Bottomonium ground state

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 η_b, 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