arXiv subjects

Richard J. Hill

Publications and source records attributed to Richard J. Hill.

64 records · Page 4Linked to original sources

arXiv2006

Comment on form factor shape and extraction of |V_ub| from B --> pi l nu

We point out that current experimental data for partial B --> pi l nu branching fractions reduce the theoretical input required for a precise extraction of |V_ub| to the form factor normalization at a single value of the pion energy. We show that the heavy-quark expansion provides a bound on the form factor shape that is orders of magnitude more stringent than conventional unitarity bounds. We find |V_ub| = (3.7 +/- 0.2 +/- 0.1) x [0.8/F_+(16 GeV^2)]. The first error is from the experimental branching fractions, and the second is a conservative bound on the residual form factor shape uncertainty, both of which will improve with additional data. Together with current and future lattice determinations of the form factor normalization this result gives an accurate, model independent determination of |V_ub|. We further extract semileptonic shape observables such as |V_ub F_+(0)| = 0.92 +/- 0.11 +/- 0.03 and show how these observables can be used to test factorization and to determine low-energy parameters in hadronic B decays.

hep-ph↗

arXiv2005

Factorization in B->V gamma Decays

The factorization properties of the radiative decays B->V gamma are analyzed at leading order in 1/m_b using the soft-collinear effective theory. It is shown that the decay amplitudes can be expressed in terms of a B->V form factor evaluated at q^2=0, light-cone distribution amplitudes of the B and V mesons, and calculable hard-scattering kernels. The renormalization-group equations in the effective theory are solved to resum perturbative logarithms of the different scales in the decay process. Phenomenological implications for the B->K* gamma branching ratio, isospin asymmetry, and CP asymmetries are discussed, with particular emphasis on possible effects from physics beyond the Standard Model.

hep-ph↗

arXiv2004

Symmetry relations for heavy-to-light meson form factors at large recoil

The description of large-recoil heavy-to-light meson form factors is reviewed in the framework of soft-collinear effective theory. At leading power in the heavy quark expansion, three classes of approximate symmetry relations arise. The relations are compared to experimental data for $D\to K^*$ and $D_s\to ϕ$ form factors, and to light-cone QCD sum rule predictions for $B\to π$ and $B\to ρ$ form factors. Implications for the extraction of $|V_{ub}|$ from semileptonic $B\to ρ$ decays are discussed.

hep-ph↗

arXiv2004

Sudakov Resummation for Subleading SCET Currents and Heavy-to-Light Form Factors

The hard-scattering contributions to heavy-to-light form factors at large recoil are studied systematically in soft-collinear effective theory (SCET). Large logarithms arising from multiple energy scales are resummed by matching QCD onto SCET in two stages via an intermediate effective theory. Anomalous dimensions in the intermediate theory are computed, and their form is shown to be constrained by conformal symmetry. Renormalization-group evolution equations are solved to give a complete leading-order analysis of the hard-scattering contributions, in which all single and double logarithms are resummed. In two cases, spin-symmetry relations for the soft-overlap contributions to form factors are shown not to be broken at any order in perturbation theory by hard-scattering corrections. One-loop matching calculations in the two effective theories are performed in sample cases, for which the relative importance of renormalization-group evolution and matching corrections is investigated. The asymptotic behavior of Sudakov logarithms appearing in the coefficient functions of the soft-overlap and hard-scattering contributions to form factors is analyzed.

hep-ph↗

arXiv2004

Loop Corrections to Heavy-to-Light Form Factors and Evanescent Operators in SCET

One-loop matching corrections are calculated for Soft-Collinear Effective Theory (SCET) operators relevant to the analysis of heavy-to-light meson form factors at large recoil. The numerical impact of radiative corrections on form factor predictions is assessed. Evanescent operators in the effective theory are studied and it is shown that even in problems of the Sudakov type, these operators can be renormalized to have vanishing matrix elements.

hep-ph↗

arXiv2003

External Operators and Anomalous Dimensions in Soft-Collinear Effective Theory

It has recently been argued that soft-collinear effective theory for processes involving both soft and collinear partons contains a new soft-collinear mode, which can communicate between the soft and collinear sectors of the theory. The formalism incorporating the corresponding fields into the effective Lagrangian is extended to include external current and four-quark operators relevant to weak interactions. An explicit calculation of the anomalous dimensions of these operators reveals that soft-collinear modes are needed for correctly describing the ultraviolet behavior of the effective theory.

hep-ph↗

arXiv2003

Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory

It is argued that soft-collinear effective theory for processes involving both soft and collinear partons, such as exclusive B-meson decays, should include a new mode in addition to soft and collinear fields. These "soft-collinear messengers" can interact with both soft and collinear particles without taking them far off-shell. They thus can communicate between the soft and collinear sectors of the theory. The relevance of the new mode is demonstrated with an explicit example, and the formalism incorporating the corresponding quark and gluon fields into the effective Lagrangian is developed.

hep-ph↗

arXiv2003

Spectator Interactions in Soft-Collinear Effective Theory

Soft-collinear effective theory is generalized to include soft massless quarks in addition to collinear fields. This extension is necessary for the treatment of interactions with the soft spectator quark in a heavy meson. The power counting of the relevant fields and the construction of the effective Lagrangian are discussed at leading order in Lambda/m_b. Several novel effects occur in the matching of full-theory amplitudes onto effective-theory operators containing soft light quarks, such as the appearance of an intermediate mass scale and large non-localities of operators on scales of order 1/Lambda. Important examples of effective-theory operators with soft light quarks are studied and their renormalization properties explored. The formalism presented here forms the basis for a systematic analysis of factorization and power corrections for any exclusive B-meson decay into light particles.

hep-ph↗

arXiv2001

Probe Brane Dynamics and the Cosmological Constant

Recently a brane world perspective on the cosmological constant and the hierarchy problems was presented. Here, we elaborate on some aspects of that particular scenario and discuss the stability of the stationary brane solution and the dynamics of a probe brane. Even though the brane is unstable under a small perturbation from its stationary position, such instability is harmless when the 4-D cosmological constant is very small, as is the case of our universe. One may also introduce radion stabilizing potentials in a more realistic scenario.

hep-th↗

arXiv2000

Order (alpha^2 Gamma, alpha^3 Gamma) Binding Effects in Orthopositronium Decay

We present a new, simplified analysis of the low-energy electron-positron interaction, and use the resulting effective theory to calculate the binding effects that contribute to the decay rate, Gamma, of orthopositronium, through Order(alpha^3 ln(alpha) Gamma). We express the total decay rate in terms of the annihilation rate for a free electron and positron at threshold, which has just recently been computed to sufficient precision. Our result corrects errors in a previous analysis.

hep-ph↗