Search for the Lepton-Number-Violating Decay Ξ − → p μ − μ −

A sensitive search for the lepton-number-violating decay ${\\ensuremath{\\Xi}}^{\\ensuremath{-}}\\ensuremath{\\rightarrow}p{\\ensuremath{\\mu}}^{\\ensuremath{-}}{\\ensuremath{\\mu}}^{\\ensuremath{-}}$ has been performed using a sample of $\\ensuremath{\\sim}{10}^{9}\\text{ }\\text{ }{\\ensuremath{\\Xi}}^{\\ensuremath{-}}$ hyperons produced in $800\\text{ }\\text{ }\\mathrm{GeV}/c$ $p$-Cu collisions. We obtain $\\mathcal{B}({\\ensuremath{\\Xi}}^{\\ensuremath{-}}\\ensuremath{\\rightarrow}p{\\ensuremath{\\mu}}^{\\ensuremath{-}}{\\ensuremath{\\mu}}^{\\ensuremath{-}})l4.0\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{\\ensuremath{-}8}$ at 90% confidence, improving on the best previous limit by 4 orders of magnitude.