  The signature of bound state formation on the lattice is of particular
interest in this talk. In the finite volume, where all states have discrete
energies, it is rather hard to distinguish between a bound state and a
scattering state if the bound state were close to threshold, i.e., like a
"loosely bound state". To study bound states in the finite volume, we calculate
the positronium spectroscopy in the Higgs phase of U(1) gauge dynamics, where
the photon is massive and then massive photons give rise to the short-ranged
interparticle force. We try to identify bound state formation on the basis of
the Luscher's finite size method, which suggests specific volume dependences of
the energy gap/shift from the threshold energy for either bound states or
scattering states.
