An amplitude analysis of $B^{-}\to J/\psi \mathrm{\Lambda }\overline{p}$ decays is performed using 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 $9{\mathrm{fb}}^{-1}$. A narrow resonance in the $J/\psi \mathrm{\Lambda }$ 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 $4338.2\pm 0.7\pm 0.4\mathrm{MeV}$ and $7.0\pm 1.2\pm 1.3\mathrm{MeV}$, where the first uncertainty is statistical and the second systematic. The spin is determined to be $1/2$ and negative parity is preferred. Because of the small $Q$-value of the reaction, the most precise single measurement of the $B^{-}$ mass to date, $5279.44\pm 0.05\pm 0.07\mathrm{MeV}$, is obtained.

• Received 24 October 2022
• Accepted 12 January 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.031901

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