The axialvector diquark is studied by using 2+1 flavor
Lattice QCD.

Being a two-quark object, diquark has a non-neutral color charge.

Hence the two-point correlators of diquark fields do not have a
particle pole due to the color confinement of QCD,

and it is not straightforward to study the diquark mass by lattice
QCD by using an exponential fit of a temporal two-point correlator.

In order to avoid this difficulty, our strategy is to regard the
diquark mass as a mass parameter of an effective quark-diquark model
which is constructed by using an extended HAL QCD method based on
equal-time quark-diquark Nambu-Bethe-Salpeter (NBS) wave functions.

We attempt to calculate the axial-vector diquark mass and the
quark-diquark potentials between a charm quark and an axial-vector
diquark in the $\Sigma_c$ baryon.

Lattice QCD Monte Carlo calculation is performed by using the 2+1
flavor QCD gauge configurations generated on $32^3\times 64$ lattice
by PACS-CS Collaboration which corresponds to the pion mass of about
700 MeV.

As a result, a quark-diquark central potential of Cornell-type and a
short-ranged spin-dependent potential are obtained.

However, from a quantitative point of view, the gound state
convergence of the NBS wave functions are not sufficient so that we
obtain a larger string tension and a smaller axial-vector diquark
mass than we have expected phenomenologically.