30 research outputs found
The Benefits of B ---> K* l+ l- Decays at Low Recoil
Using the heavy quark effective theory framework put forward by Grinstein and
Pirjol we work out predictions for B -> K* l+ l-, l = (e, mu), decays for a
softly recoiling K*, i.e., for large dilepton masses sqrt{q^2} of the order of
the b-quark mass m_b. We work to lowest order in Lambda/Q, where Q = (m_b,
sqrt{q^2}) and include the next-to-leading order corrections from the charm
quark mass m_c and the strong coupling at O(m_c^2/Q^2, alpha_s). The leading
Lambda/m_b corrections are parametrically suppressed. The improved Isgur-Wise
form factor relations correlate the B -> K* l+ l- transversity amplitudes,
which simplifies the description of the various decay observables and provides
opportunities for the extraction of the electroweak short distance couplings.
We propose new angular observables which have very small hadronic
uncertainties. We exploit existing data on B -> K* l+ l- distributions and show
that the low recoil region provides powerful additional information to the
large recoil one. We find disjoint best-fit solutions, which include the
Standard Model, but also beyond-the-Standard Model ones. This ambiguity can be
accessed with future precision measurements.Comment: 31 pages, 8 figures; Instability near minimal recoil from numerics
removed, Fig. 1 replaced and minor shifts in short distance uncertainties in
SM predictions; typos corrected and references added; main results and
conclusions unchange
Factorization at Subleading Power and Irreducible Uncertainties in Decay
Using methods from soft-collinear and heavy-quark effective theory, a
systematic factorization analysis is performed for the
photon spectrum in the endpoint region . It is proposed that, to all orders in , the spectrum obeys a
novel factorization formula, which besides terms with the structure
familiar from inclusive decay
distributions contains "resolved photon" contributions of the form and . Here and
are new soft and jet functions, whose form is derived. These
contributions arise whenever the photon couples to light partons instead of
coupling directly to the effective weak interaction. The new contributions
appear first at order and are related to operators other than
in the effective weak Hamiltonian. They give rise to
non-vanishing corrections to the total decay rate, which cannot be
described using a local operator product expansion. A systematic analysis of
these effects is performed at tree level in hard and hard-collinear
interactions. The resulting uncertainty on the decay rate defined with a cut
GeV is estimated to be approximately . It could be
reduced by an improved measurement of the isospin asymmetry to
the level of . We see no possibility to reduce this uncertainty further
using reliable theoretical methods.Comment: 63 pages, 11 Figures, Journal Versio
Regularization-scheme dependence of QCD amplitudes in the massive case
We investigate QCD amplitudes with massive quarks computed in the four-dimensional helicity scheme (FDH) and dimensional reduction at NNLO and describe how they are related to the corresponding amplitudes computed in conventional dimensional regularization. To this end, the scheme dependence of the heavy quark and the velocity-dependent cusp anomalous dimensions is determined using soft-collinear effective theory. The results are checked against explicit computations of massive form factors in FDH at NNLO. Our results complete the description of the scheme dependence of QCD amplitudes at NNLO