Precision measurement of the beta asymmetry in neutron beta decay

Precision measurements at low energies are used as indirect probes for physics beyond the standard model of particle physics. We present the result of the neutron decay spectrometer Perkeo III on the parity violating beta asymmetry in the correlation of the neutron spin and electron momentum. From this measurement we extract the ratio of axial-vector and vector coupling constants with an uncertainty of Δλ=5.5×10⁻⁴. This result is more precise than the current PDG world average by a factor of four and clarifies a long standing tension between previous measurements. A pulsed cold neutron beam at the PF1b beam site of the Institut Laue-Langevin was used to control or eliminate major sources of systematic error. We discuss the measurement and first implications of the result on the search for new physics.

The presentation will conclude with a status report on the follow-up instrument PERC, which is currently under construction at the beamline MEPHISTO of the MLZ. The aim of PERC is an improved measurement of several decay correlations in neutron beta decay by an order of magnitude.