# What are ordinary mass-terms (of neutrinos)?

What is the meaning of ordinary mass terms?

The "ordinary mass terms" in the quoted paper (see-saw mechanism) would stand for Dirac mass terms, which couple active left-handed neutrinos $$\nu_L$$ (the isospin "up" part of the electroweak $$SU(2)$$ left-handed neutrino-electron doublets) to the sterile right-handed neutrinos $$\nu_R$$ (as $$SU(2)$$-singlets): $$m (\bar{\nu}_L\nu_R + \bar{\nu}_R\nu_L).$$

The "non-ordinary mass terms" would mean Majorana mass terms, which couple the sterile right-handed neutrino $$\nu_R$$ to the charge-conjugate of itself $$\nu^c_R$$: $$M \bar{\nu}_R\nu^c_R.$$

The "ordinary" Dirac mass $$m$$ is of the eletroweak symmetry breaking scale, while the "non-ordinary" Majorana mass $$M$$ is of the much higher see-saw (or grand unification) scale . In the usual scheme of the sea-saw model, the tiny neutrino mass is the resultant effective mass $$m_{\nu}$$ with scale: $$m_{\nu}\sim \frac{m^2}{M}.$$