Determination of the Zemach radius of the proton by exciting the spin-flip in muonic atoms: The FAMU experiment.

The measure of the charge radius of the proton showed a disagreement between electronic and muonic hydrogen atoms: the so-called "proton radius puzzle". The FAMU experiment at the Rutherford Appleton Laboratory (UK) is designed to measure the Zemach radius of the proton in muonic hydrogen ({\mu $H$), to check this disagreement. A beam of negative muons produced by the ISIS synchrotron is directed to a gaseous hydrogen-oxygen target. A custom-made laser, designed to have a wavelength around $6.8 \mu m,$ is injected into the cryogenic chamber filled with the gas mixture, to stimulate resonant spin-flip in $\mu H$ atoms. These atoms promptly de-excite, acquiring $\sim$ 120 ${meV}$ of kinetic energy. As a consequence, the probability of the muon transfer to an oxygen atom is increased and the process is followed by X-ray emission around $100 {keV}$. The resonant wavelength, corresponding to the hyperfine splitting of $\mu H$, is obtained by measuring the X-ray emission at different wavelengths. Through QED calculations, the Zemach radius is then determined with a precision better than $1%$. A confirmation in the difference between the radii would point to physics beyond the Standard Model.