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| Home > Publications database > Measurement of the electromagnetic form factors of the proton via elastic scattering in the Q$^2$ range from 0.5 to 2 (GeV/c)$^2$ |
| Dissertation / PhD Thesis | GSI-2024-00487 |
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2024
Please use a persistent id in citations: urn:nbn:de:hebis:77-openscience-d78516b5-91aa-48aa-8158-5cce33f00e122
Abstract: The electric and magnetic form factor of the proton are connected to the distributionof electric charge and electric current inside the proton. As a fundamentalproperty of the proton, the knowledge of the form factors therefore allows to gaininsight into the structure of the proton. The form factors are accessible in elasticelectron-proton scattering experiments, which have been performed since the1950s. One distinguishes two types of experiments, the scattering with polarizedand unpolarized electrons. Around the turn of the millennium, it was found thatthere is a discrepancy in the electric to magnetic form factor ratio of the protonGE(Q2)/GM(Q2), when comparing the results of both methods. According to thedata from polarization experiments, the ratio follows a linear decreasing trend withincreasing momentum transfer Q2, whereas the ratio measured in unpolarized scatteringremains roughly constant. The commonly assumed explanation for this discrepancyis the contribution of the two photon exchange (TPE) to the scatteringprocess, which has a different influence on polarized and unpolarized scattering.To investigate the discrepancy in the form factor ratio and the influence of the TPE,new unpolarized elastic electron-proton scattering data were measured at the MainzMicrotron MAMI and the 3-spectrometer-facility of the A1 collaboration. The datawere taken at seven beam energies between 720MeV and 1508MeV to cover the Q2region from 0.5 (GeV/c)2 to 2 (GeV/c)2. The new data extends an already existingdata set from an earlier measurement in 2010. In total, the MAMI data set nowconsists of 1676 cross section points in the Q2 range between 0.004 (GeV/c)2 and2 (GeV/c)2. The measured cross sections were corrected for radiative processes byutilizing an extensive simulation, which calculates the radiative corrections for eachevent. Instead of using the classical Rosenbluth technique, the MAMI data set wasanalyzed via a direct fit of a form factor model to the cross section data to separatethe form factors. The fitting procedure was also used to determine the normalizationof the data, however, the fit proved to have stability issues for the data at higherbeam energies. The inclusion of external data from previous unpolarized experimentsprovided the stability needed for the fit and an excellent agreement betweenthe two parts of the MAMI data set could be demonstrated. Two different modelsfor the TPE correction were applied to the data before the fit, to investigate by howmuch the contribution shifts the form factors. The extracted form factor ratio fromthe fit was compared to the data from polarization experiments.The results showed that the MAMI data produce a linear decreasing form factorratio, independently of the applied TPE model. This trend has not been observedin previous unpolarized experiments. An overall lower electric to magnetic formfactor ratio in unpolarized electron-proton scattering could play an important rolein solving the discrepancy between polarized and unpolarized experiments.
Note: Dissertation, Johannes Gutenberg-Universität Mainz, 2023
Contributing Institute(s): Research Program(s): Experiment(s):
Appears in the scientific report 2023
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