New excited beauty-strange baryon observed

Beauty baryons are a subject of great interest at the LHC, offering unique insights into the nature of the strong interaction and the mechanisms by which hadrons are formed. While the ground states Λ_b⁰, Σ_b^±, Ξ_b^–, Ξ_b⁰, Ω_b^– were observed at the Tevatron at Fermilab and the SPS at CERN, the LHC’s higher energy and orders-of-magnitude larger integrated luminosity have allowed the discovery of more than a dozen excited beauty baryon states among the 59 new hadrons observed at the LHC so far (see LHCb observes four new tetraquarks).

Many hadrons with one c or b quark are quite similar. Interchanging heavy-quark flavours does not significantly change the physics predicted by effective models assuming “heavy quark symmetry”. The well-established charm baryons and their excitations therefore provide excellent input for theories modelling the less well understood spectrum of beauty-baryons. A number of the lightest excited b baryons, such as Λ_b(5912)⁰, Λ_b(5920)⁰, and several excited Ξ_b and Ω_b^– states, have been observed, and are consistent with their charm partners. By contrast, however, heavier excitations, such as the Λ_b(6072)⁰ and Ξ_b(6227) isodoublet (particles that differ only by an up or down quark), cannot yet be readily associated with charmed partners.

New particles

The first particle observed by the CMS experiment, in 2012, was the beauty- strange baryon Ξ_b(5945)⁰ (CERN Courier June 2012 p6). It is consistent with being the beauty partner of the Ξ_c(2645)⁺ with spin-parity 3/2⁺, while the Ξ_b(5955)^– and Ξ_b(5935)^– states observed by LHCb are its isospin partner and the beauty partner of the Ξ_c′⁰, respectively. The charm sector also suggests the existence of prominent heavier isodoublets, called Ξ_b^**: the lightest orbital Ξ_b excitations with orbital momentum between a light diquark (a pairing of a s quark with either a d or a u quark) and a heavy b quark. The isodoublet with spin-parity 1/2^– decays into Ξ_b′ π^± and the one with 3/2^– into Ξ_b^* π^±.

The CMS collaboration has now observed such a baryon, Ξ_b(6100)^–, via the decay sequence Ξ_b(6100)^– → Ξ_b(5945)⁰π^– → Ξ_b^– π⁺ π^–. The new state’s measured mass is 6100.3 ± 0.6 MeV, and the upper limit on its natural width is 1.9 MeV at 95% confidence level. The Ξ_b^– ground state was reconstructed in two channels: J/ψ Ξ^– and J/ψ Λ K^–. The latter channel also includes partially reconstructed J/ψ Σ⁰ K^– (where the photon from the Σ⁰ → Λ γ decay is too soft to be reconstructed).

If the Ξ_b(6100)^– baryon were only 13 MeV heavier, it would be above the Λ_b⁰ K^– mass threshold

The observation of this baryon and the measurement of its properties are useful for distinguishing between different theoretical models predicting the excited beauty baryon states. It is curious to note that if the Ξ_b(6100)^– baryon were only 13 MeV heavier, a tiny 0.2% change, it would be above the Λ_b⁰ K^– mass threshold and could decay to this final state. The Ξ_b(6100)^– might also shed light on the nature of previous discoveries: if it is the 3/2^– member of the lightest orbital excitation isodoublet, then the Ξ_b(6227) isodoublet recently found by the LHCb collaboration could be the 3/2^– orbital excitation of Ξ_b′ or Ξ_b^* baryons.