Stars as Laboratories for Fundamental Physics - MPP Theory Group

Two-Photon Coupling of Low-Mass Bosons 191
ing scale f a . This bound supersedes Sikivie’s (1988) previous scenario
where he tried to explain the polarization features of certain sources
by the conversion of cosmic-string-produced Nambu-Goldstone bosons
to photons in cosmic magnetic fields. This scenario would have required
C aγ ≈ 10 5 .
5.5.5 Temperature Fluctuations in the Cosmic Microwave
Background
In the presence of large-scale magnetic fields in the universe, photons of
the cosmic microwave background radiation (CMBR) could convert into
arions. The angular variations of the CMBR temperature have been
measured by the COBE satellite and other instruments to be extremely
small; a typical value is δT/T ≈ 10 −5 . Therefore, the conversion process
must not have been very efficient between the surface of last scattering
and us.
This argument has been studied in detail by Chen (1995) for photongraviton
conversion which is a very similar effect due to the two-photon
coupling vertex which the massless gravitons must have. The coupling
constant involves the inverse Planck mass. Therefore, one may also
expect interesting effects for hypothetical arions which could couple to
photons more strongly than gravitons do.
5.6 Summary of Constraints on g aγ
The astrophysical and experimental bounds on the photon coupling
of arbitrary pseudoscalars are summarized in Fig. 5.9. “Haloscope”
refers to the search for galactic axions discussed in Sect. 5.3 and so
these constraints (Fig. 5.7) apply only if the pseudoscalars are the dark
matter in our galaxy. The dotted line is the search regime for the
ongoing experiment mentioned in Sect. 5.3.
“Helioscope” refers to the search for solar axion to x-ray conversion
(Sect. 5.4.2). It is shown as a dashed line because it is not self-consistent
in that it assumes an unperturbed Sun—the area enclosed by the dashed
line is already excluded by the solar age.
“Telescope” refers to the search for decay photons from the cosmic
axion background (Sect. 12.7.2, Fig. 12.23). It is assumed that the
pseudoscalars were in thermal equilibrium in the early universe.
“Laser” refers to the birefringence and shining-light-through-walls
experiments discussed above. The most restrictive such limit is from
the rotation of the plane of polarization of a laser beam trapped in