Recent dibaryon studies at ANKE

The recent results of dibaryon studies at ANKE are being presented, along with our plans concerning further studies on the topic. TheD03resonance has been observed at ANKE in the coherentpd→pd+(ππ)0channel, accompanied by the ABC effect. An explanation of the ABC effect has been proposed, suggesting to to explain it as a kinematic cumulation of the pion pairs in theD03→D12+π→d+ (π+π)I=0cascade decay channel due to the predominantly collinearD12decay. Based on this hypothesis, a search for aD21signal in thepp→ {pp}sππreaction is suggested. Thep-wave dibaryon resonances3P2and3P0have been observed at ANKE; for3P0it is a first experimental observation. Preliminary results for a resonance peak in thepp→ {pp}sπ0reaction with the massm= 2646 ± 5 MeV and the widthΓ= 132 ± 14 MeV are being presented. Also we propose to search for the narrowNN*(1535) resonance in thepd→pdηreaction.

Observation of an "ABC" Effect in proton-proton collisions

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 quasiparticle is in the 1S0 state. At the three higher energies, the missing-mass M_{X} spectra show a strong enhancement at low M_{X}, corresponding to an Abashian-Booth-Crowe 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 M_{X} and beam energy are consistent with two-pion production being mediated through the excitation of two Delta(1232) isobars, coupled to S and D states of the initial pp system. There is no sign of any resonancelike structure in the energy dependence of the type recently observed for the pn-->dpi;{0}pi;{0} total cross section.

Observation of inverse diproton photodisintegration at intermediate energies

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 <theta_{pp}<20 degrees exhibit a steep increase with angle that is compatible with E1 and E2 multipole contributions. The ratio of the measured cross sections to those of np-->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.

Measurement of the analyzing power in p-->d-->(p p)n with a fast forward 1S0 proton pair

A measurement of the analyzing power A(y) of the p-->d--> (p p) + n reaction was carried out at the ANKE spectrometer at COSY at beam energies of 0.5 and 0.8 GeV by detection of a fast forward proton pair of small excitation energy E(pp) < 3 MeV. The S-wave dominance in the fast diproton is experimentally demonstrated in this reaction. While at T(p) = 0.8 GeV the measured analyzing power almost vanishes, it rises to nearly unity at T(p) = 0.5 GeV for neutrons emitted at theta(c.m.)(n) = 167 degrees. The results are compared with a model taking into account one-nucleon exchange, single scattering, and Delta(1232) excitation in the intermediate state. The model describes fairly well the unpolarized cross section obtained earlier and the analyzing power at 0.8 GeV; it fails to reproduce A(y) at 0.5 GeV.