Experimental limit for the charge of the free neutron

In agreement with the commonly accepted neutrality of the free neutron, its charge qₙ was found to be (-0.4±{}1.1)×{}10^-21 electron charges. This value lowers the present upper limit by more than one order of magnitude. In the experiment a beam of cold neutrons with wavelengths between 1.2 and about 3 nm was passed through a strong electric field over a length of 9 m and a beam deflection with respect to the applied field direction was searched for. In order to increase the sensitivity of the apparatus, an achromatic neutron optical imaging system consisting of curved mirrors and multislits was used.