Using proton-proton collision data collected by the Compact Muon Solenoid (CMS) experiment at CERN’s Large Hadron Collider (LHC), physicists from the CMS Collaboration have discovered a new excited beauty-strange baryon: Ξb(6100)–.
Installation of the CMS beam pipe. Image credit: CERN / CMS Collaboration.
“Many hadrons with one c or b quark are quite similar,” CMS physicists said.
“Interchanging heavy-quark flavors 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 modeling 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.”
A beauty-strange baryon called Ξb(5945)0 was observed by the CMS experiment in 2012.
“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 physicists said.
“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 Ξb(6100)−→Ξ−bπ+π− decay topology, where the Ξ−b decays to J/ψΞ− (upper) or to J/ψΛK− (lower); the numbers in blue are average decay lengths. Image credit: CMS Collaboration.
The new baryon, Ξb(6100)–, was observed via the decay sequence Ξb(6100)– → Ξb(5945)0π– → Ξb– π+ π–.
Its measured mass is 6100.3 MeV, and the upper limit on its natural width is 1.9 MeV at 95% confidence level.
“The observation of Ξb(6100)– and the measurement of its properties are useful for distinguishing between different theoretical models predicting the excited beauty baryon states,” the researchers said.
“It is curious to note that if this baryon was 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.”
“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.”
The new results were published in the June 25, 2021 issue of the journal Physical Review Letters.
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A. M. Sirunyan et al. (CMS Collaboration). 2021. Observation of a New Excited Beauty Strange Baryon Decaying to Ξ−bπ+π−. Phys. Rev. Lett 126 (25): 252003; doi: 10.1103/PhysRevLett.126.252003
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