Shape function in QED and bound muon decays

Abstract

When a particle decays in an external field, the energy spectrum of the products is smeared. We derive an analytical expression for the shape function accounting for the motion of the decaying particle and the final state interactions. We apply our result to calculate the muonium decay spectrum and comment on applications to the muon bound in an atom.

Comments: 5 pages, 3 figures v2: Corrected verrsion to appear in PRD