A remark about unitary representations of Lorentz group and an equation   for zero mass particles
  In the present note the expansion of the wave function of a massless particle (with the definite value of its helicity) over the untary irreducible representaions of the Lorentz group (defined on the light cone) is used as for the analog of the Fourier transformation for deriving of an equation in the relativistic configuration representation. 
N.B.Skachkov