  Using the CGC formalism, we calculate the two-gluon rapidity correlations of
strong colour fields in $pp$, $pA$ and $AA$ collisions, respectively. If one
trigger gluon is fixed at central rapidity, a ridge-like correlation pattern is
obtained in symmetry $pp$ and $AA$ collisions, and a huge bump-like correlation
pattern is presented in asymmetry $pA$ collisions. It is demonstrated that the
ridge-like correlation patterns are caused by the stronger correlation with the
gluon of colour source. The transverse momentum and incident energy dependence
of the ridge-like correlation pattern is also systematically studied. The ridge
is more likely observed at higher incident energy and lower transverse momentum
of trigger gluon.
