Consider two massless representations of the connected Poincare group $ISO_0(1,3)$ with helicities $s$ and $t$. What is the decomposition of their tensor product into irreducibles?

Massless representations with helicity s are defined in Wigner's classification of irreducible unitary representations of the connected Poincare group, the semidirect product $ISO_0(1,3)$ of the connected Lorentz group $SO_0(1,3)$ and the 4-dimensional translation group.