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A new method to investigate the neutron electric dipole moment


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Submitted on 1 Jan 1969

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A new method to investigate the neutron electric dipole moment

M. Forte

To cite this version:

M. Forte. A new method to investigate the neutron electric dipole moment. Re- vue de Physique Appliquée, Société française de physique / EDP, 1969, 4 (2), pp.241-242.

�10.1051/rphysap:0196900402024100�. �jpa-00243241�





Euratom CCR-Ispra, Reactor Physics Dept.

Résumé. - Une nouvelle expérience d’investigation de l’existence d’un moment électrique dipolaire du neutron (MED) est envisagée ; elle est basée sur l’interaction du MED du neutron et des moments électriques dipolaires d’une cible ferroélectrique. Les variations de la polarisation

d’un faisceau de neutrons transmis qui en résultent et la méthode de détection de la polarisation

sont décrites et confrontées avec d’autres méthodes expérimentales.



A new experiment to investigate the existence of a neutron electric dipole

moment (EDM) is considered, based on the neutron EDM interaction with the electric dipole

moments of a ferroelectric target. The changes caused in the polarization of a transmitted neutron beam and the polarization detection method are described and discussed in comparison

with other experimental methods.


I. Introduction.


The existence of a non-vanishing

electric dipole moment of elementary particles (possible

as a consequence of a small CP-invariance violation)

has not yet been confirmed by the experiments perfor-

med with the neutron [1, 2], which have set an upper limit of about 5 X 10-22 e.cm for the EDM of this

particle. Experiments like [1] look for a frequency

shift in the neutron magnetic resonance, due to the EDM interaction with a strong electric field. In [2],

a particular polarized neutron diffraction experiment

is considered, looking for an EDM interference term in the atomic form factor. We have in preparation

a new type of experiment, based on the presumable

interaction between the neutron EDM, represented

as ~7, and the electric dipole moments d of a ferro-

electric material.

An early suggestion [3] was to look for a very small

polarization term in the scattering (or total) cross-

section of polarized neutrons, but the accuracy and the

intensity required to compete with other methods

seems out of reach, at present time. Nevertheless, the dipole-dipole is a typical interaction able to produce a

very sensitive kind of effect, namely the neutron spin precession around the ferroelectric polarization vector.

The corresponding modifications in the beam polariza-

tion components and the devised analysis method will be discussed.

II. EDM scattering amplitude and phase-shift.


The amplitude for the scattering of a point dipole by

a fixed dipolar charge distribution is (in B.A.) :

We have to consider the phase-shift oc of a transmitted

polarized wave, depending on the real dipole-dipole

term (the imaginary form factor term and the successive

being not effective in forward direction). With the help

of the optical theorem, one finds ce == ~F mn-2 ~,e ~~1,

1 target thickness, ~ modulus of the ferroelectric pola- rization, assumed parallel or antiparallel to 6. It is

not essential to take into account the nuclear phase- shift, assuming no nuclear polarization. A negligible magnetic susceptibility is also assumed.

III. Outline of the experiment.


A schematic representation is provided in figure 1. An intense polarized beam is obtained by total reflection of slow

FIG. 1.


Schematic description of the EDM experiment.

reactor neutrons on a magnetized Co-Fe mirror [4],

and a similar mirror is used as a polarization analyzer.

The initial polarization can be turned adiabatically

to the desired direction. We consider incoming neu-

trons prepared in a spin state x - ~~ eigenstate of a, (complete longitudinal polarization) which may also be considered as a coherent superposition 1+ = (~~ + ç-) /2

of the eigenfunctions of 6~. The phase factors taken

1 1

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0196900402024100



by the components in traversing the ferroelectric target will modify the state into 1’ = (~+ e+iO’ + ~- e-ia) /2,

that is an eigenstate of s’ = sZ cos 2oc + ay sin 2~x, also

described by a density matrix (1 --;- s’) /2 or :

including a realistic initial polarization degree. The gain in the y polarization ag y = P, sin 2oc is detec-

ted by means of the magnetic mirror analyzer having efficiency P2 along the y direction, efficiency matrix

E _ (1 + ~(Ty)/2. The reflected intensity, propor- tional to Trace (sp’) = (cos2(x + Pl P2 sin 2Ll)/2,

will contain an asymmetry A N 2LlP1P2, which can

be put into evidence by reversing either the initial neutron polarization or the ferroelectric polarization

vector, by electrical or mechanical methods. The

longitudinal field in the target region must be weak enough (a fraction of an 0152), to limit its influence on

the spin precession, and must be turned and connected

with the transversal field of the analyzer within a

short path length (1~2 cm) to avoid depolarization

effects. The calculated correction factors are not

significantly different from unity. The overall detec-

tion efficiency of the experiment can be calibrated by comparison with the procession produced by a known

weak magnetic field applied parallel to the ferroelectric


IV. Conclusions.


The effect obtainable with a

few cm long target of some kind of ferroelectric ceramic

(possible Y values up to ~ 105 esu) is a counting rate

asymmetry of the order of 1 % for a 5 X 10-22 e . cm EDM, very favourable in comparison with previous experiments. Although the polarization term under investigation may be enhanced by some "large" inter- fering amplitude b in a cross-section measurement, the phase-shift effect is considerably more sensitive, by Ãjb orders of magnitude ( ~ 104 for slow neutrons and ordinary b values). Such an advantage suggests

to examine the possibilities of the latter method in other cases of neutron polarization interactions (with polarized nuclei, f.i.).


[1] MILLER (P. D.) et al., Phys. Rev. Letters, 1967, 19, 381.

[2] SHULL (C. G.) and NATANS (R.), Phys. Rev. Letters, 1967, 19, 384.

[3] STOPPINI (G.), private communication, march 1967.

[4] FORTE (M.), ANL-6797,I,7, p. 45,1963, and EUR-538,

e., 1964.


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