda) terms that dominate the expansion in powers of (Lambda/m_t) of the complete set of NNLO non-resonant corrections, where Lambda is a cut imposed on the invariant masses of the b W pairs that is neither too tight nor too loose (m_t Gamma_t << Lambda^2 << m_t^2).Comment: 36 pages, 6 figures. v2: minor title change and a few trivial corrections (mostly language) in accordance with the version published in PR

Two-loop electroweak next-to-leading logarithms for processes involving heavy quarks

We derive logarithmically enhanced two-loop virtual electroweak corrections for arbitrary fermion-scattering processes at the TeV scale. This extends results previously obtained for massless fermion scattering to processes that involve also bottom and top quarks. The contributions resulting from soft, collinear, and ultraviolet singularities in the complete electroweak Standard Model are explicitly extracted from two-loop diagrams to next-to-leading-logarithmic accuracy including all effects associated with symmetry breaking and Yukawa interactions.Comment: 55 pages, LaTeX, version to appear in JHEP, minor revisions of text, results unchange

Foundation and generalization of the expansion by regions

The "expansion by regions" is a method of asymptotic expansion developed by Beneke and Smirnov in 1997. It expands the integrand according to the scaling prescriptions of a set of regions and integrates all expanded terms over the whole integration domain. This method has been applied successfully to many complicated loop integrals, but a general proof for its correctness has still been missing. This paper shows how the expansion by regions manages to reproduce the exact result correctly in an expanded form and clarifies the conditions on the choice and completeness of the considered regions. A generalized expression for the full result is presented that involves additional overlap contributions. These extra pieces normally yield scaleless integrals which are consistently set to zero, but they may be needed depending on the choice of the regularization scheme. While the main proofs and formulae are presented in a general and concise form, a large portion of the paper is filled with simple, pedagogical one-loop examples which illustrate the peculiarities of the expansion by regions, explain its application and show how to evaluate contributions within this method.Comment: 84 pages; v2: comment on scaleless integrals added to conclusions, version published in JHE