The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-${\pi }^0$s produced at forward pseudorapidities ($2.6<\eta <4.0$) in $p+p$, $p+\mathrm{Al}$, and $p+\mathrm{Au}$ collisions at a center-of-mass energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back ${\pi }^0$ pairs in $p+\mathrm{Al}$ and $p+\mathrm{Au}$ collisions compared to the $p+p$ data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in $p+\mathrm{Au}$ relative to $p+\mathrm{Al}$ collisions exhibits a dependence of the saturation scale $Q_s^2$ on the mass number $A$. A linear scaling of the suppression with $A^{1/3}$ is observed with a slope of $-0.09\pm 0.01$.

• Received 19 November 2021
• Revised 12 July 2022
• Accepted 29 July 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.092501