Electron: Most accurate value for the magnetic moment

New physics? Scientists have measured the electron’s magnetic moment more precisely than ever before – and thus created the basis for more precise checks of the physical Standard Model. For the first time, their method could be precise enough to now also detect the discrepancies with current physics, which were already discovered with the heavier muon, with the electron. For this, however, contradictions in measurements of the fine structure constant would first have to be clarified.

The Standard Model of Physics has stood up to all scrutiny so far. Nevertheless, it has crucial gaps: it cannot explain the nature of dark matter, the asymmetry of matter and antimatter, or dark energy. In recent years, experimental physicists have also observed telltale deviations from theoretical predictions, including some particle decays , the mass of the W boson , and quantum transitions of certain isotopes. This could indicate “new physics” in the form of still unknown particles or forces.

Mystery of the g-factor

Another discrepancy concerns the magnetic moment of the muon, a particle belonging to the same group as the electron. In the case of this brother of the electron, which is around 200 times heavier, measurements have revealed significant deviations in the so-called g-factor of the magnetic moment. This g-factor, also known as the Landé factor, describes how the magnetic moment of a particle, which is characterized by charge, mass and spin, is influenced by quantum physical interactions.

The electron also has such a g-factor of the magnetic moment. So far, however, the precision of the measurements for this much lighter particle has not been sufficient to show possible deviations from the Standard Model for the electron as well. Because the effect visible in the muon – if it exists – would be around 40,000 times weaker in the electron. So far, this was below the detection limit for common measurement methods.

Ice-cold electron in the magnet trap

That has now changed: Physicists led by Gerald Gabrielse from Northwestern University in Illinois have developed a new method to measure the magnetic moment of the electron. The measurements achieve a relative precision of 0.13 trillionths – this allows deviations to be determined that are around 3,000 times smaller than those measured for the muon.

For the measurement, a single electron is held in suspension by a magnetic field acting from all sides. In this Penning trap, the electron is then cooled down to 50 millikelvin – a few fractions of a degree above absolute zero. The particle is temporarily brought into the low-energy ground state, but jumps to the next higher quantum state after about a second. Based on this jump, the physicists can determine the g-factor of the electron.

“Triumph for fundamental physics”

After subtracting all the systemic influencing factors and uncertainties, this results in the hitherto more precise value for the magnetic moment and the g-factor of the electron. According to the measurements, it is g/2 = 1.001 159652 18059 (13). “This new value is 2.2 times more accurate than the 14-year best reading of the electron’s magnetic moment,” the physicists write. However, it basically agrees with these earlier measurements.

“This achievement by the Northwestern University team is a triumph for fundamental physics,” comments non-study physicist Saïda Guellati-Khelifa of Sorbonne University in Paris. “The measurement method enables an unprecedentedly precise test of quantum electrodynamics (QED). The electron has never been so close to opening the window to new physics.” With the high level of precision that can now be achieved, even more precise checks of the magnetic moment and its possible deviations are within reach.

There is still a problem with the fine structure constant

The prerequisite, however, is that the theoretical values ​​are improved. So far, the predictions based on the Standard Model have suffered from the fact that there is no agreement on the value for the fine structure constant . The two most accurate measurements to date differ by 5.5 standard deviations. Depending on which of the two values ​​is used as a basis for the calculations, one arrives at two different theoretical values ​​for the magnetic moment and the g-factor of the electron.

The measured value for the g-factor determined by Gabrielse and his team lies exactly between the two theoretical values. The same applies to the fine structure constant, which can be derived from their measurements: “Our value differs by 2.2 standard deviations from the higher value and by 3.9 standard deviations from the lower,” report the physicists. In this respect, despite the precise measurements, it is difficult to prove a clear discrepancy with the standard model.

“Once the discrepancies in the fine structure constant have been resolved, our new measurement accuracy will enable more precise tests of the standard model and its possible extensions,” Gabrielse and his colleagues state. (Physical Review Letters, 2023; doi:10.1103/PhysRevLett.130.071801 )

Source: American Physical Society (APS)