Laser spectroscopy of simple muonic atoms – LASIMUS

We aim at the first laser spectroscopy of the ground state (1S) hyperfine splitting (HFS) of muonic hydrogen and the muonic helium-3 ion with a relative accuracy of 1 ppm. This yields the corresponding nuclear properties of the proton and helium-3 nucleus with 100fold improved precision. This proposal concerns the construction of the laser system, cavities and data acquisition.

Muonic atoms are exotic atoms in which the orbiting electrons are replaced by a single negative muon. As the muon is 200 times heavier than an electron, the muon’s Bohr radius is 200 times smaller, and the wave function overlap between muon and nucleus is 2003 = 8 million times larger. Muonic atoms are thus extremely sensitive to nuclear parameters, such as its size.

Even the simplest nucleus, the proton, is an extended object with an internal structure, composed of quarks and gluons. We have for the first time determined the proton’s charge radius from the measurement of the 2S Lamb shift in muonic hydrogen [Pohl, Antognini, Nez et al., Nature 466, 213 (2010)]. The charge radius quantifies the average extension of the electric charge inside the proton, and our value is ten times more accurate than the world average from the measurements in regular hydrogen and from elastic electron-proton scattering, but differs by more than 5 standard deviations.

The proposed HFS measurement will improve by a factor of 100 the so-called nuclear two-photon exchange (TPE) contribution, the elastic part of which is sensitive to the magnetization distribution inside the nucleus. This can be parametrized in terms of the so-called (magnetic) Zemach radius, which we will improve by more than a factor of 10.
The uncertainty in the Zemach radius limits the QED test in the HFS of regular hydrogen to 6 digits, while experiment (the famous „21-cm line“) has been measured with 12 digits accuracy already 50 years ago. Our measurements will thus improve the QED test in hydrogen and helium-3 by an order of magnitude.

The muonic HFS measurements will take place at the Swiss Paul-Scherrer-Institute (PSI) in the framework of the international CREMA collaboration. The goal of the present proposal is to build the mid-IR laser systems and high-rate data acquisition (DAQ) for CREMA. Novel optical parametric oscillators (OPOs) will be used to generate the large pulse energies in the mid-IR. Funded by this grant, F. Nez (LKB) provides accurate laser frequency measurement and calibration, R. Pohl (JGU) contributes the multipass-cavity for muonic atom spectroscopy and DAQ. A. Antognini (PSI) will from his existing funding provide an improved muon beam line and high-power pulsed pump laser. The OPO-based spectroscopy laser will be built and tested by all three of us through this DFG-ANR grant. The study and use of lasers in this frequency range is growing, with applications in spectroscopy, LIDAR, precise machining of glass, new bio-molecular analysis techniques and new laser therapies.