LAser Spectroscopy of sImple MUonic atomS

• Applicant: Professor Dr. Randolf Pohl
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas ; Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 407008443

• Final Report Year: 2024

Final Report Abstract

Muonic atoms are the exotic atoms in which all orbiting electrons e− are replaced by a single negative muon µ− . As the muon is 200 times more massive than the electron, the Bohr radius if about 200 times smaller, resulting in an 200^3 ≈ 10 million times larger wave function overlap of the muon with the nucleus, and a correspondinlgy 7 orders of magnitude larger sensitivity of muonic atoms to nuclear structure effects. Previous spectroscopy experiments gave a result for electric charge radius of the proton that contradicted the existing data from other methods. Our next step is spectroscopy of hyperfine structure in muonic hydrogen in its ground state which gives access to the magnetic structure of the proton. It will enable a more than 10 times more accurate QED test in the hyperfine structure of hydrogen. Within this proposal, we have developed a completely new setup including a unique laser system to produce the required large pulse energy tunable around 6.8 µm. To this end, we have developed and built a narrow-band (5 MHz), high pulse-energy (330 mJ) thin-disk laser system at 1030 nm, which can deliver light less than 1 µs after a random trigger. Two parallel OPO/OPA stages produce pulses around 3135 nm (tunable) and 2148 nm (fixed), whose difference frequency corresponds to the desired 6.8 µm light. Within this project, we have successfully built the 3.1 µm OPO/OPA branch and demonstrated the large pulse energy, narrow bandwidth and excellent beam quality. We have developed an enhancement cavity suitable to illuminate the muon stop volume. A further improvement is possible with an advanced high reflective coating currently under development. The data acquision system has been built and successfully used in our sister experiments at PSI.

Publications

• Laser excitation of the 1s-hyperfine transition in muonic hydrogen. SciPost Physics, 13(2).
  Amaro, Pedro; Adamczak, A.; Abdou, Ahmed M.; Affolter, L.; Amaro, F. D.; Carvalho, Patricia; Chen, T. -L.; Fernandes, L. M. P.; Ferro, M.; Goeldi, D.; Graf, T.; Guerra, M.; Hänsch, Theodor; Henriques, C. A. O.; Huang, Y. -C.; Indelicato, P.; Kara, O.; Kirch, Klaus; Knecht, Andreas ... & Antognini, Aldo
  
• Diffusion of muonic hydrogen in hydrogen gas and the measurement of the 1$s$ hyperfine splitting of muonic hydrogen. SciPost Physics Core, 6(3).
  Nuber, Jonas; Adamczak, A.; Abdou, Ahmed M.; Affolter, L.; Amaro, F. D.; Amaro, Pedro; Antognini, A.; Carvalho, P.; Chang, Y. -H.; Chen, T. -L.; Chen, W. -L.; Fernandes, L. M. P.; Ferro, M.; Goeldi, D.; Graf, Thomas; Guerra, M.; Hänsch, T. W.; Henriques, C. A. O.; Hildebrandt, M. ... & Zhang, J.
  
• Injection-seeded high-power Yb:YAG thin-disk laser stabilized by the Pound-Drever-Hall method. Optics Express, 31(18), 29558.
  Zeyen, Manuel; Affolter, Lukas; Abdou, Ahmed Marwan; Graf, Thomas; Kara, Oguzhan; Kirch, Klaus; Langenbach, Adrian; Marszalek, Miroslaw; Nez, François; Ouf, Ahmed; Pohl, Randolf; Rajamohanan, Siddharth; Yzombard, Pauline; Schuhmann, Karsten & Antognini, Aldo
  
• Muonic atom spectroscopy with microgram target material. The European Physical Journal A, 59(2).
  Adamczak, A.; Antognini, A.; Berger, N.; Cocolios, T. E.; Deokar, N.; Düllmann, Ch. E.; Eggenberger, A.; Eichler, R.; Heines, M.; Hess, H.; Indelicato, P.; Kirch, K.; Knecht, A.; Krauth, J. J.; Nuber, J.; Ouf, A.; Papa, A.; Pohl, R.; Rapisarda, E. ... & Wauters, F.
  
• Pound–Drever–Hall locking scheme free from Trojan operating points. Review of Scientific Instruments, 94(1).
  Zeyen, Manuel; Affolter, Lukas; Ahmed, Marwan Abdou; Graf, Thomas; Kara, Oguzhan; Kirch, Klaus; Marszalek, Miroslaw; Nez, François; Ouf, Ahmed; Pohl, Randolf; Rajamohanan, Siddharth; Yzombard, Pauline; Antognini, Aldo & Schuhmann, Karsten
  
• Compact 20-pass thin-disk multipass amplifier stable against thermal lensing effects and delivering 330 mJ pulses with M2 < 1.17. Optics Express, 32(2), 1218.
  Zeyen, Manuel; Affolter, Lukas; Abdou, Ahmed Marwan; Graf, Thomas; Kara, Oguzhan; Kirch, Klaus; Marszalek, Miroslaw; Nez, François; Ouf, Ahmed; Pohl, Randolf; Schulthess, Ivo; Rajamohanan, Siddharth; Yzombard, Pauline; Schuhmann, Karsten & Antognini, Aldo