Observation of Excited ${\mathit{\Lambda }}_b^0$ Baryons

Using $pp$ collision data corresponding to $1.0{\mathrm{fb}}^{-1}$ integrated luminosity collected by the LHCb detector, two narrow states are observed in the ${\mathit{\Lambda }}_b^0{\pi }^{+}{\pi }^{-}$ spectrum with masses $5911.97\pm 0.12(\mathrm{stat})\pm 0.02(\mathrm{syst})\pm 0.66({\mathit{\Lambda }}_b^0\mathrm{mass})\mathrm{MeV}/c^2$ and $5919.77\pm 0.08(\mathrm{stat})\pm 0.02(\mathrm{syst})\pm 0.66({\mathit{\Lambda }}_b^0\mathrm{mass})\mathrm{MeV}/c^2$. The significances of the observations are 5.2 and 10.2 standard deviations, respectively. These states are interpreted as the orbitally excited ${\mathit{\Lambda }}_b^0$ baryons, ${\mathit{\Lambda }}_b^{*0}(5912)$ and ${\mathit{\Lambda }}_b^{*0}(5920)$.

Figure 1

Invariant mass spectrum of ${\mathit{\Lambda }}_c^{+}{\pi }^{-}$ combinations. The points with error bars are the data, and the fitted ${\mathit{\Lambda }}_b^0\to {\mathit{\Lambda }}_c^{+}{\pi }^{-}$ signal and three background components (${\mathit{\Lambda }}_b^0\to {\mathit{\Lambda }}_c^{+}{K}^{-}$, partially reconstructed, and random background) are shown with different fill styles.

Figure 2

Invariant mass spectrum of (a) ${\mathit{\Lambda }}_b^0{\pi }^{+}{\pi }^{-}$ and (b) ${\mathit{\Lambda }}_b^0{\pi }^{\pm }{\pi }^{\pm }$ combinations. The points with error bars are the data, the solid line is the fit result, and the dashed line is the background contribution.

Figure 3

Invariant mass of the two pions from ${\mathit{\Lambda }}_b^{*0}(5920)\to {\mathit{\Lambda }}_b^0{\pi }^{+}{\pi }^{-}$ decay. The points with the error bars are background-subtracted data, and the solid histogram is the result of phase-space decay simulation.