I keep seeing in the literature that the axion is a force-carrier. I assume this is because it has spin 0, and all integer-spin particles are force-carriers. But I cannot find anywhere what force that is. I know it interacts with electromagnetism and gravity, but is it actually a mediator of a new force?
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If the axion couples to matter particles, the matter particles may exchange virtual axions. The exchange of virtual particles is the mechanism behind the known quantum-mechanical forces. Photon exchange is the core mechanism of quantum electrodynamics; the exchange of pions and other mesons is a good model for the long-range part of the strong force, while gluon exchange becomes a more parsimonious explanation at shorter distances; the weak force is mediated by the exchange of virtual W and Z bosons.
If the axion doesn't couple to the known matter particles, but does couple to other bosons in the standard model, then there would still be an axion-mediated force between matter particles. This force wouldn't be due to exchange at "tree level," which we forbade in the previous sentence, but instead via higher-order diagrams where the axions couple to virtual particle loops surrounding the matter particles.
We don't have any experimental evidence for such a "fifth force," but it's not for want of searching.
For a historical analogy, consider the search for the weak neutral current after its proposal in the late 1960s. Most people know about the winner of the race, which was the direct detection of the Z boson in the high-energy accelerator at CERN in 1983. But there were also a number of efforts to isolate the weak neutral current by making precision measurements of parity violation in low-energy systems. There is a similar race happening today to search for the axion, with a number of direct and indirect searches.
rob
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