Electric dipole moments (EDM) are one of the keys to understand the origin of our
Universe. The Universe as we know it has a microscopic net baryon number - about
0.2 baryons per cubic meter, or ∼ 10-10 of the density of relic photons. In 1967 Andrei
Sakharov formulated three conditions for baryogenesis:
- Early in the evolution of the universe, the baryon number conservation must be
violated sufficiently strongly,
- the C and CP invariances, and T invariance thereof, must be violated, and
- at the moment when the baryon number is generated, the evolution of the universe
must be out of thermal equilibrium.
CP violation in kaon decays is known since 1964, it has been observed in B-decays
and charmed meson decays. The Standard Model (SM) accommodates CP violation via
the phase in the Cabibbo-Kobayashi-Maskawa matrix. CP and P violation entail nonvanishing
P and T violating electric dipole moments (EDMs) of elementary particles
d = dS . Although extremely successful in many aspects, the SM has at least two weaknesses:
neutrino oscillations do require extensions of the SM and, most importantly, the SM
mechanisms fail miserably in the expected baryogenesis rate. Simultaneously, the SM
predicts an exceedingly small electric dipole moment of nucleons 10-33 <
dn < 10-31
e·cm, way below the current upper bound for the neutron EDM, dn
< 2.9 x 10-26 e·cm,
and also beyond the reach of future EDM searches. In the quest for physics beyond the
SM one could follow either the high energy trail or look into new methods which offer very
high precision and sensitivity. Supersymmetry is one of the most attractive extensions of
the SM and S. Weinberg emphasized in 1992: "Endemic in supersymmetric (SUSY)
theories are CP violations that go beyond the SM. For this reason it may be that the next
exciting thing to come along will be the discovery of a neutron electric dipole moment."
The SUSY predictions span typically 10-29 < dn < 10-24
e·cm and precisely this range is targeted in the new generation of EDM searches.
There is consensus among theorists that measuring the EDM of the proton, deuteron
and helion is as important as that of the neutron. Furthermore, it has been argued some 25
years ago that T-violating nuclear forces could substantially enhance nuclear EDMs.
At the moment, there are no significant direct upper bounds available on dp or dd.
Non-vanishing EDMs give rise to the precession of the spin of a particle in an electric
field. In the rest frame of a particle
dS/dt* = μS x B* + d x E*
where in terms of the lab frame fields
E* = γ(E + β x B )
B* = γ(B - β x E )
While ultra-cold electrically neutral atoms and neutrons can conveniently by stored in
traps, the EDM of charged particle can only be approached with storage rings. EDM
searches of charged fundamental particles have hitherto been impossible, because of the
absence of the required new class of electrostatic storage rings. An ambitious quest
for a measurement of the EDM of the proton with envisioned sensitivity down to
dp ∼10-29 e·cm is under development at BNL. The principal idea is to store protons with
longitudinal polarization in a purely electrostatic ring: the EDM would cause a precession
around the radial electric field and thus lead to a build-up of transverse polarization which
could be measured by standard polarimetry.
Before jumping into construction of dedicated storage rings, it is imperative to test
technical issues at existing facilities. Here we review several ideas for precursor experiments
which could be performed at COSY subject to very modest additions to the existing
machine. In a magnetic ring like COSY, the stable polarization axis in the absence of
longitudinal magnetic fields is normal to the ring plane, and at the heart of the most
promising proposal is a radio-frequency electric field (RFE) spin flipper which would rotate
the spin into the ring plane. The resulting EDM-generated P and T non-invariant
in-plane polarization which can be determined from the up-down asymmetry of the scattering
of stored particles on the polarimeter. Unless show stoppers like false spin rotations
1Jülich Electric Dipole moment Investigations.
via the magnetic moment pop up, one could theoretically aim for an upper bound for the
deuteron of dd < 10-24 e·cm, which would be as valuable as the existing upper bounds on dn.