Precision Measurement of the Mass and Lifetime of the ${\mathbf{\Xi }}_b^{-}$ Baryon

We report on measurements of the mass and lifetime of the ${\mathrm{\Xi }}_b^{-}$ baryon using about 1800 ${\mathrm{\Xi }}_b^{-}$ decays reconstructed in a proton-proton collision data set corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$ collected by the LHCb experiment. The decays are reconstructed in the ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}_c^0{\pi }^{-}$, ${\mathrm{\Xi }}_c^0\to p{K}^{-}{K}^{-}{\pi }^{+}$ channel and the mass and lifetime are measured using the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ mode as a reference. We measure $M({\mathrm{\Xi }}_b^{-})-M({\mathrm{\Lambda }}_b^0)=178.36\pm 0.46\pm 0.16\mathrm{MeV}/c^2$, $({\tau }_{{\mathrm{\Xi }}_b^{-}}/{\tau }_{{\mathrm{\Lambda }}_b^0})=1.089\pm 0.026\pm 0.011$, where the uncertainties are statistical and systematic, respectively. These results lead to a factor of 2 better precision on the ${\mathrm{\Xi }}_b^{-}$ mass and lifetime compared to previous best measurements, and are consistent with theoretical expectations.

Figure 1

Invariant mass spectrum, along with the fit projections, for (left) ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ and (right) ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}_c^0{\pi }^{-}$ candidates.

Figure 2

Ratio of the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ to the ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}_c^0{\pi }^{-}$ selection efficiencies as a function of decay time. The uncertainties are due to the finite size of the simulated samples.

Figure 3

Corrected yield ratio, $N_{\text{cor}}({\mathrm{\Xi }}_b^{-})/N_{\text{cor}}({\mathrm{\Lambda }}_b^0)$ in bins of decay time, along with the exponential fit. The uncertainties are statistical only.