Compilation of coupling constants and low-energy parameters

The electron capture decay ${}^{163}\text{Ho}\text{→}{}^{163}\text{Dy}{}^{\mathrm{<mtext>H</mtext>}}\text{+ν}{}_{\mathrm{<mtext>e</mtext>}}$ occurs with a record low energy release, Q ～ 2.6 keV. The daughter Dy^H atom has an electron hole, H, and predominantly decays by electron ejection Dy^H→Dy^H₁H₂+e^?. We investigate the neutrino mass sensitivity of the electron spectrum in the overall process Ho→Dy^H₁H₂+e^?+ν_e. In this spectrum, the fraction of events sensitive to a fixed non-zero neutrino mass in one to two orders of magnitude smaller than in the standard case of tritium β decay. But the electron energies in ¹⁶³Ho decay are considerably smaller than in ³H decay (Q ～ 18 keV). This suggests experiments whose energy resolution could be much better than that of the magnetic spectrometers conventionally used in the tritium case.