Observation of the bottomonium ground state in the decay Upsilon(3S) -> gamma eta_b

TL;DR

The results of a search for the bottomonium ground state etab(1S) in the photon energy spectrum with a sample of (109+/-1) million of Upsilon(3S) recorded at the Upsilon-3S energy with the BABAR detector at the PEP-II B factory at SLAC are reported.

Abstract

We report the results of a search for the bottomonium ground state eta_b(1S) in the photon energy spectrum with a sample of (109 +/- 1) million of Upsilon(3S) recorded at the Upsilon(3S) energy with the BABAR detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at E_gamma = 921.2 {+2.1} {-2.8} (stat) +/- 2.4(syst) MeV with a significance of 10 standard deviations. We interpret the observed peak as being due to monochromatic photons from the radiative transition Upsilon(3S) -> gamma eta_b(1S). This photon energy corresponds to an eta_b(1S) mass of $9388.9 {+3.1} {-2.3} (stat) +/- 2.7(syst) MeV/c2. The hyperfine Upsilon(1S)-eta_b(1S) mass splitting is 71.4 {+2.3} {-3.1} (stat) +/- 2.7(syst) MeV/c2. The branching fraction for this radiative Upsilon(3S) decay is estimated to be (4.8 +/- 0.5(stat) +/- 1.2 (syst)) x 10^(-4).

Figures (2)

• Figure 1: Inclusive photon energy spectrum in the below-$焇(4S)$ data, with the non-peaking background subtracted.The peak at 1.03 GeV is from the ISR process $e^+e^- \rightarrow\xspace \gamma_{ISR} \, 焇(1S)$. The superimposed histogram corresponds to a fit with a CB function.
• Figure 2: (a) Inclusive photon spectrum in the region $0.50<E_\gamma<1.1$$\mathrm{\,Ge V}$. The component PDFs determined from the fit are overlaid on the data points. A prominent $\chi_{bJ}(2P)$ peak is clearly seen. The dashed line corresponds to the non-peaking background component. (b) Inclusive photon spectrum after subtracting the non-peaking background, with PDFs for $\chi_{bJ}(2P)$ peak (solid), ISR $焇(1S)$ (dot), $\eta_b$ signal (dash) and the sum of all three (solid). (c) Inclusive photon spectrum after subtracting all components except the $\eta_b$ signal. The CB function shape describes the data points well.