τdecay puzzle

Directly measured $\tau$ decay branching ratios $B(\tau \to e\nu \overline{\nu })$, $B(\tau \to \mu \nu \overline{\nu })$, $B(\tau \to \frac{{\nu }_{\tau }\pi }{K})$, and $B(\tau \to {\nu }_{\tau }{\pi }^{-}{\pi }^0)$ are compared with standard-model predictions parametrized in terms of the $\tau$ mass and lifetime. For current averages, $m_{\tau }={1784.1}_{-3.6\mathrm{}}^{+2.7\mathrm{}}$ MeV and ${\tau }_{\tau }=3.04\mathrm{}\pm 0.07\times {10}^{-13\mathrm{}}$ s, the experimental branching ratios are found to be systematically smaller than theory, thereby suggesting that significant reductions in the $\tau$ lifetime or mass (perhaps both) are likely. If ${\tau }_{\tau }$ and $m_{\tau }$ are correct, a heavy fourth-generation neutrino with $\frac{m}{{\nu }_4}\gtrsim 45.3$ GeV and mixing ${sin}^2{\theta }_{34}\simeq 0.06$ could be the source of the discrepancy.