The spin magnetic moment of a particle is proportional to q/2m, where q and m are charge and mass of the particle respectively. So if an elementary particle is neutral (like neutrinos or photons), then how is there any magnetic moment, despite the fact that they have a non-zero spin (1/2 for neutrinos, 1 for photons)? I can understand why particles like neutrons have a magnetic moment, as it's a composite particle of other charged particles (quarks), but that's of course not the case for elementary particles.
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@NiharKarve Thanks for the suggestion. It did actually answer a lot of my confusion. But one question remains: according to several people here, the photon does actually have a zero magnetic moment. Why can't the same logic be used for the photon as for the neutrinos to predict a small magnetic moment? Is it because of its zero mass (Danu explained that if you impose zero mass for the neutrinos, then the magnetic moment will actually vanish)? – User3141 Jul 08 '21 at 09:57
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The Z boson has a mass but no magnetic moment, for structural reasons of the SM. But, in principle, neutrino loops could induce a very-very-very-very small one. – Cosmas Zachos Jul 08 '21 at 15:18