Neutron EDM (K.
Green)
A group at RAL together with a group from
Sussex is carrying out an experiment to measure the EDM of the
neutron, using very slow, ultra-cold, neutrons from the high flux
beam reactor at ILL.
For particles to have electric dipole moments
(EDM), the forces concerned in their structure must be asymmetric
with regard to Parity (P) and time reversal (T). The Standard
Model of the electroweak interaction gives a very small value for
the neutron EDM . However extensions to the Standard Model
invariably give much larger values which should be measurable.
The experiment seeks to measure a shift in the Larmor Nuclear
Magnetic Resonance (precession) frequency of polarised neutrons,
stored in a 18l insulating bottle within a weak magnetic field, when
they are subjected to an external electrical field gradient. An
active programme of technical work is in progress to give
significantly improved upper limits on the value of the neutron
EDM.
EDM Apparatus
Figure a): A schematic of the apparatus which is being
developed and used at the Institut Laue Langevin (ILL) in
Grenoble, France to study the static properties of free neutrons.
Nuclear Magnetic Resonance (NMR) methods are used to investigate
energy differences of 10-22 electron-volts as an
indication of fundamental interactions which violate the
symmetries of Space and Time. The ILL provides a unique source of
ultra cold neutrons (UCN) which can be stored in the apparatus as
a 'free gas of neutrons' and subjected to experimental
observation for many hundreds of seconds.
Neutron Counts per Cycle
Neutron spin precession frequency in HZ
Figure b): A typical
double pulse NMR signal from 10,000 polarised neutrons stored in
the apparatus for 150 secs and which is used to search for an
electric dipole moment (EDM) to the neutron. The double pulse
interference pattern between the freely processing neutrons and a
coherent, oscillating electro-magnetic field is analogous to the
well known double-slit interference pattern in optics. A shift in
the interference pattern of one linewidth would correspond to a
magnetic field change of 1 micro-gauss or an EDM of 10-22
e cm.
Further information about the experiment can be
found at: http://hepnts1.rl.ac.uk/EDM/
Further Reading:
- Performance of an Atomic Mercury
Magnetometer in the Neutron EDM Experiment. K. Green, P. G. Harris, P. Iaydjiev et al,
Nucl. Instr. and Meth. B404
(1998) 381
- A Search for the Electric Dipole Moment of
the Neutron. K. F. Smith et
al., Phys. Lett. B234 (1990) 191
- Fundamental Physics with Ultracold
Neutrons. J. M. Pendlebury,
Ann. Rev. Part. Sci., 43 (1993)
687