How old are electrons

How big is the proton?

Garching / Villigen (Switzerland) - Extremely precise measurements on exotic hydrogen atoms provide a smaller value for the radius of the proton than measurements on ordinary hydrogen. The discrepancy is seven times larger than the measurement error, reports an international team of researchers in the journal "Science". So far, there is no explanation for this problem, which similar measurements by the same team had indicated almost three years ago. The effect may point to new physics beyond the current Standard Model, the researchers said.

Experiment with laser beams

The so-called charge radius of the proton is 0.8409 femtometers - this is the result of the experiments at the Paul Scherrer Institute in Villigen, Switzerland, carried out by Aldo Antognini from the Max Planck Institute for Quantum Optics in Garching and his colleagues. The currently generally accepted value is significantly higher at 0.8775 femtometers. A femtometer is a millionth part of a millionth of a millimeter. According to the researchers, the probability that the deviation is due to measurement errors is around one in four billion.

The previous value is mainly based on laser spectroscopic investigations of hydrogen. Hydrogen is the most simply structured element: its atoms each consist of a proton, which is orbited by an electron. The electron can only assume certain energy states, which in turn depend slightly on the radius of the proton. Measurements of the transitions between the energy levels therefore enable the size of the proton to be determined.

To get even more precise results, Antognini and his colleagues replaced the electron with a muon. Muons are elementary particles that are similar to the electron, but two hundred times heavier. Due to the greater mass, the muon moves closer to the proton than the electron and therefore reacts more strongly to its size. In contrast to their first measurements published in 2010, the researchers have once again improved the method. You are now measuring two different transitions in the muonic hydrogen and thereby almost doubling the accuracy.

The question now is whether the discrepancy found is due to systematic experimental errors, or whether the fundamentals of quantum electrodynamics, one of the cornerstones of the standard model of physics, must be called into question. Only further experiments can provide an answer to this. The research group is already planning tests with muonic helium this year.