Observation of Polarized Positrons from an Undulator-Based Source

An experiment (E166) at the Stanford Linear Accelerator Center (SLAC) has demonstrated a scheme in which a multi-GeV electron beam passed through a helical undulator to generate multi-MeV, circularly polarized photons which were then converted in a thin target to produce positrons (and electrons) with longitudinal polarization above 80% at 6 MeV. The results are in agreement with Geant4 simulations that include the dominant polarization-dependent interactions of electrons, positrons and photons in matter

Reduction of Quark Mass Scheme Dependence in B bar -> Xs gamma at the NNLL Level

The uncertainty of the theoretical prediction of the B {yields} X{sub s}{gamma} branching ratio at NLL level is dominated by the charm mass renormalization scheme ambiguity. In this paper we calculate those NNLL terms which are related to the renormalization of m{sub c}, in order to get an estimate of the corresponding uncertainty at the NNLL level. We find that these terms significantly reduce (by typically a factor of two) the error on BR(B {yields} X{sub s}{gamma}) induced by the definition of m{sub c}. Taking into account the experimental accuracy of around 10% and the future prospects of the B factories, we conclude that a NNLL calculation would increase the sensitivity of the observable B {yields} X{sub s}{gamma} to possible new degrees of freedom beyond the SM significantly

Towards the NNLL Precision in the Decay Bˉ→Xsγ\bar B \rightarrow X_s \gamma

The present NLL prediction for the decay rate of the rare inclusive process {bar B} {yields} X{sub s}{gamma} has a large uncertainty due to the charm mass renormalization scheme ambiguity. We estimate that this uncertainty will be reduced by a factor of 2 at the NNLL level. This is a strong motivation for the on-going NNLL calculation, which will thus significantly increase the sensitivity of the observable {bar B} {yields} X{sub s}{gamma} to possible new degrees of freedom beyond the SM. We also give a brief status report of the NNLL calculation

NNLL QCD Contribution of the Electromagentic Dipole Operator to \bar B --> X_s gamma

We present an independent calculation of that part of the O({alpha}{sub s}{sup 2}) contribution to dipole operator {Omicron}{sub 7}. Using a different method, we find complete agreement mass ambiguity appearing at NLL accuracy. the complete NNLL prediction of {Lambda}({bar B} {yields} X{sub s}{gamma}) which will resolve the charm quark the decay width {Lambda}({bar B} {yields} X{sub s}{gamma}) which arises from the self-interference term of the electromagnetic with a previous calculation. This NNLL contribution is an important ingredient fo

Studies of nucleon resonance structure in exclusive meson electroproduction

Studies of the structure of excited baryons are key factors to the N* program at Jefferson Lab (JLab). Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever achieved, with a kinematic reach up to Q2 = 12 GeV 2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with Quantum Chromodynamics (QCD)-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the nonperturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD. I. G. Aznauryan... K. Tsushima... et al