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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The cross section for inclusive multipion production in the pp → ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1 S0 state. At the three higher energies the missing mass MX spectra show a strong enhancement at low MX , corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with MX and beam energy are consistent with two-pion production being mediated through the excitation of two ∆(1232) isobars, coupled to S-and D-states of the initial pp system. The ABC effect is a sharp enhancement of the twopion invariant mass spectrum near its threshold that has puzzled hadron physicists for almost fifty years. First observed in a pd → 3 HeX missing-mass experiment [1], and subsequently confirmed using a deuteron beam [2], it was seen most spectacularly in the dd → 4 HeX reaction [3,4]. The ABC typically manifests itself as a peak at an invariant mass M X ≈ 310 − 330 MeV/c 2 and it was first even speculated that it might be a scalar meson with isospin I = 0. However, the fact that the mass and width varied with experimental conditions [2] suggests that the ABC is a kinematic enhancement. This is consistent with the smooth behavior of the isoscalar ππ phase shifts at low energies. To understand the nature of the ABC it is necessary to investigate the effect in simpler systems.The ABC showed up very clearly in the zero-degree momentum spectrum of the deuteron from np → dX [5].Here it leads to two peaks, i.e. forward and backward production in the c.m. system, though the momentum spread of the neutron beam degraded the missing-mass resolution. This has recently been overcome by measuring the quasi-free pd → dπ 0 π 0 p sp reaction semi-exclusively at beam energies of T p = 1.03 and 1.35 GeV, with just the spectator proton p sp escaping detection [6]. The only published study of the ABC effect in proton-proton collisions was carried out at T p = 1.52 and 1.81 GeV [7], though this concentrated on possible substructure.Studies of the energy dependence of the pd → 3 HeX and dd → 4 HeX cross sections showed that the ABC effect is most prominent for energies where the maximum missing mass is about 600 MeV/c 2 [3]. Since this corresponds to the mass difference between two ∆(1232) isobars and two nucleons, it is tempting to suppose that the two-pion production is mediated by the excitation and decay of two separate ∆ isobars. Such a model with π [8] and then π + ρ exchange [9] had semi-quantitative success in describing the existing np → dX data. The dominant contribution to the cross section arises when the two ∆ are in a relative S-wave. For the production of an I ππ = 0 pion pair, isospin conservation requires I ∆∆ = 0. It follows from the generalized Pauli principle that the two isobars must then couple to a total spin of S ∆∆ = 1 or 3. In contrast, for pp → ∆∆ → ppππ, I...