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Authors: Lin, Dexu
Title: Measurement of proton electromagnetic form factors using the initial-state-radiation process e+e- ppgamma at BESIII
Online publication date: 27-Sep-2017
Language: english
Abstract: The electromagnetic structure of the proton can be studied by measuring the electromagnetic form factors. In the time-like kinematic region, several experiments have been performed in the last two decades. However, limited statistics in most cases only allowed for extraction of the effective form factor of the proton. Only few experiments have been able to extract the ratio of the electric and magnetic form factors of the proton, with uncertainties larger than 11%. The results of the ratio from different experiments show strong discrepancies in the momentum-transferred region close to the proton-antiproton pair ($pbar{b}$) production threshold. In this thesis, the proton electromagnetic form factors are studied via the initial-state-radiation process $e^{+}e^{-}to pbar{p}gamma$. The total luminosity used amounts to 7.41 fb$^{-1}$, collected at seven center-of-mass energies between 3.773 and 4.600 GeV with the BESIII spectrometer at the BEPCII collider. Two scenarios can be distinguished depending on whether the radiated photon is detected (tagged case, around 12% of the events) or not (untagged case, around 40% of the events). In the analysis with the radiated photon tagged, the ratio of proton form factors is extracted for six bins of the momentum transfer from 1.877 to 3.0 GeV/c. The statistical and systematic uncertainties range from 18.5% to 33.6% and 4.5% to 15.6%, respectively. The cross section of $e^{+}e^{-}to pbar{p}$ and the effective form factor of the proton are measured in bins of 25 MeV/c width from 1.877 to 2.3 GeV/c, and in bins of 50 MeV/c width from 2.3 to 3.0 GeV/c, with statistical uncertainties at the 10% level. In the analysis with the radiated photon untagged, the cross section and the effective form factor are measured from 2.0 to 3.8 GeV/c with uncertainties at the few-percent level. The ratio is extracted for four bins from 2.0 to 3.0 GeV/c, with statistical and systematic uncertainties from 24.8% to 35.4% and 7.1% to 14.8%, respectively. The measurements presented in this work are essential for further understanding of the $e^{+}e^{-}to pbar{p}$ process and the proton electromagnetic form factors in a wide momentum-transferred region. Close to the $pbar{p}$ threshold, the ratio of proton form factors confirms the results from BABAR and CMD-3, conflicting with PS170. At higher momentum transfers, the ratio of proton form factors is consistent with existing measurements. The cross section and the effective form factor are in a good agreement with previous measurements at the few-percent precision level.
DDC: 000 Allgemeines
000 Generalities
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 08 Physik, Mathematik u. Informatik
Place: Mainz
URN: urn:nbn:de:hebis:77-diss-1000015218
Version: Original work
Publication type: Dissertation
License: In Copyright
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Extent: XX, 147 Seiten
Appears in collections:JGU-Publikationen

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