Leptonic Pion Decay And Physics Beyond The Electroweak Standard Model

The ratio of branching ratios in leptonic pion decay $R_{\pi} \equiv (\Gamma(\pi^- \to e \nu_e))/(\Gamma(\pi^- \to \mu \nu_\mu))$ is a powerfully sensitive probe of new interactions beyond the electroweak standard model. This is due to the chirality suppression of the standard model amplitude for the decay, which results in a precise prediction for the ratio, and suppressed amplitudes for new contributions to interfere with. We calculate, including QCD corrections, the contributions to $R_{\pi}$ arising from a broad selection of standard model extensions to which it is sensitive, including: R-parity violating interactions in supersymmetric theories, theories with light (electroweak scale) leptoquark degrees of freedom, non-minimal models of extra doublet Higgs bosons, models in which the quarks and leptons are composite both with and without supersymmetry, and models with strong TeV scale gravitational interactions. Comparing with existing measurements of $R_{\pi}$ we provide limits on each of these classes of models; our calculations also represent state of the art theoretical benchmarks against which the results from the upcoming round of leptonic pion decay experiments may be compared.Comment: 31 pages, 3 figure

Translational perspectives on perfusion-diffusion mismatch in ischemic stroke

Magnetic resonance imaging has tremendous potential to illuminate ischemic stroke pathophysiology and guide rational treatment decisions. Clinical applications to date have been largely limited to trials. However, recent analyses of the major clinical studies have led to refinements in selection criteria and improved understanding of the potential implications for the risk vs. benefit of thrombolytic therapy. In parallel, preclinical studies have provided complementary information on the evolution of stroke that is difficult to obtain in humans due to the requirement for continuous or repeated imaging and pathological verification. We review the clinical and preclinical advances that have led to perfusion–diffusion mismatch being applied in phase 3 randomized trials and, potentially, future routine clinical practice

New very high resolution radar studies of the Moon

As part of an effort to further understand the geologic utility of radar studies of the terrestrial planets, investigators at the Hawaii Institute of Geophysics are collaborating with NEROC Haystack Observatory, MIT and the Jet Propulsion Laboratory in the analysis of existing 3.8 and 70 cm radar images of the Moon, and in the acquisition of new data for selected lunar targets. The intent is to obtain multi-polarization radar images at resolutions approaching 75 meters (3.8 cm wavelength) and 400 meters (70 cm wavelength) for the Apollo landing sites (thereby exploiting available ground truth) or regions covered by the metric camera and geochemical experiments onboard the command modules of Apollos 15, 16 and 17. These data were collected in both like- and cross-polarizations, and, in the case of the 70 cm data, permit the phase records to be used to assess the scattering properties of the surface. The distribution of surface units on the Moon that show a mismatch between the surface implied by like- and cross-polarized scattering data is being analyzed, based on the scattering models of Evans and Hagfors