Analysis of the reaction pi +d-->pp to 500 MeV

Triangle singularity in the K−d → pΣ− reaction and its relevance to find the K̅N amplitude below threshold

We study for the first time the pΣ− → K−d and its time reversal reaction close to threshold and show that they are driven by a triangle mechanism, with the Λ(1405), a proton and a neutron as intermediate states, which develops a triangle singularity close to the K̅d threshold. As the main result, we find that the cross section, well within measurable range, is very sensitive to different models that, while reproducing K̅N observables above threshold, provide different extrapolations of the K̅N amplitudes below threshold. The observables of this reaction will provide new constraints on the theoretical models, leading to more reliable extrapolations of the K̅N amplitudes below threshold and to more accurate predictions of the Λ(1405) state of lower mass.

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.

Δ-Isobar contribution to the pion production in the reaction pp → {pp}sπ0

ANKE@COSY data on the cross section of the reaction pp → {pp}sπ0, where {pp}s is the proton pair in the 1S 0 state at small excitation energy Epp = 0 – 3 MeV, obtained at beam energies 0.5 - 2.0 GeV are analyzed within the one-pion exchange model. The model involves the subprocess π0 p → π0 p and accounts for the final state pp-interaction. A broad maximum observed in the cross section of the reaction pp → {pp}sπ0 at 0.5 - 1.4 GeV in the forward direction is explained by this model as a dominant contribution of the isospin $\cfrac{3}{2}$ in the π0 p-scattering. The second bump in data at 2 GeV is underpredicted within this model by one order of magnitude. An explicit excitation of the Δ(1232)-isobar using the box-diagram is also considered in the region of the first maximum.

Observation of resonance-like behavior of the pp → {pp}sπ0 reaction around √s = 2.65 GeV

The pp → {pp}sπ0 reaction, where {pp}s denotes a diproton, i.e. an unbound interacting proton pair in the 1S0 state, has been studied in order to obtain the forward differential cross section dσ/dΩ at 11 energy values in the region of 0.8–2.8 GeV. A resonance-like peak with the energy E0 = 2.647 ± 0.005 GeV and the width Γ = 0.26 ± 0.03 GeV has been observed in the energy dependence of the differential cross section dσ/dΩ at zero angle. The slope of the angular dependence for the energies in the peak region is different compared with the energies around it. Possible implications on this phenomenon are discussed.

Interaction of eta mesons with nuclei

Back in the mid-1980s, a new branch of investigation related to the interaction of eta mesons with nuclei came into existence. It started with the theoretical prediction of possible exotic states of eta mesons and nuclei bound by the strong interaction and later developed into an extensive experimental program to search for such unstable states as well as understand the underlying interaction via eta-meson producing reactions. The vast literature of experimental as well as theoretical works that studied various aspects of eta-producing reactions such as the π(+)n → ηp, pd → (3)Heη, p (6)Li → (7)Be η and γ (3)He → η X, to name a few, had but one objective in mind: to understand the eta-nucleon (ηN) and hence the η-nucleus interaction which could explain the production data and confirm the existence of some η-mesic nuclei. In spite of these efforts, there remain uncertainties in the knowledge of the ηN and hence the η-nucleus interaction. Therefore, this review is an attempt to bind together the findings in these works and draw some global and specific conclusions which can be useful for future explorations.The ηN scattering length (which represents the strength of the η-nucleon interaction) using different theoretical models and analyzing the data on η production in pion, photon and proton induced reactions was found to be spread out in a wide range, namely, 0.18 ≤ Re aηN ≤ 1.03 fm and 0.16 ≤ Rm aηN ≤ 0.49 fm. Theoretical searches of heavy η-mesic nuclei based on η-nucleus optical potentials and lighter ones based on Faddeev type few-body approaches predict the existence of several quasibound and resonant states. Although some hints of η-mesic states such as (3)(η)He and (25)(η)Mg do exist from previous experiments, the promise of clearer signals for the existence of η-mesic nuclei lies in the experiments to be performed at the J-PARC, MAMI and COSY facilities in the near future. This review is aimed at giving an overall status of these efforts.