A very light axion (sometimes called invisible axion) was invented in 1979 and
has been known to work as a cold dark matter candidate since 1983. It is a most
bizzarre particle we encounter, in a sense that it is created when the
temperature of the universe is extremely high around 10^12 GeV but its mass
is very tiny or its Compton wave length is macroscopic around cm level. The
particles in the Particle Data book are far from this strange behavior.
Probably, it is the most light pseudo-Goldstone boson (aside from the
quintessence). The global symmetry defining the Goldstone boson is a Peccei-
Quinn symmetry. The way this symmetry is broken is explicit at one loop quark
diagrams, and its coupling is to the so-called gluon anomaly. This itself is
a remarkable fact since many other symmetries are protected by higher order
loops. Furthermore. the axion is needed in constructing a successful
superstring model. Namely, the superstring gives a theoretical ground root
for the very light axion.
If this axion is not present in Nature, then particle physics will be in
a dilemma: How should we solve the strong CP problem? There are some
solutions under the name of calculability, but these calculable solutions
do not give a strong CP solution automatically. And the most attractive
cold dark matter candidate is removed from the Particle Data book.
On the other hand, if it is observed, it would be a strong support toward
the last 25 years activity in theoretical particle physics: instanton physics,
the strong CP solution, and the axion cold dark matter in the universe.
In the literature, two kinds of very light axions are reported: KSVZ axion
and DFSZ axion. These axions differ in the method how the PQ charge is
assigned to a singlet Higgs field, in the KSVZ the singlet couples to
heavy quarks and in the DFSZ the singlet couples to Higgs doublets. But the
common feature is that the PQ charge carrying Higgs singlet is the main source
of the spontaneous symmetry breaking. The vacuum expectation value of this
singlet has a narrow window now
10^9 GeV < F_a < 10^12 GeV.
Already, 20 years have passed since the invention of the very light axion, and
soon its existence or absence will be probed by the ongoing experiments.
Relevant references and the current situation can be found in
J. E. Kim, Phys. Rev. D58, 055006 (1998).
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