Observation of a New ${\mathrm{\Xi }}_b^{-}$ Resonance

From samples of $pp$ collision data collected by the LHCb experiment at $\sqrt{s}=7$, 8 and 13 TeV, corresponding to integrated luminosities of 1.0, 2.0 and $1.5{\mathrm{fb}}^{-1}$, respectively, a peak in both the ${\mathrm{\Lambda }}_b^0{K}^{-}$ and ${\mathrm{\Xi }}_b^0{\pi }^{-}$ invariant mass spectra is observed. In the quark model, radially and orbitally excited ${\mathrm{\Xi }}_b^{-}$ resonances with quark content $bds$ are expected. Referring to this peak as ${\mathrm{\Xi }}_b(6227{)}^{-}$, the mass and natural width are measured to be $m_{{\mathrm{\Xi }}_b(6227{)}^{-}}=6226.9\pm 2.0\pm 0.3\pm 0.2\mathrm{MeV}/c^2$ and ${\mathrm{\Gamma }}_{{\mathrm{\Xi }}_b(6227{)}^{-}}=18.1\pm 5.4\pm 1.8\mathrm{MeV}/c^2$, where the first uncertainty is statistical, the second is systematic, and the third, on $m_{{\mathrm{\Xi }}_b(6227{)}^{-}}$, is due to the knowledge of the ${\mathrm{\Lambda }}_b^0$ baryon mass. Relative production rates of the ${\mathrm{\Xi }}_b(6227{)}^{-}\to {\mathrm{\Lambda }}_b^0{K}^{-}$ and ${\mathrm{\Xi }}_b(6227{)}^{-}\to {\mathrm{\Xi }}_b^0{\pi }^{-}$ decays are also reported.

Figure 1

Invariant mass spectra for (top) ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$, (middle) ${\mathrm{\Lambda }}_c^{+}$ from ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\mu }^{-}X$, and (bottom) ${\mathrm{\Xi }}_c^{+}$ from ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{\mu }^{-}X$ candidate decays. The left column is for 7, 8 TeV and the right is for 13 TeV data. Fits are overlaid, as described in the text. Here, the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\mu }^{-}X$ mode has been prescaled by a factor of 10.

Figure 2

Spectra of mass differences for ${\mathrm{\Xi }}_b(6227{)}^{-}$ candidates, reconstructed in the final states (top) ${\mathrm{\Lambda }}_b^0{K}^{-}$, with ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$, (middle) ${\mathrm{\Lambda }}_b^0{K}^{-}$, with ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\mu }^{-}X$, and (bottom) ${\mathrm{\Xi }}_b^0{\pi }^{-}$, with ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{\mu }^{-}X$, along with the results of the fits. The left column is for 7, 8 TeV and the right is for 13 TeV data. The symbol $M^{*}$ represents the mass after the constraint $(p_{H_c^{+}}+p_{{\mu }^{-}}+p_{\mathrm{miss}}{)}^2=m_{H_b^0}^2$ is applied, as described in the text.