Electric Dipole Moment of a Nucleon

A violation of time-reversal invariance of any of the elementary particle interactions would imply the existence of a static electric-dipole moment for all these particles. We make here a theoretical estimate of the electric-dipole moment (${\mu }_e$) of a bare nucleon, assuming the existence of an intermediate vector boson ($W$) and a small violation of $T$ invariance for the weak interactions. With the existing experimental upper limit for $T$ violation from polarized neutron decay, we find that ${\mu }_e\lesssim \frac{e}{m_N}\times {10}^{-7\mathrm{}}$. Since a value for ${\mu }_e$ somewhat smaller than this upper limit seems to be experimentally observable, such an experiment would provide a good test for $T$ invariance as well as for the existence of $W$ mesons.