Observation of the Decay ${\Xi }^0\to {\Sigma }^{+}{\mu }^{-}{\overline{\nu }}_{\mu }$

The ${\Xi }^0$ muon semileptonic decay has been observed for the first time with nine identified events using the KTeV beam line and detector at Fermilab. The decay is normalized to the ${\Xi }^0$ beta decay mode and yields a value for the ratio of decay rates $\Gamma ({\Xi }^0\to {\Sigma }^{+}{\mu }^{-}{\overline{\nu }}_{\mu })/\Gamma ({\Xi }^0\to {\Sigma }^{+}{e}^{-}{\overline{\nu }}_e)$ of $[{1.8}_{-0.5}^{+0.7}(\mathrm{stat})\pm 0.2(\mathrm{syst})]\times {10}^{-2}$. This is in agreement with the SU(3) flavor symmetric quark model.

Figure 1

Reconstructed $\mathrm{p}{\pi }^0$ invariant mass and $p_t^2$ of the ${\Sigma }^{+}{\mu }^{-}$ for events passing the ${\Xi }^0$ muon decay selection criteria.

Figure 2

Reconstructed $\mathrm{p}{\pi }^0$ invariant mass and $p_t^2$ of the ${\Sigma }^{+}{\mu }^{-}$ for MC ${\Xi }^0\to {\Sigma }^0\gamma$ events passing the ${\Xi }^0$ muon decay selection criteria. Ten times the measured number of ${\Xi }^0$ decays were generated.

Figure 3

Reconstructed $\mathrm{p}{\pi }^0$ invariant mass distribution for 1999 data events passing the ${\Xi }^0\to {\Sigma }^{+}{e}^{-}{\overline{\nu }}_e$ selection criteria.

Figure 4

Data–MC simulation comparison of ${\Sigma }^{+}{e}^{-}$ invariant mass distribution for events passing the ${\Xi }^0$ beta decay selection criteria. Data are shown in dots; MC simulation in histogram.