Leptonic Photons and Nucleosynthesis
  Should $U(1)$ long-range forces be associated to electron, muon and/or tau quantum number then their ''fine structure constants" are seen to be bound by nucleosynthesis data to be less than about $1.7 \times 10^{-11}$. For $\tau$ and $\mu$ this is the best upper limit up to date 
J. A. Grifols and E. Masso