Measurement of the Σ− Charge Radius at Selex

The charge radii of Σ and π have been determined by direct elastic scattering on shell electrons. The measurement was performed in the framework of the Selex (E781) charm hadroproduction experiment at Fermilab which employs a 600 GeV/c high-intensity Σ/π beam and a three-stage magnetic spectrometer covering 0.1 ≤ xF ≤ 1.0. Scattering angles and momenta of both hadron and electron were measured with high precision using silicon microstrip detectors, thus allowing for a segmented solid target. Two TRDs provided full particle identification. A preliminary result for the Σ− charge radius for a four-momentum transfer squared of 0.03 GeV/c ≤ Q ≤ 0.16 GeV/c will be reported. In a parallel analysis the π charge radius has been determined for 0.03 GeV/c ≤ Q ≤ 0.2 GeV/c, and is found to be consistent with previous experiments.