The ηN and ηNN states

Signature of the ηNN configurations in coherent πphotoproduction on the deuteron

“…The isospin dependence of the S 11 (1535) photo excitation amplitude was taken according to the relation [12] σ(γp → ηp) σ(γn → ηn) ≈ 0.67 . The parameters, appearing in the expressions (1) through (5), are listed in Table I. They are chosen in such a way that, on the one hand, the reactions γN → αN and π − p → ηn are well reproduced in the S 11 channel as presented in our previous work [13,14].…”

“…It is worth noting that in some work the ηN N interaction is predicted to generate even a bound state in the quasideuteron configuration (J π ; T ) = (1 − ; 0). Results provided by more sophisticated models [5,6] show, however, that the fundamental ηN interaction is likely to be too weak for yielding binding of the ηN N system, so that only a virtual (antibound) state can be generated. In the context of the present discussion two points are relevant: (a) Although the pole 'recedes' to the nonphysical region, it remains quite close to the zero energy.…”

“…Parameters of the ηN scattering matrix in Eqs. (1) through(5). The values of Λα, γππ, and M0 are in MeV.…”

Close-to-threshold meson production in hadronic interactions

“…Small relative momenta in the final state effectively limit the number of partial waves contributing, simplifying the theoretical interpretation of experimental results. It should be noted, that already three-body final states require -in principle -a three-body Faddeev like approach, which has not been accomplished so far [16,17]. However, as first described by Watson [18], with two strongly interacting particles in the final state, the energy dependence of the total cross section close-to-threshold is essentially determined by the (three-body) phase-space and the energy dependence of the on-shell final state interaction (FSI).…”

“…gives the S s partial cross section of equation (16). On the basis of the formula (22) the kinematically available phase-space volume (V ps ) can be as easily calculated as the excess energy Q. Close-to-threshold -in the range of a few tens of MeV -the non-relativistic approximation differs only by a few per cent from the full solution given in equation (21), which in fact with an up-to-date computer can be solved numerically with little effort.…”