  It is well known that the particles in a beam of Boson obeying Bose-Einstein
statistics tend to cluster (bunching effect), while the particles in a
degenerate beam of Fermion obeying Fermi-Dirac statistics expel each other
(anti-bunching effect). Here we investigate, for the first time, the
statistical correlation effect for the composite Boson, which is formed from a
spin singlet entangled electron pair. By using nonequilibrium Green's function
technique, we obtain a positive cross correlation for this kind of the
composite Boson when the external voltage is smaller than the gap energy, which
demonstrates that a spin singlet entangled electron pair looks like a composite
Boson. In the larger voltage limit, the cross correlation becomes negative due
to the contribution of the quasiparticles. At large voltages, the oscillation
between Fermionic and Bosonic behavior of cross correlation is also observed in
the strong coupling regime as one changes the position of the resonant levels.
Our result can be easily tested in a three-terminal
normal-superconductor-superconductor (N-S-S) hybrid mesoscopic system.
