A report from the CMS experiment
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 Λb0, Σb±, Ξb–, Ξb0, Ωb– 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 Λb(5912)0, Λb(5920)0, and several excited Ξb and Ωb– states, have been observed, and are consistent with their charm partners. By contrast, however, heavier excitations, such as the Λb(6072)0 and Ξb(6227) isodoublet (particles that differ only by an up or down quark), cannot yet be readily associated with charmed partners.
The first particle observed by the CMS experiment, in 2012, was the beauty- strange baryon Ξb(5945)0 (CERN Courier June 2012 p6). It is consistent with being the beauty partner of the Ξc(2645)+ with spin-parity 3/2+, while the Ξb(5955)– and Ξb(5935)– states observed by LHCb are its isospin partner and the beauty partner of the Ξc′0, respectively. The charm sector also suggests the existence of prominent heavier isodoublets, called Ξb**: the lightest orbital Ξb 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 Ξb′ π± and the one with 3/2– into Ξb* π±.
The CMS collaboration has now observed such a baryon, Ξb(6100)–, via the decay sequence Ξb(6100)– → Ξb(5945)0π– → Ξb– 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 Ξb– 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 Ξb(6100)– baryon were only 13 MeV heavier, it would be above the Λb0 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 Ξb(6100)– baryon were only 13 MeV heavier, a tiny 0.2% change, it would be above the Λb0 K– mass threshold and could decay to this final state. The Ξb(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 Ξb(6227) isodoublet recently found by the LHCb collaboration could be the 3/2– orbital excitation of Ξb′ or Ξb* baryons.
CMS Collab. 2021 arXiv:2102.04524 (submitted to Phys. Rev. Lett.).