P. Amaro, A. Adamczak, M. Abdou Ahmed, L. Affolter, F. D. Amaro, P. Carvalho, T. L. Chen, L. M. P. Fernandes, M. Ferro, D. Goeldi, T. Graf, M. Guerra, T. W. Hänsch, C. A. O. Henriques, Y. C. Huang, P. Indelicato, O. Kara, K. Kirch, A. Knecht, F. Kottmann, Y. W. Liu, J. Machado, M. Marszalek, R. D. P. Mano, C. M. B. Monteiro, F. Nez, J. Nuber, A. Ouf, N. Paul, R. Pohl, E. Rapisarda, J. M. F. dos Santos, J. P. Santos, P. A. O. C. Silva, L. Sinkunaite, J. T. Shy, K. Schuhmann, S. Rajamohanan, A. Soter, L. Sustelo, D. Taqqu, L. B. Wang, F. Wauters, P. Yzombard, M. Zeyen, A. Antognini
SciPost Phys. 13, 020 (2022) ·
published 15 August 2022

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The CREMA collaboration is pursuing a measurement of the groundstate hyperfine splitting (HFS) in muonic hydrogen ($\mu$p) with 1 ppm accuracy by means of pulsed laser spectroscopy to determine the twophotonexchange contribution with $2\times10^{4}$ relative accuracy. In the proposed experiment, the $\mu$p atom undergoes a laser excitation from the singlet hyperfine state to the triplet hyperfine state, then is quenched back to the singlet state by an inelastic collision with a H$_2$ molecule. The resulting increase of kinetic energy after the collisional deexcitation is used as a signature of a successful laser transition between hyperfine states. In this paper, we calculate the combined probability that a $\mu$p atom initially in the singlet hyperfine state undergoes a laser excitation to the triplet state followed by a collisionalinduced deexcitation back to the singlet state. This combined probability has been computed using the optical Bloch equations including the inelastic and elastic collisions. Omitting the decoherence effects caused by the laser bandwidth and collisions would overestimate the transition probability by more than a factor of two in the experimental conditions. Moreover, we also account for Doppler effects and provide the matrix element, the saturation fluence, the elastic and inelastic collision rates for the singlet and triplet states, and the resonance linewidth. This calculation thus quantifies one of the key unknowns of the HFS experiment, leading to a precise definition of the requirements for the laser system and to an optimization of the hydrogen gas target where $\mu$p is formed and the laser spectroscopy will occur.
SciPost Phys. Proc. 5, 030 (2021) ·
published 6 September 2021

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A number of experiments with muons are limited by the poor phase space quality of the muon beams currently available. The muCool project aims at developing a phasespace cooling method to transform a surface μ+ beam with 4 MeV energy and 1 cm size into a slow muon beam with eV energy and 1 mm size. In this process the phase space is reduced by a factor of 10^9 − 10^10 with efficiencies of 2 · 10^−5 − 2 · 10^−4 . The beam is then reaccelerated to keVMeV energies. Such a beam opens up new avenues for research in fundamental particle physics with muons and muonium atoms as well as in the field of μSR spectroscopy.
SciPost Phys. Proc. 5, 021 (2021) ·
published 6 September 2021

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The energy levels of hydrogenlike atomic systems are shifted slightly by the complex structure of the nucleus, in particular by the finite size of the nucleus. These energy shifts are vastly magnified in muonic atoms and ions, i.e. the hydrogenlike systems formed by a negative muon and a nucleus. By measuring the 2S2P energy splitting in muonic hydrogen, muonic deuterium and muonic helium, we have been able to deduce the p, d, 3He and 4He nuclear charge radii to an unprecedented accuracy. These radii provide benchmarks for hadron and nuclear theories, lead to precision tests of bound state QED in regular atoms and to a better determination of the Rydberg constant.