arXiv subjects

Thomas Becher

Publications and source records attributed to Thomas Becher.

80 records · Page 5Linked to original sources

arXiv2002

Continuum methods in lattice perturbation theory

We show how methods of continuum perturbation theory can be used to simplify perturbative lattice calculations. We use the technique of asymptotic expansions to expand lattice loop integrals around the continuum limit. After the expansion, all nontrivial dependence on momenta and masses is encoded in continuum loop integrals and the only genuine lattice integrals left are tadpole integrals. Using integration-by-parts relations all of these can be expressed in terms of a small number of master integrals. Four master integrals are needed for bosonic one loop integrals, sixteen in QCD with Wilson or staggered fermions.

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arXiv2002

Constraints on Light Bottom Squarks from Radiative B-Meson Decays

The presence of a light b-squark (with mass about 4 GeV) and gluino (with mass about 15 GeV) might explain the observed excess in b-quark production at the Tevatron. Though provocative, this model is not excluded by present data. The light supersymmetric particles can induce large flavor-changing effects in radiative decays of B mesons. We analyse the decays B->X_s gamma and B->X_{sg} in this scenario and derive restrictive bounds on the flavor-changing quark-squark-gluino couplings.

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arXiv2002

The Asymptotic Expansion of Lattice Loop Integrals Around the Continuum Limit

We present a method of computing any one-loop integral in lattice perturbation theory by systematically expanding around its continuum limit. At any order in the expansion in the lattice spacing, the result can be written as a sum of continuum loop integrals in analytic regularization and a few genuine lattice integrals (``master integrals''). These lattice master integrals are independent of external momenta and masses and can be computed numerically. At the one-loop level, there are four master integrals in a theory with only bosonic fields, seven in HQET and sixteen in QED or QCD with Wilson fermions.

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arXiv2002

Baryon Chiral Dynamics

After contrasting the low energy effective theory for the baryon sector with one for the Goldstone sector, I use the example of pion nucleon scattering to discuss some of the progress and open issues in baryon chiral perturbation theory.

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arXiv2001

Flavor-Change with Ultra-Light Sbottom and Gluinos

Implications of a 2 - 5.5 GeV sbottom and 12 - 16 GeV gluino masses for rare B decay phenomenology are discussed. An effective Hamiltonian is constructed in which the gluinos are integrated out and a \tilde{b} squark remains among the light flavor degrees of freedom. Restrictive constraints come from $b \to s γand b \to sg, but they allow a substantially enhanced inclusive b decay rate into charmless hadronic final states, and {\cal O}(10%) direct CP asymmetries in $B \to X_s γand B^\pm \to K^0 π^\pm decays, which are an order of magnitude larger than in the Standard Model. New contributions to B_s mixing are negligible but significant effects in B_d mixing may be possible.

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arXiv2001

Improved Determination of |V_{ub}| from Inclusive Semileptonic B-Meson Decays

We reduce the perturbative uncertainty in the determination of |V_{ub}| from inclusive semileptonic B decays by calculating the rate of B -> X_u l nu events with dilepton invariant mass q^2>(M_B-M_D)^2 at subleading order in the hybrid expansion, and to next-to-leading order in renormalization-group improved perturbation theory. We also resum logarithmic corrections to the leading power-suppressed contributions. Studying the effect of different b-quark mass definitions we find that the branching ratio after the cut is Br(B -> X_u l nu)=(20.9+-4.0)|V_{ub}|^2, where the dominant error is due to the uncertainty in the b-quark mass. This implies that |V_{ub}| can be determined with a precision of about 10%.

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arXiv2001

Two-Loop Renormalization of Heavy--Light Currents at Order 1/m_Q in the Heavy-Quark Expansion

We present exact results, at next-to-leading order in renormalization-group improved perturbation theory, for the Wilson coefficients appearing at order 1/m_Q in the heavy-quark expansion of heavy-light current operators. To this end, we complete the calculation of the corresponding two-loop anomalous dimension matrix. Our results are important for determinations of |V_{ub}| using exclusive and inclusive semileptonic B decays. They are also relevant to computations of the decay constant f_B based on a heavy-quark expansion.

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arXiv2001

Asymptotics of QCD Factorization in Exclusive Hadronic Decays of B Mesons

Using the renormalon calculus, we study the asymptotic behavior of the perturbative expansion of the hard-scattering kernels entering the QCD factorization formula for the nonleptonic weak decays B->D M, where M is a light meson. In the ``large-beta_0 limit'', the kernels are infrared finite and free of endpoint singularities to all orders of perturbation theory. The leading infrared renormalon singularity corresponding to a power correction of order Lambda/m_b vanishes if the light meson has a symmetric light-cone distribution amplitude. We calculate the Borel transforms and the corresponding momentum distribution functions of the hard-scattering kernels, and resum the series of O(beta_0^{n-1} alpha_s^n) corrections to explore the numerical significance of higher-order perturbative and power corrections. We also derive explicit expressions for the O(beta_0 alpha_s^2) contributions to the kernels, and for the renormalon singularities corresponding to power corrections of order (Lambda/m_b)^2. Finally, we study the limit m_c->0 relevant to charmless hadronic decays such as B->pi pi.

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