Measurement of the branching ratios of the decays Xi0 --> Sigma+ e- nubar and anti-Xi0 --> anti-Sigma+ e+ nu

TL;DR

This paper reports precise measurements of the branching ratios for Xi0 and anti-Xi0 decays into Sigma+ e- nu and anti-Sigma+ e+ nu, respectively, using data from the NA48/1 experiment, and derives the CKM matrix element |Vus| and the form factor ratio g1/f1.

Contribution

First measurement of these specific Xi0 decay branching ratios with high precision, enabling determination of |Vus| and the form factor ratio g1/f1.

Findings

Measured BR(Xi0 --> Sigma+ e- nubar) = (2.51+-0.03(stat)+-0.09(syst))E-4

Measured BR(anti-Xi0 --> anti-Sigma+ e+ nu) = (2.55+-0.14(stat)+-0.10(syst))E-4

Determined |Vus| = 0.209+0.023-0.028 and g1/f1 = 1.20+-0.05.

Abstract

From 56 days of data taking in 2002, the NA48/1 experiment observed 6316 Xi0 --> Sigma+ e- nubar candidates (with the subsequent Sigma+ --> p pi0 decay) and 555 anti-Xi0 --> anti-Sigma+ e+ nu candidates with background contamination of 215+-44 and 136+-8 events, respectively. From these samples, the branching ratios BR(Xi0 --> Sigma+ e- nubar)= (2.51+-0.03stat+-0.09syst)E(-4) and BR(anti-Xi0 --> anti-Sigma+ e+ nu)= (2.55+-0.14stat+-0.10syst)E(-4) were measured allowing the determination of the CKM matrix element |Vus| = 0.209+0.023-0.028. Using the Particle Data Group average for |Vus| obtained in semileptonic kaon decays, we measured the ratio g1/f1 = 1.20+-0.05 of the axial-vector to vector form factors.