  We consider systems of two-component fermions with unequal masses and
interacting via a short-range attractive potential. We discuss the case where
the two-component fermions form a shallow dimer with large scattering length.
The three-fermion and four-fermion systems with such properties are universal
and charazteried by the two-fermion scattering length $a_{\text{ff}}$ and the
ratio of the mass of spin-$\uparrow$ fermion to the mass of spin-$\downarrow$
fermion, $m_{\uparrow}/m_{\downarrow}$. In this study using lattice effective
field theory we analyze fermion-dimer and dimer-dimer systems, and calculate
the universal fermion-dimer and dimer-dimer scattering lengths for various
values of the mass ratio $m_{\uparrow}/m_{\downarrow}$. We find that these
universal scattering lengths increase logarithmically with the mass ratio
$m_{\uparrow}/m_{\downarrow}$.
