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Proton still less than I thought

A new experiment on laser spectroscopy of hydrogen confirmed that the radius of the proton differs from that predicted by theory. Will have to revise the theory , not yet clear.


The vacuum chamber used for measuring the frequency of radiation of hydrogen.

Hydrogen is the simplest of all chemical elements. It consists of a proton and electron. Quantum electrodynamics allows to calculate its energy levels with a precision of 12 digits.

Due to this, the hydrogen plays an important role in our understanding of nature. On studies of hydrogen-based determination of certain fundamental constants, for example, the radius of the proton and Rydberg constants used for the calculations of energy levels and frequencies of radiation of atoms.

Therefore, great attention of physicists attracted held in Switzerland, the experiment 2010, in which the value of the radius of the proton was found to be 4% less than previously measured and predicted by the theory. This result was obtained using laser spectroscopy of muonic hydrogen in which the electron in the atom is replaced by another elementary particle, the muon, which is 200 times heavier than electron.

In laser spectroscopy of atoms irradiated by the laser, they absorb the laser light and excited. Excited atoms themselves radiate, and this radiation is studied. Due to the fact that the muon is much heavier than the electron, it is located in the atom closer to the nucleus better and “feels” the size of the proton. Because of this, the radius of the proton by seven orders of magnitude stronger effect on the spectral lines of the muonic hydrogen than normal hydrogen.

This allowed to determine the radius of the proton with high precision. In the discussions of the 2010 result, some physicists have even suggested that it might be related to beyond the Standard Model, the modern theory of elementary particles. That is, the so-called “new physics”, which has long been looking for researchers around the world.

To resolve the issues was necessary new measurements with high precision and, if possible, using different experimental approaches. And now, after seven years of physics at the Institute for quantum optics in them. Max Planck (Germany) with the participation of Russian researchers from the Physical Institute named after P. N. Lebedev, Russian Academy of Sciences and the Russian quantum centre (SKOLKOVO) conducted a high-precision spectroscopic measurements for ordinary hydrogen and confirmed the 2010 results for the radius of the proton and Rydberg constant. The results were published in the journal “Science”.

The accuracy of determining the frequency of radiation up to 15 digits. This high precision is made possible by the invention of the “optical comb”, which is the head of the laser spectroscopy division of Professor Theodor Hans was awarded the Nobel prize in physics in 2005. The researchers overcame a number of other difficulties.

Compared to the previous experiments, which used atoms at room temperature in the new experiment, the atoms had a significantly lower temperature of 5.8 the Kelvin and consequently a much lower speed. This, together with the use of special methods, strongly suppressed the Doppler frequency shifts, which accounted for the largest source of noise in the measurements.

Another source of uncertainty in this experiment was the so-called quantum interference – the mutual influence of the radiation of the atoms at different frequencies. The fact that it is impossible to obtain the radiation corresponding to only a single energy transition of the electron in the atom. To describe the effect of quantum interference, the researchers conducted a complex numerical simulation, which allowed to improve the accuracy of the determination of the frequencies, clearing them from the “quantum noise”.

The reason that the radius of proton less than predicted by the theory, is still unknown, although the similarity of the results for muonic and ordinary hydrogen makes the authors doubt that this is due to the “new” physics. The investigation will continue.

According to the materials of the Institute of quantum optics to them. Max Planck

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