  We study the initial, high-energy scatterings in heavy ion collisions using
the saturation/Color Glass Condensate framework. We focus on two-particle
long-range rapidity correlations which are modeled as two-gluon correlations.
We calculate the two-gluon production cross section using the saturation
framework in the heavy-light ion regime, including all-order saturation effects
in the heavy nucleus while considering only two-orders in the light ion. The
two-gluon production cross section generates four types of long-range in
rapidity correlations: (i) geometric correlations, (ii) Hanbury Brown and Twiss
(HBT) like correlations accompanied by a back-to-back maximum, (iii) near-side
correlations, and (iv) away-side azimuthal correlations. The geometric
correlations (i) are due to the fact that nucleons are correlated by simply
being confined within the same nucleus. Correlations (iii) and (iv) have
exactly the same amplitudes along with azimuthal and rapidity shapes: one
centered around $\Delta \phi =0$ and the other one centered around $\Delta \phi
=\pi$ (here $\Delta \phi$ is the azimuthal angle between the two produced
gluons). The geometry dependence of the correlation function leads to stronger
azimuthal near- and away-side correlations in the tip-on-tip U+U collisions
than in the side-on-side U+U collisions, an exactly opposite behavior from the
correlations generated by the elliptic flow of the quark-gluon plasma: a study
of azimuthal correlations in the U+U collisions may help to disentangle the two
sources of correlations. Finally we rewrite our result for the two-gluon
production cross-section in a $k_T$-factorized form resulting in an expression
involving a convolution of one- and two-gluon Wigner distributions over the
transverse momenta and impact parameters. This differs from the
$k_T$-factorized forms used in the literature.
