A test of charge conservation. See the “Note on Testing Charge Conservation and the Pauli Exclusion Principle” following this section in our 1992 edition (Physical Review D45 S1 (1992), p.$~$VI.10).
Most of these experiments are one of three kinds: Attempts to observe (a)$~$the 255.5 keV gamma ray produced in ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \nu}_{{{e}}}}{{\mathit \gamma}}$, (b)$~$the (K)$~$shell x$~$ray produced when an electron decays without additional energy deposit, e.g., ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \nu}_{{{e}}}}{{\overline{\mathit \nu}}_{{{e}}}}{{\mathit \nu}_{{{e}}}}$ (``disappearance” experiments), and (c)$~$nuclear de-excitation gamma rays after the electron disappears from an atomic shell and the nucleus is left in an excited state. The last can include both weak boson and photon mediating processes. We use the best ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \nu}_{{{e}}}}{{\mathit \gamma}}$ limit for the Summary Tables.
Note that we use the mean life rather than the half life, which is often reported.