Precision Measurement Of The Neutron’s Beta Asymmetry Using Ultra-Cold Neutrons

Abstract

Abstract.A measurement of A b , the correlation between the electron momentum and neutron (n) spin (the beta asymmetry) in n beta-decay, together with the n lifetime, provides a method for extracting fundamental parameters for the charged-current weak interaction of the nucleon. In particular when combined with decay measurements, one can extract the V ud element of the CKM matrix, a critical element in CKM unitarity tests. By using a new SD 2 super-thermal source at LANSCE, large fluxes of UCN (ultra-cold neutrons) are expected for the UCNA project. These UCN will be 100% polarized using a 7 T magnetic field, and directed into the b spectrometer. This approach, together with an expected large reduction in backgrounds, will result in an order of magnitude reduction in the critical systematic corrections associated with current n b-asymmetry measurements. This paper will give an overview of the UCNA A b measurement as well as an update on the status of the experiment.