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A reasonablytouipact apparatus employing solenoid fields, Gerenkov counters, drift chambers, and time-of-flight capabi ties is described. Good particle identification can be obtained through J combination of time-of-flight information and Cerenkov thresholds fo n, k, p up to about 3.5 GeV. I. Central Detector for Momentum Measurement and la-tification of Low Momentum Hadrons. iVe use here an ordinary solenoid type detector provic g a longi tudinal field. This will need compensating coils to get a /B-dS.=o along the beam. Here the stress is put on low momentum particles with good separation of n, K, P rather than a specially good momentum resolution. This would be needed to make an exact energy momentum balance which, in any case, would be very difficult with a total energy of 30 GeV. Instead of a plain solenoid, one can think of Helmholtz coils,with the difficulty that the field is no longer uniform which makes track recognition more difficult. On the other hand, it provides free space which can be useful for neutral detection. *•*.. The setup , which is described here, (figure 1 and 2), deals with Wl*> detection of charged particles. One can add a neutral detector outside ^5s which would provide a complete determination of the multiplicity over the solid angle. Nevertheless, the inside radius of this detectoT for neutrals would be 1.4 m which makes it rather expensive.

expect, large values of /> L are less likely with increasing multiplicity. A typical plot of a transverse momentum spectrum and its fitted curve is shown in Fig. 1(b). Figure 2 shows the systematic trends observed in both a and b as the visible multiplicity changes. In order to plot true multiplicity, one would have to increase the values along the abscissa to reflect the number of neutrons and 7r 0, s that were not included in this presentation.The recent achievement of higher luminosity 1 (counting rate per unit cross section) in the Orsay storage ring (AGO) made possible the investigation of the electron-positron annihilation e + + e~ -~ 7r + + 7r"" in the region of the p° resonance. In this Letter, we present a preliminary measurement of the cross section a^ of this reaction at the energy of 775 MeV.Electron-positron pairs from large-angle Bhabha scattering and 7r + 7r~" pairs were detected in the same experimental setup (Fig. 1). We were thus able to monitor the 7r + 7r"~ events Commission under Contract No. AT(11-1)-881, tPresent address:

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The differential cross section for the reaction y+p-^w+n was measured at 19 photon energies between 300 and 750 MeV in the laboratory frame, for pion angles between 0° and 130° in the cm. system. The pions were analyzed in angle and momentum with a magnetic spectrometer and detected by a counter telescope. The 0° measurements could be achieved, in spite of the excessive positron rate, owing to a mass-spectrometer arrangement. No direct indication for the electromagnetic excitation of the Pn resonance (1466 MeV) was found. Comparison is made with theoretical calculations of w + photoproduction.