Measurement performed at e+e- colliders (best results from ALEPH, OPAL, CLEO).

Talk

In tau decays (overall, including muons and electrons) only 15.18% have 3 prongs, 0.1% have 5 prongs.
When you measure the neutrino mass you want to enhance the sensitivity in the endpoint region. Decays with many hadrons in the final state are the best, since they leave only very little phase space available for the neutrino. And if the neutrino has sizeable mass, the effect on the endpoint is bigger.

Backgrounds: Bhabha (i.e. e+e- -> e+e-), e+e- -> hadrons, e+e- -> e+e- hadrons.

Two (non independent) variables are used, whose endpoint regions are sensitive to neutrino mass: E_beam-E_h (since we can assume that the tau energy is the same as the beam) and M_h, where h is the hadronic system associated to a single tau.
These two variables form a triangle, whose region of maximum sensitivity is close to the vertex (E_h=1,M_h=m_τ).

Main systematics: ECAL (for π0's), momentum measurement (for the charged prongs), and spectral function (uncertainty on the ρ, ρ', a₁ resonances).