What weighs an electron?

Accurate measurement of the electron mass allows more accurate testing of the physical standard model

The electron is elusive: the position of an electron can only be represented as a probability of residence in the form of such orbitals. © Pérez / CC_by-sa 3.0
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The electron is one of the most important elementary particles, because it influences the behavior of all atoms and molecules in the universe. Now German researchers have succeeded in determining its mass as accurately as never before. This will allow future even more precise tests of the physical standard model and could even help with dark matter research, as the scientists report in the journal Nature.

The electron is one of the basic building blocks of matter, because only this negatively charged particle completes the positive nucleus of an atom. At the same time, the electron also holds the atoms bound together in molecules and gives them their typical properties. The lightest among the electrically charged elementary particles also forms a basis of the standard model of physics. The mass of the electron is one of the possibly fundamental constants of this model and influences many other parameters.

How do you weigh an elementary particle?

The problem with this is that the mass of the electron is so small that it would not be directly measurable with any scale in the world. Instead, one has to resort to indirect methods. So far, these measurements were usually done in a so-called Penning trap. The electron is trapped in a strong magnetic field and placed in a circular motion. The characteristics of these movements, the so-called cyclotron frequency, can be measured and compared with that of charged particles of known mass.

In a Penning trap (left), a magnetic field (black arrow) forces a carbon-12 core with a single electron onto a screwed-in circuit (right). Simplified, one can imagine this as a circular path (green). From the orbital frequency, the precise mass of the carbon-12 nucleus can be determined with one electron. From the mass of the five-fold charged carbon ion and the spin of the electron spin (black line, right), a quantum-mechanical relationship yields the electron mass. Sven Sturm / MPI for Nuclear Physics

"In a Formula 1 race on a circuit, viewers can count how often a race car races past and use the track length to estimate their speed, " explains Project Manager Sven Sturm from the Max Planck Institute for Nuclear Physics in Heidelberg. Similarly, this also works in the Penning trap, whereby the physicists additionally detect the tiny spin of the electron spin in the magnetic field.

A bag in the Airbus

But all elementary particles useful as comparative weights are much heavier than the electron. "The proton or the neutron, for example, is two thousand times heavier, " explains Klaus Blaum, director at the MPI for Nuclear Physics. "That would be like weighing a rabbit with an elephant as a counterweight." In the absence of an alternative, the official reference value for the electron mass is based on these measurements. The relative uncertainty of this value is 4 x 10 -10. display

In this Penning trap physicists determine the mass of an electron. Sven Sturm / MPI for Nuclear Physics

Sturm and his colleagues have now modified the measurement with the Penning trap. Instead of comparing the cyclotron frequency of the electron with that of another particle, they shot off five of its six electrons from one carbon atom and combined the core and the electron. The advantage here: The mass of the carbon nucleus is well known and allows it in combination with the electron to determine its mass more accurately than before. "Calculated on an Airbus A-380 alone we could determine by weighing whether a mosquito is on board as a stowaway, " according to Sturm.

New tests of the standard model possible

With these measurements, the researchers came to a mass of the electron of 0.000548579909067 atomic mass units. An atomic mass unit is defined as the twelfth part of a carbon-12 atom. The relative accuracy for the new value is 3x10 -11. This is 13 times more accurate than previously possible. "This result prepares the stage for future ultra-high-precision tests of the standard model at low energies, " the researchers note.

For example, the fine-structure constant alpha could be better determined it reflects the strength of the electromagnetic interaction. "It basically describes everything we see, " explains Blaum. "Because it plays a central role in the interaction between light and matter." The Rydberg constant, which is important for the calculation of atomic spectra, is also defined by the electron mass. The more detailed knowledge of these constants opens up new ways to learn more about the fundamental interactions of matter. (Nature, 2014; doi: 10.1038 / nature13026)

(Nature, 20.02.2014 - NPO)