Peculiarities of the Weyl-Wigner-Moyal formalism for scalar charged particles

A description of scalar charged particles, based on the Feshbach-Villars fo rmalism, is proposed. Particles ape described by an object that is a Wigner function in usual coordinates and momenta and a density matrix in the char ge variable. It is possible to introduce the usual Wigner function for a la rge class of dynamical variables. Such an approach explicitly contains a me asuring device frame. From our point of view it corresponds to the Copenhag en interpretation of quantum mechanics. It is shown how physical properties of such particles depend on the definition of the coordinate operator. The evolution equation for the Wigner function of a single-charge state in the classical limit coincides with the Liouville equation. Localization peculi arities manifest themselves in specific constraints on possible initial con ditions.