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Title: What mechanism generates neutrino masses?
Plain: Neutrinos are at least a million times lighter than the electron. They could get mass like other particles but with a tiny coupling, or through a seesaw with very heavy partner particles.
Precise: Identify the origin of $m_{\nu} < \sim 0.1\ \mathrm{eV}$: Dirac Yukawas with $y \sim 1e-12$ plus right-handed neutrinos, the type I, II or III seesaw ($m_{\nu} \sim y^2 v^2 / M$), or radiative and low-scale mechanisms. An answer is the mechanism together with its scale, confirmed by a signature such as lepton number violation or heavy neutral leptons.
Settled by: Observation of neutrinoless double beta decay plus identification of the associated new states or interactions at colliders or in rare decays.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 1998