Stars as Laboratories for Fundamental Physics - MPP Theory Group

182 Chapter 5
However, an ongoing experimental project at the Institute for Nuclear
Physics in Novosibirsk may be able to improve the helioscope
significantly. The conversion magnet has been gimballed so that it can
track the Sun, providing much longer exposure times. First results can
be expected for late 1995—see Vorobyov and Kolokolov (1995) for a
status report.
Another possibility would be to use the straight sections of the
beam pipe of the LEP accelerator at CERN as an axion helioscope.
Hoogeveen and Stuart (1992) have calculated the times and dates of
alignment with the Sun. They proposed an experimental setup that
might allow one to reach a sensitivity in g aγ down to 4×10 −10 GeV −1 ,
which would be very impressive, but still far from the globular-cluster
bound Eq. (5.23).
Finally, Paschos and Zioutas (1994) proposed to use a single crystal
as a detector where the Primakoff conversion of solar axions is coherently
enhanced over the electric fields of many atoms. Put another way,
one would expect a strong enhancement via Bragg scattering. Even
with this improvement, however, it does not seem possible to beat the
bound from globular-cluster stars.
5.4.3 Shining Light through Walls
Instead of using the solar axion flux one can make one’s own by shining
a laser beam through a long transverse magnetic field region where it
develops an axion component. Then the laser beam is blocked while
the weakly interacting axions traverse the obstacle. In a second magnet
they are back-converted into photons so that one “shines light through
walls” (Anselm 1985; Gasperini 1987; van Bibber et al. 1987). Instead
of a freely propagating beam one may use resonant cavities on either
side of the wall which are coupled by the axion field (Hoogeveen and
Ziegenhagen 1991). Another possibility to improve the sensitivity is to
use squeezed light (Hoogeveen 1990).
An actual experiment was performed by Ruoso et al. (1992) who
used two superconducting magnets of length 440 cm each with a field
strength of 3.7 T. The light beam was trapped in a resonant cavity
in the first magnet, allowing for about 200 traversals; the incident
laser power was 1.5 W. At the end of the second magnet photons were
searched for by a photomultiplier. For an axion mass m < a ∼ 10 −3 eV an
upper bound g aγ < 0.7×10 −6 GeV −1 was found.