The high energy spectrum of internal positrons from radiative muon capture on nuclei

Abstract

The Mu2e and COMET collaborations will search for nucleus-catalyzed muon conversion to positrons (${\mu }^{-}\to e^{+}$) as a signal of lepton number violation. A key background for this search is radiative muon capture where either: 1) a real photon converts to an $e^{+}{e}^{-}$ pair "externally" in surrounding material, or 2) a virtual photon mediates the production of an $e^{+}{e}^{-}$ pair "internally". If the $e^{+}$ has an energy approaching the signal region then it can serve as an irreducible background. In this work we describe how the near end-point internal positron spectrum can be related to the real photon spectrum from the same nucleus, which encodes all non-trivial nuclear physics.

Comments: 7 pages + 7 pages appendix, 4 figures | Error bands added to figures, algebraic error in appendix A corrected (only influences error estimates)