Eta and eta' meson masses from Nf=2+1+1 twisted mass lattice QCD
  We determine mass and flavour content of eta and eta' states using Nf=2+1+1 Wilson twisted mass lattice QCD. We describe how those flavour singlet states need to be treated in this lattice formulation. Results are presented for two values of the lattice spacing, a~0.08 fm and a~0.09 fm, with a range of light quark masses corresponding to values of the pion mass from 270 to 500 MeV and fixed bare strange and charm quark mass values. 
Konstantin Ottnad, Carsten Urbach, Chris Michael and Siebren Reker
hep-lat