Measurement of the Timelike Electromagnetic Form Factors of the Neutron at the BESIII Experiment with the Process e+e− → nn
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Abstract
The investigation of the fundamental properties of the nucleon is one of the most important topics in modern hadron physics. The precise knowledge of its internal structure in
terms of quark and gluon degrees of freedom, accessible through the electromagnetic
probe, is crucial for the understanding of the strong interaction. This structure can be
studied through the measurement of the electromagnetic form factor (FF). Starting with
the groundbreaking work of Hofstadter, many experiments successfully extended the
picture of the spatial electromagnetic distribution densities of protons and neutrons in
the spacelike region with a negative momentum transfer q2 < 0. In contrast, the experimental situation of the FF in the timelike region (q2 > 0) is less clear. In particular, the
investigation on the timelike FF of the neutron has been done only by three experiments
up to date. Moreover, limited statistics permitted to determine only the effective FF. While
in the case of the proton, at least several experiments measured the magnetic FF and the
electromagnetic FF ratio, the neutron FF remains mainly unknown.
In this thesis, the first measurement of the magnetic FF |Gn M(q2)| and the electromagnetic
FF ratio of the neutron |Gn E(q2)|/|Gn M(q2)|, determined at five center-of-mass energies from
the final state angular distribution in the reaction e^+e^− → nn, is reported. The analyzed
data set covers the center-of-mass energy range between 2.00 and 3.08 GeV with a total
integrated luminosity of 651 pb^-1 and was collected at the BESIII experiment at the BEPCII
collider in Beijing, China. The Born cross section and the effective FF are analyzed from
18 data sets, using Monte Carlo simulation with dedicated event generators for the determination of the selection efficiency. Corrections of the selection efficiency determined
from the Monte Carlo simulation are obtained with data-driven methods. Radiative QED
corrections are implemented.
The results from this work are a significant addition to the database, extending the knowledge of the electromagnetic FF of the neutron in a wide momentum transfer region and
therefore contribute to the understanding of the nucleon in the framework of strong interaction. They will serve as a new reference in every physics context where FFs are needed
as input, for example helping to distinguish between a variety of phenomenological and
theory-inspired parametrizations for the electromagnetic structure of the nucleon, or
serving as input parameters for a new generation of Monte Carlo based event generators.