The observables and localized states of a dirac particle

It is shown that for a non-zero mass Dirac particle, only one of the four position operators recently discovered by the authors has eigenstates (describing the localized states of the particle) which obey the regularity condition imposed by Newton and Wigner. However, this condition is not applicable to the states of a particle with zero rest mass, and in such a case the multiplicity of possible position (and other) operators persists