Radiative (anti)neutrino energy spectra from muon, pion, and kaon decays

Abstract

To describe low-energy (anti)neutrino fluxes in modern coherent elastic neutrino-nucleus scattering experiments as well as high-energy fluxes in precision-frontier projects such as the Enhanced NeUtrino BEams from kaon Tagging (ENUBET) and the Neutrinos from STORed Muons (nuSTORM), we evaluate (anti)neutrino energy spectra from radiative muon (µ − → e − νe ν µ (γ), µ + → e + ν e νµ (γ)), pion π 2 (π − → µ − νµ (γ), π + → µ + ν µ (γ)), and kaon K 2 (K − → µ − νµ (γ), K + → µ + ν µ (γ)) decays. We compare detailed O (α) distributions to the well-known tree-level results, investigate electron mass corrections and provide energy spectra in analytic form. Radiative corrections introduce continuous and divergent near the endpoint component on top of the monochromatic tree-level meson-decay spectra which can change the flux-averaged cross section at 6 × 10 −5 level for the scattering on 40 Ar nucleus with (anti)neutrinos from the pion decay at rest. Radiative effects modify the expected (anti)neutrino fluxes from the muon decay around the peak region by 3 − 4 permille which is a precision goal for the next-generation artificial neutrino sources.