Axions, on the other hand, are amenable to direct detection [109], although it is challenging to fully explore the whole of the
theoretically available parameter space. Among particles proposed
to solve the CP violation problem, the axion comes in two
varieties, which have fairly well defined properties [75]. Axions can be converted completely into photons in what is
essentially a two-photon interaction. In experiments to detect
galactic dark-matter axions the second photon is provided by an
intense ambient electromagnetic field. The photon created has an
energy equal to the total energy of the axion (rest mass plus
kinetic energy). As noted earlier, the dark matter energy density
at the position of the Earth is about
. The preferred mass range for the axion is between
and
, although there is a second window between 2 and 5
[139]. The lower limit of the preferred mass range keeps
, while the upper limit prevents excessive energy-loss mechanisms
in stars and supernovae due to axion production and loss. If the
galactic dark matter is axions, then their local density is
between
and
. With a virial velocity distribution (
), the flux through a terrestrial detector is enormous, but
unfortunately the two-photon conversion process is very weak. In
an ambient 6 Tesla field each axion has a conversion probability
around
per second, and the photon produced has an energy in the
microwave region (2-200 GHz). Such an experiment requires a
tuned high-Q cavity, tunable over the projected axion mass/energy
range, with a sensitivity of around
. Two early experiments of this type [93,
84,
63] have been followed by a number of second generation
instruments [109], and the preferred axion mass window has been closed over a
very small range at its lowest end (
to
) at the 90% confidence level for KSVZ axions [62]. A variant on the tuned cavity technique is to incorporate
Rydberg atoms into the cavity where the
to
transition is also resonant with the cavity [145]. In addition to the direct dark matter axion searches, there
are a number of experiments looking for evidence of axion
existence, such as axion telescopes pointed at the Sun [87] and torsion balance instruments looking for short-range weak
force spin-coupling interactions of the type mediated by the
axion [122,
98,
114,
134]. These have yet to achieve sufficient sensitivity.
Neutralinos have received by far the most attention and there are an enormous range of techniques being used to search for these particles [119, 132, 6]. The basic questions that need to be addressed to assess the feasibility of detection of WIMPs in the halo of our Galaxy are:
![]() |
Experimental Searches for Dark Matter
Timothy J. Sumner http://www.livingreviews.org/lrr-2002-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |