New measurement confirms discrepancy to the default value of the proton radius

Deviation confirmed: One of the basic building blocks of matter is smaller than thought so far - the proton. New hydrogen measurements have shown that the proton radius must be four to five percent smaller than the official standard value. This confirms similar discrepancies in earlier measurements and suggests that some physical constants must be corrected, as the researchers report in the magazine "Science".

The Proton is obviously smaller than official standard values have indicated so far. © NPO / thinkstock

The proton is one of the fundamental building blocks of matter, because together with the neutron it forms the atomic nucleus. At the same time, important natural constants are based on the proton, including the Rydberg constant. With it, for example, spectral lines can be assigned to certain elements. It is correspondingly important to know exactly the mass and diameter of the proton.

Puzzling discrepancies

But this is precisely the problem: in the last few years, several measurements aroused doubts about the official values ​​for the proton. In 2010, physicists determined a proton radius by means of the laser spectroscopy of a proton-muon compound, which was about four percent below the standard value of 0.88768 femtometers.

Because the muon is 200 times heavier than the electron, the proton gets much closer and literally senses its extent, explains project manager Randolf Pohl from the University of Mainz. This results in the high precision with which we could determine the proton radius.

In 2016, a measurement with deuterium nuclei plus muon also showed smaller values ​​for the proton radius. Finally, at the beginning of 2017, researchers also identified mass deviations from the standard value.

Testing of the hydrogen

But how reliable are these measurements? Does the official default value need to be changed? In order to answer this question, a decisive measurement has so far been made: the determination of the proton radius at the "normal" hydrogen atom - without replacement of the electron by the heavy muon. Axel Beyer of the Max Planck Institute for Quantum Optics (MPQ) in Garching and his colleagues have now succeeded in this measurement.

For their experiments, the researchers analyzed the spectral lines generated by two different energy transitions of the hydrogen atom - this doubling is necessary to make the values ​​reliable. The first measurement was already carried out in 2011 with the so-called 1S-2S transition measurement. In this case the electron, after excitation, falls back into the ground state, producing a particularly sharp, thin spectral line.

Smaller radius confirmed

In the current experiment, Beyer and his colleagues analyzed the 2S-4P transition, a change between two excited states. For this purpose they irradiated the hydrogen atoms cooled down to a few degrees above the absolute zero point by a laser with a wavelength of 468 nanometers. If the electrons excited to 4P level, and then fall back to the lower 2S state, they emit photons whose energy content allows conclusions on the proton size.

The result: Both spectral measurements together confirm that the proton could be smaller than previously assumed. Even for ordinary hydrogen, the proton radius results in values ​​that are up to five percent below the standard value: Beyer and his colleagues are currently at 0.8335 femtometers instead of the previously valid 0.88768 femtometers.

This corresponds to a difference of 3.3 standard deviations to the hydrogen world data, explains project manager Thomas Udem of the MPQ.

Need to change default values?

In addition, our measurement is almost as accurate as all other previous experiments on regular hydrogen, Udem says.

He and his colleagues have also analyzed their data over a period of three years to rule out measurement errors and falsifications. The evidence for a smaller proton thus increases considerably. The physicists stress, however, that further measurements - also with other methods - must follow to be sure.

However, the Committee of Data for Science and Technology (CODATA) has already pricked up their ears. It is the official board that sets the default values ​​for physical constants.

We will take this result very seriously, says Krzysztof Pachucki, member of CODATA.

He considers it quite probable that the official value for the proton radius and the Rydberg constant will be changed if the next revision is pending - this could already be the case in 2018.

Source: Science, 2017; doi: 10.1126 / science.aah6677