# Lepton universality

1. Jan 24, 2013

### JoePhysicsNut

I'd just like to clarify something I read on Wikipedia:

"The branching ratio of the common purely leptonic tau decays are:

17.82% for decay into a tau neutrino, electron and electron antineutrino;
17.39% for decay into a tau neutrino, muon and muon antineutrino.

The similarity of values of the two branching ratios is a consequence of lepton universality."

Just to be clear, lepton universality insures the couplings of tau to mu and electron are identical, however, there is a slight difference between the BRs and cross sections due to the availability of phase space. BR_mu is slighly less than BR_el because the mu is heavier so the density of states term in the cross section expression is smaller. Is this correct?

2. Jan 24, 2013

Yes, it is.