Speaker
Description
Observables of neutron decay are, among others, the $\beta$-asymmetry $A$ and the Fierz interference term $b$. Through precision measurements of $A$ we have access to the CKM matrix element $V_{ud}$, while a non-zero Fierz term $b$ would imply the existence of scalar or tensor interactions beyond the V-A theory of the Standard Model.
The currently most precise direct determinations of $A$ and $b$ from $\beta$-spectrum measurements were obtained with the PERKEO III experiment at the ILL PF1b facility. These results are based on the 2009 measurement campaign, using a combined fit to the experimental $\beta$-asymmetry. To control major systematic effects, we used a pulsed beam of cold neutrons. This beam is guided into the 2 m long decay volume of the experiment, in which some of the neutrons decay. The charged particles from the decay follow the magnetic field toward one of two scintillation detectors with PMT readout.
A subsequent measurement campaign in 2019/20 aimed to measure the electron spectrum from unpolarized neutrons to extract an improved limit for the Fierz interference term $b$. This method offers higher statistical sensitivity but is systematically challenging. We present experimental details as well as the status of the ongoing analysis. Our current focus lies on characterizing the readout electronics and incorporating a comprehensive model of secondary particles induced by our calibration device into the analysis chain.
