We report on measurements of the differential cross section dσ/d and the first measurement of the analyzing power A y in the (1232) excitation energy region of the reaction pp → {pp} s π 0 where {pp} s is a 1 S 0 proton pair. The experiment has been performed with the ANKE spectrometer at COSY-Jülich. The data reveal a peak in the energy dependence of the forward {pp} s differential cross section, a minimum at zero degrees of its angular distribution, and a large analyzing power. The results present a direct manifestation of two two-baryon resonance-like states with J P = 2 − and 0 − and an invariant mass of 2.2 GeV/c 2 .

The fundamental reaction pp-->{pp}_{s}gamma, where {pp}_{s} is a proton pair with low excitation energy, has been observed with the ANKE spectrometer at COSY-Jülich for proton beam energies of T_{p}=0.353, 0.500, and 0.550 GeV. This is equivalent to photodisintegration of a free 1S0 diproton for photon energies E_{gamma} approximately T_{p}/2. The differential cross sections measured for c.m. angles 0 degrees gammad is on the 10;{-3}-10;{-2} level. The increase of the pp-->{pp}_{s}gamma cross section with T_{p} might reflect the influence of the Delta(1232) excitation.

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Abstract. Hard bremsstrahlung production in proton-proton collisions has been studied with the ANKE spectrometer at COSY-Jülich in the energy range of 353-800 MeV by detecting the final proton pair {pp} s from the pp → {pp} s γ reaction with very low excitation energy. Differential cross sections were measured at small diproton c.m. angles from 0• to 20 • and the average over this angular interval reveals a broad peak at a beam energy around 650 MeV with a FWHM ≈ 220 MeV, suggesting the influence of ∆(1232)N intermediate states. Comparison with deuteron photodisintegration shows that the cross section for diproton production is up to two orders of magnitude smaller, due largely to differences in the selection rules.PACS numbers: 25.40. Ep, 25.20.Dc

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