Unique characteristics and performance of a mid-infrared laser source for muonic hydrogen spectroscopy and proton Zemach radius measurement: The laser of the FAMU experiment
Year: 2026
Authors: Baruzzo M., Fasci E., Moretti L., Subrez-Vargas J.J., Cabrera H., Toci G., Gianfrani L., Pizzolotto C., Mocchiutti E., Danailov M.B., Vacchi A.
Autors Affiliation: INFN, Sez Trieste, Trieste, Italy; INFN, Sez Napoli, Naples, Italy; Univ Campania Luigi Vanvitelli, Dipartimento Matemat & Fis, Caserta, Italy; Abdus Salam Int Ctr Theoret Phys, Trieste, Italy; CNR, INO, Sesto Fiorentino, Italy; Elettra Sincrotrone, Trieste, Italy; Univ Udine, Dipartimento Sci Matemat Informat & Fis, Udine, Italy.
Abstract: The FAMU experiment, aimed at improving the precision in determining the proton Zemach radius via muonic hydrogen spectroscopy, is currently in the second half of its planned data-taking campaigns. Conducted at the ISIS facility of the Rutherford Appleton Laboratory (UK), the experimental method requires an intense pulsed muon beam to generate muonic hydrogen, which is then excited using a custom-designed laser system to excite the hyperfine splitting in the muonic hydrogen 1S state. Developed by the INFN Trieste section in collaboration with University of Campania and Elettra-Sincrotrone Trieste, the laser is unique in its class, providing high energy, narrow linewidth, and precise tunability in the mid-infrared range around 6.78 mu m, corresponding to an energy of 0.182 eV. The laser operates via a Difference Frequency Generation (DFG) process within a BaGa4Se7 nonlinear crystal, combining a commercial 1064 nm Nd:YAG pump with a custombuilt tunable 1262 nm Cr:forsterite-based laser source. To ensure long-term stability and remote operation, a dedicated control system was developed, integrating energy, wavelength, and beam position sensors with active feedback piezoelectric actuators. Throughout four completed data taking runs, the mid-IR radiation, with linewidth of 15 pm, has featured an excellent energy stability, with pulse energies exceeding 1.5 mJ and a standard deviation of 5%, a central wavelength reproducibility of 3 pm together with a tunability step of 10 pm.
Journal/Review: Nuovo Cimento C-Colloquia and Communications in Physics
Volume: 49 (1-2) Pages from: 78-1 to: 78-5
