Leipzig 2002 – scientific programme

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T: Teilchenphysik

T 106: Test des Standardmodells

T 106.1: Talk

Monday, March 18, 2002, 14:00–14:25, HS 8

The Standard Model and neutron β-decay — •Michael Kreuz, Hartmut Abele, Dirk Dubbers, Ulrich Mayer, Daniela Mund, and Christian Vogel — Physikalisches Institut, Universität Heidelberg,, Philosophenweg 12, 69120 Heidelberg

We have measured neutron decay parameters at the Institut Laue-Langevin in Grenoble: Firstly, the β-asymmetry A₀, and secondly, the neutrino-asymmetry B in the decay of free polarized neutrons. A measurement of the β-asymmetry together with the world average of the neutron lifetime τ determines the first element |V_ud| of the quark-mixing-CKM-matrix. With this value and the particle data group values for |V_ud| and |V_ub|, the unitarity condition for the first row of the CKM-matrix deviate from unity by 3.0 σ standard deviations and conflicts the prediction of the Standard Model of particle physics. We compare our result with other sources of information for |V_ud|, like W physics at LEP. The recent measurement of the neutrino-asymmetry is sensitive to the admixture of right handed currents in the electro weak interaction. The experiment was installed at the PF1 cold neutron beam position at the High Flux Reactor at the Institut Laue-Langevin, Grenoble. Cold neutrons are obtained from a 25 K deuterium cold moderator near the core of the 57 MW uranium reactor. The neutrons are guided via a 60 m long neutron guide to a high performance experiment: The degree of neutron polarization was P = 99% with a spin flip efficiency of f = 99.7(1)%. The decay count rate was high at low background. The neutrino momentum was reconstructed by proton and electron detectors.