Radiative transitions in charmonium from $N_f=2$ twisted mass lattice   QCD
  We present a study for charmonium radiative transitions: $J/\psi\rightarrow\eta_c\gamma$, $\chi_{c0}\rightarrow J/\Psi\gamma$ and $h_c\rightarrow\eta_c\gamma$ using $N_f=2$ twisted mass lattice QCD gauge configurations. The single-quark vector form factors for $\eta_c$ and $\chi_{c0}$ are also determined. The simulation is performed at a lattice spacing of $a= 0.06666$ fm and the lattice size is $32^3\times 64$. After extrapolation of lattice data at nonzero $Q^2$ to 0, we compare our results with previous quenched lattice results and the available experimental values. 
Ying Chen, De-Chuan Du, Bao-Zhong Guo, Ning Li, Chuan Liu, Hang Liu,   Yu-Bin Liu, Jian-Ping Ma, Xiang-Fei Meng, Zhi-Yuan Ni...
hep-lat hep-ph