The FAMU experiment at RIKEN RAL for a precise measure of the proton radius

The goal of the FAMU experiment at RIKEN RAL is the measure of the hyperfine splitting of the ground state of the muonic hydrogen, to allow a determination of the proton Zemach radius with a precision better than $5 \times 10^{-3}$ . The comparison of this measurement with the ones done with ordinary hydrogen may help to solve the so-called "proton radius puzzle", triggered by the $6 σ$ discrepancy in the proton charge radius value as extracted from muonic Lamb shift and the value based on e-p scattering and ordinary hydrogen spectroscopy.