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Each experiment, or set, consists of measuring the cross-section (F) as an angle (phi) is changed. Since phi is an angle measurement, its range is from 0° to 360°. The figure below is an example of F vs phi for a fixed kinematic setting.
phi
At each angle that is used, the cross-section is measured multiple times. The mean of these values is recorded in the ‘F’ column of the data file, and the standard deviation is recorded in the ‘errF’ column.
Different experiments represent different environments feature-spaces in which these measurements are performed. The environment fearture-space of an experiment is determined by the kinematic variables k, QQ, x_b, and t. Each set of values of these variables represents an environment (also called a kinematic setting) for an experiment to be run in.
The cross-section is a known function of the Compton Form Factors, the kinematic variables, phi, and a couple other values. The other values are F1, F2, and dvcs, which are all known functions of the kinematic variables, and their values are provided in each data file. The CFFs are all unknown functions of the kinematic variables: we know that they are functions of the kinematic variables, but we do not know their exact formulations. In each experiment, we are fixing the kinematic variables, varying phi, and measuring F. The values of F1, F2, and dvcs are known, so the only unknowns left are the Compton Form Factors. Our goal in this experiment is to determine the values of the CFFs using the other values. We are focusing on three Compton Form Factors: ReH, ReE, and ReH? . ReHtilde
There are two kinds of fits that are used to determine the Compton Form Factors: local fits and global fits. A local fit’s end goal is to find the numerical value of one of the CFFs in a certain kinematic setting. A global fit’s end goal is to find the equation that relates the values of the kinematic variables to the value of one of the Compton Form Factors. A local fit is a guess of the value of a CFF in a certain kinematic setting, while a global fit is a guess of the equation for a CFF that can be used to calculate the value of the CFF in any kinematic setting.
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