Observation of a $J\mathrm{/}\psi \mathrm{\Lambda }$ resonance consistent with a strange pentaquark candidate in $B^{-}\to J\mathrm{/}\psi \mathrm{\Lambda }\bar{p}$ decays

An amplitude analysis of $B^{-}\to J\mathrm{/}\psi \mathrm{\Lambda }\bar{p}$ decays is performed using about 4400 signal candidates selected on a data sample of $pp$ collisions recorded at center-of-mass energies of 7, 8 and 13 TeV with the LHCb detector, corresponding to an integrated luminosity of 9 fb${}^{-1}$. A narrow resonance in the $J\mathrm{/}\psi \mathrm{\Lambda }$ system, consistent with a pentaquark candidate with strangeness, is observed with high significance. The mass and the width of this new state are measured to be $4338.2\pm 0.7\pm 0.4$MeV and $7.0\pm 1.2\pm 1.3$MeV, where the first uncertainty is statistical and the second systematic. The spin is determined to be $1\mathrm{/}2$ and negative parity is preferred. Due to the small $Q$-value of the reaction, the most precise single measurement of the $B^{-}$ mass to date, $5279.44\pm 0.05\pm 0.07$MeV, is obtained.