Stars as Laboratories for Fundamental Physics - MPP Theory Group

Two-Photon Coupling of Low-Mass Bosons 181
By adjusting the gas pressure within the magnetic field volume one
can make the photon and axion degenerate and thus enhance the transition
rate (van Bibber et al. 1989). This applies, in particular, to
solar axions which have keV energies so that the corresponding photon
dispersion relation in low-Z gases is “particle-like” with the plasma
frequency being the effective mass.
If there is a gradient of the gas density, for example near a star, or
if the gas density and magnetic field strength change in time as in the
expanding universe, suitable conditions allow for resonant axion-photon
conversions in the spirit of the neutrino MSW effect (Yoshimura 1988;
Yanagida and Yoshimura 1988).
For the magnetic conversion of pseudoscalars in the galactic magnetic
field one must worry about density fluctuations of the interstellar
medium which can be of order the medium density itself. In this case
Eq. (5.29) is no longer valid because it was based on the assumption of
spatial homogeneity of all relevant quantities. Carlson and Garretson
(1994) have derived an expression for the conversion rate in a medium
with large random density variations. They found that it can be significantly
suppressed relative to the naive result.
5.4.2 Solar Axions
An axion helioscope experiment was performed by Lazarus et al. (1992)
who used a vacuum pipe of 6 ′′ diameter which was placed in the bore of
a dipole magnet of 72 ′′ length (1.80 m); the field strength was 2.2 T. The
helioscope was oriented so that its long axis pointed along the azimuth
of the setting Sun. This provided a time window of approximately
15 min every day during which the line of sight through the vacuum
region pointed directly to the Sun. As a detector they used an x-ray
proportional chamber at the end of the pipe.
Data were taken on several days with He at different pressures in
the pipe. At 1 atm helium provides a plasma mass of about 0.3 keV to
x-rays. For vacuum, a 3σ bound of g aγ < 3.6×10 −9 GeV −1 for m a <
0.050 eV was found. For a helium pressure of 55 Torr the limit was
3.9 in the same units, applicable to 0.050 < m a /eV < 0.086, and for
100 Torr it was 3.4, applicable to 0.086 < m a /eV < 0.110. These
bounds assume an axion flux as given by Eq. (5.21) which in turn
assumes an unperturbed Sun. Unfortunately, this assumption is not
consistent because the present-day age of the Sun already requires the
bound Eq. (5.22).