Abstract
The discrepancy between the proton charge radius extracted from the muonic hydrogen Lamb shift measurement and the best present value obtained from the elastic scattering experiments, remains unexplained and represents a burning problem of today's nuclear physics: after more than 50 years of research the radius of a basic constituent of matter is still not understood. This paper presents a summary of the best existing proton radius measurements, followed by an overview of the possible explanations for the observed inconsistency between the hydrogen and the muonic-hydrogen data. In the last part the upcoming experiments, dedicated to remeasuring the proton radius, are described.
Highlights
The discrepancy between the proton charge radius extracted from the muonic hydrogen Lamb shift measurement and the best present value obtained from the elastic scattering experiments, remains unexplained and represents a burning problem of today’s nuclear physics: after more than 50 years of research the radius of a basic constituent of matter is still not understood
Its radius has been determined by various electron scattering experiments and many atomic Lamb shift measurements. Both approaches gave consistent results. Their average does not agree with the findings of recent very precise Lamb shift measurements in muonic-hydrogen [2, 3], which report a new value for the proton charge radius, 8σ away from the previously accepted value
The spectroscopy of the hydrogen atom plays an important role in the development of the modern physics, because it can be used to precisely test the predictions of the theory of Quantum Electrodynamics (QED)
Summary
"How big is the proton?" is one of the fundamental physics questions. The proton has been studied since the early days of experimental hadronic physics [1]. Its radius has been determined by various electron scattering experiments and many atomic Lamb shift measurements (see Figure 1) Their average does not agree with the findings of recent very precise Lamb shift measurements in muonic-hydrogen [2, 3], which report a new value for the proton charge radius, 8σ away from the previously accepted value. This discrepancy, known as the proton radius puzzle, represents an important open question of today’s nuclear physics
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