Développement et optimisation d'un système de polarisation de ...

tel-00726870, version 1 - 31 Aug 2012
28 1.1. The Electric Dipole moment
1.1.2 EDM of atoms
Two kinds of atoms can be defined in terms of their electronic outer layer: the paramagnetic
atoms, and the diamagnetic atoms.
Paramagnetic atoms
Paramagnetic atoms have at least one single electron in the outer shell. In this configuration,
the main contribution to the EDM comes from the electron EDM. This is due to the motion of
the electrons. If the electrons were static, the Schiff’s screening theorem [23] would predict a
zero contribution from the single electron EDM. In fact, this theorem states that the electric
charges of an atom are placed in a way that the external electric fields are canceled. Heavy
atoms (high Z) with small angular momentum (J) and large polarizability (α, Sec. 1.2.2) are
the ones with the largest EDM [2]. Indeed, we have :
dpar ∼
10α2Z 3
de
(1.3)
J(J + 1/2)(J + 1) 2
The electron EDM can be extracted from the measurement of the EDM of these atoms.
The current value: de = (6.9 ± 7.4) × 10 −28 e cm comes from the measurement with the 205 Tl
atom [24].
Diamagnetic atoms
In contrast to paramagnetic atoms, diamagnetic atoms do not have a single electron on the
outer layer: their angular momentum is zero. The measurement of the atom EDM probes the
nuclear EDM and the proton EDM can be extracted from this. The best limit on dp comes from
the measurement of 199 Hg EDM and is dp < 5.4 × 10 −24 e cm [25].
1.1.3 The Neutron EDM
Since 1950 [5], the measured upper limit on dn has been decreased by 6 orders of magnitude
(Fig. 1.3). Physicists have used two kinds of neutrons: cold or very cold neutron beams and
ultra-cold neutrons (Sec. 1.2) stored in a vessel. For the first type of experiment, the sensitivity
is limited by the relativistic v × E effect [26]. Inside an electric field E, a neutron with velocity
vn experiences an effective magnetic field:
B = E ∧ vn
c 2
With a cold neutron mean velocity of 150 m.s −1 , the v ×E gives a false EDM of 2 · 10 −24 e cm [9].
For the second type of experiments, the neutron velocity is below 8 m.s −1 , which decreases
the v × E systematic effect. Over the last 50 years, the neutron densities have continuously
increased. This allowed the reduction of the statistical uncertainty leading to the upper limit
|dn| < 2.9 · 10 −26 e cm (90 % C.L) [3]. This result was obtained with the apparatus we are
using.
There is 6 collaborations around the world which try to improve the sensitivity on the upper
limit on the neutron EDM. The expected schedules to reach new sensitivities for each of these
experiments is given in Fig. 1.4.
(1.4)