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要旨

We present a scenario where neutrino masses and Dark Matter are related due to a global $U(1)_{B-L}$ symmetry. Specifically we consider neutrino mass generation via the Zee--Babu two-loop mechanism, augmented by a scalar singlet whose VEV breaks the global $U(1)_{B-L}$ symmetry. In order to obtain a Dark Matter candidate we introduce two Standard Model singlet fermions. They form a Dirac particle and are stable because of a remnant $Z_2$ symmetry. Hence, in this model the stability of Dark Matter follows from the global $U(1)_{B-L}$ symmetry. We discuss the Dark Matter phenomenology of the model, and compare it to similar models based on gauged $U(1)_{B-L}$. We argue that in contrast to the gauged versions, the model based on the global symmetry does not suffer from severe constraints from $Z'$ searches.