NA4-PREN: Proton Radius European Network

The proton is the primary building block of the visible Universe, but some of its most elementary properties are not well understood. For example, its “size”, i.e. the root-mean-square (rms) charge radius of the proton (Rp) has for years been believed to be about 0.88 fm, with 1% uncertainty. Two independent methods, elastic electron-proton scattering, and precision spectroscopy of atomic hydrogen yielded consistent results. In 2010, however, a measurement of Rp using the exotic “muonic hydrogen atom”, resulted in a 10 times more accurate, but a 4% (corresponding to 5 standard deviations) smaller radius: Rp = 0.84 fm. Within the Standard Model, which assumes lepton universality, both probes must yield the same radius.

This so-called “proton radius puzzle” (PRP) has sparked great activity in both experiment and theory. The 2017 NuPECC Long Range Plan (LRP) thus calls for a dedicated, cross-disciplinary program involving experimental hadron and atomic physics, supported by renewed efforts in hadron and particle theory, as well as lattice QCD (LRP, page 62).

Many ideas have been put forward to explain this striking discrepancy, such as incorrect radius extraction from scattering data, inaccurate hadronic corrections, or new physics effects such as lepton nonuniversality which would imply physics beyond the Standard Model. None of these are however widely accepted.
This motivates a reinforced world-wide effort, both experimental and theoretical. It combines atomic spectroscopy and lepton scattering, each involving both electrons and muons. The international effort on this matter concerns several leading groups in Europe. The network proposed here constitutes the critically missing forum between atomic spectroscopy and lepton scattering communities. It will provide the ideal framework to develop synergies, to draw common strategies and to enhance constructive collaborative theoretical and experimental research activities in order to converge together in attempting to solve the proton charge radius puzzle.

Besides, it will offer an environment of excellence for the training of students and young scientists in fields as diverse as atomic spectroscopy and lepton scattering, both in experiment and theory. Advances in theory are mandatory to guide, to interpret and to model experiments. This network will allow for a closer collaboration and fruitful exchanges between all theorists involved.

The framework of the proposed European network represents a unique and valuable opportunity to associate, for the first time, physicists involved in the understanding of hadron structure to shed new light on the actual proton charge radius puzzle, which has become even more intriguing given the recent results obtained on atomic hydrogen.