First observation of Σ -- e- elastic scattering in the hyperon beam experiment WA89 at CERN

We have investigated the elastic scattering of high energy Σ ^- off electrons from carbon and copper targets using the CERN hyperon beam. Scattering events are identified using a maximum likelihood technique exploring the kinematical relations of the scattered particles. The observed Q ²-distribution of these events suggests a mean square charge radius of the Σ ^- of < r^{2_ch>(Σ ^-)} = 0.91 +/- 0.32 (stat.) +/- 0.4 (syst.) fm ². This constitutes the first measurement of < r^{2_ch>(Σ ^-)} and demonstrates the possibility to also measure the charge radii of other hyperons.