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−ν¯eνμ(γ),μ+→e+νeν¯μ(γ)), pion πℓ2 (π−→μ−ν¯μ(γ),π+→μ+νμ(γ)), and kaon Kℓ2 (K−→μ−ν¯μ(γ),K+→μ+νμ(γ)) decays. We compare detailed O(α) distributions to the well-known tree-level results, investigate electron-mass corrections and provide energy spectra in analytical form. Radiative corrections introduce continuous and divergent spectral components near the endpoint, 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 Ar40 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 next-generation artificial neutrino sources.