Production and interaction of the η meson with nucleons and nuclei

. We report on the status of the search for η -mesic nuclei and the studies of the interaction of the η meson with nucleons. Recently we have completed the analysis of the new WASA-at-COSY data on the production of the η meson with polarized proton beam. New results on the analyzing power for the (cid:4) pp → pp η reaction with more than an order of magnitude improved precision shed a new light on the production mechanism of the η meson in nucleon-nucleon collisions. Also, the latest results of the search for η -mesic nuclei are discussed.

In this contribution we discuss the η production mechanism in the interaction with nucleons and the search for the η-mesic nuclei in the context of the recent experimental results from WASA-at-COSY collaboration.

η production mechanism in the interaction with nucleons
The measurements of the large total cross-section of NN → NNη reaction near the η production threshold [7][8][9][10][11][12][13][14][15][16][17][18] motivated the two-step η production model proposed in [19]. In this scenario one of the proton is firstly excited through the exchange of a single meson and forms the S 11 resonance, which in the second step deexcites via the emission of η and nucleon. In principle, the excitation to S 11 state can occur by exchanging π, η, ω or/and ρ mesons. The measurements of total crosssection isospin dependence by WASA/PROMICE and COSY-11 showed that the η production in the total isosinglet state is much higher that in the isotriplet state [18,20]. This result strongly suggest the isovector meson exchange, reducing the candidates to π and ρ particles [19,21,22]. To further distinguish between the π and ρ meson exchange models, the determination of the analysing power in the polarization measurement was required. The first measurement by COSY-11 gave a indication in favour of the pseudoscalar exchange model, although due to the limited statistics the decisive conclusions could not be done [23][24][25]. The WASA-at-COSY performed a high-statistics measurement of the pp → ppη with the polarized beam. The spin of the polarization was flipped from cycle to cycle. The data was gathered for two separated beam momenta 2026 MeV/c and 2188 MeV/c, which correspond to excess energy over the η production threshold of 15 MeV and 72 MeV, respectively. More details of the analysis can be found in [26].

Search for the η-mesic nuclei
The recent reviews on the search for mesic nuclei can be found in [3,[27][28][29][30][31][32][33]. The WASA-at-COSY collaboration [34] performed three dedicated experiments with aim to search for the η-mesic nuclei in 4 He and 3 He systems in the deuteron-deuteron and proton-deuteron collisions, respectively [35][36][37][38][39]. The choice of the light nuclei was motivated by both theoretical considerations (see e.g. [40,41]) as well as the earlier measurements by SATURNE, ANKE and COSY-11 [42][43][44][45][46], that provided strong experimental hints for the existence of the bound state in the 3 He − η and 4 He − η systems. The main experimental idea for the 4 He is based on the measurement of the excitation function of the dd → 3 HeNπ reaction for energies in the vicinity of the η production threshold and on the selection of events with low 3 He center-of-mass (CM) momenta. In the case of existence of the 4 He − η bound state we expect to observe a resonance-like structure in the excitation function below the threshold for the production of the 4 He − η system. The 3 He state is investigated in proton on deuteron collisions. The details can be found in [47].

Summary
The latest preliminary results from the 2010 experiment in 4 He−η system, do not confirm the existence of the η-mesic nuclei. The preliminary value of the upper limit obtained from the simultaneous fit, taking into account the isospin dependence of the dd → ( 4 He-η) bound → 3 Henπ 0 and dd → ( 4 Heη) bound → 3 Hepπ − excitation functions , of order of few nb can be compared to the theoretical estimate of 4 nb [48]. In case of the 3 He − η system, the analysis is ongoing. The current experimental upper limit for the production of pd → ( 3 He-η) bound → pppπ − comes from the COSY-11 measurement and is equal to about 270 nb [49]. Due to the high statistics the expected sensitivity in current WASA analysis is of order of 10 nb, which, taking into account the theoretical estimate of 80 nb [50], should be sufficient to confirm or rule out the hypothesis of existence of the 3 He − η mesic nuclei.
The WASA-at-COSY determined the analysing power in the pp → ppη reaction with two order of magnitude higher precision than the previous COSY-11 measurement. The preliminary results of the angular dependency of the analysing power is in disagreement with the prediction by both the pseudoscalar and the vector exchange models. For higher energy (Q =72 MeV), the Ps-Pp interference is clearly observed.