The axion () is a hypothetical elementary
particle postulated by the Peccei–Quinn
theory in 1977 to resolve the strong CP problem
in quantum chromodynamics (QCD). If axions
exist and have low mass within a specific
range, they are of interest as a possible
component of cold dark matter.
== History ==
=== 
Prediction ===
As shown by Gerard 't Hooft, strong interactions
of the standard model, QCD, possess a non-trivial
vacuum structure that in principle permits
violation of the combined symmetries of charge
conjugation and parity, collectively known
as CP. Together with effects generated by
weak interactions, the effective periodic
strong CP-violating term, Θ, appears as a
Standard Model input – its value is not
predicted by the theory, but must be measured.
However, large CP-violating interactions originating
from QCD would induce a large electric dipole
moment (EDM) for the neutron. Experimental
constraints on the currently unobserved EDM
implies CP violation from QCD must be extremely
tiny and thus Θ must itself be extremely
small. Since a priori Θ could have any value
between 0 and 2π, this presents a “naturalness”
problem for the standard model. Why should
this parameter find itself so close to 0?
(Or, why should QCD find itself CP-preserving?)
This question constitutes what is known as
the strong CP problem.
One simple solution exists: If at least one
of the quarks of the standard model is massless,
Θ becomes unobservable. However, empirical
evidence strongly suggests that none of the
quarks are massless.
In 1977, Roberto Peccei and Helen Quinn postulated
a more elegant solution to the strong CP problem,
the Peccei–Quinn mechanism. The idea is
to effectively promote Θ to a field. This
is accomplished by adding a new global symmetry
(called a Peccei–Quinn symmetry) that becomes
spontaneously broken. This results in a new
particle, as shown by Frank Wilczek and Steven
Weinberg, that fills the role of Θ, naturally
relaxing the CP-violation parameter to zero.
This hypothesized new particle is called the
axion. The original Weinberg–Wilczek axion
was ruled out. Current literature discusses
the mechanism as the "invisible axion", which
has two forms: KSVZ (Kim–Shifman–Vainshtein–Zakharov)
and DFSZ (Dine–Fischler–Srednicki–Zhitnitsky).
=== Searches ===
It had been thought that the invisible axion
solves the
strong CP problem without being amenable to
verification by experiment. Axion models choose
coupling that does not appear in any of the
prior experiments. The very weakly coupled
axion is also very light because axion couplings
and mass are proportional. The situation changed
when it was shown that a very light axion
is overproduced in the early universe and
therefore excluded. The critical mass is of
order 10−11 times the electron mass, where
axions may account for the dark matter. The
axion is thus a dark-matter candidate, as
well as a solution to the strong CP problem.
Furthermore, in 1983, Pierre Sikivie wrote
down the modification of Maxwell's equations
from a light stable axion and showed that
axions can be detected on Earth by converting
them to photons, using a strong magnetic field,
the principle of the ADMX. Solar axions may
be converted to X-rays, as in CAST. Many experiments
are searching laser light for signs of axions.A
mass value between 50 and 1,500 µeV for the
axion was reported in a paper published in
November 2016 (Borsanyi, S. et al.). The result
was calculated by simulating the formation
of axions during the post-inflation period
on a supercomputer.
=== Maxwell's equations with axion modifications
===
If magnetic monopoles exist then there is
a symmetry in Maxwell's equations where the
electric and magnetic fields can be rotated
into each other with the new fields still
satisfying Maxwell's equations. Luca Visinelli
showed that the duality symmetry can be carried
over to the axion-electromagnetic theory as
well. Assuming the existence of both magnetic
charges and axions, Maxwell's equations read
If magnetic monopoles do not exist, then the
same equations hold with the density
ρ
m
{\displaystyle \rho _{m}}
and current
J
m
{\displaystyle \mathbf {J} _{m}}
replaced by zero. Incorporating the axion
has the effect of rotating the electric and
magnetic fields into each other.
where the mixing angle
ξ
{\displaystyle \xi }
depends on the coupling constant
κ
{\displaystyle \kappa }
and the axion field strength
θ
{\displaystyle \theta }
By plugging the new values for electromagnetic
field
E
′
{\displaystyle \mathbf {E'} }
and
B
′
{\displaystyle \mathbf {B'} }
into Maxwell's equations we obtain the axion-modified
Maxwell equations above. Incorporating the
axion into the electromagnetic theory also
gives a new differential equation – the
axion law – which is simply the Klein-Gordon
Equation (the quantum field theory equation
for massive spin-zero particles) with an
E
⋅
B
{\displaystyle \mathbf {E} \cdot \mathbf {B}
}
source term.
A term analogous to the one that would be
added to Maxwell's equations to account for
axions also appears in recent (2008) theoretical
models for topological insulators giving an
effective axion description of the electrodynamics
of these materials. This term leads to several
interesting predicted properties including
a quantized magnetoelectric effect. Evidence
for this effect has recently been given in
THz spectroscopy experiments performed at
the Johns Hopkins University.
== Experiments ==
The Italian PVLAS experiment searches for
polarization changes of light propagating
in a magnetic field. The concept was first
put forward in 1986 by Luciano Maiani, Roberto
Petronzio and Emilio Zavattini. A rotation
claim in 2006 was excluded by an upgraded
setup. An optimized search began in 2014.
Another technique is so called "light shining
through walls", where light passes through
an intense magnetic field to convert photons
into axions, that pass through metal. Experiments
by BFRS and a team led by Rizzo ruled out
an axion cause. GammeV saw no events in a
2008 PRL. ALPS-I conducted similar runs, setting
new constraints in 2010; ALPS-II will run
in 2019. OSQAR found no signal, limiting coupling
and will continue.
Several experiments search for astrophysical
axions by the Primakoff effect, which converts
axions to photons and vice versa in electromagnetic
fields. Axions can be produced in the Sun's
core when X-rays scatter in strong electric
fields. The CAST solar telescope is underway,
and has set limits on coupling to photons
and electrons. ADMX searches the galactic
dark matter halo for resonant axions with
a cold microwave cavity and has excluded optimistic
axion models in the 1.9-3.53 μeV range. It
is amidst a series of upgrades and is taking
new data, including at 4.9-6.2 µeV. Other
experiments of this type include HAYSTAC,
CULTASK, and ORGAN. HAYSTAC recently completed
the first scanning run of a haloscope above
20 µeV.Resonance effects may be evident in
Josephson junctions from a supposed high flux
of axions from the galactic halo with mass
of 0.11 meV and density 0.05 GeV⋅cm−3
compared to the implied dark matter density
0.3±0.1 GeV⋅cm−3, indicating said axions
would not have enough mass to be the sole
component of dark matter. The ORGAN experiment
plans to conduct a direct test of this result
via the haloscope method.Dark matter cryogenic
detectors have searched for electron recoils
that would indicate axions. CDMS published
in 2009 and EDELWEISS set coupling and mass
limits in 2013. UORE and XMASS also set limits
on solar axions in 2013. XENON100 used a 225-day
run to set the best coupling limits to date
and exclude some parameters.Axion-like bosons
could have a signature in astrophysical settings.
In particular, several recent works have proposed
axion-like particles as a solution to the
apparent transparency of the Universe to TeV
photons. It has also been demonstrated in
a few recent works that, in the large magnetic
fields threading the atmospheres of compact
astrophysical objects (e.g., magnetars), photons
will convert much more efficiently. This would
in turn give rise to distinct absorption-like
features in the spectra detectable by current
telescopes. A new promising means is looking
for quasi-particle refraction in systems with
strong magnetic gradients. In particular,
the refraction will lead to beam splitting
in the radio light curves of highly magnetized
pulsars and allow much greater sensitivities
than currently achievable. The International
Axion Observatory (IAXO) is a proposed fourth
generation helioscope.Axions may be produced
within neutron stars, by nucleon-nucleon bremsstrahlung.
The subsequent decay of axions to gamma rays
allows constraints on the axion mass to be
placed from observations of neutron stars
in gamma-rays using the Fermi LAT. From an
analysis of four neutron stars, Berenji et
al. obtained a 95% CL upper limit on the axion
mass of 0.079 eV.
== Possible detection ==
It was reported in 2014 that evidence for
axions may have been detected as a seasonal
variation in observed X-ray emission that
would be expected from conversion in the Earth's
magnetic field of axions streaming from the
Sun. Studying 15 years of data by the European
Space Agency's XMM-Newton observatory, a research
group at Leicester University noticed a seasonal
variation for which no conventional explanation
could be found. One potential explanation
for the variation, described as "plausible"
by the senior author of the paper, is the
known seasonal variation in visibility to
XMM-Newton of the sunward magnetosphere in
which X-rays may be produced by axions from
the Sun's core. This interpretation of the
seasonal variation is disputed by two Italian
researchers, who identify flaws in the arguments
of the Leicester group that are said to rule
out an interpretation in terms of axions.
Most importantly, the scattering in angle
assumed by the Leicester group to be caused
by magnetic field gradients during the photon
production, necessary to allow the X-rays
to enter the detector that cannot point directly
at the sun, would dissipate the flux so much
that the probability of detection would be
negligible.In 2013, Christian Beck suggested
that axions might be detectable in Josephson
junctions; and in 2014, he argued that a signature,
consistent with a mass ≈110 μeV, had in
fact been observed in several preexisting
experiments.In 2016 a theoretical team from
MIT devised a possible way of detecting axions
using a strong magnetic field. The magnetic
field need be no stronger than that produced
in a MRI scanning machine and it should show
a slight wavering variation that is linked
to the mass of the axion. The experiment is
now being implemented by experimentalists
at the university. Another approach being
used by the University of Washington uses
a strong magnetic field to detect the possible
weak conversion of axions to microwaves.
== Properties ==
=== 
Predictions ===
One theory of axions relevant to cosmology
had predicted that they would have no electric
charge, a very small mass in the range from
10−6 to 1 eV/c2, and very low interaction
cross-sections for strong and weak forces.
Because of their properties, axions would
interact only minimally with ordinary matter.
Axions would also change to and from photons
in magnetic fields.
=== Supersymmetry ===
In supersymmetric theories the axion has both
a scalar and a fermionic superpartner. The
fermionic superpartner of the axion is called
the axino, the scalar superpartner is called
the saxion or dilaton.
They are all bundled up in a chiral superfield.
The axino has been predicted to be the lightest
supersymmetric particle in such a model. In
part due to this property, it is considered
a candidate for dark matter.
=== Cosmological implications ===
Inflation suggests that axions were created
abundantly during the Big Bang. Because of
a unique coupling to the instanton field of
the primordial universe (the "misalignment
mechanism"), an effective dynamical friction
is created during the acquisition of mass
following cosmic inflation. This robs all
such primordial axions of their kinetic energy.
If axions have low mass, thus preventing other
decay modes (since there's no lighter particles
to decay into), theories predict that the
universe would be filled with a very cold
Bose–Einstein condensate of primordial axions.
Hence, axions could plausibly explain the
dark matter problem of physical cosmology.
Observational studies are underway, but they
are not yet sufficiently sensitive to probe
the mass regions if they are the solution
to the dark matter problem. High mass axions
of the kind searched for by Jain and Singh
(2007) would not persist in the modern universe.
Moreover, if axions exist, scatterings with
other particles in the thermal bath of the
early universe unavoidably produce a population
of hot axions.Low mass axions could have additional
structure at the galactic scale. If they continuously
fall into galaxies from the intergalactic
medium, they would be denser in "caustic"
rings, just as the stream of water in a continuously-flowing
fountain is thicker at its peak. The gravitational
effects of these rings on galactic structure
and rotation might then be observable. Other
cold dark matter theoretical candidates, such
as WIMPs and MACHOs, could also form such
rings, but because such candidates are fermionic
and thus experience friction or scattering
among themselves, the rings would be less
pronounced.
Axions would also have stopped interaction
with normal matter at a different moment than
other more massive dark particles. The lingering
effects of this difference could perhaps be
calculated and observed astronomically.
João G. Rosa and Thomas W. Kephart suggested
that axion clouds formed around unstable primordial
black holes might initiate a chain of reactions
that irradiate electromagnetic waves, allowing
their detection. When adjusting the mass of
the axions to explain dark matter, the pair
discovered that the value would also explain
the luminosity and wavelength of fast radio
bursts, being a possible origin for both phenomena
