Two structures are observed close to the kinematic threshold in the $\Xi_b^0 \pi^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 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 $J^P = \frac{ 1}{ 2}^+$ and $J^P=\frac{ 3}{ 2}^+$ states, denoted $\Xi_b^{ \prime -}$ and $\Xi_b^{ *-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(\Xi_b^{ \prime -}) - m(\Xi_b^0) - m(\pi^{ -}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(\Xi_b^{ *-}) - m(\Xi_b^0) - m(\pi^{ -}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $\Gamma(\Xi_b^{ *-}) = 1.65 \pm 0.31 \pm 0.10$ 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 $\Gamma(\Xi_b^{ \prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.
Categories: hep-ex