Timelike electromagnetic form factors of hyperons at large $q^2$

Abstract

In the last few years there has been considerable progress in the study of the electromagnetic form factors of baryons in the timelike region, through electron-positron scattering, with increasing squared transfer momentum $q^2$. The modulus of the electric ($G_E$) and magnetic ($G_M$) form factors has been measured for nucleons, hyperons and other baryons at BaBar, CLEO, Belle and BESIII. The novel measurements motivated the extension of a covariant quark model, developed to the spacelike region ($q^2 \le 0$), to the timelike region, without any further parameter fitting. The extension is based on asymptotic relations derived from analyticity and unitarity, valid for the large-$q^2$ region. We use the model to make predictions for the effective form factor $|G|$ (combination of $G_E$ and $G_M$) and the ratio $|G_E/G_M|$ for spin 1/2 hyperons at large $q^2$ (above 10 GeV$^2$). Our calculations are in good agreement with the data from CLEO and BESIII for $\Lambda$, $\Sigma^+$ and $\Xi^-$ above $q^2=15$ GeV$^2$. Upcoming data for $\Sigma^0$, $\Sigma^-$ and $\Xi^-$ at large $q^2$ may be used to further test our predictions. We also compare our model calculations with the scarce available data for $|G_E/G_M|$. We conclude that the present $q^2$ range is not large enough to test our calculations, but that a more definitive test can be made by experiments above $q^2=20$ GeV$^2$.