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Measuring mini-masses – What does an elektron weigh?



Electrons buzz around an atomic nucleus and are very small and lightweight. To measure the mass of an electron as precisely as possible is an important research goal, e. g. to test the standard model. In an experiment with GSI participations scientists now succeeded in improving the accuracy substantially.

To measure the electron's mass scientists used a Penning trap at the university in Mainz. In the trap charged particles can be captured in a magnetic field. The particles rotate in the magnetic field and their mass can be determined from the rotational frequency. However, measuring a single electron is difficult and afflicted with a large error. Thus scientists use a trick: They couple the electron to a carbon nucleus whose behaviour in the magnetic field is well known.

While the rotational frequency of the bound electron in the trap now doesn't play any role anymore, another effect comes to the fore. We know this effect from the nursery: a toy top has a rotation round its axis. In addition we can observe that when a force is applied to it, the top will tilt and in total rotate around an invisible axis in the centre of the tilted position. This behaviour is called precession. The electrons own rotation, also know as the spin, precesses in the trap similar to the top. By measuring the precession frequency one can determine its mass.

The value for the electron mass found in the experiments is 0,000548579909067(14)(9)(2) atomic units. The numbers in the brackets give the errors the measurement is afflicted with despite its high precision. This result improves previous measurements by a factor of 13 in accuracy. "At the moment we are constructing an even larger trap to further reduce the errors", explains Dr. Wolfgang Quint from GSI's atomic physics department, who took part in the setup of the trap in Mainz. "To test quantum electrodynamics we also plan further experiments on heavy highly charged ions at the GSI experiment HITRAP."

Precise measurements of the electron mass play an important role in the tests of the standard model of elementary particles and their interactions. The number contributes, for example, to the structure and the properties of atoms and molecules and affects the fine structure constant and the tests of quantum electrodynamics. The experiment is a collaboration of the Max-Planck-Institute for Nuclear Physics and the International Max Planck Research Schoolfor Quantum Dynamics in Heidelberg, the Johannes-Gutenberg-University in Mainz, the ExtreMe Matter Institutes EMMI and the GSI Helmholtzzentrum für Schwerionenforschung GmbH.

Publication in the science journal "Nature"

Precessing electron
The electron (green) rotates around the carbon atom. Its spin precesses in the magnetic field of the trap.
Picture: GSI Helmholtzzentrum für Schwerionenforschung