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M. A. Samuel

Publications and source records attributed to M. A. Samuel.

3 recordsLinked to original sources
arXiv1997

Beyond the Narrow Resonance Approximation: Decay Constant and Width of the First Pion-Excitation State

We consider the first pion excitation as a sub-continuum resonance in the pseudoscalar channel, and we obtain parameters characterizing this resonance through a global fit of the Borel-parameter dependence of the field-theoretical pseudoscalar Laplace sum rule to its hadronic (pion + pion-excitation + QCD-continuum) content. Our analysis incorporates finite-width deviations from the narrow resonance approximation, instanton effects, and higher-loop perturbative contributions to the pseudoscalar correlator. We obtain the following values (uncertainties reflect 90% confidence levels): mass $M_Π= 1.15 \pm 0.28 GeV$, width $Γ_Π= < 0.48 GeV$, decay constant $r \equiv [F_ΠM_Π^2 / f_πm_π^2]^2 = 4.7 \pm 2.8$.

hep-ph↗
arXiv1993

Zeroing in on more photons and gluons

We discuss radiation zeros that are found in gauge tree amplitudes for processes involving multi-photon emission. Previous results are clarified by examples and by further elaboration. The conditions under which such amplitude zeros occur are identical in form to those for the single-photon zeros, and all radiated photons must travel parallel to each other. Any other neutral particle likewise must be massless (e.g. gluon) and travel in that common direction. The relevance to questions like gluon jet identification and computational checks is considered. We use examples to show how certain multi-photon amplitudes evade the zeros, and to demonstrate the connection to a more general result, the decoupling of an external electromagnetic plane wave in the ``null zone". Brief comments are made about zeros associated with other gauge-boson emission.

hep-ph↗