In the final run of its fruitful decade-long operation, the PEPII electron–positron collider at SLAC has revealed the lowest-energy state of bottomonium, the heaviest family of mesons. All the bottomonium mesons are bound quark–antiquark pairs of the bottom quark b. And they all have masses near 10 GeV, roughly the mass of a boron atom. Until now, the only known bottomonium mesons have had the spins of the two spin-½ quarks aligned to form triplet spin-1 configurations. Their lower-energy singlet spin-0 hyperfine partners had to exist, but 30 years of looking for them had been in vain. So there was almost no empirical information about the spin dependence of the strong force between b quarks. Quantum chromodynamics, the relevant theory, should in principle predict the hyperfine mass splitting. But QCD predictions are notoriously hard to calculate. Computational tricks based on QCD are presumed to do best with the heaviest quarks. Because top quarks are too short-lived to form mesons or any other particles, b-quark states afford the best opportunity for comparing theory with measurement. Having now discovered the spin-singlet ground state, labeled ηb, the BaBar detector collaboration at PEPII has measured its mass to lie just 71±4 MeV below that of Y(1S), the lowest-mass spin-triplet state, whose discovery in 1977 first revealed the existence of the b quark. BaBar’s measurement of the hyperfine mass splitting provides an important validation of the lattice-QCD computational technique that predicted 61±14 MeV. (A. Aubert et al., http://arxiv.org/abs/0807.1086.) — Bertram M. Schwarzschild
Bottomonium ground state
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This is an interesting development. The ever growing number of known composite particles points to the richness of the strong nuclear force and the complexity of QCD.
I have often wondered if, given all of the decay sequences known to mankind for the entire set of known fundamental particles and known composite particles, whether or not there is anyway that such sub-sequences can be combined or any of the various short lived particles, composite and/or simple, be colocated in spacetime in such a manner that useful exothermic reactions can be facilitated, perhaps for energy production, novel spatial-temporal effects, or yet to be discovered particles produced without the need for extremely high particle energies such as the 14 TeV of the LHC at CERN.
There are now so many known decay sequences and there are becoming so many known composite particles that the mind becomes boggled by the complexity of the thermodynamic degrees of freedom that exist in particle physics interactions.
In a way, I am pleased by all of this richness and complexity and feel like a kid in a candy shop every time a new particle or element/isotope is discovered. I get the feeling that there will always be new ground for particle physics to cover.