Synopsis: Strange Beauty

A new baryon has been discovered by the CMS collaboration at the Large Hadron Collider.

One would think that by now, apart from big game like the Higgs boson, high-energy physicists would have bagged nearly all of the beasts on the particle horizon. There are still a few trophies to be taken, among them a group of baryons, each of which comprises one strange quark, one $b$ quark (meaning bottom or beauty), and a third quark. When the third member of the trio is an up or down quark, the particle is known as a ${\mathrm{\Xi }}_{b}$ baryon. The Tevatron at Fermilab observed particle decays consistent with some of these states, but a full accounting was not available. Now, in a paper in Physical Review Letters, the CMS team at the Large Hadron Collider at CERN report their observation of a new particle that they conclude is likely to be the ${\mathrm{\Xi }}_{b}^{*0}$.

New particles are nearly always found by watching what they turn into, and the CMS collaboration observed a cascade of decays from ${\mathrm{\Xi }}_{b}^{*0}$ to ${\mathrm{\Xi }}_{b}^{-}$ to $J/\psi$ to muons, pions, and other bits and pieces. Careful reconstruction based on data from proton-proton events at the LHC puts the particle mass at $5945\phantom{\rule{0.333em}{0ex}}\text{MeV}$, with a statistical significance of more than $5$ standard deviations.

Nailing down further properties of the ${\mathrm{\Xi }}_{b}^{*0}$ remains a challenge, but the observation of a new member of the $b$-baryon family will help physicists understand how quarks interact in composite particles and shows that there is still room on the trophy wall for particle discoveries. – David Voss

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