Testing the neutrality of matter by acoustic means in a spherical resonator

Abstract

Measurements to test the neutrality of matter by acoustic means are reported. The apparatus is based on a spherical capacitor filled with gaseous SF${}_{6}$ excited by an oscillating electric field. The apparatus has been calibrated measuring the electric polarizability. Assuming charge conservation in the $\ensuremath{\beta}$ decay of the neutron, the experiment gives a limit of ${\ensuremath{\epsilon}}_{\mathrm{p}\ensuremath{-}\mathrm{e}}\ensuremath{\lesssim}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ for the electron-proton charge difference, the same limit holding for the charge of the neutron. Previous measurements are critically reviewed and found to be inaccurate.