Abstract
We search for new charmless decays of neutral b-hadrons to pairs of charged hadrons with the upgraded Collider Detector at the Fermilab Tevatron. Using a data sample corresponding to 1 fb-1 of integrated luminosity, we report the first observation of the B0(s) --> K- pi++ decay, with a significance of 8.2 sigma, and measure B(B0(s) --> K- pi++) =5.0 +/- 0.7 (stat) +/- 0.8 (syst)) x 10-6. We also report the first observation of charmless b-baryon decays in the channels Lambda0(b) --> p pi+- and Lambda0_s --> pK- with significances of 6.0 sigma and 11.5 sigma respectively, and we measure B(Lambda0(b) --> p pi+-) =3.5 +/- 0.6 (stat) +/- 0.9 (syst)) x 10-6 and B(Lambda0(b) --> pK-) =5.6 +/- 0.8 (stat) +/- 1.5 (syst)) x 10-6. No evidence is found for the decays B0 --> K+ K- and B0(s) --> pi++ pi+-, and we set an improved upper limit B(B0(s) --> pi++ pi+-) < 1.2 x 10-6 at the 90% confidence level. All quoted branching fractions are measured using B(B0 --> K+ pi+-) as a reference.