Testing the neutrality of matter by acoustic means in a spherical   resonator
  New 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 $\beta$ decay of the neutron, the experiment gives a limit of $\epsilon_\text{p-e}\lesssim1\cdot10^{-21}$ for the electron-proton charge difference, the same limit holding for the charge of the neutron. Previous measurements are critically reviewed and found incorrect: the present result is the best limit obtained with this technique. 
G. Bressi, G. Carugno, F. Della Valle, G. Galeazzi, G. Ruoso, G.   Sartori
physics.atom-ph hep-ph physics.class-ph