Reanalysis of the β−ν¯ₑ Angular Correlation Measurement from the aSPECT Experiment with New Constraints on Fierz Interference

Abstract

On the basis of revisions of some of the systematic errors, we reanalyzed the electron-antineutrino angular correlation (a coefficient) in free neutron decay inferred from the recoil energy spectrum of the protons which are detected in 4π by the aSPECT spectrometer. With a=−0.104 02(82) the new value differs only marginally from the one published in 2020. The experiment also has sensitivity to b, the Fierz interference term. From a correlated (b,a) fit to the proton recoil spectrum, we derive a limit of b=−0.0098(193) which translates into a somewhat improved 90% confidence interval region of −0.041≤b≤0.022 on this hypothetical term. Tighter constraints on b can be set from a combined [shown as superscript (c)] analysis of the PERKEO III (β asymmetry) and aSPECT measurement which suggests a finite value of b with b(c)=−0.0181±0.0065 deviating by 2.82σ from the standard model.