Observation of B+->(K)over-bar(0) K+ and B-0 -> K-0 (K)over-bar(0)

We report observations of the b→d penguin-dominated decays B+→K̄0K+ and B0→K0K̄0 in 316fb-1 of e+e- collision data collected with the BABAR detector. We measure the branching fractions B(B+→K̄0K+)=(1.61±0.44±0.09)×10-6 and B(B0→K0K̄0)=(1.08±0.28±0.11)×10-6 and the CP-violating charge asymmetry ACP(K̄0K+)=0.10±0.26±0.03. Using a vertexing technique previously employed in several analyses of all-neutral final states containing kaons, we report the first measurement of time-dependent CP-violating asymmetries in B0→KS0KS0, obtaining S=-1.28-0.73-0.16+0.80+0.11 and C=-0.40±0.41±0.06. We also report improved measurements of the branching fraction B(B+→K0π+)=(23. 9±1.1±1.0)×10-6 and CP-violating charge asymmetry ACP(K0π+)=-0.029±0.039±0.010. © 2006 The American Physical Society.