X(3872) in lattice QCD with exact chiral symmetry
  We investigate the mass spectrum of $ 1^{++} $ exotic mesons with quark content $ (\c\q\cbar\qbar) $, using molecular and diquark-antidiquark operators, in quenched lattice QCD with exact chiral symmetry. For the molecular operator $ \{(\qbar\gamma_i\c)(\cbar\gamma_5\q)- (\cbar\gamma_i\q)(\qbar\gamma_5\c) \} $ and the diquark-antidiquark operator $ \{(\q^T C \gi \c)(\qbar C \gamma_5 \cbar^T)-(\qbar C \gi^T \cbar^T)(\q^T C \gamma_5 \c) \} $, both detect a resonance with mass around $ 3890 \pm 30 $ MeV in the limit $ m_q \to m_u $, which is naturally identified with $ X(3872) $. Further, heavier exotic meson resonance with $ J^{PC} = 1^{++} $ is also detected, with quark content $ (\c\s\cbar\sbar) $ around $ 4100 \pm 50 $ MeV. 
TWQCD Collaboration: Ting-Wai Chiu, Tung-Han Hsieh