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Precision Measurements of Fundamental Constants

Final Report Summary - MEFUCO (Precision Measurements of Fundamental Constants)

Within the project MEFUCO fundamental constants determining the basic structure of the universe have been determined with improved precision. These constants cannot be predicted by theory, but their precise determination is required to enable the comparison of theoretical models with experimental observations at the highest possible level. The improvement of these quantities beyond the present level of accuracy represents a significant challenge for modern metrology. This requires novel extensive experimental developments like, e.g. new manipulation, cooling, storage, and low-noise detection techniques for the studied particles. Such developments have been performed within the project, including among others the implementation of phase-sensitive detection techniques of the ion motion in a Penning trap, resulting in a factor of 5-10 improvement in the achieved accuracy for g-factor and mass measurements as well as the development of low-noise highly-stable voltage sources to provide the ion trapping potentials.

The development of a new phase-sensitive measurement technique for the reduced cyclotron frequency enabled g-factor determinations with unprecedented uncertainties. This was first demonstrated by measuring the g-factor of hydrogenlike silicon with a relative statistical uncertainty of 4∙10-11 [S. Sturm et al., Phys. Rev. A 87 (2013) 030501]. Considering the limitation of the experimental precision by the uncertainty of the electron mass, in the converse argument the electron mass was determined from a measurement of the g-factor of hydrogenlike carbon. For this system, the theoretical g-factor provided by the group of Ch. Keitel is known with a relative precision of 3.5∙10-12 and the uncertainty of the ion mass, arising primarily from the electron binding energies, is 1∙10-13. The required frequency ratio was measured with a precision of 3∙10-11, hereby improving the uncertainty of the electron mass by a factor of 13 [S. Sturm et al., Nature 506, 467 (2014)].

Besides the g-factor of the electron bound in hydrogenlike ions, lithiumlike ions are of special interest to test the relativistic many-electron calculations. To this end, the g-factor of the valence electron bound in lithiumlike silicon 28Si11+ has been measured to a precision of 1.1∙10-9 [A. Wagner et al., Phys. Rev. Lett. 110 (2013) 033003] as well as the g-factor of the valence electron in two lithiumlike calcium isotopes 40,48Ca17+ [F. Köhler et al., Nature Comm. 7 (2016) 10246]. They are in excellent agreement with the theoretical value and their comparison yields the most stringent test of relativistic many-electron calculations up to date.

Precise comparisons of the fundamental properties of matter and antimatter provide stringent tests of CPT symmetry, the most fundamental symmetry of the Standard Model of particle physics. One test which has not yet been performed with high precision is the direct comparison of the magnetic moments of the proton and the antiproton. Several important milestones were achieved within this ERC project. Among others the so-called double Penning trap technique was developed [A. Mooser et al., Phys. Lett. B 723 (2013) 78] and discrete spin transition resolution of a single proton achieved [A. Mooser et al., Phys. Rev. Lett. 110 (2013) 140405]. By using this technique a first direct high-precision measurement of the magnetic moment of the proton and improving the 42 year old best value by more than a factor of 3 has been achieved [A. Mooser et al., Nature 509, 596 (2014)]. By comparing the proton-to-antiproton charge-to-mass ratio at a precision level of 69 parts per trillion the most stringent test to date of CPT symmetry on the baryonic sector date could be performed [S. Ulmer et al., Nature 524 (2015) 196] using the BASE (Baryon Antibaryon Symmetry Experiment) facility at the Antiproton Decelerator (AD) of CERN. A high-precision comparison at the 10-9 level of the proton-to-antiproton g-factors is presently ongoing.