Charge Radius Experiment with Muonic Atoms
The protonAtomic nuclei are composed of protons and neutrons. In atoms, electrons orbit around these nuclei. These particles (protons, neutons, electrons) are the main components of matter on earth. Although the electron is considered as a sizeless particle, the proton – which is made up of three quarks – has a spatial extension.
Until recently, only two methods were used to measure its radius, both related to interaction between one proton and one electron. One is the study of electron-proton collisions at high energy, the other is the spectroscopy of atomic hydrogen (the simplest atom, composed of one proton and one electron). The value obtained by these methods, the one used by physicists, is about 0.88 fm (1 femtometer = \( 10^{-15}\) m).