Neutron electric dipole moment

The pseudovector four-potential B may still contribute to other effects in the microscopic domain. For example, it would predict that a particle, such as a neutron, would have an electric dipole moment, whose value is proportional to the term in the Dirac Hamiltonian 2,a E [12]. However, after much experimental investigation into the possibility of the neutron electric dipole moment, it has not been found [15]—that is, in the context of this theory, the parameter if it were nonzero, must be too small (the order of 10-13) for this effect to be observed. 

After discovery of the combined charge and space parity violation, or CP-violation, in iT°-meson decay [7], the search for the electric dipole moments (EDMs) of elementary particles has become one of the most fundamental problems in physics [6, 8, 9, 10, 1]. A permanent EDM is induced by the weak interaction that breaks both the space symmetry inversion and time-reversal invariance [11]. Considerable experimental effort has been invested in probing for atomic EDMs induced by EDMs of the proton, neutron and electron, and by P,T-odd interactions between them. The best available restriction for the electron EDM, de, was obtained in the atomic T1 experiment [12], which established an upper limit of de < 1.6 X 10 e-cm, where e is the charge of the electron. The benchmark upper limit on a nuclear EDM is obtained in atomic experiment on i99Hg [13], ]dHgl < 2.1 X 10 e-cm, from which the best restriction on the proton EDM, dp < 5.4 x 10 " e-cm, was also recently obtained by Dmitriev Sen kov [14] (the previous upper limit on the proton EDM was obtained in the TIE experiment, see below). 

Progress in precision studies and shortage of data on possible extension of the Standard Model of weak, electromagnetic and strong interactions have produced a situation where a number of experiments to search for so-called new physics have been performed or planned in atomic physics. Among such experiments are a search for an electric dipole moment of an electron and a neutron, search for variation of fundamental constants and violation of Lorentz invariance, etc. 

CP or T violation divide into (a) those that involve weak interactions or parity violation, and (b) those that involve processes otherwise allowed by the strong or electromagnetic interactions. In class (a) the most sensitive are probably the searches for an electric dipole moment of the neutron, measured to be < 1.0 X 10 e cm, and the electron (—0.18 0.16) x 10 e cm. A nonzero value requires both P and T violation. Class (b) includes the search for C violation in r] decay, believed to be an electromagnetic process, e.g., as measured by r( 7 all) < 5 x 10 , and searches for T... 

Baker, C.A. et al.. Improved experimental limit on the electric dipole moment of the neutron, Phys. Rev. Lett., 97,131801, 2006. 

The expectation value of the property A at the space-time point (r, t) depends in general on the perturbing force F at all earlier times t — t and at all other points r in the system. This dependence springs from the fact that it takes the system a certain time to respond to the perturbation that is, there can be a time lag between the imposition of the perturbation and the response of the system. The spatial dependence arises from the fact that if a force is applied at one point of the system it will induce certain properties at this point which will perturb other parts of the system. For example, when a molecule is excited by a weak field its dipole moment may change, thereby changing the electrical polarization at other points in the system. Another simple example of these nonlocal changes is that of a neutron which when introduced into a system produces a density fluctuation. This density fluctuation propagates to other points in the medium in the form of sound waves. 

The neutron is a particle of mass m = 1.0098 dalton, with a zero electrical dipole (or smaller than 10 16 esu), a spin 1/2 and a magnetic moment which is equal to — 1.91 nuclear magneton, where... 

Figure 14. Nilsson diagram for odd neutrons close to N = 82. The Fermi levels for N = 83, 85, and 87 indicate a successive filling of the/)/2 shell. On the right, the experimental magnetic dipole and electric quadrupole moments are compared with the results from particle-rotor calculations assuming deformations of e = 0.10 and 0.15. The trend of the quadrupole moments reproduces the increase of coupling to the collective motion, while the discrepancy in the trend for the magnetic moments is understood as a change of core polarization.

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