Search for the Lepton-Number-Violating Decay $\Xi^- \to p \mu^- \mu^-$
  A sensitive search for the lepton-number-violating decay $\Xi^-\to p \mu^-\mu^-$ has been performed using a sample of $\sim10^9$ $\Xi^-$ hyperons produced in 800 GeV/$c$ $p$-Cu collisions. We obtain $\mathcal{B}(\Xi^-\to p \mu^-\mu^-)< 4.0\times 10^{-8}$ at 90% confidence, improving on the best previous limit by four orders of magnitude. 
HyperCP Collaboration: D. Rajaram, R. A. Burnstein, A. Chakravorty, Y.   C. Chen, W.-S. Choong, K. Clark, E. C. Dukes, C. Du...