  The STAR Collaboration reports measurements of back-to-back azimuthal
correlations of di-$\pi^0$ produced at forward pseudorapidity ($2.6<\eta<4.0$)
in $p+$$p$, $p+$$\mathrm{Al}$ and $p+$$\mathrm{Au}$ collisions at a
center-of-mass energy per nucleon-nucleon pair of 200 GeV. A clear suppression
of the correlated away-side yields is observed in $p+$$\mathrm{Au}$ and for the
first time in $p+$$\mathrm{Al}$ collisions, compared with the $p$$+$$p$ data.
The enhanced suppression found in $p+$$\mathrm{Au}$ with respect to
$p+$$\mathrm{Al}$ collisions exhibits the saturation scale ($Q_s^2$) dependence
on the mass number $A$. The observed suppression of back-to-back pairs as a
function of event activity and transverse momentum points to non-linear gluon
dynamics arising at high parton densities.
