NLL Corrections for B-Meson Radiative Exclusive Decays

We calculate the next-to-leading corrections to the branching ratio of exclusive $B\to K^* \gamma$ decay. The renormalization scale dependence is reduced compared to the leading logarithmic result but there remains a dependence on a cutoff parameter of the hadronic model. The calculated corrections increase the predicted branching ratio by about 10%, but it remains in agreement with the experimental value.Comment: 16 pages, 8 figure

Photon Energy Spectrum in B→XsγB \to X_s \gamma and Comparison with Data

A comparison of the inclusive photon energy spectrum in the radiative decay \BGAMAXS, measured recently by the CLEO collaboration, with the standard model is presented, using a $B$-meson wave function model and improving earlier perturbative QCD-based computations of the same. The dependence of the photon energy spectrum on the non-perturbative model parameters, $p_F$, the $b$-quark Fermi momentum in the $B$ hadron, and $m_q$, the spectator quark mass, is explicitly shown, allowing a comparison of these parameters with the ones obtained from the analysis of the lepton energy spectrum in semileptonic $B$ decays. Taking into account present uncertainties, we estimate \BBGAMAXS = (2.55 \pm 1.28) \times 10^{-4} in the standard model, assuming \absvts/\absvcb= 1.0. Comparing this with the CLEO measurement \BBGAMAXS = (2.32 \pm 0.67) \times 10^{-4} implies \absvts/\absvcb= 1.1 \pm 0.43, in agreement with the CKM unitarity.Comment: 16 pages (including 5 Figures as encapsulated ps-files: use epsf and rotate

Two-loop SQCD corrections to Higgs-quark-quark couplings in the generic MSSM

In this article we compute the two-loop SQCD corrections to Higgs-quark-quark couplings in the generic MSSM generated by diagrams involving squarks and gluinos. We give analytic results for the two-loop contributions in the limit of vanishing external momenta for general SUSY masses valid in the MSSM with general flavour-structure. Working in the decoupling limit (M_SUSY >> v) we resum all chirally enhanced corrections (related to Higgs-quark-quark couplings) up to order \alpha_s^(n+1) tan(\beta)^n. This resummation allows for a more precise determination of the Yukawa coupling and CKM elements of the MSSM superpotential necessary for the study of Yukawa coupling unification. The knowledge of the Yukawa couplings of the MSSM superpotential in addition allows us to derive the effective Higgs-quark-quark couplings entering FCNC processes. These effective vertices can in addition be used for the calculation of Higgs decays into quarks as long as M_SUSY > M_Higgs holds. Furthermore, our calculation is also necessary for consistently including the chirally enhanced self-energies contributions into the calculation of FCNC processes in the MSSM beyond leading order. At two-loop order, we find an enhancement of the SUSY threshold corrections, induced by the quark self-energies, of approximately 9% for \mu=M_SUSY compared to the one-loop result. At the same time, the matching scale dependence of the effective Higgs-quark-quark couplings is significantly reduced.Comment: 35 pages, 5 figures, typos correcte

Exclusive Radiative Decays of B Mesons

We present within the Standard Model the exclusive radiative decays B -> K*/rho gamma and B_(s/d) -> gamma gamma in QCD factorization based on the heavy-quark limit m_b >> Lambda_QCD. For the decays with a vector meson in the final state we give results complete to next-to-leading order in QCD.Comment: 4 pages, contributed to QCD 02: High-Energy Physics International Conference in Quantum Chromodynamics, Montpellier, France, 2-9 July 200