Centrality and rapidity dependence of inclusive jet production in $\sqrt{s_\mathrm{NN}} = 5.02$ TeV proton--lead collisions with the ATLAS detector

The ATLAS Collaboration has measured the centrality and rapidity dependence of inclusive jet production in $\sqrt{s_\mathrm{NN}} = 5.02$ TeV proton-lead ($p$+Pb) collisions and the jet cross-section in $\sqrt{s} = 2.76$ TeV proton-proton collisions in datasets corresponding to an integrated luminosity of $27.8$ nb$^{-1}$ and $4.0$ pb$^{-1}$, respectively, recorded at the Large Hadron Collider in 2013. The $p$+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval $3.2 <\eta <4.9$ in the direction of the lead beam. Jets were reconstructed using the anti-$k_t$ algorithm with $R=0.4$ from energy deposits measured in the ATLAS calorimeter. Results are presented for the double differential per-collision yields as a function of jet rapidity and transverse momentum for both minimum bias and centrality-selected $p$+Pb collisions. The central-to-peripheral ratio $R_\mathrm{CP}$ and the nuclear modification factor $R_\mathrm{pPb}$ are evaluated to probe deviations in the jet rate from the geometric expectation. The total yield of jets in minimum bias $p$+Pb collisions is consistent with a small $p_\mathrm{T}$-dependent excess above the expectation. The ratios of jet spectra between different centrality selections, corrected for the corresponding partonic luminosities, show a centrality-dependent modification of jet production at all $p_\mathrm{T}$ at forward rapidities and at large $p_\mathrm{T}$ at mid-rapidity. When compared to the expectation from independent nucleon-nucleon collisions, this modification appears as a suppression of jets in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton-nucleus collisions. Furthermore, the violations may have a simple dependence on the hard parton-parton kinematics.