First measurement of proton's charge form factor at very low $Q^2$ with initial state radiation

TL;DR

This paper introduces a novel experimental method using initial-state radiation in electron-proton scattering to measure the proton's charge form factor at very low momentum transfer, providing new data in an unexplored $Q^2$ range.

Contribution

The study presents the first measurement of the proton charge form factor at extremely low $Q^2$ using an innovative ISR technique validated at MAMI.

Findings

First measurement of $G_E^p$ at $Q^2$ between 0.001 and 0.004 (GeV/c)^2

Validation of the ISR method with dedicated experiment at MAMI

New data extends understanding of proton structure at low momentum transfer

Abstract

We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor ($G_E^p$) at extremely small $Q^2$. The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of $G_E^p$ for $0.001\leq Q^2\leq 0.004 (GeV/c)^2$.