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 $\sim {10}^9{\Xi }^{-}$ hyperons produced in $800\mathrm{GeV}/c$ $p$-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 4 orders of magnitude.

Figure 1

A possible diagram for the decay ${\Xi }^{-}\to p{\mu }^{-}{\mu }^{-}$; ${\nu }_m$ represents a Majorana neutrino.

Figure 2

Plan view of the HyperCP spectrometer. C1–C8 are multiwire proportional chambers. Note that the $z$ scale is compressed by a factor of 10 compared to the $x$ scale.

Figure 3

$p{\mu }^{-}{\mu }^{-}$ invariant-mass distribution before full application of selection requirements; note broadened and shifted peaks due to $\pi$-$\mu$ misidentification.

Figure 4

$p{\mu }^{-}{\mu }^{-}$ invariant-mass distribution after application of all selection requirements; inset: $p{\pi }^{-}{\pi }^{-}$ invariant-mass distribution from the normalization sample.

Figure 5

Comparison of normalization-mode Monte Carlo calculation (dashed histogram) and data (solid histogram) distributions in (a) $z_{\mathrm{vertex}}$ and (b) $x_{\mathrm{target}}$.