  This paper presents a measurement of forward-forward and forward-central
dijet azimuthal angular correlations and conditional yields in proton-proton
($pp$) and proton-lead ($p$+Pb) collisions as a probe of the nuclear gluon
density in regions where the fraction of the average momentum per nucleon
carried by the parton entering the hard scattering is low. In these regions,
gluon saturation can modify the rapidly increasing parton distribution function
of the gluon. The analysis utilizes 25 pb$^{-1}$ of $pp$ data and 360 $\mu
\mathrm{b}^{-1}$ of $p$+Pb data, both at $\sqrt{s_{\rm NN}}$ = 5.02 TeV,
collected in 2015 and 2016, respectively, with the ATLAS detector at the LHC.
The measurement is performed in the center-of-mass frame of the nucleon-nucleon
system in the rapidity range between $-$4.0 and 4.0 using the two highest
transverse momentum jets in each event, with the highest transverse momentum
jet restricted to the forward rapidity range. No significant broadening of
azimuthal angular correlations is observed for forward-forward or
forward-central dijets in $p$+Pb compared to $pp$ collisions. For
forward-forward jet pairs in the proton-going direction, the ratio of
conditional yields in $p$+Pb collisions to those in $pp$ collisions is
suppressed by approximately 20%, with no significant dependence on the
transverse momentum of the dijet system. No modification of conditional yields
is observed for forward-central dijets.
