Shrinking the proton: Laser scan

Size matters

An international research team has confirmed the surprisingly small radius of the proton using laser spectroscopy on muonic hydrogen. There are three ways to determine the proton charge radius. The historical one is from electron-proton elastic scattering. The second, is from high-precision spectroscopy in H, that is, by measuring, using lasers, transition frequencies in H. A more precise determination of these radii can be achieved by a third method: laser spectroscopy at muonic hydrogen'.

The new results fuel the debate as to whether the discrepancies observed can be explained by standard physics, for example an incomplete understanding of the systematic errors that are inherent to all measurements, or whether they are due to new physics, or a higher complexity than previously assumed for the structure of the proton.

Proton complexity

Hydrogen comprises a single positively charged proton "orbited" by a negatively charged electron. This model has stood spectroscopists in good stead since it was first proposed in 1913 by Niels Bohr. The energy levels in a hydrogen atom can be predicted from quantum electrodynamics. The extended nature of the proton being composed by quarks and gluons, slightly affects the energy levels in hydrogen and muonic hydrogen. Thus by measuring the shift of the energy level is possible to deduce the electric and magnetic charge radii of the proton.

Research by a team at the Paul Scherrer Institut (PSI), in Villigen, Switzerland, has replaced hydrogen's electron with a negatively charged muon - with its mass 200 times that of the electron - to measure the proton charge radius in this exotic form of hydrogen. The larger mass of the muon means its atomic orbit is much closer to the proton than that of the electron, which means that the muon's energy level has a much greater effect on the proton's charge radius than does an electron. However, the value obtained by Aldo Antognini and colleagues is much smaller than the one determined in regular hydrogen or through electron-proton-scattering. The work has also used laser spectroscopy of muonic hydrogen to obtain the magnetic radius of the proton. The work was carried out in collaboration with teams at the Max Planck Institute of Quantum Optics (MPQ) in Garching near Munich, Germany, the Swiss Federal Institute of Technology ETH Zurich the University of Fribourg, the Institut für Strahlwerkzeuge (IFSW) of the Universität Stuttgart, and Dausinger & Giesen GmbH, Stuttgart, also in Germany and at the University of Coimbra and Aveiro in Portugal and the LKB in Paris, France.

Shrinking protons

New disk laser technology developed by the Institut für Strahlwerkzeuge (IFSW) of the Universität Stuttgart allows the short-lived muons - they decay within two millionths of a second - to be observed and the charge radius to be measured. In the latest experiment, the energy shift gives a value of the electric charge radius of the proton at 0.84087(39) femtometres, which was in good agreement with the figure obtained by the team in 2010, but is 1.7 times more precise. The results also allow a value of 0.87(6) femtometres for the magnetic radius to be obtained, which is in agreement with previous measurements, with the same degree of accuracy. Future improvements in the system will allow more precise measurements for the magnetic radius to be obtained.

Measurements in the first half of the twentieth century confirmed the Dirac equation and laid the foundations for modern spectroscopic studies. However, the deviation from predicted energy splitting observed in 1947 using microwave spectroscopy fuelled the development of quantum electrodynamics, the team explains, and during the following decades applied increasing pressure on experimentalists to obtain a more precise value for the proton radius. The current work does not yet solve the puzzle but does provide improved foundations on which to build future studies.