Muonic atoms are formed when negative muons come to rest and subsequently get captured by a nearby atom, where it behaves like a heavy atomic electron. During the cascades down to the lowest atomic orbital it, Auger electrons and x-rays are emitted. Due of its large mass of about 200 times the electron mass, the muon resides 200 times closer to the atomic nucleus. Therefore this exotic atom is and excellent system to study nuclear finite size effects and short range interactions.

We perform high-precision muonic x-ray spectroscopy experiments at the Paul Scherrer Institute in Switzerland on a wide range of nuclei, combining High-Purity Germanium (HPGe) or Magnetic Magnetic Calorimeters (MMC) x-ray detectors with novel target methods and stat-of-the-art digitizing and pulse analysis techniques to achieve the highest accuracy on the transition energies. In addition, large acceptance of the HPGe array allows us to observe x-ray in the cascade with a low branching ratio.

The most straight forward observable is the nuclear charge radius, which is typically derived from the 2p-1s transition energy after taking into account the necessary bound-state QED and nuclear polarizabilty contributions. This radius then serves as a benchmark for nuclear structure calculations, benchmarks laser spectroscopy measurements, or pins down the finite size contribution of precision standard model tests.

We perform high-precision muonic atom spectroscopy with variety of techniques. A few selected transitions in muonic hydrogen and helium are accessible for laser spectroscopy, where the highest accuracy can be reached. For more information, see https://www.agpohl.physik.uni-mainz.de/. The transition energies of the light muonic atoms from helium to neon are in the range of 20-200 keV, where the resolution of a solid state detector is insufficient to precisely determine the nuclear finite size effect. In collaboration with colleagues at the Kirchhoff-Institut für Physik in Heidelberg, we deployed novel Magnetic Metallic Callorimeters (MMCs) at a muon facility for the first time.

The combination of exceptional resolving power and broad energy acceptance makes these devices ideally suited for high-precision exotic atom spectroscopy measurements. In particular, the Quartet collaboration aims to determine absolute nuclear charge radii from lithium to neon with a relative precision of 0.1%.

First measurements with stable lithium isotopes demonstrated the capability to determine muonic x-ray energies to precision better then 1 eV under the conditions at a secondary muon beamline. An dedicated MMC design improved the the robustness of the detector against various backgrounds.

Some explanation about the program, and what we do in Mainz.

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