Evidence of a $J/\psi \varLambda$ structure and observation of excited $\varXi^-$ states in the $\varXi_b^-\to J/\psi \varLambda K^-$ decay

TL;DR

This paper reports the first evidence of a structure in the $J/psi Lambda$ invariant mass distribution, suggesting a possible charmonium pentaquark with strangeness, and observes excited $Xi^-$ states with improved measurements.

Contribution

It provides the first evidence of a $J/psi Lambda$ structure and measures properties of excited $Xi^-$ states in $Xi_b^-$ decays, using LHCb data.

Findings

Evidence of a $J/psi Lambda$ structure consistent with a pentaquark.

Observation of excited $Xi^-$ states $Xi(1690)^-$ and $Xi(1820)^-$.

Measured masses and widths with improved precision.

Abstract

First evidence of a structure in the $J/\psi \varLambda$ invariant mass distribution is obtained from an amplitude analysis of $\varXi_b^- \to J/\psi \varLambda K^-$ decays. The observed structure is consistent with being due to a charmonium pentaquark with strangeness. Its mass and width are determined to be $4458.8\pm2.9\,^{+4.7}_{-1.1}$MeV and $17.3\pm6.5\,^{+8.0}_{-5.7}$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 $\varXi^-$ states, $\varXi(1690)^-$ and $\varXi(1820)^-$, are seen for the first time in a $\varXi_b^-$ 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.