Measurement of the branching ratios of the decays Xi(0) -> Sigma(+)e(-)(v)over-bar (e) and (Xi)over-bar(0) -> (Sigma)over-bar(+) e(+)v(e)

From 56 days of data taking in 2002, the NA48/1 experiment observed 6316 Xi(0) -> Sigma(+)e(-)nu(e) candidates (with the subsequent Sigma(+) -> p pi(0) decay) and 555 (Xi(0)) over bar -> (Sigma(+)) over bare+nu(e) candidates with background contamination of 215 +/- 44 and 136 +/- 8 events, respectively. From these samples, the branching ratios BR(Xi(0) -> Sigma(+)e(-)nu(e)) = (2.51 +/- 0.03(stat) +/- 0.09(syst)) x 10(-4) and BR((Xi(0)) over bar -> (Sigma(+)) over bare(+)nu(e)) = (2.55 +/- 0.14(stat) +/- 0.10(syst)) x 10(-4) were measured allowing the determination of the CKM matrix element vertical bar V-us vertical bar = 0.209(-0.028)(+0.023) Using the Particle Data Group average for vertical bar V-us vertical bar obtained in semileptonic kaon decays, we measured the ratio g(1)/f(1) = 1.20 +/- 0.05 of the axial-vector to vector formfactors. (c) 2006 Elsevier B.V. All rights reserved.