The proton charge radius extracted from the Initial State Radiation experiment at MAMI

We report on a comprehensive reinterpretation of the existing cross-section data for elastic electron-proton scattering obtained by the initial-state radiation technique, resulting in a significantly improved accuracy of the extracted proton charge radius. By refining the external energy corrections we have achieved an outstanding description of the radiative tail, essential for a detailed investigation of the proton finite-size effects on the measured cross-sections. This development, together with a novel framework for determining the radius, based on a regression analysis of the cross-sections employing a polynomial model for the form factor, led us to a new value for the charge radius, which is $(0.870 pm 0.014_mathrm{stat.}pm 0.024_mathrm{sys.} pm 0.003_mathrm{mod.}),mathrm{fm}$.