Experimental limit for the charge of the free neutron

Abstract

The neutron charge has been measured to be $q_n=(-1.5\mathrm{}\pm 2.2)\times {10}^{-20\mathrm{}}$ electron charges at a confidence level of 90%. This value lowers the known limit by two orders of magnitude. In the experiment, slow neutrons of 20-Å wavelength passed a strong electric field of 10-m length. The deflection of the neutron beam was measured with respect to reversal of the field. For an increase in sensitivity the beam was focused by a neutron lens to a sharp image in the detector plane. Over a long run time the deflection of the neutron beam due to the electric field was less than 2×${10}^{-2\mathrm{}}$ μm. The result on the neutron charge is in agreement with the commonly accepted neutrality of the neutron. The implications of a hypothetical neutron charge are discussed and an improved apparatus is proposed which could lower the limit on the free neutron's charge by up to two orders of magnitude.