Observation of Two New ${\mathrm{\Xi }}_b^{-}$ Baryon Resonances

Two structures are observed close to the kinematic threshold in the ${\mathrm{\Xi }}_b^0{\pi }^{-}$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P={(1/2)}^{+}$ and $J^P={(3/2)}^{+}$ states, denoted ${\mathrm{\Xi }}_b^{\prime -}$ and ${\mathrm{\Xi }}_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m({\mathrm{\Xi }}_b^{\prime -})-m({\mathrm{\Xi }}_b^0)-m({\pi }^{-})=3.653\pm 0.018\pm 0.006\mathrm{MeV}/c^2$, $m({\mathrm{\Xi }}_b^{*-})-m({\mathrm{\Xi }}_b^0)-m({\pi }^{-})=23.96\pm 0.12\pm 0.06\mathrm{MeV}/c^2$, $\mathrm{\Gamma }({\mathrm{\Xi }}_b^{*-})=1.65\pm 0.31\pm 0.10\mathrm{MeV},$ where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $\mathrm{\Gamma }({\mathrm{\Xi }}_b^{\prime -})<0.08\mathrm{MeV}$ at 95% confidence level. Relative production rates of these states are also reported.

Figure 1

Distribution of $m_{\text{cand}}({\mathrm{\Xi }}_b^0)$ for ${\mathrm{\Xi }}_b^0{\pi }_s^{-}$ candidates passing the full selection apart from the $m_{\text{cand}}({\mathrm{\Xi }}_b^0)$ requirement. Inset: The subset of candidates that lie in the $\delta m$ signal regions of $3.0<\delta m<4.2\mathrm{MeV}/c^2$ and $21<\delta m<27\mathrm{MeV}/c^2$.

Figure 2

Distribution of the mass difference, $\delta m$, for ${\mathrm{\Xi }}_b^0{\pi }_s^{-}$ candidates in data. The points with error bars show right-sign candidates in the ${\mathrm{\Xi }}_b^0$ mass signal region, and the hatched histogram shows wrong-sign candidates with the same selection. The curve shows the nominal fit to the right-sign candidates. Inset: detail of the region $2.0–5.5\mathrm{MeV}/c^2$.