  The low-energy scattering of ortho positronium (Ps) by ortho Ps has been
studied in a full quantum mechanical coupled-channel approach. In the singlet
channel (total spin $s_T=0$) we find S- and P-wave resonances at 3.35 eV (width
0.02 eV) and 5.05 eV (width 0.04 eV), respectively and a binding of 0.44 eV of
Ps$_2$. The scattering length for $s_T=0$ is 3.95 \AA and for $s_T=2$ is 0.83
\AA . The small $s_T=2$ scattering length makes the spin-polarized ortho Ps
atoms an almost noninteracting ideal gas which may undergo Bose-Einstein
condensation.
