Investigation of the $^3$He--$\eta $ System with Polarised Beams at ANKE

Abstract

Differential and total cross sections for the dp → 3 He η reaction have been measured with both unpolarised and tensor/vector polarised deuteron beams at the COSY-ANKE experiment near threshold. The data have been taken by using a continuous beam energy ramp up to an excess energy Q of 10 MeV with essentially 100% acceptance of the detection system. The data allowed the determination of the energy dependence of the tensor analysing power t 20 and hence of the two s-wave production amplitudes at threshold. The observed very weak energy dependence of these quantities supports the hypothesis that the rapid energy variation of the scattering amplitude for this reaction is dominantly given by a strong s-wave η 3 He final state interaction, which is discussed to be a signal for the existence of a possible quasi-bound eta-mesic state.

Few-Body Aspects of the Near Threshold Pseudoscalar Meson Production

“…It grows from zero up the the value of about 400 nb over the range of 1 MeV of excess energy and next keeps almost constant in the excess energy range of about 10 MeV. This enhancement may be assigned to the 3 He − η interaction, because it is also observed in the photo-production reactions [91][92][93], thus it is independent on the initial channel, and because the s-wave production amplitude is fairly energy independent [78,94,95]. Moreover, the asymmetry in the angular distribution of the η meson emission [82,83] indicates strong changes of the phase of the s-wave production amplitude with energy, as expected from the occurrence of the bound or virtual η 3 He state [96].…”

QCD symmetries and the η and η ′ in nuclei

“…There is presently vigorous experimental [7,15,16,17,18,19] and theoretical [1,20,21,22,23] activity aimed at understanding the η and η ′ in medium and to search for evidence of possible η and η ′ bound states in nuclei. QCD inspired models of the η and η ′ nucleus systems are constructed with different selections of "good physics input": how they treat confinement, chiral symmetry and axial U(1) dynamics.…”

QCD Symmetries in $\eta $- and $\eta '$-Mesic Nuclei

“…The binding energies and in-medium masses of the η and η ′ are sensitive to the flavour-singlet component in the mesons and hence to the non-perturbative glue associated with axial U(1) dynamics [4]. There is presently a vigorous experimental programme to search for evidence of these bound states with ongoing experiments at COSY to look for possible η bound states in Helium [5] and new experiments at ELSA [6] and GSI/FAIR [7] to look for possible η ′ bound states in Carbon.…”

“…COSY searches are focussed on possible η bound states in 3 He and 4 He [5]. Eta bound states in Helium require a large η−nucleon scattering length with real part greater than about 0.9 fm [9].…”