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][1] 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. 


  [1]: https://en.wikipedia.org/wiki/Wigner%27s_classification