Protons Are Even Smaller Than Expected

This result might bring the fundamental theory of the interaction of the light with the matter into question or imply a change of knowledge about the well known fundamental constants.
The proton is one of the three basic modules of the matter. It builds the nucleus together with the neutron that is circled by an electron. Chemical elements are defined by their number of protons in the nucleus. Hydrogen is the simplest chemical element. Its nucleus consists of a single proton that is circled by an electron. Many fundamental physical questions were answered by determining the properties of hydrogen. For measuring the radius, the scientists created a new variation of hydrogen by replacing the electron with a muon.  Hence, instead of an electron, a negatively charged muon circles the proton in the nucleus. Muons are similar to electrons but 200times heavier. Following the rules of the quantum mechanics, muons circle the proton on an orbit that is 200times narrower. The quantities of muons therefore depend on the proton radius to a larger extent than electrons in a regular hydrogen atom, as a muon can "feel" the extent of the proton and adjust its circles. The physicists succeeded in quantifying this effect with a laser especially developed for this and a muon source developed by the PSI and in determining the proton radius very accurately.

Already during the 70s, scientists hit upon the idea of using muonic hydrogen for measuring the proton radius. But it has been a long way to put it into practice, due to many difficulties, such as a fast decay of the muonic atoms

First measurements already took place in the years 2002, 2003, and 2007 but the breakthrough was achieved on 5th July 2009, after a 3 month building phase and 3 weeks of measuring. The measured value is  0,84184 femtometres (1 femtometre = 0.000 000 000 000 001 metre) which is not only a value that is 10 times more accurate but also conflicts with the value so far recognised that is 0,8768 femtometres.
The discussions on the cause are still going on. The results might even question the physical theory of the quantum electrodynamics. However, before that happens, the calculations of the theorists need to be examined. 

For 2012 the scientists plan to examine muonic helium spectroscopically to define the radius of its nucleus. This new venture might give a hint on which interpretation is correct.

Original Publication
Randolf Pohl, Aldo Antognini, François Nez, Fernando D. Amaro, François Biraben, João M. R. Cardoso, Daniel S. Covita, Andreas Dax, Satish Dhawan, Luis M. P. Fernandes, Adolf Giesen, Thomas Graf, Theodor W. Hänsch, Paul Indelicato, Lucile Julien, Cheng-Yang Kao, Paul Knowles, José A. M. Lopes, Eric-Olivier Le Bigot, Yi-Wei Liu, Livia Ludhova, Cristina M. B. Monteiro, Françoise Mulhauser, Tobias Nebel, Paul Rabinowitz, Joaquim M. F. dos Santos, Lukas A. Schaller, Karsten Schuhmann, Catherine Schwob, David Taqqu, João F. C. A. Veloso & Franz Kottmann "The size of the proton" Nature, Doi:10.1038/nature09250, 8 July 2010



Source: Max-Planck-Institut für Quantenoptik