My professor mentioned that a particle with an intrinsic magnetic quadrupole moment would be CP violating in an analogous manner to how a particle with an electric dipole would be evidence for new sources of CP violation. Are then any active or past experiments which are trying to measure the magnetic quadruple moment of particles like the electron for example? I know of at least two ongoing experiments to measure the electric dipole moment of the electron and neutron, but I'm wondering if anyone is studying magnetic quadrupole moments in the same way.
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Do a Google search with quotes "electric dipole of the electron" , the experiments come up. Do "magnetic quadrupole of the electron" and nothing comes up. – anna v Feb 17 '23 at 06:05
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My searches actually turned up this interesting paper which proposes methods to measure energy shifts in heavy polar molecules due to a nonzero MQM: https://arxiv.org/pdf/2210.17506.pdf . I am curious if any similar experiments are in the works – klippo Feb 17 '23 at 06:16
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A spin-half particle cannot have an intrinsic quadrupole moment, because the two-state system does not have enough degrees of freedom. See this answer for a hand-waving explanation, which is related to the Wigner-Eckart theorem.
In a comment, you link to this preprint, which discusses magnetic quadrupole moments in a number of odd-$A$ nuclei with spin $3/2$ or higher.
rob
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