axionlike

Axionlike particles, or ALPs, are hypothetical light, neutral pseudoscalar bosons predicted by extensions of the Standard Model that feature spontaneously broken global symmetries. Like the QCD axion, ALPs are pseudo-Nambu-Goldstone bosons, but unlike the QCD axion they do not require a fixed relationship between their mass and their coupling to standard-model fields. As a result, ALPs can have a broad range of masses and couplings, most commonly described by a two-photon coupling gaγ and a mass ma, with possible additional couplings to fermions or nucleons.

The dominant interaction is the aγγ coupling gaγ a Fμν F̃μν. In magnetic fields this leads to oscillations between photons and ALPs, enabling laboratory experiments and astrophysical probes. ALPs could be produced in stars and in the early universe. Experimental approaches include light-shining-through-walls experiments, which search for photon→ALP→photon conversions; helioscopes aimed at detecting solar ALPs; and haloscopes that seek to detect dark-matter ALPs with resonant cavities in strong magnetic fields.

Cosmological and astrophysical considerations constrain ALPs in wide regions of mass and coupling. They can affect stellar cooling, supernova dynamics, and the cosmic gamma-ray spectrum, and in some models may contribute to the dark matter density through non-thermal production mechanisms such as misalignment or string decay.

Status: no conclusive detection to date. The parameter space is being probed by a range of experiments and observations; notable efforts include CAST and planned next-generation helioscopes, as well as diverse laboratory searches and haloscope experiments. The ALP hypothesis remains open, with ongoing programs expanding sensitivity to new regions of mass and coupling.