abstract

New experimental data are presented on the energy dependence of the total cross-sections for the \(np\to d\eta \) and quasi-two-body \(pp\to pp\eta \) reactions, where the final diproton is detected at very low excitation energy. Differential cross-sections of \(pp\to pp\eta \) away from threshold show the influence of higher partial waves and a partial wave decomposition is attempted.

abstract

The nuclear states of \(\bar {K}\) and \(\eta \) are bound by a similar mechanism — the excitations of nucleons to \({\mit \Lambda }(1405)\) and \(N^*(1535)\) resonant states. The observed large differences in binding energies are understood in terms of separation of the involved energies and the resonance positions. The other experimental findings: broad \(\bar {K}\)-mesic and narrow \(\eta \)-mesic states are more difficult to understand. A phenomenological model for \(\eta \)–\(N\) interactions is used to explain the suppression of the \(\eta \) absorption in light nuclei.

abstract

In this paper, we briefly introduce the interests and the recent research activities on the \(\eta \)-mesic nuclei. We also mention the activities on the \(\eta ' (958)\)-mesic nucleus formation.

abstract

The \(\eta \) meson can be bound to atomic nuclei. Experimental search is discussed in the form of final state interaction for the reactions \(dp\,\to \,{^3{\rm He}}\,\eta \) and \(dd\,\to \,{^4{\rm He}}\,\eta \). For the latter case tensor polarized deuterons were used in order to extract the \(s\)-wave strength. For both reactions complex scattering lengths are deduced: \(a_{{}{^3{\rm He}}\eta } = \left [{\pm \left ( {10.7 \pm 0.8_{ - 0.5}^{ + 0.1} } \right ) + i \, \left ( {1.5 \pm 2.6_{ - 0.9}^{ + 1.0} } \right )}\right ]\) fm and \(a_{^4{\rm He}\eta } = \left [ { \pm \left ( {3.1 \pm 0.5} \right ) + i\, \left ( {0 \pm 0.5} \right )} \right ]\) fm. In a two-nucleon transfer reaction under quasi-free conditions, \(p^{27}{\rm Al}\,\to \, {^3{\rm He}}X\), was investigated. The system \(X\) can be the bound \(^{25}{\rm Mg}\otimes \eta \) at rest. When a possible decay of an intermediate \(N^*(1535)\) is required, a highly significant bump shows up in the missing mass spectrum. The data give for a bound state a binding energy of \(13.3 \pm 1.6\) MeV and a width of \(\sigma =4.4\pm 1.3\) MeV.

abstract

The experimental data of the COSY-GEM Collaboration for the recoil-free transfer reaction \(p\,(^{27}\)Al,\(^{3}\)He)\(\pi ^{-} p' X\), leading to the formation of bound state of eta \((\eta \)) meson in \(^{25}\)Mg nucleus, is reanalyzed in this paper. In particular, predicted values of binding energy and half-width of the \(\eta \)-mesic nucleus \(^{25}\)Mg\(_{\eta }\), given by different theoretical approaches, are compared with the ones obtained from the experimental missing mass spectrum. It is found that the spectrum can be explained reasonably well if interference effect of another process, where \(\eta \) is not bound in \(^{25}\)Mg but is scattered by the nucleus and emerge as a pion, is taken into account. The data also indicate that the interaction between \(N^{*}\)(1535) and a nucleus is attractive in nature.

abstract

\(\eta \) bound states in nuclei are sensitive to the flavour-singlet component in the \(\eta \). The bigger the singlet component, the more attraction and the greater the binding. \(\eta \)–\(\eta '\) mixing plays an important role in understanding the value of the \(\eta \)-nucleon scattering length \(a_{\eta N}\). Working with the Quark Meson Coupling model, we find a factor of two enhancement from mixing relative to the prediction with a pure octet \(\eta \).

abstract

Photoproduction of \(\eta \) mesons off light nuclei (\(d,\,^3\)He, \(^7\)Li) has been measured at the tagged photon beam of the Mainz MAMI accelerator with the combined Crystal Ball/TAPS detection system. Special attention was given to the threshold behavior of the reactions in view of possible indications for the formation of (quasi-) bound \(\eta \)-nucleus states, so-called \(\eta \)-mesic nuclei. A very strong threshold enhancement of coherent \(\eta \) photoproduction off \(^3\)He was found and coherent \(\eta \) photoproduction off \(^7\)Li was observed for the first time. Preliminary results will be discussed.

abstract

The merits of searching for \(\eta \)-mesic nuclei by use of the (\(\pi ,\,N\)) reaction at J-PARC is discussed, by comparison with an old experiment with the same reaction at BNL in 1980’s. The importance of a pilot experiment using deuterium target is also stressed for a better understanding of the background which should exist even after the coincidence of the decay particles of \(\eta \)-mesic nuclei.

abstract

The differential and total cross-sections for the \(dp\to \,^{3}{\rm He}\,\eta \) reaction have been measured in a high precision high statistics COSY–ANKE experiment near threshold using a continuous beam energy ramp up to an excess energy \(Q\) of 11.3 MeV with essentially 100% acceptance. The kinematics allowed the mean value of \(Q\) to be determined to about 9 keV. Evidence is found for the effects of higher partial waves for \(Q\gt 4\,\)MeV. The very rapid rise of the total cross-section to its maximum value within 0.5 MeV of threshold implies a very large \(\eta \,^3\)He scattering length and hence the presence of a quasi-bound state extremely close to threshold. In addition, differential and total cross-sections have been measured at excess energies of 19.5, 39.4, and 59.4 MeV over the full angular range. While at 19.5 MeV the results can be described in terms of \(s\)- and \(p\)-wave production, by 59.4 MeV higher partial waves are required. Including the 19.5 MeV point together with the near-threshold data in a global \(s\)- and \(p\)-wave fit gives a poorer overall description of the data though the position of the pole in the \(\eta ^3\)He scattering amplitude, corresponding to the quasi-bound or virtual state, is hardly changed.

abstract

We review status and perspectives of the search for the light \(\eta \)-mesic nuclei using COSY-11, WASA-at-COSY and COSY-TOF detector systems.