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Martin Gorbahn

Publications and source records attributed to Martin Gorbahn.

43 records · Page 3Linked to original sources

Effective Hamiltonian for Non-Leptonic |Delta F| = 1 Decays at NNLO in QCD

We compute the effective hamiltonian for non-leptonic |Delta F| = 1 decays in the standard model including next-to-next-to-leading order QCD corrections. In particular, we present the complete three-loop anomalous dimension matrix describing the mixing of current-current and QCD penguin operators. The calculation is performed in an operator basis which allows to consistently use fully anticommuting gamma_5 in dimensional regularization at an arbitrary number of loops. The renormalization scheme dependences and their cancellation in physical quantities is discussed in detail. Furthermore, we demonstrate how our results are transformed to a different basis of effective operators which is frequently adopted in phenomenological applications. We give all necessary two-loop constant terms which allow to obtain the three-loop anomalous dimensions and the corresponding initial conditions of the two-loop Wilson coefficients in the latter scheme. Finally, we solve the renormalization group equation and give the analytic expressions for the low-energy Wilson coefficients relevant for non-leptonic B meson decays beyond next-to-leading order in both renormalization schemes.

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The supersymmetric Higgs sector and B-Bbar mixing for large tan beta

We match the Higgs sector of the most general flavour breaking and CP violating minimal supersymmetric standard model (MSSM) onto a generic two-Higgs-doublet model, paying special attention to the definition of tan beta in the effective theory. In particular no tan beta-enhanced loop corrections appear in the relation to tan beta defined in the DRbar scheme in the MSSM. The corrections to the Higgs-mediated flavour-changing amplitudes which result from this matching are especially relevant for the B_d and B_s mass differences dM_s,d for minimal flavour violation, where the superficially leading contribution vanishes. We give a symmetry argument to explain this cancellation and perform a systematic study of all Higgs-mediated effects, including Higgs loops. The corrections to dM_s are at most 7% for mu>0 and M_A < 600 if constraints from other observables are taken into account. For mu<0 they can be larger, but are always less than about 20%. Contrary to recent claims we do not find numerically large contributions here, nor do we find any tan beta-enhanced contributions from loop corrections to the Higgs potential in B^+ -> tau^+ nu or B -> X_s gamma. We further update supersymmetric loop corrections to the Yukawa couplings, where we include all possible CP-violating phases and correct errors in the literature. The possible presence of CP-violating phases generated by Higgs exchange diagrams is briefly discussed as well. Finally we provide improved values for the bag factors P^VLL_1, P^LR_2, and P^SLL_1 at the electroweak scale.

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Electroweak Corrections to the Charm Quark Contribution to K+ -> pi+ nu nu-bar

We compute the leading-log QED, the next-to-leading-log QED-QCD, and the electroweak corrections to the charm quark contribution relevant for the rare decay K+ -> pi+ nu nu-bar. The corresponding parameter P_c(X) is increased by up to 2% with respect to the pure QCD estimate to P_c(X) = 0.372 +- 0.015 for m_c(m_c)= (1.286 +- 0.013) GeV, alphas(M_Z) = 0.1176 +- 0.0020 and |V_us| = 0.2255. For the branching ratio we find B(K+ -> pi+ nu nu-bar) = (8.5 +- 0.7)*10^{-11}, where the quoted uncertainty is dominated by the CKM elements.

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From transition magnetic moments to majorana neutrino masses

It is well known that a majorana mass induces a (small) transition magnetic moment. The converse is also true; in this paper we estimate the loop contribution of transition magnetic moments $[μ]_{αβ}$ to the neutrino mass matrix $[m]_{αβ}$. We show that for hierarchical neutrino masses, the contribution of $[μ]_{e τ}$ to $[m]_{e τ}$ can exceed the experimental value of $[m]_{e τ}$.

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Three-Loop Mixing of Dipole Operators

We calculate the complete three-loop O(alpha_s^3) anomalous dimension matrix for the dimension-five dipole operators that arise in the Standard Model after integrating out the top quark and the heavy electroweak bosons. Our computation completes the three-loop anomalous dimension matrix of operators that govern low-energy |Delta F| = 1 flavor-changing processes, and represents an important ingredient of the next-to-next-to-leading order QCD analysis of the Bbar -> X_s gamma decay.

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Anomalous Dimension Matrix for Radiative and Rare Semileptonic B Decays up to Three Loops

We compute the complete O(alphas^2) anomalous dimension matrix relevant for the b to s gamma, b to s gluon and b to s l+l- transitions in the standard model and some of its extensions. For radiative decays we confirm the results of Misiak and Munz, and of Chetyrkin, Misiak and Munz. The O(alphas^2) mixing of four-quark into semileptonic operators is instead a new result and represents one of the last missing ingredients of the next-to-next-to-leading-order analysis of rare semileptonic B meson decays.

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Higgs Sector of the Minimal Left-Right Symmetric Model

We perform an exhaustive analysis of the most general Higgs sector of the minimal left-right symmetric model (MLRM). We find that the CP properties of the vacuum state are connected to the Higgs spectrum: if CP is broken spontaneously, the MLRM does not approach the Standard Model in the limit of a decoupling left-right symmetry breaking scale. Depending on the size of the CP phases scenarios with extra non-decoupling flavor-violating doublet Higgses or very light SU(2) triplet Higgses emerge, both of which are ruled out by phenomenology. For zero CP phases the non-standard Higgses decouple only if a very unnatural fine-tuning condition is fulfilled. We also discuss generalizations to a non-minimal Higgs sector.

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