Neutral top-pion and $t\gamma(z)$ production at the HERA and THERA   colliders
  In the context of top-color assisted technicolor ($TC2$) models, we calculate the contributions of the neutral top-pion $ \pi_{t}^{0}$ to $t\gamma$ and $tz$ production via the processes $ ep\to \gamma c \to t\gamma $ and $ ep\to \gamma c \to tz$ at the $HERA$ and $THERA$ colliders. Our results show that the cross sections $\sigma_{t\gamma}(s)$ and $\sigma_{tz}(s)$ are very small at the $HERA$ collider with $\sqrt{s}=320GeV $. However, in most of the parameter space, $\sigma_{t\gamma}(s)$ or $\sigma_{tz}(s)$ is in the range of about $0.1pb \sim 1 pb$ at the $THERA$ collider with $\sqrt{s}=1000GeV$. 
Chongxing Yue, Dongqi Yu, Zhengjun Zong