Evidence for the eta_b(1S) Meson in Radiative Upsilon(2S) Decay

TL;DR

This paper reports the first evidence of the eta_b(1S) meson in radiative Upsilon(2S) decay, measuring its mass and branching fraction with data from the BABAR detector, advancing understanding of bottomonium states.

Contribution

First experimental observation of eta_b(1S) in Upsilon(2S) decay, providing precise measurements of its mass and decay rate.

Findings

Observed a photon energy peak at 609.3 MeV indicating eta_b(1S) presence.

Measured eta_b(1S) mass as approximately 9394 MeV/c^2.

Determined the branching fraction for Upsilon(2S) to gamma eta_b(1S) decay.

Abstract

We have performed a search for the eta_b(1S) meson in the radiative decay of the Upsilon(2S) resonance using a sample of 91.6 million Upsilon(2S) events recorded with the BABAR detector at the PEP-II B factory at the SLAC National Accelerator Laboratory. We observe a peak in the photon energy spectrum at E_gamma = 609.3 {+4.6} {-4.5} (stat) +/- 1.9 (syst) MeV, corresponding to an eta_b(1S) mass of 9394.2 {+4.8} {-4.9} (stat) +/- 2.0 (syst) MeV/c^2. The branching fraction for the decay Upsilon(2S) --> gamma eta_b(1S) is determined to be [3.9 +/-1.1 (stat) {+1.1} {-0.9} (syst)] x 10^(-4). We find the ratio of branching fractions B[Upsilon(2S) --> gamma eta_b(1S)]/B[Upsilon(3S) --> gamma eta_b(1S)] = 0.82 +/- 0.24 (stat) {+0.20}{-0.19} (syst).