A PRODUCTION VIA ?p -Aax BY LINEARLY POLARIZED PHOTONS AT 2.8 AND 4.7 GEV”

The production of A( 1236) by linearly polarized photons at 2.8 and 4.7 GeV was studied using a hydrogen bubble chamber. At 2,8 GeV the cross sections for yp -A*n(yp -A’-r’, A’--+ pn-) were found to be 3.7kO.4 pb (0.5ttO.2 ,ub); at 4.7 GeV, l.O&Oelpb (0.16&0.09~b)., Measurement of the parity asymmetry, P o-, for A* production with It I < 0.5 GeV’ yielded the values -0 0 27-40 ., 12 and -0.53-10.15 at 2.8 and 4.7 GeV respectively, whereas pure one-pion exchange (OPE) would lead to Po. = 1. The gauge invariant OPE model of Stichel and Scholz, with the inclusion of absorption corrections, accounts well for the A* differential cross sections up to It I s 0.3 GeV2 and is in agreement with the Au spin density matrix and Per for It I 2 0.1 GeV2, *Work supported in part by the U.S. Atomic Energy Commission and in part by the Nationa. Science Foundation, **Visitor from Laboratoire Interuniversitaire des Hautes Energies, Brussels, Belgium. ***On leave from Max-Plan&-Institut fur Physik und Astrophysik, Munich, Germany. ton leave from Weizmann Institute, Rehovoth, Israel, t?On leave from Brookhaven National Laboratory, Upton, New York. tt?On leave from DESY, Hamburg, Germany, (Submitted to Phys. Rev, Letters.) Previous photoproduction experiments using bubble chambers 192 and the SLAC-spectrometer3 have studied the reaction YP -+-an (1) where A is the A(1236) nucleon resonance, and have shown that for photon energies Ey,2 GeV the differential cross section do-/dt (t is the square of the fourmomentum transfer between incoming proton and outgoing A) is proportional to l/E; in common with a number of two-body photoproduction processes, 4 Such an energy dependence would be expected for processes dominated by one-pion exchange (OPE), but OPE leads to a zero cross section in the forward direction in contrast to experiment; 3 it is also not gauge invariant. These difficulties are overcome by a gauge-invariant extension of the OPE model proposed by Stichel and Scholz, 5 The angular correlations in A production by polarized photons provide a further check of the gauge invariant OPE model6 and a test of relations based on vector dominance (VDM) 0 7 We exposed the 82” hydrogen bubble chamber at SLAC to the linearly polarized Compton backscattered laser beam at 2.8 and 4.7 GeV, and studied A production in the reaction p--*+57(2) At the two energies, 2854 and 2910 events of reaction (2) were obtained. Details of the beam, exposure and our analysis procedure for reaction (2) have been published. 8,9 Results. In Fig. 1 we show the r*p mass spectra for reaction (2) D At both energies a clear -t-lA signal is found; some A0 production may also be present. The shaded distributions are for events selected with it I < 0.4 GeV2 and M+n> 1.0 GeV so as to remove most of the p” reflection and to minimize other backgrounds 0 Corrections for A* production due to contamination from wideangle electron-positron pair production and for scanning losses of events with -2short recoil protons (proton momenta < 0,14 GeV/c) 8 were found to be negligible from a Monte-Carlo simulation., The solid curves in Fig. 1 were obtained from a maximum likelihood fit to the entire Dalitz plot assuming A* , ,A’, p” production, and a phase space background. The p” mesons were assumed to have their spin aligned along their direction of motion in the overall c 0 m. s 0 9 and to have a dipion mass spectrum described by our previously determined parameterization8 which gave a good fit to the data, The Soding model, which also fits the 7r+nmass distributions well, 8 was not used because the Drell terms in this model already contain contributions to A production. In order to state a total A cross section, u(Am), we must choose a reasonable parameterization of the A production amplitude Tn. In place of the usual BreitWigner forms, e.g., those discussed by Jackson, 10 which describe the A shape well near resonance but fail far away, 11 we feel it is more meaningful to use a purely phenomenological form lo derived from the experimental phase shifts i!~~~: sill2 6 ,TAj2 x E(G3 = rh (M:M”>” + (MAl?(M))2 (3) where I’(M) follows from tan 633 = MAl?(M)/(Mi-M2) and MA = 1.236 GeV. The values of 633 have been taken from a phase shift analysis. 12 If instead the second part of Eq. (3) is used together with a conventional parameterization for r(M) as was done, e.g., by Boyarski et al.,’ -one finds a value of cr(A?r) larger by -20%. In Table 1 the total cross sections for production of A* and A0 (pndecay mode only) are given for the two energies. Figure 2 shows the differential cross sections dr/dt for A* production obtained from an independent maximum likelihood fit as described above for each t-interval. Also shown are the measurements of Boyarski et al. 3 at Ey=5.0 GeV. Measurements in the backward direction have -been made by Anderson et al, 13 -The A* angular distributions have been analyzed in terms of the A spin density matrix in the Gottfried-Jackson frame. The z axis is taken as the direction of the -3incident proton in the A rest frame; the y axis is defined as the normal to the production plane ($ oc $ x $7 O The electric vector E of the photon makes an angle @ with the production plane: COS~J=$~(~X $), sin+=?* EhO The decay angles 0 and @ are the polar and azimuthal angles of the outgoing proton in the A rest sysdecay angular distribution is then given by 14 : W(cos ~,~,@)=4~ I pi3 sin2 8 -I(l/2 -pi3)(l/3+ cos2 I9 1 -2/>3 Re pi1 cos 4 sin 26 2/$3 Re p&l cos 2$ sin’ 8 [ 1 2 1 -Pycos 26 p33 sin 6+ pll ( 1/3+cos2 8 ) -2/$3 Re pi.,. cos 4 sin 20 -2/h Re pi_1 cos 24 sin2 8