Neutral Pion-like Resonances at Photon Colliders
  Two photons can annihilate into a neutral pion-like resonance via the anomaly coupling, just like $\pi^0\gamma\gamma$ in QCD. In some strongly interacting electroweak symmetry breaking models, e.g., technicolor type models, there often exist neutral pion-like resonances. TeV photon colliders have a strong capability to discover such particles, because the standard model background in photon scattering goes through box diagrams and is therefore highly suppressed. In this study, we perform a signal-background comparison. We show that $e^+ e^-$ linear colliders running in $\gamma\gamma$ mode can discover such neutral-pion-like resonances with a decent sensitivity. 
Kingman Cheung and Hsin-Wu Tseng (NTHU)