Further swampland constraint on Dirac neutrino.

Recent studies on swampland conjectures (e.g. non-SUSY AdS conjecture or AdS distance conjecture) predict that the (lightest) neutrino must be Dirac and the mass must be cosmologically small m < c Λ cc 1 / 4 ∼ 1 0 meV. The Dirac neutrino naturally accompanies a U (1) symmetry that can be embedded in the anomaly-free U (1) B − L global symmetry of the standard model. We point out that the swampland conjectures applied to a circle compactification with the U (1) symmetry twisting lead to further constraints. In particular, the U (1) symmetry must be broken down to ℤ 4 , ℤ 8 or ℤ 1 0 in the case of normal hierarchy, and ℤ 4 in the case of inverted hierarchy, providing evidence for the absence of continuous global symmetry in quantum gravity. We also predict a more constrained upper bound of the neutrino mass for each case. [ABSTRACT FROM AUTHOR]