Thin-disk laser for muonic atoms spectroscopy

• Applicant: Professor Dr. Thomas Graf
• Co-Applicants: Dr. Aldo Antognini; Professor Dr. Klaus Kirch
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2012 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 209386059

At the Paul Scherrer Institute, Switzerland, we have recently measured the 2S-2P energy difference in muonic hydrogen (an atom formed by a negative muon and a proton) by means of laser spectroscopy, allowing us to determine the rms charge radius of the proton. The new proton radius value of 0.84184(67) fm is 10 times more precise than previously obtained. However, it disagrees by 4 standard deviations from the value extracted from hydrogen spectroscopy and also by 5 standard deviations from electron-proton scattering data. The origin of the discrepancies is not yet known. It may come either from hydrogen-like atoms theory, or from a problem in the determination of the most accurately know fundamental constant (the Rydberg constant), or from an unexpected proton shape, or it occurs from uncalculated or new effects.The variance of the various proton radius values has led to a very alive discussion in various field of physics: particle and nuclear physics (proton structure, new physics, scattering analysis), in atomic physics (hydrogen energy level theory, fundamental constants) and fundamental theories (bound-state problems, QED, effective field theories). Several new experiments and proposals have also been triggered worldwide.In order to shed some light onto this proton radius conundrum we plan to measure several transition frequencies between the 2S and the 2P states in muonic He3 and He4 ions. Beside helping to solve the proton radius puzzle these measurements will result in the best determination of the alpha-particle and helion rms charge radii to a relative accuracy of at least 0.03% corresponding to an absolute precision of 0.0005 fm. These radii represent interesting and precise parameters to check few-nucleon ab-initio calculations, effective field nuclear theories and potentials.Furthermore when combined with the 1S-2S transition in He+ these measurements will give rise to the most accurate test of the crucial QED terms in hydrogen-like atoms.The main goal of this project described in this application is the development of a thin-disk laser suited for the measurements of several 2S-2P transitions in muonic helium. The challenge for this work is the realization of a thin-disk laser with at least 150 mJ pulse energy (with 20 ns pulse width), a delay between trigger and emission of the pulse smaller than 400 ns, random triggerable to repetition rates up to 1kHz.Beside being necessary for precision spectroscopy of muonic helium this laser system may find an interesting application in the field of muon spin resonance and holds the potential for the development of new thin-disk laser technologies.A thin-disk laser system has been already developed in a collaboration between A. Antognini, ETHZ (former Max Planck Institut für Quantenoptik) and the IFSW for the muonic hydrogen Lamb shift experiment. This development has enabled the measurement in beam time 2009 at PSI of the proton radius. Together with the low-energy beam line it was the most challenging development necessary for this measurement.With this project our collaboration would like to improve the pulse energy, stability and reliability of this thin-disk laser system, particularly in view of the muonic helium Lamb shift experiment (this experiment has been approved by the international research committee for particle physics at the PSI). With this project we would like to develop a thin-disk laser oscillator delivering pulses up to 60 mJ energy (ca 20 ns long) with delays of about 250 ns. This is a factor of 6 increase in energy compared with the present set-up. Moreover we will advance the performance of our multi-pass amplifier to output pulse energies of 150 mJ (a factor of 3 increase). These developments could find application in high power (kW power) cw and fs regimes and contribute to thin-disk technology advances.Such a laser with high pulse energy and short delay is also suited for future muon spin resonance applications in line with new proposals at PSI.For this project, which is in the framework of a DACH collaboration between ETHZ and IFSW, we ask the support for 2 PhD students: one for each involved institute. The students will work not only at the development of the thin-disk laser and related technologies but also at spectroscopy of muonic helium.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Participating Person: Dr.-Ing. Andreas Voß