Pauli Terms Must Be Absent In Dirac Equation
  It should be of interest, whether Dirac's equation involves all 16 basis elements of his Clifford algebra $Cl_D.$ These include the 6 `tensorial' $\sigma^{\mu\nu}$ with which the `Pauli terms' are formed. We find that these violate a basic axiom of any *-algebra, when Dirac's $\Psi$ is canonical. Then the Dirac operator is spanned only by the 10 elements $1,i\gamma_5,\gamma^\mu,\gamma^\mu\gamma_5$ (which don't form a basis of $Cl_D$ because the $\sigma^{\mu\nu}$ are excluded). 
Kurt Just and James Thevenot