Searches for Λ0b and Ξ0b Decays to K0S p π− and K0S p K− Final States with First Observation of the Λ0b → K0S p π− Decay

A search for previously unobserved decays of beauty baryons to the final states K0S p π− and K0S p K− is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb^−1 of pp collisions. The Λ0b → anti-K0 p π− decay is observed with a significance of 8.6 σ, with branching fraction B(Λ0b → anti-K0 p π−) = (1.26 ± 0.19 ± 0.09 ± 0.34 ± 0.05) × 10^−5, where the uncertainties are statistical, systematic, from the ratio of fragmentation fractions f_Λ0b/f_d, and from the branching fraction of the B0 → K0 π+ π− normalisation channel, respectively. A first measurement is made of the CP asymmetry, giving A_CP(Λ0b → anti-K0 p π−) = 0.22 ± 0.13(stat) ± 0.03(syst). No significant signals are seen for Λ0b → K0S p K− decays, Ξ0b decays to both the K0S p π− and K0S p K− final states, and the Λ0b → D− s(→ K0S K−) p decay, and upper limits on their branching fractions are reported.