Recent studies in nuclear physics and astrophysics are pointing to the important
role of three-baryon forces. They, however, suffer from huge uncertainties
to this date, and the direct determination from lattice QCD is highly desirable.
In my talk, after giving a brief review for the recent progress in lattice calculation
for two-nucleon forces, I present the lattice QCD study for three-nucleon forces.
We utilize the Nambu-Bethe-Salpeter (NBS) wave function for both of two- and
three-nucleon forces on the same footing. The calculation is quite challenging,
since the computational cost becomes enormous because of the enlarged degrees
of freedom. Identification of genuine three-nucleon forces is also a nontrivial matter.
A new procedure to overcome these problems is discussed in detail. The lattice
calculations are performed with Nf=2 clover fermions with a large quark mass
corresponding to m(pion)= 1.13GeV. The results for the three-nucleon force in the
triton channel are presented. Important physical implications and future extensions of
our methodology are also discussed.
