Massive neutrinos are guaranteed to have nonzero electromagnetic moments and,
since there are at least three neutrino species, these dipole moments define a
matrix. Here, we estimate the current upper bounds on all independent neutrino
electromagnetic moments, concentrating on Earth-bound experiments and
measurements with solar neutrinos, including the very recent results reported
by XENONnT. We make no simplifying assumptions and compare the hypotheses that
neutrinos are Majorana fermions or Dirac fermions. In particular, we fully
explore constraints in the Dirac-neutrino parameter space. Majorana and Dirac
neutrinos are different; for example, the upper bounds on the magnitudes of the
elements of the dipole moment matrix are weaker for Dirac neutrinos, relative
to Majorana neutrinos. The potential physics reach of next-generation
experiments also depends on the nature of the neutrino. We find that a
next-generation experiment two orders of magnitude more sensitive to the
neutrino electromagnetic moments via νμ elastic scattering may
discover that the neutrino electromagnetic moments are nonzero if the neutrinos
are Dirac fermions. Instead, if the neutrinos are Majorana fermions, such a
discovery is ruled out by existing solar neutrino data, unless there are more
than three light neutrinos.Comment: 20 pages, 3 figures, 2 table