$\eta'$-$\eta_c$-mixing with improved stochastic estimators

Charmonia are flavour singlet mesons and thus in principle contributions from disconnected quark line diagrams might affect their masses, either directly or via mixing with other flavour singlet channels. We present a first study that takes both effects into account. We employ improved stochastic all-to-all propagator techniques (including new methods) to calculate the diagrams that appear within the mixing matrix between the $\eta'$ and the $\eta_c$. The runs are initially performed on $N_f=2$ $16^3\times 32$ configurations with the non-perturbatively improved Sheikholeslami-Wilson action, both for valence and sea quarks.