One-family Dirac-Majorana mass matrix ($\mathcal{M}=\left(\begin{array}{cc}{M}_L& m\\ m& M_R\end{array}\right)$)

In an all-left-handed Weyl basis $({\nu }_L,{\nu }_R^c)$, the symmetric matrix has left and right Majorana entries $M_L,M_R$ and off-diagonal Dirac entry $m$. Real eigenvalues can have signs; physical Majorana masses are nonnegative after appropriate field rephasings.