  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 $6
\sigma$ 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.
