Localizability, gauge symmetry and Newton-Wigner operator for massless particles

Abstract: The notion of position operator for massless spinning particles is discussed in some detail. The noncommutativity of coordinates is related to the gauge symmetry following from the freedom in definition of standard state in Wigner's construction of induced representations of Poincare group. The counterpart of Newton-Wigner operator is discussed. It is explained why the Newton-Wigner construction works only for helicity $ \mid\lambda\mid\leq1/2$.