Matter effects on $\eta$ and $\eta'$ mesons
  We show how the nuclear medium affects the masses of the $\eta$ and $\eta'$ mesons. The change should be easily detectable for dense matter and/or strong $\eta(\eta')N\bar N$ coupling. We also find that the $\eta-\eta'$ mixing angle is less in magnitude in the nuclear matter than in vacuum. 
Vivek Kumar Tiwari and Anirban Kundu