Introduction

The well-known hydrogen atom is the simplest of all atoms: it consists only of a single proton which is orbited by one electron. While the negative electron is an elementary particle, and it is assumed to be point-like, the proton is made up from several quarks. Protons are therefore extended objects. They also carry a positive charge. 

Muons are like electrons, but only heavier. Muons are therefore also elementary particles, like electrons. But muons only live for 2.2 microseconds and they are 200 times heavier than electrons. This is what makes them attractive for us. 

Muonic hydrogen is an exotic hydrogen atom, where a muon (instead of an electron) orbits the proton. Because the muon is 200 times heavier than the electron, the muon's orbit is 200 times closer to the proton in muonic hydrogen than that of the electron in regular hydrogen. 

This 200 times smaller orbit means that the muon "feels" the size of the proton: certain muon orbits are significantly perturbed by the size of the positive charge distribution of the proton. By measuring the perturbation of the muon orbit using a laser, it is possible to determine the size of the proton.

Hydrogen-like atoms (i.e. atoms with one negative particle orbiting a positive "nucleus") have served as very successful probes of the basic laws of physics. It is the detailed investigation of the spectral lines emitted by hydrogen and helium atoms that led to the development of quantum physics:

• Niels Bohr created quantization rules in order to explain the existence of stable discrete energy levels which were a mystery in classical physics.
• Modern quantum mechanics, a revolutionary concept, was introduced by Heisenberg and Schrödinger in 1925. Bound states are described in a self-consistent scheme.
• The Dirac equation unifies the concepts of special relativity and quantum mechanics. It describes the main experimental features of the hydrogen spectrum, in particular the fine structure.
• A discrepancy between the predictions of the Dirac equation and the experimental data, the Lamb shift, was found in 1947. It initiated the development of relativistic quantum electrodynamics (QED) which is a cornerstone of the Standard Model of particle physics, which describes all the forces of nature except for gravity.
• Today, QED is the theory which is experimentally verified to the highest accuracy, but challenging questions about the subtle properties of bound-state systems still remain open.

Our measurement of the muonic hydrogen Lamb shift has to be conceived as a progress in the investigation of the hydrogen atom. In fact, when combined with the the measured transition frequencies in hydrogen, the proton radius inferred from the measurement of the muonic hydrogen Lamb shift will provide the most precise test of bound-state QED in the hydrogen atom to this date. Our measurement is thus likely to spur additional investigations of the fundamental theory of the electromagnetic interaction (quantum electrodynamics), a theory that links charged particles and photons (and hence light), which are some of the most important building blocks of our universe.