The proton size

The charge radius of the proton can be determined using two different experimental techniques: measurements of electron–proton elastic scattering cross sections and high-resolution spectroscopy of the hydrogen atom.A decade ago, the precision of the atomic spectroscopy method was greatly improved using muonic hydrogen atoms, wherein the electron is replaced by a muon. However, the measured value of the proton radius was in disagreement with previous determinations, giving rise to the ‘proton-radius puzzle’.The latest results from refined scattering and spectroscopy experiments agree with the muonic value, leading to questions regarding the estimation of systematic uncertainties in previous scattering and ordinary hydrogen experiments.More measurements are needed to confirm or disprove this trend. Various projects are in progress, aiming to improve some aspects of existing techniques or using new approaches.The charge radius of the proton can be determined using two different experimental techniques: measurements of electron–proton elastic scattering cross sections and high-resolution spectroscopy of the hydrogen atom.A decade ago, the precision of the atomic spectroscopy method was greatly improved using muonic hydrogen atoms, wherein the electron is replaced by a muon. However, the measured value of the proton radius was in disagreement with previous determinations, giving rise to the ‘proton-radius puzzle’.The latest results from refined scattering and spectroscopy experiments agree with the muonic value, leading to questions regarding the estimation of systematic uncertainties in previous scattering and ordinary hydrogen experiments.More measurements are needed to confirm or disprove this trend. Various projects are in progress, aiming to improve some aspects of existing techniques or using new approaches.