Search for radiative decays of Υ(1S) into η and η′

We report on a search for the radiative decay of Υ(1S) to the pseudoscalar mesons η and η′ in (21.2±0.2)×106 Υ(1S) decays collected with the CLEO III detector at the Cornell Electron Storage Ring. The η meson was reconstructed in the three modes η→γγ, η→π+π-π0, or η→π0π 0π0. The η′ meson was reconstructed in the mode η′→π+π-η with η decaying through any of the above three modes, and also η′→γρ0, where ρ0→π+π-. Five out of the seven submodes are found to have very low backgrounds. In four of them we find no signal candidates and in one [Υ(1S)→γη′, η′→π+π-η, η→π+π-π0] there are two good signal candidates, which is insufficient evidence to claim a signal. The other two submodes (η→γγ and η′→γρ) are background limited, and show no excess of events in their signal regions. We combine the results from different channels and obtain upper limits at the 90% C.L. which are B(Υ(1S)→γη)<1.0×10-6 and B(Υ(1S) →γη′)<1.9×10-6. Our limits are an order of magnitude tighter than the previous ones and below the predictions made by some theoretical models.