Search for B0s → hh decays at the Υ(5S) resonance

We have searched for B 0 s → hh decays, where h stands for a charged or neutral kaon, or a charged pion. These results are based on a 23.6 fb -1 data sample collected with the Belle detector on the Y(5S) resonance at the KEKB asymmetric-energy e + e - collider, containing 1.25 × 10 6 Br ( * ) s B ( * ) s events. We observe the decay B 0 s → K + K - and measure its branching fraction, B(B 0 s → K + K - ) = [3.8 +1.1 -0.9 (stat) ± 0.5(syst) ± 0.5(f s )] × 10 -5 . The first error is statistical, the second is systematic, and the third error is due to the uncertainty in the B 0 s production fraction in e + e - → bb events. No significant signals are seen in other decay modes, and we set upper limits at the 90% confidence level: B(B 0 s → K - π + ) < 1.2 × 10 -5 , B(B 0 s → π + π - )< 2.6×10 -5 , and B(B 0 s → K 0 K 0 ) < 6.6 × 10 -5 .