The to date most accurate measurement of the weak axial vector coupling constant gA has just been published as Editors' suggestion in PRL . This fundamental constant relates the weak decay of nucleons to that of quarks: On the level of elementary fermions (leptons and quarks), weak interaction is described by vector coupling and axial vector coupling, equal in magnitude and of opposite sign (V-A theory, where "-" reads minus). But quarks are bound in nucleons and strong interaction renormalizes the weak axial vector current for neutron decay. This is quantified by the weak axial vector coupling constant gA. Although gA can be calculated in lattice QCD, calculations only reach 1% accuracy today and are actually benchmarked by experiments.
In particle physics, gA is needed together with the neutron lifetime to calculate the element Vud of the CKM quark mixing matrix. With the new result, the accuracy of Vud from neutron data approaches that from nuclear decays where the latter require specific nuclear corrections. The unitarity of the CKM matrix is very sensitive to new physics beyond the standard model of particle physics. Its test provides constraints that currently cannot be obtained from other precision tests and collider experiments. Furthermore, the new result has already been used to exclude exotic (dark) decays as explanation for the so-called neutron lifetime anomaly. But gA is also an important input in other fields: It is needed to calculate the rate of the primary reaction of the proton-proton chain in the sun and the solar neutrino flux, the abundance of light elements after primordial nucleosynthesis, the formation of neutron stars, or the efficiency of neutrino detectors.
The unprecedented accuracy of the new result became possible thanks to the PERKEO III spectrometer developed at Heidelberg University, the use of a pulsed cold neutron beam with high intensity at the instrument PF1B, and the continuous improvements of tools for neutron polarization analysis with 3He spin filters by the ILLs' neutron optics group. The experiment was performed and analyzed in a collaboration of scientists from Heidelberg University, the ILL, and the Technical Universities of Munich and Vienna.
The result was obtained in a blind analysis, where the experimental asymmetry, the beam polarization and a leading spectrometer-specific correction were determined by independent teams, the ILL scientists being responsible for the polarization analysis. Several technical services of the ILL contributed to the success of the experiment, in particular Hall d'Essais, Installations Electriques, Radioprotection, and Amenagement Bâtiments et Sites d’Expériences.
 B. Märkisch et al., Phys. Rev. Lett. 122, 242501 (2019). DOI:10.1103/PhysRevLett.122.242501