Two-Body B Meson Decays to $\eta$ and $\eta^{'}$ -- Observation of $B\to \eta${'}K$

In a sample of 6.6 million produced B mesons we have observed decays B → η ′ K , with branching fractions B ( B + → η ′ K + ) = (6 . 5 +1 . 5 − 1 . 4 ± 0 . 9) × 10 − 5 and B ( B 0 → η ′ K 0 ) = (4 . 7 +2 . 7 − 2 . 0 ± 0 . 9) × 10 − 5 . We have searched with com-parable sensitivity for 17 related decays to final states containing an η or η ′ meson accompanied by a single particle or low-lying resonance. Our upper limits for these constrain theoretical interpretations of the B → η ′ K signal.