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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 $m < c Λ^{1/4}_{\text{cc}}\sim 10 \ \text{meV}$. 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 $\mathbb{Z}_4$,$\mathbb{Z}_8$ or $\mathbb{Z}_{10}$ in the case of normal hierarchy, and $\mathbb{Z}_4$ 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.