The Proton Just Got Smaller

In a discovery that could poke holes in the theory of quantum electrodynamics (QED), scientists have discovered that the proton, one of the smallest building blocks of all matter, is actually 4 percent smaller than previously thought.

Part of the laser facility needed for the experiment for the determination of the radius of the proton. Here, invisible infrared laser pulses are transformed into green laser light. (PSI/F. Reiser)

The threatened theory describes how light and matter interact and has provided many successful and highly precise predictions of atomic properties. The precision of both the theory and atomic spectroscopy has advanced to the point where accurate knowledge of the size of the proton (specifically, its charge radius) is the limiting factor for comparing experiment with theory. The currently accepted value for the proton radius, based mostly on spectroscopy of the hydrogen atom, is known to an accuracy of only 1 percent.

Randolf Pohl and an international team of researchers at Paul Scherrer Institute (PSI) in Villigen have improved this accuracy by a factor of 10, performing a technically challenging experiment that has only recently become feasible. Replacing the electron in hydrogen by its heavier counterpart, the muon, increases the effect of the proton radius on the measured atomic spectrum. The resulting, more accurate, value for the radius differs from the previously accepted value by an amount that cannot be explained. According to the researchers, the result seems either to require a change in the previously well accepted value of the Rydberg constant (which plays an important role in the hydrogen spectrum) or there is a problem with QED itself.

PSI scientists Aldo Antognini (left) and Franz Kottmann in PSI’s experimental hall. Here, the experiment for the determination of the radius of the proton is performed. (PSI/M. Fischer)

The path that the muon travels around the proton has a radius 200 times smaller than that followed by the electron in a hydrogen atom, and, consequently, the characteristics of the muon path are more closely dependent on the dimensions of the proton. A specially developed laser allowed the investigators to measure features of the muon path, enabling them to determine the radius of the proton. These experiments could be undertaken only at PSI because it is the only location in the world at which a muon beam of sufficient intensity can be generated.

“We were actually aiming to measure the recognized value of the proton radius more accurately so that QED could be checked more closely. We had no idea that we would find a discrepancy between the recognized values and our measurements,” said Franz Kottmann, a researcher who has been part of the project from the beginning.

Antognini (left) and Kottmann in the “laser hut.” Here, the laser light for the experiment for the determination of the radius of the proton is produced. (PSI/M. Fischer)

The result, however, differed significantly from the currently accepted value for the proton radius: 0.84184 fm (1 fm = 0.000 000 000 000 001 m) instead of 0.8768 fm — a difference far too large to be explained by measurement inaccuracies.

“Either the most precise theory in physics or the most accurately determined physical constant — the Rydberg constant — is wrong,” said physicist Aldo Antognini. “Others will have to establish where the error lies, but our next experiment, in which we will be using helium rather than hydrogen, should provide some important pointers to the right direction.

“It would not have been possible to do this experiment anywhere else [than the PSI accelerator facility], because this is the only muon beam with enough intensity.” A particularly intense beam is needed to produce enough muon atoms for the experiment. Even so, the team said the measurements took several weeks to carry out, both day and night.

“All the equipment had to be developed and constructed from scratch for this experiment. This is why it took more than ten years from its beginning to its end result,” Kottmann said. “The idea for the experiment was put forward at PSI 30 years ago, but we didn’t have the technical resources at that stage to actually carry it through.”

The project is a cooperative effort among establishments from various countries that contributed their expertise in fields such as accelerator physics, atomic physics, and laser and detector technology. The most significant of these are:

    •Paul Scherrer Institute, Villigen, Switzerland
    •Institute for Particle Physics, Swiss Federal Institute of Technology (ETH) Zurich
    •Max-Planck Institute of Quantum Optics, Garching, Germany
    •Laboratoire Kastler Brossel, Paris
    •Department of Physics, University of Coimbra, Portugal
    •Institute für Strahlwerkzeuge, University of Stuttgart, Germany
    •Dausinger & Giesen GmbH, Stuttgart, Germany
    •Department of Physics, University of Fribourg, Switzerland

For more information, visit: muhy.web.psi.ch/wiki