Search for the Fierz Interference Term with PERKEO III

by Anna Schubert (TUM)

Thursday 3 Jul 2025, 09:30 → 10:30 Europe/Berlin

E18/ENE Seminar Room 3268 (TUM PH)

Measurements of the free neutron decay enable a variety of tests of the Standard Model of particle physics. Observables of the decay are, among others, the beta asymmetry $A$ and the Fierz interference term $b$. From precision measurements of $A$ the CKM matrix element $V_{ud}$ may be determined, while a non-zero Fierz term $b$ would imply the existence of scalar and tensor interactions beyond the V-A theory of the Standard Model.

The currently most precise values for these neutron decay parameters were obtained by measurements with the PERKEO III experiment at the ILL PF1b facility with a polarized neutron beam. To this end, the electron and/or proton energy spectrum was measured. To control major systematic effects, a pulsed beam of cold neutrons was used. 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 are guided by a magnetic field towards one of two scintillation detectors with PMT readout. 

I present experimental details of the 2019/2020 campaign aiming to measure the electron spectrum from unpolarized neutrons to extract an improved limit for the Fierz interference term, as well as the status of the ongoing analysis.