Investigation of initial state effects in p+Pb collisions at ATLAS via measurement of dijet production

Proton-lead collisions at LHC energies offer unique possibilities to investigate the nuclear modifications of parton distribution functions (PDF) over a wide kinematic range. In this talk, we will present measurements of the dijet per-event yields at center-of-mass energy 8.16 TeV in p+Pb collisions. The per-event yields of unfolded dijet pairs will be presented in terms of kinematic variables that allow for full characterization of the partonic scattering system, i.e. the average $p_T$ of the dijet, $\,p_{T,Avg} = (p_{T,1} + p_{T,2})/2$, the boost of the dijet system, $y_b = (y^{CM,1} +y^{CM,2})/2$, and the half-rapidity separation between the jets, $y^{*}= |y^{CM,1} −y^{CM,2}| /2$. Initial state effects are investigated by constructing central-to-peripheral ratios of dijet yields, $R_{CP}$. We present $R_{CP}$ results comparing central (0-20%) collisions with peripheral (60-90%) ones rescaled by the nuclear thickness function $T_{AB}$, in order to equalize the two based on pure geometrical expectations. The $R_{CP}$ shows a clear dependence on the Bjorken-x of the parton extracted from the proton in the hard-scattering, $x_p$, while no clear trend is observed while displaying the results as a function $x_Pb$. These results will help in understanding the implications introduced by the initial state kinematics in p+Pb collisions.