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

TL;DR

This paper reports the first observation of the eta_b(1S) bottomonium ground state via radiative decay from Upsilon(3S), measuring its mass, hyperfine splitting, and decay branching fraction with high significance.

Contribution

The study provides the first experimental evidence of eta_b(1S) in Upsilon(3S) decay, including precise measurements of its mass and decay properties.

Findings

Observed a peak at 921.2 MeV in photon spectrum with 10 sigma significance.

Measured eta_b(1S) mass as approximately 9389 MeV/c^2.

Determined the hyperfine splitting to be about 71 MeV.

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).