Direct Comparison of using a Z-Transformation instead of the traditional $Q^2$ for for Extraction of the Proton Radius from $e-p$ Scattering Data

Abstract

A discrepancy in the determination of the proton's charge radius, $r_p$, between muonic hydrogen spectroscopy versus classic atomic spectroscopy and electron scattering data has become known as the proton radius puzzle. Extractions of $r_p$ from electron scattering data require determination of the slope of the proton's charge form factor, $G_E^p$, in the limit of $Q^2\rightarrow0$ through fitting and extrapolation. Some works have presented the $Z$-transformation fitting technique as the best choice for this type of extraction due to the true functional form of $G_E^p$ being mathematically guaranteed to exist within the parameter-space of the fit function. In this work, we test this claim by examining the mathematical bias and variances introduced by this technique as compared to the more traditional $Q^2$ fits using statistically sampled $G_E^p$ parameterizations with known input radii. Our tests conclude that the quality of the $Z$-transformation technique depends on the range of data used. In the case of new experiments, the fit function and technique should be selected in advance by generating realistic pseudodata and assessing the power of different techniques.