Axion
An axion (/ˈæksiɒn/) is a hypothetical elementary particle originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve 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.
As shown by Gerard 't Hooft,[2] strong interactions of the standard model, QCD, possess a non-trivial vacuum structure[a] that in principle permits violation of the combined symmetries of charge conjugation and parity, collectively known as CP.
[b] In 1977, Roberto Peccei and Helen Quinn postulated a more elegant solution to the strong CP problem, the Peccei–Quinn mechanism.
This results in a new particle, as shown independently by Frank Wilczek[5] and Steven Weinberg,[6] that fills the role of Θ, naturally relaxing the CP-violation parameter to zero.
Wilczek named this new hypothesized particle the "axion" after a brand of laundry detergent because it "cleaned up" a problem,[7][8] while Weinberg called it "the higglet".
[13][14][15] There are two distinct scenarios in which the axion field begins its evolution, depending on the following two conditions: Broadly speaking, one of the two possible scenarios outlined in the two following subsections occurs: If both (a) and (b) are satisfied, cosmic inflation selects one patch of the Universe within which the spontaneous breaking of the PQ symmetry leads to a homogeneous value of the initial value of the axion field.
However, other bounds that come from isocurvature modes severely constrain this scenario, which require a relatively low-energy scale of inflation to be viable.
[16][17][18] If at least one of the conditions (a) or (b) is violated, the axion field takes different values within patches that are initially out of causal contact, but that today populate the volume enclosed by our Hubble horizon.
The proper treatment in this scenario is to solve numerically the equation of motion of the PQ field in an expanding Universe, in order to capture all features coming from the misalignment mechanism, including the contribution from topological defects like "axionic" strings and domain walls.
[28] He showed that these axions could be detected on Earth by converting them to photons, using a strong magnetic field, motivating a number of experiments.
[35] In 2019, a team at the Max Planck Institute for Chemical Physics of Solids published their detection of an axion insulator phase of a Weyl semimetal material.
[59][60] It has also been demonstrated that, in the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., magnetars), photons will convert much more efficiently.
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.
[64] The emerging photons lie in the GHz frequency range and can be potentially picked up in radio detectors, leading to a sensitive probe of the axion parameter space.
[65] A novel, alternative strategy consists in detecting the transient signal from the encounter between a neutron star and an axion minicluster in the Milky Way.
From an analysis of four neutron stars, Berenji et al. (2016) obtained a 95% confidence interval upper limit on the axion mass of 0.079 eV.
In 2016, a theoretical team from Massachusetts Institute of Technology devised a possible way of detecting axions using a strong magnetic field that need be no stronger than that produced in an MRI scanning machine.
[72][73] Resonance effects may be evident in Josephson junctions[74] from a supposed high flux of axions from the galactic halo with mass of 110 μeV and density 0.05 GeV/cm3[75] compared to the implied dark matter density 0.3±0.1 GeV/cm3, indicating said axions would not have enough mass to be the sole component of dark matter.
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.
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.
[85] 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.
[86] In 2020, the XENON1T experiment at the Gran Sasso National Laboratory in Italy reported a result suggesting the discovery of solar axions.
[88] New observations made in July 2022 after the observatory upgrade to XENONnT discarded the excess, thus ending the possibility of new particle discovery.
[89][90] One theory of axions relevant to cosmology had predicted that they would have no electric charge, a very small mass in the range from 1 μeV/c2 to 1 eV/c2,[1] and very low interaction cross-sections for strong and weak forces.
[91] 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.
[citation needed] Ultralight axion (ULA) with m ~ 10−22 eV/c2 is a kind of scalar field dark matter that seems to solve the small scale problems of CDM.
A single ULA with a GUT scale decay constant provides the correct relic density without fine-tuning.
[citation needed] João G. Rosa and Thomas W. Kephart suggested that axion clouds formed around unstable primordial black holes might initiate a chain of reactions that radiate electromagnetic waves, allowing their detection.
[citation needed] In 2020, it was proposed that the axion field might actually have influenced the evolution of the early Universe by creating more imbalance between the amounts of matter and antimatter – which possibly resolves the baryon asymmetry problem.
