Updated: Wednesday, July 09, 2008

BaBar observes the η_b, the ground state of the b-anti-b quark system

July 8, 2008

The inclusive photon spectrum from Υ(3S) decays (points) in the region 0.50 < Eγ < 1.1 GeV, after subtracting all background components. The ηb signal is clearly seen. The histogram is a Gaussian with a power-law tail fitted to the signal and describes the data well.

Yesterday the BABAR Collaboration reported the first observation of the  ground state of the b-anti-b quark system ("bottomonium") to Physical Review Letters. The ground state, called the η_b  ("eta-sub-b"), was seen in the decay Υ(3S) → γη_b. The search for the η_b(1S), the pseudoscalar partner of the Υ(1S), examined the photon energy spectrum from a sample of (109 ± 1) million Υ(3S) decays recorded at the Υ(3S) energy with the BABAR detector at the PEP-II B-factory at SLAC. A peak in the photon energy spectrum was found at a photon energy E_γ = [921.2^+2.1−2.8 (stat.) ± 2.4 (syst.)] MeV with a significance of 10 standard deviations (see figure). The Collaboration interprets the observed peak as being due to monochromatic photons from the radiative transition Υ(3S) → γη_b(1S). The photon energy corresponds to an η_b(1S) mass of [9388.9^+3.1_−2.3 (stat.) ± 2.7 (syst.)] MeV/c². The hyperfine Υ(1S)-η_b(1S) mass splitting is [71.4^+2.3_−3.1 (stat) ± 2.7(syst)] MeV/c². The branching fraction for this radiative Υ(3S) decay is estimated to be [4.8 ± 0.5 (stat) ± 1.2 (syst)] x 10⁻⁴.

Further studies of the η_b(1S) are underway. At least seven other states, predicted by theory  to lie below the Υ(4S) B-anti-B threshold, still await discovery.

—Ray F. Cowan,  BABAR webmaster

Content: Cowan
Page: McDunn

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