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  • Elementary quantum mechanic...
    Baym, Gordon; Beck, D. H.

    Proceedings of the National Academy of Sciences - PNAS, 07/2016, Volume: 113, Issue: 27
    Journal Article

    The neutron, in addition to possibly having a permanent electric dipole moment as a consequence of violation of time-reversal invariance, develops an induced electric dipole moment in the presence of an external electric field. We present here a unified nonrelativistic description of these two phenomena, in which the dipole moment operator, D → , is not constrained to lie along the spin operator. Although the expectation value of D → in the neutron is less than 10−13 of the neutron radius, rn , the expectation value of D → 2 is of order r n 2 . We determine the spin motion in external electric and magnetic fields, as used in past and future searches for a permanent dipole moment, and show that the neutron electric polarizability, although entering the neutron energy in an external electric field, does not affect the spin motion. In a simple nonrelativistic model we show that the expectation value of the permanent dipole is, to lowest order, proportional to the product of the time-reversal-violating coupling strength and the electric polarizability of the neutron.