We report differential cross sections for iT'p elastic scattering near 180° at incident pion momenta of 8 and 16 GeV/c. For | u\ < 0.5 (GeV/c) 2 the formula dv/du = Aexp(Bu) fits the data very well. At 8 GeV/c, A = 3.75 ±0.35 Mb/(GeV/c) 2 and B = 3.16 ±0.24 (GeV/ c)~2. Atl6GeV/c, A-0.91 ±0.13 \xb/(GeV/c) 2 andB =4.23±0.40 (GeV/cP 2 . A Reggepole model fit to these data gives for the A 6 trajectory a(u) = (-0.06±0.12) + (0.8±0.3)w.Several years ago the importance of precise measurements of backward ir~p elastic scattering at high energy was stressed as a test of the Regge model. 1 Since a doubly charged baryon must be exchanged in this model, only the Ag trajectory contributes and the characteristic Regge "shrinkage" was predicted. Experiments at that time and more recently 2 have shown a "backward peak" in high-energy (5-to 16-GeV) Tr~p elastic scattering at pion scattering angles near 180°. Recent attempts to fit these data in terms of Regge-pole 3 and quark models, 4 and to systematize the energy dependence of two-body reaction cross sections, 5 have emphasized the need for more accurate data at the highest available pion energies. We have, therefore, measured the differential cross section for backward elastic scattering, viz., lr a)~+P(2) -~£<3) +7r <4)~, at incident pion momenta of 8.0 and 16.0 GeV/c with improved precision. (The subscripts are assigned for the purpose of labeling dynamical pa-rameters hereafter.) This experiment, performed at the Brookhaven alternating-gradient synchrotron (AGS), covers a range of the fourmomentum transfer squared (u) given by -0.40^^+0.12 (GeV/c) 2 at 8.0 GeV/c and -0.73^^-0.10 (GeV/c) 2 at 16.0 GeV/c,where w = (^1-/> 3 ) 2 and^-is the four momentum of particle i. The method employed was the missing-mass technique. By measuring p x and/> 3 with high precision, it was not necessary to measure p 4 as in previous experiments. 2 Elastic events are those for which the square of the missing mass, W 2 -[ §i + %""^3fy lies in the peak at W 2 =m 7r 2 as seen in a typical missing-mass spectrum such as shown in Fig. 2(a).The apparatus is shown schematically in Fig. 1. The negative pions were produced at the F-9 target (beryllium) of the AGS near 0° to the 28-GeV/ c internal proton beam. The beam transport sys-RECOIL SPECTROMETER w~ K" p BEAM C SYSTEM 0 BENDING MAGNETS QUADRUPOLE MAGNETS S^ . 4 RECOIL ^V-^PROTON NOT TO SCALE FIG. 1. Schematic layout of experiment.A valid trigger was defined by PIT AT S^^^CS^ where P = signal from momentum hodoscope, 7r = signal from beam Cherenkov system indicating that the beam particle was a pion, A = signal from angle hodoscope, T=veto by aperture counter, and S t Sfi d CS A = signal indicating a scattered proton in the spectrometer. 1529

Targets made of C, Al, Cu, Pb, and U were exposed to .irf, .ir-, and proton beams of 9.92 and 19.85 GeV/c (for p-Pb only) at the Brookhaven AGS. A magnetic spectrometer with spark chambers was used to detect elastically scattered particles in the Coulomb-nuclear interference region (5-35 mrad). Differential cross sections are presented and compared with an optical model, taking full account of multiple scattering in the target.

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