FAMU

Recent measurements of electron–proton scattering and muonic hydrogen spectroscopy have produced discrepant results regarding the proton charge radius. The FAMU experiment (Fisica Atomi MUonici) aims to contribute to resolving this puzzle by measuring the Zemach radius of the proton through hyperfine splitting measurements in muonic atoms.

The FAMU detector consists of a vessel containing the gas mixture in which muon captures occur, using a muon beam from the RIKEN-RAL laboratories (UK). The characteristic X-rays emitted following the de-excitation of muonic atoms are detected by a set of photodetectors (HPGe, LaBr₃), with an energy resolution better than 3%.

The FAMU-Pavia group has been responsible for the construction of the muon beam monitor and the LaBr₃ photodetectors with SiPM readout. After several technical runs in the years 2015–2018, the final data-taking campaign of FAMU has been ongoing since 2023.