Mysteriously Shrinking Proton Continues to Puzzle Physicists

Mysteriously Shrinking Proton Continues to Puzzle Physicists

The new measurements of the proton loading radius, made by physicists from the American laboratory Jefferson, showed that the particle size can be about 5% lower than the generally accepted value: approximately 0.83 femtometers versus 0.88 femtometers.

The new result was obtained by measuring the dispersion of electrons, and this is the third measurement method that gives a smaller radius of the proton. It seems that the “mystery of the radius of the proton”, emerged eight years ago, is becoming more enigmatic.

According to the CODATA commission, which follows the results of the measurements of the fundamental constants, the proton loading radius is 0.8751 (61) femtometers. This value shows how the beam of negatively charged particles is dispersed in the proton: the larger the charge radius, the greater the proportion of dispersed particles. One of the methods for its measurement is the ultraprecision spectroscopy of electronic transitions in a hydrogen atom.

In 2010, measurement data were obtained for the load radius, which was carried out not in hydrogen, but in exotic muonic atoms, the electrons were replaced by muons. It turned out that, in this case, the value of the radius – 0.8418 femtometer – differs from a previous measurement by 4%. Thus was born the “mystery of the radius of protons”.

In October 2017, the new experiments in spectroscopy of hydrogen atoms showed a value of 0.8335 (91) femtometer, which, within the error limit, coincides with the results of the muon hydrogen. Then, the scientists declared that it was too early to talk about a solution to the enigma, since only one measurement had been obtained.

The new value obtained in the Jefferson laboratory by PRad fits into this series. Scientists have used the spectrometric method to measure the elastic dispersion of electrons by protons, at an angle of only 0.6 degrees. The preliminary results were presented at a conference of the American Physical Society and the Physical Society of Japan in Hawaii.

According to Science News, this time the scientists again obtained a value of approximately 0.83 femtómetros. Therefore, now both the data on electron scattering and the data on hydrogen spectrometry of 2017 coincided with the muon hydrogen spectrometry. Although in 2010 the dispersion data obtained in the Mainz experiment gave 0.879 (8) femtometers.

In May, a team of scientists managed to measure the pressure inside a proton, its value is 10 35  pascals. This is the same to a billion times the pressure that is at the bottom of the Marianas Trench, this is also equal to 10 times the pressure inside a neutron star, which is the densest object known in the universe. The details were published in Nature.

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