First determination of the proton electric form factor at very small momentum transfer using initial state radiation
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Abstract
The proton radius puzzle came into existence because of different results of the electric radius of the proton depending whether muonic hydrogen or normal hydrogen was used. To understand this when concerning scattering experiments, new data with low momentum transfer (Q^2) are needed to determine the electric form factor. Therefore a new method based on initial state radiation was used. At the Mainz Microtron a pilot experiment was performed, using two spectrometers. To be able to compare the results of this new method with already existing results, the experiment was performed at three beam energies and the two highest could be compared with existing data. During the analysis the data were compared with a simulation. When the current parametrization of the form factor was implemented in the simulation, it matched the new data within about 1%. Therefore the usage of this new method was justified. Moreover the form factor was determined as low as Q^2=1.3·10^(−3) (GeV/c)^2 . With these results the following value of the charge radius of the proton was determined:
r_E = (0.810 ± 0.035_stat. ± 0.074_syst. ± 0.003_mod.) fm
In a succeeding experiment this new method will be used to determine the form factor with smaller uncertainties as low as Q^2≈1·10^(−4) (GeV/c)^2.