= Gauge-invariant right-handed neutrino Majorana mass
{title2=$-\tfrac12M\,\overline{\nu_R^c}\nu_R+\text{Hermitian conjugate}$}
A <right-handed neutrino> is a singlet of the entire <Standard Model> gauge group. Its same-chirality bilinear with the field obtained by <charge conjugation> therefore admits the displayed renormalizable <Majorana mass term>, in addition to a Higgs-induced <Dirac mass term>. It violates <lepton number> by two units. Analogous terms for electrically charged Standard Model fermions carry nonzero <electric charge>; quarks also have nontrivial <color charge>. The presence of a right-handed field permits mass but does not force a nonzero mass parameter.
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