Measurement of the branching fraction for B→η′K and search for B→η′π+

We report measurements for two-body charmless B decays with an eta' meson in the final state. Using 11.1X10^6 BBbar pairs collected with the Belle detector, we find BF(B^+ ->eta'K^+)=(79^+12_-11 +-9)x10^-6 and BF(B^0 -> eta'K^0)=(55^+19_-16 +-8)x10^-6, where the first and second errors are statistical and systematic, respectively. No signal is observed in the mode B^+ -> eta' pi^+, and we set a 90% confidence level upper limit of BF(B^+-> eta'pi^+) < 7x10^-6. The CP asymmetry in B^+- -> eta'K^+- decays is investigated and a limit at 90% confidence level of -0.20<Acp<0.32 is obtained.