Colloquia

We report on a precise measurement of the electron-antineutrino angular correlation (the “a” coefficient) in free neutron beta-decay. The “a” coefficient is inferred from the recoil energy spectrum of the protons which are detected in the aSPECT spectrometer. The measurement is substantially more precise than what has been achieved previously, and its implications will be discussed. An alternative analysis is presented that allows for Beyond Standard Model Physics, specifically a non-zero value for the Fierz term “b”. The result constitutes the most precise determination of “b” in free neutron beta decay, and is consistent with the Standard Model prediction. In the Standard Model, however, the aSPECT result for “a” is inconsistent by about 3 sigma with the value for the beta asymmetry “A” recommended by the Particle Data Group. A combined analysis of the results from PERKEO III, which dominates the beta asymmetry average, and aSPECT, now allowing for a non-zero Fierz term, reconciles the two. It finds a more precise value for the Fierz term, consistent with the one obtained from aSPECT data alone, but about 3 sigma away from the Standard Model value, and suggests scalar and tensor interactions in free neutron beta decay.