On "dynamical mass" generation in Euclidean de Sitter space

We consider the perturbative treatment of the minimally coupled, massless, self-interacting scalar field in Euclidean de Sitter space. Generalizing work of Rajaraman, we obtain the dynamical mass m^2 \propto sqrt{lambda} H^2 of the scalar for non-vanishing Lagrangian masses and the first perturbative quantum correction in the massless case. We develop the rules of a systematic perturbative expansion, which treats the zero-mode non-perturbatively, and goes in powers of sqrt{lambda}. The infrared divergences are self-regulated by the zero-mode dynamics. Thus, in Euclidean de Sitter space the interacting, massless scalar field is just as well-defined as the massive field. We then show that the dynamical mass can be recovered from the diagrammatic expansion of the self-energy and a consistent solution of the Schwinger-Dyson equation, but requires the summation of a divergent series of loop diagrams of arbitrarily high order. Finally, we note that the value of the long-wavelength mode two-point function in Euclidean de Sitter space agrees at leading order with the stochastic treatment in Lorentzian de Sitter space, in any number of dimensions.Comment: LaTeX, 15 page

Bˉ→Xsγ\bar B\to X_s \gamma with a warped bulk Higgs

We study the decay $\bar B\to X_s\gamma$ in Randall-Sundrum models with an IR-localised bulk Higgs. The two models under consideration are a minimal model as well as a model with a custodial protection mechanism. We include the effects of tree- and one-loop diagrams involving 5D gluon and Higgs exchanges as well as QCD corrections arising from the evolution from the Kaluza-Klein scale to the typical scale of the decay. We find the RS corrections to the branching fraction can be sizeable for large Yukawas and moderate KK scales $T$; for small Yukawas the RS contribution is small enough to be invisible in current experimental data.Comment: 20 pages, 5 figures(V2: references added, matches version published in Nuclear Physics B

The top-quark's running mass

We discuss the direct determination of the running top-quark mass from measurements of the total cross section of hadronic top-quark pair-production. The theory predictions in the MSbar scheme are very stable under scale variations and show rapid apparent convergence of the perturbative expansion. These features are explained by studying the underlying parton dynamics.Comment: 6 pages, 2 figures; to appear in Proceedings of the 9th International Symposium on Radiative Corrections, RADCOR 2009, Ascona, Switzerland, October 200

Lepton flavour violation in RS models with a brane- or nearly brane-localized Higgs

We perform a comprehensive study of charged lepton flavour violation in Randall-Sundrum (RS) models in a fully 5D quantum-field-theoretical framework. We consider the RS model with minimal field content and a "custodially protected" extension as well as three implementations of the IR-brane localized Higgs field, including the non-decoupling effect of the Kaluza-Klein (KK) excitations of a narrow bulk Higgs. Our calculation provides the first complete result for the flavour-violating electromagnetic dipole operator in Randall-Sundrum models. It contains three contributions with different dependence on the magnitude of the anarchic 5D Yukawa matrix, which can all be important in certain parameter regions. We study the typical range for the branching fractions of mu -> e gamma, mu -> 3e, mu N -> e N as well as tau -> mu gamma, tau -> 3 mu and the electron electric dipole moment by a numerical scan in both the minimal and the custodial RS model. The combination of mu -> e gamma and mu N -> e N currently provides the most stringent constraint on the parameter space of the model. A typical lower limit on the KK scale T is around 2 TeV in the minimal model (up to 4 TeV in the bulk Higgs case with large Yukawa couplings), and around 4 TeV in the custodially protected model, which corresponds to a mass of about 10 TeV for the first KK excitations, far beyond the lower limit from the non-observation of direct production at the LHC.Comment: 64 pages, 21 figures, LaTeX, v2: electron EDM analysis added, matches published versio

- XSummer - Transcendental Functions and Symbolic Summation in Form

Harmonic sums and their generalizations are extremely useful in the evaluation of higher-order perturbative corrections in quantum field theory. Of particular interest have been the so-called nested sums,where the harmonic sums and their generalizations appear as building blocks, originating for example from the expansion of generalized hypergeometric functions around integer values of the parameters. In this Letter we discuss the implementation of several algorithms to solve these sums by algebraic means, using the computer algebra system Form.Comment: 21 pages, 1 figure, Late

Scattering amplitudes for e^+e^- --> 3 jets at next-to-next-to-leading order QCD

We present the calculation of the fermionic contribution to the QCD two-loop amplitude for e^+e^- --> q qbar g.Comment: 5 pages, 4 figures, espcrc2.sty (included), Talk given at QCD '02, Montpellier, France, 2-9th July 200