Observation of a new excited beauty strange baryon decaying to $\Xi^-_\mathrm{b} \pi^+ \pi^-$

TL;DR

This paper reports the discovery of a new excited baryon state, the $ ext{Xi}_b$(6100)$^-$, observed in proton-proton collisions at 13 TeV, consistent with an orbitally excited $ ext{Xi}_b$ baryon with specific quantum numbers.

Contribution

First observation of the $ ext{Xi}_b$(6100)$^-$ resonance, expanding knowledge of bottom baryon spectroscopy and decay properties.

Findings

Discovery of a narrow resonance at 6100.3 MeV

Upper limit on natural width is 1.9 MeV

Resonance consistent with an excited $ ext{Xi}_b$ baryon with $J^P=3/2^-$

Abstract

The $\Xi^-_\mathrm{b} \pi^+ \pi^-$ invariant mass spectrum is investigated with an event sample of proton-proton collisions at $\sqrt{s} = $ 13 TeV, collected by the CMS experiment at the LHC in 2016-2018 and corresponding to an integrated luminosity of 140 fb$^{-1}$. The ground state $\Xi^-_\mathrm{b}$ is reconstructed via its decays to J$/\psi$ $\Xi^-$ and J$/\psi$ $\Xi^-$ $\Lambda$ K$^-$. A narrow resonance, labeled $\Xi_\mathrm{b}$(6100)$^-$, is observed at a $\Xi^-_\mathrm{b} \pi^+ \pi^-$ invariant mass of 6100.3 $\pm$ 0.2 (stat) $\pm$ 0.1 (syst) $\pm$ 0.6 ($\Xi^-_\mathrm{b}$) MeV, where the last uncertainty reflects the precision of the $\Xi^-_\mathrm{b}$ baryon mass. The upper limit on the $\Xi_\mathrm{b}$(6100)$^-$ natural width is determined to be 1.9 MeV at 95% confidence level. Following analogies with the established excited $\Xi_\mathrm{c}$ baryon states, the new $\Xi_\mathrm{b}$(6100)$^-$ resonance and its decay sequence are consistent with the orbitally excited $\Xi^-_\mathrm{b}$ baryon, with spin and parity quantum numbers $J^P$ $=$ 3/2$^-$.