Thursday, July 29, 2010


The proton, one of the primary components of matter, could be smaller than previously thought. This is the surprising result experimentally established by an international collaboration of physicists, in which the Laboratoire Kastler Brossel (ENS Paris / UPMC / CNRS) is actively involved. This new measurement of the radius of the proton, obtained with an extreme accuracy, could call into question certain predictions of quantum electrodynamics, one of the fundamental theories of quantum physics, or even the value of the Rydberg constant (the most accurate physical constant to date).

Published in Nature on 8 July, this work is featured on the journal's cover.

The nuclei of atoms are made up of protons and neutrons, around which electrons orbit. These three elements (protons, neutrons and electrons) constitute practically all of the Earth's matter. Whereas the electron is considered as a “sizeless” particle, the proton, which is composed of quarks, is an extended object. Until now, only two methods have been used to measure its radius. Based on the study of the interactions between a proton and an electron, they focus either on the collisions between an electron and a proton or on the hydrogen atom (constituted of an electron and a proton). The value obtained, and that used by physicists, is 0.877 femtometers (+/- 0.007).

In order to determine the radius of protons more accurately, the physicists used “muonic hydrogen” in which the electron is replaced by a muon, a negatively charged elementary particle. “This idea goes back to the 1970s”, explains François Nez, CNRS researcher at the Laboratoire Kastler Brossel (LKB). “However, techniques needed to improve in order to make it possible.” The hydrogen atom, which is the simplest of existing atoms, has often been the best object for studying fundamental questions in physics. But why replace the electron by a muon? Negatively charged, a muon is 200 times heavier than an electron. Therefore, according to the laws of quantum physics, it should move 200 times nearer the proton than an electron in “normal” hydrogen does. The muon is “much more sensitive” to the size of the proton than an electron. Consequently, its atomic binding energy is highly dependent on the size of the proton. The measurement of this energy allows scientists to determine the radius of the proton in a much more accurate manner (0.1 % accuracy) than measurements using electrons (around 1 % accuracy).

To achieve this, an infrared laser had to be specifically designed. The six LKB researchers, from CNRS and UPMC, mainly provided their expertise in its manufacturing, essentially with regard to the “titanium-sapphire” part of the laser chain. The objective was to design a laser in which the emission wavelength (in other words the color of the laser light) can be adjusted at will. Since a muon disintegrates in 2 millionths of a second, it is necessary to be able to carry out the measurement on the muonic hydrogen during this very short lapse of time. The laser shot therefore needs to be triggered very rapidly (in around 1 millionth of a second). A first measurement campaign at the end of 2002 allowed the experimental set up developed by LKB to be put through its paces. LKB was also responsible for measuring the emission wavelength of the complete laser system. This involved targeting the different wavelengths absorbed by the muonic hydrogen one by one, making it possible to deduce the energy of the muon around the proton and thus the size of the proton.

After several series of measurements conducted with the accelerator of the Paul Scherrer Institute (PSI) in Switzerland, where the beam of muons is particularly intense , the researchers obtained an unexpected value for the radius of the proton. In fact, this result differs from that obtained using electrons. It amounts to 0.8418 femtometers (+/- 0.0007) instead of 0.877 femtometers for measurements using electrons. “We did not envisage that there could be any divergence between known values and our measurements”, points out LKB director Paul Indelicato. This difference is far too big to be put down to measurement inaccuracies and the team of scientists is currently attempting to explain this discrepancy. It could call into question the most accurately tested theory in physics, namely the theory of quantum electrodynamics, which is one of the cornerstones of modern physics. Another possibility is that the current value of the Rydberg constant, the physical constant determined with the greatest accuracy so far, could need to be revised. The researchers plan to repeat this experiment in the near future with muonic helium (instead of hydrogen), which could shed new light on these unexpected results.


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