Evidence of a $J/\ddot{I}ˆ\Lambda$ structure and observation of excited ${\Xi }^{-}$ states in the ${\Xi }_b^{-}\to J/\ddot{I}ˆ\Lambda K^{-}$ decay

Abstract

First evidence of a structure in the $J/\ddot{I}ˆ\Lambda$ invariant mass distribution is obtained from an amplitude analysis of ${\Xi }_b^{-}\to J/\ddot{I}ˆ\Lambda K^{-}$ decays. The observed structure is consistent with being due to a charmonium pentaquark with strangeness. Its mass and width are determined to be $4458.8\pm 2.9{}_{-1.1}^{+4.7}$MeV and $17.3\pm 6.5{}_{-5.7}^{+8.0}$MeV, where the quoted uncertainties are statistical and systematic, respectively. The structure is also consistent with being due to two resonances. In addition, the narrow excited ${\Xi }^{-}$ states, $\Xi (1690{)}^{-}$ and $\Xi (1820{)}^{-}$, are seen for the first time in a ${\Xi }_b^{-}$ decay, and their masses and widths are measured with improved precision. The analysis is performed using $pp$ collision data corresponding to a total integrated luminosity of 9fb${}^{-1}$, collected with the LHCb experiment at centre-of-mass energies of $7$, $8$ and $13$TeV.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2020-039.html (LHCb public pages)