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.