Lifetime of Tau lepton is really short

The tau (and the muon) leptons are not as known in our everyday life as the electron. The reason for this is their extremely short lifetime. Tau being the most massive of the three leptons decays after a very small fraction of a second. It decays into a tau neutrino and the W boson, which is a force carrier of the weak interaction.

The average lifetime of a W boson is even shorter than the halflife of the tau, and it rapidly decays into other particles, in this case, an electron and an electron antineutrino. The electron and the neutrinos are stable particles.