Dijet azimuthal correlations and conditional yields in $p\!p$ and $p$+Pb collisions at $\sqrt{s_{_\text{NN}}}$~=~5.02 TeV with the ATLAS detector

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 for possible gluon-density saturation in regions where the momentum fraction of a parton compared to a nucleon in the lead nucleus 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~$\mu$b$^{-1}$ of $p$+Pb data, both at $\sqrt{s_{_\text{NN}}}~=~5.02~\mathrm{TeV}$, 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 center-of-mass rapidity range between -4.0 and 4.0 using the two highest transverse momentum jets in each event. The highest transverse momentum jet is restricted to the forward rapidity range where it is possible to probe the region where the momentum fraction of a parton compared to a nucleon in the lead nucleus is low. No significant broadening of azimuthal angular correlations is observed for forward--forward or forward--central dijets in $p$+Pb compared to $pp$ collisions within the uncertainties. The ratio of conditional yields of forward--forward jet pairs in the proton-going direction in $p$+Pb collisions compared to $pp$ collisions is suppressed by approximately $20\%$, with no significant dependence on the transverse momentum of the dijets system. No modification of conditional yields is observed for forward--central dijets.