Complete measurement of $\hat{I}›$ electromagnetic form factors

Abstract

The exclusive process $e^{+}{e}^{-}\to \hat{I}›\bar{\hat{I}}›$, with $\hat{I}›\to p\ddot{I}{€}^{-}$ and $\bar{\hat{I}}›\to \bar{p}\ddot{I}{€}^{+}$, has been studied at $\sqrt{s}=$ 2.396 GeV for measurement of the $\hat{I}›$ electric and magnetic form factors, $G_E$ and $G_M$. A data sample, corresponding to an integrated luminosity of 66.9 pb${}^{-1}$, was collected with the BESIII detector for this purpose. A multi-dimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio $R=|G_E/G_M|$ and, for the first time for any baryon, the relative phase $\hat{I}”\hat{I}¦=\hat{I}{¦}_E-\hat{I}{¦}_M$. The resulting values are obtained using the recent and most precise measured value of the asymmetry parameter $\hat{I}{\pm }_{\hat{I}}›$ = $0.750\pm 0.010$ to be $R=0.96\pm 0.14(stat.)\pm 0.02(sys.)$ and $\hat{I}”\hat{I}¦=37^{\mathrm{o}}\pm 12^{\mathrm{o}}(stat.)\pm 6^{\mathrm{o}}(sys.)$, respectively. In addition, the cross section is measured with unprecedented precision to be $\ddot{I}ƒ=118.7\pm 5.3(stat.)\pm 5.1(sys.)$ pb, which corresponds to an effective form factor of $|G|=0.123\pm 0.003(stat.)\pm 0.003(sys.)$. The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon time-like electromagnetic form factors.

7 pages, 3 figures, Submitted to Phys. Rev. Lett