Fermion nonminimal gravitational coupling and the solar‐neutrino problem

In this work, a universal magnetic‐dipole interaction between massive fermions is considered, with the coupling mediated by the local spacetime curvature. The strong principle of equivalence is not valid for fermions because of their intrinsic spin. Hence, the associated principle of ’’minimal gravitational coupling’’ for the Dirac equation coupled to electromagnetic fields in the presence of gravity is an assumption which is unsupported by either theory or experiment. We show that relaxing the arbitrary minimal‐coupling constraint leads to a simple kind of nonminimal gravitational coupling (NMGC) which can generate curvature‐dependent magnetic‐moment effects, in background gravitational fields, for fermions coupled electromagnetically. Application of this model to the case of solar neutrinos yields a simple explanation of the low terrestrial neutrino flux in terms of sufficient neutrino energy loss (via multiple neutrino‐electron magnetic elastic scattering in the solar plasma) to account for the very low detection rate on earth. Terrestrial tests of this type of NMGC effect for neutrinos would appear in high‐energy neutrino‐nucleon scattering, in terms of anomalous (charge‐dependent) neutrino‐deuteron interactions, which could not be explained by charge‐indenpendent neutral‐current models alone.