Using a newly developed technique, scientists at the Max Planck Institute
for Nuclear Physics (MPIK) in Heidelberg have measured the very small
difference in the magnetic properties of two isotopes of highly charged neon
in an ion trap with previously inaccessible accuracy. Comparison with
equally extremely precise theoretical calculations of this difference allows
a record-level 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 some of the most fundamental building blocks of the matter we
know. They are characterized by some very distinctive properties, such as
their negative charge and the existence of a very specific intrinsic angular
momentum, also called spin. As a charged particle with spin, each electron
has a magnetic moment that aligns itself 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 precise matches of theory and
experiment in physics to date. However, the magnetic moment of the electron
changes as soon as it is no longer a "free" particle, i.e., unaffected by
other influences, but instead is bound to an atomic nucleus, for example.
The slight changes of the g-factor can be calculated by means of QED, which
describes the interaction between electron and nucleus in terms of an
exchange of photons. High-precision measurements allow a sensitive test of
this theory.
"With our work, we have now succeeded in investigating these QED predictions
with unprecedented resolution, and partially, for the first time," reports
group leader Sven Sturm. "To do this, we looked at the difference in the
g-factor for two isotopes of highly charged neon ions that possess only a
single electron." These are similar to hydrogen, but with 10 times higher
nuclear charge, enhancing the QED effects. Isotopes differ only in the
number of neutrons in the nucleus when the nuclear charge is the same.
20Ne9+ and 22Ne9+ with 10 and 12 neutrons, respectively, were investigated.
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 magnetic field of 4 Tesla in a nearly perfect vacuum. The aim of
the measurement is to determine the energy needed to flip the orientation of
the "compass needle" (spin) in the magnetic field. To do this, the exact
frequency of the microwave excitation required for this purpose is looked
for. However, this frequency also depends on the exact value of the magnetic
field. To determine this, the researchers exploit the motion 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 tiny fluctuations of the magnetic field limit previous
measurements to about 11 digits of accuracy.
The idea of the new method is to store the two ions to be compared, 20Ne9+
and 22Ne9+ simultaneously in the same magnetic field in a coupled motion. In
such a motion, the two ions always rotate opposite each other on a common
circular path with a radius of only 200 micrometers," explains Fabian Heiße,
Postdoc at the ALPHATRAP experiment.
As a result, the fluctuations of the magnetic field have practically
identical effects on both isotopes, so there is no influence on the
difference of the energies searched for. Combined with the measured magnetic
field, the researchers were able to determine the difference of the
g-factors of both isotopes with record accuracy to 13 digits, an improvement
by a factor of 100 compared to 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 performed with similar accuracy in
Christoph Keitel's department at MPIK. "In comparison with the new
experimental values, we confirmed that the electron does indeed interact
with the atomic nucleus via the exchange of photons, as predicted by QED,"
explains group leader Zoltán Harman. This has now been resolved and
successfully tested for the first time by the difference measurements on the
two neon isotopes. Alternatively, assuming the QED results are known, the
study allows the nuclear radii of the isotopes to 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 interaction of the ion with dark matter," states postdoc
Vincent Debierre.
"In the future, the method presented here could allow for a number of novel
and exciting experiments, such as the direct comparison of matter and
antimatter or the ultra-precise determination of fundamental constants,"
states first author Dr. Tim Sailer.
Reference:
Tim Sailer et al, Measurement of the bound-electron g-factor difference in
coupled ions, Nature (2022).
DOI: 10.1038/s41586-022-04807-w
Tags:
Physics