出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Theoretical predictions for exclusive semileptonic decays of heavy mesons are essential for determining CKM matrix elements and constraining new physics. On the lattice, one must treat the heavy quark in an effective theory, as 1=M a. At high recoil, discretization errors from the large 3-momentum of the final meson in the rest frame of the heavy meson are significant. A technique to avoid these errors is to give the heavy meson a significant “external” momentum in the lattice frame. The decay meson momentum is then not large on the lattice and a wider q2 region can be covered. While boosting to a moving frame increases the discretization error for the quarks in a B meson by _ (g21)L2 QCD, the discretization error of the final-state light meson behaves like _ 12 jpFj 2, and using q2 = (pB pF)2, one can pick an optimal v to minimize the total error. The key fact in applying the boosted frame technique is that almost all of the heavy meson momentum is carried by the heavy quark whilst the dynamics of the heavy quark inside the heavy meson remain non-relativistic. This leads to a generalization of Non-Relativistic Quantum Chromodynamics (NRQCD) to the case of moving heavy quarks, called moving NRQCD (mNRQCD). We have carried out extensive perturbative and non-perturbative tests of the formalism, which show that the decay constants of both heavy-light and heavy-heavy mesons can be calculated with small systematic errors (although at the cost of increasing statistical errors) up to much larger momenta than with NRQCD.
