Lars Hofer

Recursive generation of one-loop amplitudes in the Standard Model

A bstractWe 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.

Resummation of tan β-enhanced supersymmetric loop corrections beyond the decoupling limit

We study the Minimal Supersymmetric Standard Model with Minimal Flavour Violation for the case of a large parameter tan ? and arbitrary values of the supersymmetric mass parameters. We derive several resummation formulae for tan ?-enhanced loop corrections, which were previously only known in the limit of supersymmetric masses far above the electroweak scale. Studying first the renormalisation-scheme dependence of the resummation formula for the bottom Yukawa coupling, we clarify the use of the sbottom mixing angle in the supersymmetric loop factor ?b. As a new feature, we find tan ?-enhanced loop-induced flavour-changing neutral current (FCNC) couplings of gluinos and neutralinos which in turn give rise to new effects in the renormalisation of the Cabibbo-Kobayashi-Maskawa matrix and in FCNC processes of B mesons. For the chromomagnetic Wilson coefficient C8, these gluino-squark loops can be of the same size as the known chargino-squark contribution. We discuss the phenomenological consequences for the mixing-induced CP asymmetry in Bd ? KS. We further quote formulae for Bs ? ?+?? and Bs?s mixing valid beyond the decoupling limit and find a new contribution affecting the phase of the Bs?s mixing amplitude. Our resummed tan ?-enhanced effects are cast into Feynman rules permitting an easy implementation in automatic calculations.

Complete resummation of chirally-enhanced loop-effects in the MSSM with non-minimal sources of flavor-violation

In this article we present the complete resummation of the leading chirallyenhanced corrections stemming from gluino-squark, chargino-sfermion and neutralino-sfermion loops in the MSSM with non-minimal sources of flavor-violation. We compute the finite renormalization of fermion masses and the CKM matrix induced by chirality-flipping self-energies. In the decoupling limit MSUSY ≫ v, which is an excellent approximation to the full theory, we give analytic results for the effective gaugino(higgsino)-fermion-sfermion and the Higgs-fermion-fermion vertices. Using these vertices as effective Feynman rules, all leading chirally-enhanced corrections can consistently be included into perturbative calculations of Feynman amplitudes. We also give a generalized parametrization for the bare CKM matrix which extends the classic Wolfenstein parametrization to the case of complex parameters λ and A.

COLLIER - A fortran-library for one-loop integrals

We introduce the fortran-library COLLIER for the numerical evaluation of one-loop scalar and tensor integrals in perturbative relativistic quantum field theories. Important features are the implementation of dedicated methods to achieve numerical stability for 3- and 4-point tensor integrals, the support of complex masses for internal particles, and the possibility to choose between dimensional and mass regularization for infrared singularities. COLLIER supports one-loop N-point functions up to currently N=6 and has been tested in various NLO QCD and EW calculations.

Lepton-flavour violating

LHCb has reported deviations from the Standard Model in

B

b\to s\mu^+\mu^-

decays in generic

transitions for which a new neutral gauge boson is a prime candidate for an explanation. As this gauge boson has to couple in a flavour non-universal way to muons and electrons in order to explain

Z^\prime

R_K

models

, it is interesting to examine the possibility that also lepton flavour is violated, especially in the light of the CMS excess in

Phenomenological consequences of radiative flavor violation in the MSSM

h\to\tau^\pm\mu^\mp