Measurement of the absolute branching fractions of J/ψ →γη and η decay modes

Based on a data sample of (1.0087±0.0044)×1010 J/ψ events collected by the BESIII detector at the BEPCII accelerator, the absolute branching fraction (BF) of the decay J/ψ→γη is measured with high precision using events in which the radiative photon converts to e+e-. Using the measured absolute BF of J/ψ→γη, the absolute BFs of four dominant η decay modes are measured for the first time. The results are B(J/ψ→γη)=(1.067±0.005±0.023)×10-3, B(η→γγ)=(39.86±0.04±0.99)%, B(η→π0π0π0)=(31.96±0.07±0.84)%, B(η→π+π-π0)=(23.04±0.03±0.54)%, and B(η→π+π-γ)=(4.38±0.02±0.10)%, where the first and second uncertainties are statistical and systematic, respectively. The results are consistent with the world average values within two standard deviations.