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The Sigma^- mean squared charge radius has been measured in the space-like Q^2 range 0.035-0.105 GeV^2/c^2 by elastic scattering of a Sigma^- beam off atomic electrons. The measurement was performed with the SELEX (E781) spectrometer using the Fermilab hyperon beam at a mean energy of 610 GeV/c. We obtain <r^2> = (0.61 +/- 0.12 (stat.) +/- 0.09 (syst.)) fm^2. The proton and pi^- charge radii were measured as well and are consistent with results of other experiments. Our result agrees with the recently measured strong interaction radius of the Sigma^-.
Total systematic errors are given.
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CROSS SECTION ON NEUTRON CALCULATED FROM DEUTERIUM MEASUREMENTS USING THE NUCLEON SPECTATOR MODEL.
Cross-sections for the photoproduction of positive pions in hydrogen have been measured at the 1.1 GeV Frascati electron synchrotron for photon energiesE γ between 500 and 800 MeV and for π+ c.m. angles of θ=30o, 90o. The cross-sections exhibit a smooth behavior as a function of energy forE γ=(500÷600) MeV. No immediate evidence is found of a contribution of theP 11 resonance.
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New results are presented on the differential cross-section for the reaction α+p→π0+p, at energies between 600 and 1000 MeV, and c.m. pion angles Θ*π=40° and Θ*π=60°. The present data, together with that at Θ*π=40° already published (11), show an angle-independent position of the second resonance at about 750 MeV. Rather flat angular distributions in the forward c.m. hemisphere are also favoured by these data. On comparing the cross-sections obtained when detecting both the neutral pion and the recoil proton, and when detecting only the latter, estimates of the background of «ghost protons» are obtained, in agreement with the empirical curve proposed in ref. (11).
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The final results of an experimental investigation of the reaction γ+n→p+π− performed with a deuterium bubble chamber at the 1 GeV Frascati electrosynchrotron are presented. Total and differential cross-sections on neutrons are extracted by means of the spectator model, the reliability of which has been checked by numerous tests and is extensively discussed. The problems of a possible isotensor component in the electromagnetic current, the time-reversal invariance of the electromagnetic interactions and the photoproduction of the Roper resonance are considered in detail.
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THIS HADRON PAIR CROSS SECTION PROVIDES ONLY AN UPPER LIMIT TO THE PION FORM FACTOR ABOVE 1.5 GEV SINCE KAON PRODUCTION IS NOT DISTINGUISHED.
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