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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?

ZenFox42
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    You should consider adding any references where you read these claims, it's likely they fit into someone's specific theory they are trying to build. – Triatticus Sep 20 '23 at 16:40
  • To whoever closed this question : I'm sorry, but I don't know how to add more details or clarity. Regardless of references, this seems like a very straight-forward question. 1) The axion has 0 spin. 2) All integer-spin particles are force-carriers. 3) What force does the axion mediate? Can you please describe what further details or clarity would help to re-open the question? – ZenFox42 Sep 21 '23 at 13:44
  • To whoever closed this question : also, Google is telling me that there's another posted answer to this question that starts with "The axion indeed a spin-0 particle, but it's not a force-carrier. Instead, the axion, if it exists, would be a very light, weakly-interacting particle." Even if you don't re-open the question, could you please make this answer visible, it sounds like it might be promising. I would be very grateful if I could see it, thanks! – ZenFox42 Sep 21 '23 at 13:49
  • @ZenFox42 There is a second answer which was deleted by its author; we generally do not undelete material by user request. – rob Sep 21 '23 at 14:19
  • @rob Ok, understandable. Can you please tell me what to add or change about my original question to get it re-opened? – ZenFox42 Sep 22 '23 at 10:19

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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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  • I think I just figured out the problem : the axion is an integer-spin particle, but it is created thru the mechanism of a scalar field with a potential and a broken symmetry, like the Higgs boson. Are bosons created this way not force-carriers? – ZenFox42 Sep 21 '23 at 14:32
  • I don't understand your question. I am confirming what you wrote above that you have read in the literature: if an axion exists, then virtual axions would mediate a(n as-yet-undiscovered) force, regardless of whether non-virtual axions are stable particles. – rob Sep 21 '23 at 23:13
  • I'm now flipping my opinion. I did a more complete search, and found that the only sources that said the axion was a force-carrier were "popular science" webpages, I couldn't find anything academic that stated that. Then I found a Stack post here : "Is the Higgs Boson a Force Carrier" that confirmed it was NOT. Then I noticed the similarities between how the Higgs is mathematically produced and how the axion is mathematically produced. There are so many similarities between them that I now think the axion is also not a force carrier, for the same reason the Higgs isn't. What do you think? – ZenFox42 Sep 22 '23 at 10:16
  • Please use links rather than titles or vague descriptions, so that we can be sure we are talking about the same thing. I think you're referring to this answer stating that the Higgs doesn't meditate a fundamental force. There are clearer answers to this effect here and here. Unlike the heavy Higgs, a low-mass axion would mediate an undiscovered long-range Yukawa interaction, which is interesting whether "fundamental" or not. – rob Sep 22 '23 at 14:46
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    Sorry, I thought comments were text-only, I didn't even try to put a link in one. And yes, your first link is the post I was referring to. Just FYI, the link in your second "here" is the same as the first one. However, thanks for all your responses, they have shown me that I'm thinking about this too simplistically. I'm Googling [low-mass "axion" mediate "long-range" "Yukawa interaction"] for more information. – ZenFox42 Sep 22 '23 at 15:55