Ultralow $Q$ values for neutrino mass measurements

We investigate weak nuclear decays with extremely small kinetic energy release ($Q$ value) and thus extremely good sensitivity to the absolute neutrino mass scale. In particular, we consider decays into excited daughter states, and we show that partial ionization of the parent atom can help to tune $Q$ values to $\ll 1$ keV. We discuss several candidate isotopes undergoing ${\beta }^{\pm }$, bound state $\beta$, or electron capture decay and come to the conclusion that a neutrino mass measurement using low-$Q$ decays might only be feasible if no ionization is required and if future improvements in isotope production technology, nuclear mass spectroscopy, and atomic structure calculations are possible. Experiments using ions, however, are extremely challenging because of the large number of ions that must be stored. New precision data on nuclear excitation levels could help to identify further isotopes with low-$Q$ decay modes and possibly less challenging requirements.

Figure 1

An example for tuning of a $Q$ value by ionization. Note that energy levels in the decay ${}^{194}\mathrm{Ir}\to {}^{194}{\mathrm{Pt}}^{*}$ shown here have an uncertainty of $O(1.6\hspace{0.5em}\text{keV})$, so the figure is to be understood as an illustration of the principal idea only.

Figure 2

Relative effect of nonzero $m_{\nu }$ on the decay rate in $\mathrm{b}\beta$ and EC decay.