The theoretician Steven Bass is a guest scientist at SMI since the second half of 2013, His theoretical work focuses on η’-nucleon and nucleus interactions, proton spin structure and the vacuum energy puzzle with main focus during 2013 on the η’-nucleus system described here.

Measurements of the η - and η’- (as well as pion and kaon) nucleon and nucleus systems are sensitive to dynamical chiral and axial U(1) symmetry breaking in low energy QCD. While pions and kaons are would-be Goldstone bosons associated with chiral symmetry, the isosinglet η and η’ mesons are too massive by about 300-400 MeV for them to be pure Goldstone states. They receive extra mass from non-perturbative gluon dynamics associated with the QCD axial anomaly. OZI violation is also expected to enter the η’ nucleon interaction. How do the  gluonic degrees of freedom that contribute to the mass also contribute to the interaction of these mesons with nucleons and how does this gluonic part change in nuclei?

Several general features can be deduced from QCD about the scattering lengths and possible mass shifts of the η’ in nuclear media. η -η’ mixing increases the octet relative to singlet component in the η’, reducing the binding through increased strange quark component in the η’ wavefunction. Without the gluonic mass contribution the η’ would be a strange quark state after eta-η’ mixing, with the eta a light quark state degenerate with the pion mirroring the situation with isoscalar ω and Φ vector mesons.

To the extent that coupling to nucleons and nuclear matter is induced by light-quark  components in the meson, then any observed scattering length and mass shift is induced by the QCD axial anomaly that generates part of the η’ mass. With finite gluon anomaly contribution to the η’ mass there is no vanishing Weinberg-Tomozawa relation for the η’ nucleon scattering length in the chiral limit. General QCD arguments suggest that the gluonic mass contribution to the eta and η’ mesons decreases in the nuclear medium and the medium acts to partially neutralise axial U(1) symmetry breaking by gluonic effects.