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

**Authors:**M. Mihovilovič, A. B. Weber, P. Achenbach, T. Beranek, J. Beričič, J. C. Bernauer, R. Böhm, D. Bosnar, M. Cardinali, L. Correa, L. Debenjak, A. Denig, M. O. Distler, A. Esser, M. I. Ferretti Bondy, H. Fonvieille, J. M. Friedrich, I. Friščić, K. Griffioen, M. Hoek, S. Kegel, Y. Kohl, H. Merkel, D. G. Middleton, U. Müller, L. Nungesser, J. Pochodzalla, M. Rohrbeck, S. Sánchez Majos, B. S. Schlimme, M. Schoth, F. Schulz, C. Sfienti, S. Širca, S. Štajner, M. Thiel, A. Tyukin, M. Vanderhaeghen, M. Weinriefer

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 $0.001\leq Q^2\leq 0.004 (GeV/c)^2$.

**Comments:**6 pages, 3 figures

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**Affiliations:**

^{1}CERN,

^{2}LBNL

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