Experimental limit for the charge of the free neutron

Abstract

The neutron charge has been measured to be ${q}_{n}=(\ensuremath{-}1.5\ifmmode\pm\else\textpm\fi{}2.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}$ 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-\AA{} 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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ \ensuremath{\mu}m. 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.