  The doubly charm tetraquark with flavor $cc\bar u\bar d$ and isospin
$I\!=\!0$ is investigated by calculating the $DD^*$ scattering amplitude with
lattice QCD. The simulation is done on CLS ensembles with dynamical $u/d,s$
quarks and $m_\pi\simeq 280~$MeV for two charm quark masses, one slightly
larger and one slightly lower than the physical value. The scattering
amplitudes for partial waves $l=0,1$ are extracted near-threshold via the
L\"uscher's method by considering systems with total momenta
$PL/(2\pi)=0,1,\sqrt{2},2$ on two spatial volumes. A virtual bound state pole
in the $DD^*$ scattering amplitude with $l=0$ is found $9.9_{-7.1}^{+3.6}~$MeV
below $DD^*$ threshold for the charm quark mass closer to the physical value.
This pole is likely related to the doubly charm tetraquark discovered by LHCb
less than $1~$MeV below $D^0D^{*+}$ threshold. Future lattice simulations
closer to the continuum limit and physical quark masses would be valuable to
establish this connection systematically.
