Few-Body Aspects of the Near Threshold Pseudoscalar Meson Production

Abstract

During the last decade large samples of data have been collected on the production of the groundstate pseudoscalar mesons in collisions of proton or deuteron beam with hydrogen or deuterium target. These measurements have been performed in the vicinity of the kinematical threshold for meson production where only a few partial waves in both initial and final state are expected to contribute to the production process. This simplifies significantly the interpretation of the data, yet still appears to be challenging due to the three or four particle final state systems with a complex hadronic potential. We review experiments and phenomenology of the near threshold production of the ground-state mesons in the few-body final states as for example: nucleus-meson and nucleon-nucleon-meson, and report on the status of the search of the mesic-nuclei (a meson-nucleus bound states). Experimental advantages of measurements close to the kinematical threshold are discussed, and general features of the production mechanism of the η and η mesons in the nucleon-nucleon collisions are presented emphasising results of measurements of spin and isospin dependence of the production cross sections.

Unresolved Issues in the Search for Eta-mesic Nuclei

“…Conclusions about the existence of eta mesic states based on scattering length approaches for the final state should however be taken with some caution as will be discussed in the next section. Some hope of shedding light on the existence of these light nuclear states lies in the ongoing efforts made by the WASA collaboration at COSY [19,20,21,22,23].…”

Constraining the optical potential in the search for η-mesic 4He

“…They provide only signals which might be interpreted as indications of the η-mesic nuclei. The interested reader can find recent reviews on the η mesic bound states searches in Refs [4,5,14,16,39,40,41,42,43,44].…”

Search for $\eta $-mesic Nuclei with WASA-at-COSY

“…An upper limit for the formation and decay of the bound state in the process dd → ( 4 He-η) bound → 3 He pπ − at the 90% confidence level, was determined from 20 nb to 27 nb for the bound state width ranging from 5 MeV to 35 MeV, respectively [25]. During the second experiment, in November 2010, two channels of the η-mesic helium decay were registered: dd → ( 4 He-η) bound → 3 He pπ − and dd → ( 4 He-η) bound → 3 He nπ 0 → 3 He nγγ [26][27][28] in the excess energy range from −70 MeV to 30 MeV.…”