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. In a pursuit of reconciling the puzzle an experiment is underway at MAMI, which exploits the radiative tail of the elastic peak to study the properties of electromagnetic processes and to extract the proton charge form factor $ \left( {\mathop G\nolimits_E^p } \right) $ at extremely small Q2. This paper reports on the latest results of the first such measurement performed at the three-spectrometer facility of the A1-Collaboration, which led to a precise validation of radiative corrections far away from elastic line and provided measurements of $ \mathop G\nolimits_E^p $ for 0.001 ≤ Q2 ≤ 0.017 (GeV/c)2.
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
In this paper we describe a new kind of nuclear scattering experiment which has a potential to provide precise formfactor data at Q2 < 0.004 GeV2/c2 and ensure a more robust extraction of the proton charge radius from the nuclear experiments
The measurements at Q2 < 0.004 GeV2/c2 are extremely difficult due to the kinematical limitations of the available apparatuses. To reach this kinematical region, the presented experiment uses the elastic data, but exploits the information about the GEp that is hidden inside the radiative tail of the elastic peak
Summary
To examine many different Feynman diagrams and the current consensus is that a missing higher order term is not responsible for the discrepancy, because they all contribute much less than the needed 300 μV [8]. This could suggest a hidden problem with a leading (vacuum polarization) term, or it could even be an indication of a fundamental problem with QED. A promising candidate for a mediator particle is a U(1) gauge boson that moderates the interaction between the dark matter and the standard model particles. In this paper we describe a new kind of nuclear scattering experiment which has a potential to provide precise formfactor data at Q2 < 0.004 GeV2/c2 and ensure a more robust extraction of the proton charge radius from the nuclear experiments
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