  The well-known Landau-Yang (LY) theorem on the decay of a neutral particle
into two photons is generalized for analyzing the decay of a neutral or charged
particle into two identical massless particles of any spin. Selection rules
categorized by discrete parity invariance and Bose/Fermi symmetry are worked
out in the helicity formulation. The general form of the Lorentz-covariant
triple vertices are derived and the corresponding decay helicity amplitudes are
explicitly calculated in the Jacob-Wick convention. After checking the
consistency of all the analytic results obtained by two complementary
approaches, we extract out the key aspects of the generalized LY theorem.
