Using a newly developed technique, scientists at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg measured the very small difference in the magnetic properties of two highly charged neon isotopes in an ion trap with precision previously unattainable. Comparison with equally extremely accurate theoretical calculations of this difference allows for a record-breaking test of quantum electrodynamics (QED). The agreement of the results is an impressive confirmation of the Standard Model of physics, allowing conclusions regarding the properties of nuclei and setting limits for new physics and dark matter.
Electrons are among the most fundamental building blocks of matter that we know. They are characterized by very particular properties, such as their negative charge and the existence of a very specific intrinsic angular momentum, also called spin. As a spin-charged particle, each electron has a magnetic moment that aligns in a magnetic field similar to a compass needle. The strength of this magnetic moment, given by the so-called g-factor, can be predicted with extraordinary accuracy by quantum electrodynamics. This calculation agrees with the experimentally measured g-factor to within 12 digits, one of the most accurate matches in physics theory and experiment to date. However, the magnetic moment of the electron changes as soon as it is no longer a “free” particle, that is to say unaffected by other influences, but bound to an atomic nucleus, for example. Slight changes in the g-factor can be calculated using QED, which describes the interaction between the electron and the nucleus in terms of photon exchange. High precision measurements allow a sensitive test of this theory.
“With our work, we have now succeeded in studying these QED predictions with unprecedented resolution, and partially, for the first time,” reports group leader Sven Sturm. “To do this, we looked at the g-factor difference for two highly charged neon ion isotopes that have only one electron.” These are similar to hydrogen, but with a 10 times higher nuclear charge, enhancing QED effects. Isotopes differ only in the number of neutrons in the nucleus when the nuclear charge is the same. 20not9+ and 22not9+ with 10 and 12 neutrons, respectively, were studied.
The ALPHATRAP experiment at the Max Planck Institute for Nuclear Physics in Heidelberg provides a specially designed Penning trap to store single ions in a strong 4 Tesla magnetic field in near-perfect vacuum. The purpose of the measurement is to determine the energy required to reverse the orientation of the “compass needle” (spin) in the magnetic field. To do this, the exact frequency of the microwave excitation necessary for this purpose is sought. However, this frequency also depends on the exact value of the magnetic field. To determine this, the researchers exploit the movement of ions in the Penning trap, which also depends on the magnetic field.
Despite the very good temporal stability of the superconducting magnet used here, unavoidable minute fluctuations in the magnetic field limit previous measurements to about 11 digits of precision.
The idea of the new method is to store the two ions to be compared, 20not9+ and 22not9+ simultaneously in the same magnetic field in a coupled movement. In such a motion, both ions always rotate opposite each other on a common circular trajectory with a radius of only 200 micrometers”, explains Fabian Heiße, Postdoc at the ALPHATRAP experiment.
Because of this, fluctuations in the magnetic field have practically identical effects on the two isotopes, so there is no influence on the difference in the energies sought. Combined with the measured magnetic field, the researchers were able to determine the difference in the g-factors of the two isotopes with record 13-digit accuracy, a 100-fold improvement over previous measurements and thus the most accurate comparison of two g -factors worldwide. The resolution achieved here can be illustrated as follows: If, instead of the g-factor, the researchers had measured Germany’s highest mountain, the Zugspitze, with such precision, they would be able to recognize individual additional atoms on the summit by the height of the mountain.
The theoretical calculations were carried out with similar precision in the department of Christoph Keitel at the MPIK. “In comparison with the new experimental values, we have confirmed that the electron does indeed interact with the atomic nucleus via photon exchange, as predicted by QED,” explains group leader Zoltán Harman. This has now been solved and successfully tested for the first time by difference measurements on the two isotopes of neon. Alternatively, assuming the QED results are known, the study allows the nuclear rays of the isotopes be determined more precisely than previously possible by a factor of 10.
“Conversely, the agreement between the results of theory and experiment allows us to constrain new physics beyond the known standard model, such as the strength of the ion’s interaction with black matter“, specifies the postdoc Vincent Debierre.
“In the future, the method presented here could enable a number of new and exciting experiments, such as the direct comparison of matter and antimatter or the ultra-precise determination of fundamental constants,” says the first author. , Dr. Tim Sailer.
Tim Sailer et al, Measurement of g-factor difference of bound electrons in coupled ions, Nature (2022). DOI: 10.1038/s41586-022-04807-w
Max Planck Institute for Microstructural Physics
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