  The lowest-lying glueballs are investigated in lattice QCD using $N_f=2$
clover Wilson fermion on anisotropic lattices. We simulate at two different and
relatively heavy quark masses, corresponding to physical pion mass of
$m_\pi\sim 938$ MeV and $650$ MeV. The quark mass dependence of the glueball
masses have not been investigated in the present study. Only the gluonic
operators built from Wilson loops are utilized in calculating the corresponding
correlation functions. In the tensor channel, we obtain the ground state mass
to be 2.363(39) GeV and 2.384(67) GeV at $m_\pi\sim 938$ MeV and $650$ MeV,
respectively. In the pseudoscalar channel, when using the gluonic operator
whose continuum limit has the form of $\epsilon_{ijk}TrB_iD_jB_k$, we obtain
the ground state mass to be 2.573(55) GeV and 2.585(65) GeV at the two pion
masses. These results are compatible with the corresponding results in the
quenched approximation. In contrast, if we use the topological charge density
as field operators for the pseudoscalar, the masses of the lowest state are
much lighter (around 1GeV) and compatible with the expected masses of the
flavor singlet $q\bar{q}$ meson. This indicates that the operator
$\epsilon_{ijk}TrB_iD_jB_k$ and the topological charge density couple rather
differently to the glueball states and $q\bar{q}$ mesons. The observation of
the light flavor singlet pseudoscalar meson can be viewed as the manifestation
of effects of dynamical quarks. In the scalar channel, the ground state masses
extracted from the correlation functions of gluonic operators are determined to
be around 1.4-1.5 GeV, which is close to the ground state masses from the
correlation functions of the quark bilinear operators. In all cases, the mixing
between glueballs and conventional mesons remains to be further clarified in
the future.
