  Model-independent considerations are presented for the calculation of
Bose-Einstein correlation functions and momentum distributions which describe
boson-emitting systems containing a central part surrounded by a large halo. If
the characteristic geometrical size of the halo is sufficiently large, the
contributions of central part and the halo to the invariant momentum
distribution are shown to be separable. The momentum-dependence of the
intercept parameter of the correlation function plays a central role. Almost
all high energy reactions including lepton-lepton, lepton hadron, hadron-hadron
and nuclear reactions are shown to be interpretable as boson emitting systems
with large halo. The results are applied to certain high energy heavy ion data
at CERN SPS. New insights are obtained for the description of low transverse
momentum enhancement of pions.
