Searches for lepton flavour violation and baryon number violation at LHCb

We report on the first searches for lepton flavour violating $\tau^-$ decays at a hadron collider. These include searches for the lepton flavour violating decay $\tau^-\to\mu^+\mu^-\mu^-$ and the lepton flavour and baryon number violating decays $\tau^-\to\bar{p}\mu^+\mu^-$ and $\tau^-\to p\mu^-\mu^-$. Using $1.0$ fb$^{-1}$ of data collected at $\sqrt{s} = 7$ TeV by LHCb in 2011 upper limits of $\cal{B}(\tau^-\to\mu^+\mu^-\mu^-) < 7.8 (6.3) \times 10^{-8}$, $\cal{B}(\tau^-\to\bar{p}\mu^+\mu^-) < 4.5 (3.4) \times 10^{-7}$ and $\cal {B}(\tau^-\to p\mu^-\mu^-) < 6.0 (4.6) \times 10^{-7}$ are set at 95 % (90 %) confidence level.