Pseudoscalar neutral mesons in hot and dense matter
  The behavior of neutral pseudoscalar mesons $\pi^0, \eta$ and $\eta'$ in hot and dense matter is investigated, in the framework of the three flavor Nambu-Jona-Lasinio model. Three different scenarios are considered: zero density and finite temperature, zero temperature and finite density in a flavor asymmetric medium with and without strange valence quarks, and finite temperature and density. The behavior of mesons is analyzed in connection with possible signatures of restoration of symmetries. In the high density region and at zero temperature it is found that the mass of the $\eta'$ increases, the deviation from the mass of the $\eta$ being more pronounced in matter without strange valence quarks. 
Pedro Costa, M. C. Ruivo and Yu. L. Kalinovsky