A team of thirty-two researchers at the Max-Planck-Institut für Quantenoptik (MPQ) in Germany have found that the proton, the primary building block of our universe, has a four percent smaller radius than previously measured (from 0.8768 to 0.8418 femtometre). The accuracy of this measurement is ten times better than that of previous studies (CODATA) (DOI:10.1038/nature09250).

Normally, the size of the proton can be measured by observing its interaction with an electron, which can only occupy discrete energy levels. The experiment is usually done with hydrogen atoms, whose nucleus consists of a single proton orbited by one electron.

In the first energy level, also referred to as the first shell, the electron can only move in one sphere-shaped orbital, known as the s-orbital. Here, there is a small probability of finding the electron exactly at the nucleus. This means that the positive charge of a hydrogen nucleus can be partially cancelled out by the negative charge of the electron. The charge cancellation decreases the average attractive force of the nucleus and, hence, the energy of the electron.

However, in the second shell, the electron can be found in both a spherical s-orbital (2S orbital) and a dumbbell p-orbital (2P orbital), in which the electron is never located in the nucleus. Therefore, charges do not cancel in the same way as in the s-orbital.

The 2S orbital and the 2p orbital have slightly different energy levels. A laser can then be used to supply a tiny amount of energy that precisely matches the energy level difference so that the electron jumps from the s- to the p-orbital. From the energy difference, the proton size can be calculated using relativistic and quantum electrodynamic corrections to the energies of the 2P and 2S orbitals.

In the recent experiment led by Randolf Pohl at MPQ, the team created muonic hydrogen atoms that have muons (instead of electrons) orbiting the nucleus. Because the muon has the same electronic charge but is about 200 times heavier than the electron, the attractive force between the nucleus and the muon is larger than that between the nucleus and the electron. This leads to a smaller atomic size and a larger interaction between the proton and the muon, making the muon more likely to be found in the s-orbital. Additionally, there is a bigger energy shift between orbitals, making the muon more sensitive to the proton's size. In the normal hydrogen, this shift is not big enough to measure.

The authors are astonished with the result: "We didn't imagine that there would be a gap between the known measures of the proton and our own," says Paul Indelicato and "it's a very serious discrepancy ... there is really something seriously wrong someplace," says Ingo Sick. "Something is missing, this is very clear," agrees Carl Carlson.

Carlson suggests a possible explanation: previously undetected particles are changing the interaction of the muon and the protons. But it could also be an error in calculations or poor quantum mechanical approximations.

If true, the results would change the Rydberg constant, which measures the "highest wavenumber of any photon that can be emitted from the hydrogen atom." Consequently, the calculations for the spectral lines of the elements would now shift. "Since the Rydberg constant is the most accurately determined fundamental constant so far, it is as solid as a rock," says Randolf Pohl, who assumes that the error has been made somewhere in the calculations. If this was not the case, the slightly shifted proton radius would trigger an earthquake in quantum mechanical physics.

1. Brumfiel, G. The proton shrinks in size: tiny change in radius has huge implications. Nature News, DOI:10.1038/news.2010.337 (7 July 2010)

2. France-Presse, A. Shrunken proton leaves scientists stunned. COSMOS magazine (8 July 2010)

3. PhysOrg News. Particle Physics: Honey, I shrunk the proton. (7 July 2010) http://www.physorg.com/news197727820.html

4. Pohl, R. et al. The size of the proton. Nature 466, 213-216, DOI:10.1038/nature09250 (8 July 2010)

Author: Molly Hjorth Jensen

Reviewed by: Mai Truong, Renee Gilberti, and Yangguang Ou

Published by: Maria Huang