FAMU - Measuring the muonic hydrogen ground state hyperfine splitting to determine the Zemach radius of the proton

The experiment will measure the hyperfine splitting (hfs) in the ground state of muonic hydrogen with a relative accuracy better than 10-5. An intense, pulsed, low-energy, negative muon beam and an appropriate mid-infrared laser source are required. The measurement will contribute to the following key challenging aspects:• determination with an accuracy of 1% or better of the Zemach radius of the proton rZ, the first moment of the convolution of its electric charge and magnetic moment distributions;• resolution of the ambiguities about the electromagnetic structure of proton: the needed completion to a cycle of high-precision exotic atom counterparts of the historical measurements of the Lamb shift and the hyperfine splitting in ordinary hydrogen. A step of great importance as fundamental tests of QED.