Exploring the proton structure with the FAMU experiment: detector performance and first physics results

My PhD Thesis work has been focused on the activity of the FAMU experiment, aimed at measuring the ground state hyperfine splitting in the muonic hydrogen atom (μH), in order to extract an independent measurement of the Zemach radius of the proton. The field of proton radii re-gained interest in the last 15 years thanks to a new measurement of the proton charge radius, about 8σ away from the CODATA2014 accepted value, measured by the CREMA experiment at PSI by evaluating the Lamb Shift in μH. While the charge radius is only dependent on the electric form factor, the Zemach radius takes into account of both the electric and magnetic internal structure of the proton. The FAMU experiment is being carried out at the RIKEN-RAL Port 1 muon beamline at the ISIS Neutron and Muon Source in Didcot, United Kingdom. The μH atoms are created by injecting a negative muon beam in a pressurised target containing hydrogen with a contamination of oxygen. A custom-made laser system with variable wavelength is injected in the chamber to excite the spin flip in μH atoms. The observable consists in an excess of delayed muonic oxygen (μO) X-rays resulting from an enhanced muon transfer probability following the transition, and the aim of the experiment is to look for a resonance in this observable as a function of the laser wavelength. The spectral region of interest for the experiment is 100-200 keV. For this reason, the detection setup consists in a set of scintillating crystals surrounding the pressurised target, complemented by a germanium detector for inter-calibration. The main detector studied in this work is the muon beam monitor, which is the focus of Chapter 3. It consists in a scintillating fibre hodoscope which is currently being used also as a flux-meter thanks to the extensive calibration and simulation work hereby presented. The calibration consisted in low-flux measurements to extract the response of a single particle and compare it to the simulated response function, to construct a high-flux response function used for the muon flux estimation. The simulation work was carried out in Geant4 and compared to a model developed with FLUKA-CERN. Finally, the FAMU data analysis workflow is presented in Chapter 4, with a specific focus on the muon beam normalisation and quality control carried out with the FAMU beam monitor. The data selection and data analysis on the scintillating detectors is also presented and coupled with the information from the hodoscope to fully characterise and double-check the incoming beam and the setup itself. The ongoing and future steps of data analysis towards the final resonance plot are also presented and discussed. In Conclusion, the work developed for my PhD Thesis consisted in a wide overview on the FAMU experiment and its hardware and analysis framework. In particular, I developed an innovative method for the extraction of the absolute flux of a high intensity muon beam based on data from a custom beam monitor, initially designed by INFN Pavia and Milano-Bicocca only as a beam shape monitor. This information was fundamental in developing the analysis tools I developed for the normalisation of FAMU data. Lastly, I spent part of my PhD work analysing FAMU LaBr3 detector data within the FAMU Analysis Group in order to develop the method to extract the observable, i.e. the number of delayed μO X-rays as a function of the injected laser wavelength. This observable is linked to the μH ground state hyperfine splitting, and therefore to the final goal of the proton Zemach radius.