Improved determination of the beta-neutrino angular correlation coefficient a in free neutron decay using the aSPECT experiment

The “Standard Model of particle physics” (SM) successfully explains an enormous number of experimental results and a wide variety of phenomena. Despite its great success the SM is incomplete. For example, it doesn’t include the gravitational force and the baryon-asymmetry in our universe (matter over antimatter) is still an unsolved puzzle. An unique opportunity to investigate the SM is the weak decay of the free neutron, n -> pev + 782.3 keV. This is described as purely left-handed, vector-axialvector (V − A) interaction within the framework of the SM. The low decay energy and the absence of nuclear structure allows for an excellent theoretical interpretation. Information gained by high precision measurements of observables in free neutron decay can be used to test for physics beyond the SM, e.g., to search for tensor (T ) and scalar (S) interactions. In example, the measurement of different angular correlation coefficients allows for the determination of the ratio of the weak coupling constants, lambda = gA/gVwhich allows sensitive checks of the model’s validity and limits. The aSPECT experiment is a retardation spectrometer built to measure the proton energy spectrum in free neutron beta-decay. From the shape of the spectrum, the beta-neutrino angular correlation coefficient a can be derived and thus lambda(a). In 2013, aSPECT had a successful beam time at the Institut Laue-Langevin, Grenoble (France). Different parameter settings of the spectrometer helped to trace instrumental systematics. Supportive follow-up measurements were conducted, e.g. to determine the spatial and temporal work function fluctuations of the electrodes, which is a source of one of the main systematic errors. These measurements were used as input for electromagnetic field computations and particle tracking simulations for a correction of systematics. The data of the runs in the individual configurations were combined and analysed using a multi-dimensional fit with a as free fit parameter. This thesis gives a full representation of the almost completed analysis serving as draft for the final publication. Special focus is given on the systematic effects that could only be quantified by sophisticated electromagnetic field and particle tracking simulations. With this thesis, the aSPECT experiment, after 19 years of its proposal, comes to an end and can report on an improved determination of the angular correlation coefficient a: a = −0.10476(85) . This corresponds to da/ a ~ 0.8 %, which is an improvement of a factor of 3.3 in comparison to the recent PDG 2018 average. Since one of the systematics has to be reviewed, this result is preliminary, but no significant change is expected