Messung der Lebensdauer des $\Xi^{0}$ -Hyperons mit dem NA48-Detektor

One of the main characteristics of particles is the lifetime. The mean lifetime of the Xi0 hyperon, which can be determined theoretically from the Xi- lifetime by using the Delta I=1/2 rule, has been measured a couple of times. The most recent measurement from 1977 has a relative uncertainty of 5%, which could be improved by usind data from new experiments like NA48/1. The Xi0 lifetime is an important input parameter in the determination of the matrix element Vus of the Cabibbo-Kobayashi-Maskawa matrix in semileptonic Xi0 decays. In 2002 a high intensity data acquisition was performed by the NA48/1 collaboration, in which about 10^9 Xi0 decay candidates were recorded. From this sample 192000 events of the decay "Xi0 to Lambda pi0" were reconstructed and a subsample of 107000 could be used to determine the lifetime. The lifetime was determined by comparison of measured and simulated data in ten energy bins to avoid systematic effects. The result has a higher precision than older measurements. It differs from the world average (tau=(2.90+-0.09)*10^(-10)s) by (+4.99+-0.50(stat)+-0.58(syst))% corresponding to 1.7 standard deviations. The lifetime is tau=(3.045+-0.015(stat)+-0.017(syst))*10^(-10)s. The lifetime of the Anti-Xi0 could be measured using the same data sample. The result is tau=(3.042+-0.045(stat)+-0.017(syst))*10^(-10)s.