A new method to investigate the neutron electric dipole moment

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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�

241.

A NEW METHOD TO INVESTIGATE THE NEUTRON ELECTRIC DIPOLE MOMENT

M. FORTE,

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.

Abstract.

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.

REVUE DE PHYSIQUE ?~PPI,IQU~~ ^{TOME 4,} JUIN 1969,

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

242

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

vector.

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.).

REFERENCES

[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.