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We have measured relative cross sections in the angular range of 26° to 65° at 8.5 and 30 eV. Our 30-eV data show a monotonic increase with decreasing scattering angle, in disagreement with three theoretical calculations which predict a structure of the cross section in this range, but in qualitative agreement with a prediction of Joachain and Potvliege, derived from optical-potential calculations at and above 100 eV. At 8.5 eV, below all inelastic thresholds, our data agree very well with the theoretical results of McEachran and Stauffer as well as Nakanishi and Schrader but disagree with other calculations.PACS numbers: 34.90.+q Atomic-physics scattering experiments with positrons are performed in order to aid in the development of electron and positron scattering theory. Positron-atom scattering is an interesting test case for theory because of the absence of exchange with atomic electrons which complicates the theory of electron-atom scattering at low energies.Low-energy beams of monoenergetic positrons are obtained by moderation of fast positrons (from p decay or pair creation) in a metal and reacceleration of the positrons which emerge from the moderator with nearly thermal energies. Despite considerable improvements in source and moderator technology, positron experiments are still much more difficult than corresponding electron experiments because the available intensities are orders of magnitude smaller. For many years only totalscattering cross sections could be measured in gas-cell transmission experiments. Recently, however, more sophisticated partial-cross-section measurements have become feasible. 1,2 In a pioneering experiment, Coleman and McNutt 3 derived absolute differential elastic e + -Ar cross sections for forward-scattering angles from time-of-flight spectra of a gas-target transmission experiment with longitudinal magnetic guiding field. This method is restricted to energies below the first inelastic threshold because above it the time-of-flight analysis cannot unambiguously distinguish between loss of axial velocity due to wide-angle scattering and due to energy loss. More versatile but also more difficult are positron-atom crossed-beam experiments on differential scattering as have been pursued by the experimental groups at Wayne State University and Universitat Bielefeld for several years. The Wayne State group (Hyder et al.) published their first results on e+-Ar elastic scattering form 100 to 300 eV in 1986, 4 and later Kauppila and Stein reported measurements at lower energies. 5 We present here our first results, of which a part was recently reported. 6 Four theoretical groups have obtained results for low energies by means of different methods. McEachran and Stauffer employed an adiabatic polarized-orbital approximation without the need to fit to experimental data. The other three groups performed model potential calculations: The polarization potentials of Nakanishi and Schrader 8 and Datta et al. 9 contain one effective radius and an experimental value for the polarizability of...

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