Andrea Vacchi (INFN Trieste)
FAMU will realize the first measurement of the hyperfine splitting (hfs) in the 1S state of muonic hydrogen ΔE_hfs(μ^-p)_1S – providing crucial information on proton structure and muon-nucleon interaction – this by using an intense pulsed muon beam, and an on-purpose developed high-energy mid infrared tunable laser. FAMU initiates a new class of experiments representing a significant leap forward in the quality of spectroscopic measurements in muonic atoms. Specifically it will provide r_Z, the Zemach radius of the proton with higher precision than previously possible, disentangling discordant theoretical values and will quantify any level of discrepancy that may exist between values of r_Z as extracted from normal and muonic hydrogen atoms. It will set a cornerstone result about not yet explained anomalies on the charge r_ch radius of the the proton. The Zemach radius r_Z and the charge radius r_ch are the only proton shape-related values that can be directly extracted from experimental data, and r_Z is the only one that gives information about the proton’s magnetic dipole moment distribution. FAMU’s theoretical and experimental effort will establish new limits on the proton structure parameters measuring the muonic hydrogen transition ΔE_hfs(μ^-p)_1S with unprecedented precision δλ/λ < 10^(-5).