Inclusive jet production in ultrarelativistic proton-nucleus collisions

Abstract

High-$p_\mathrm{T}$ processes in proton- and deuteron-nucleus collisions at TeV energies are the best presently available way to study the partonic structure of the nucleus in a high-density regime. Jet production over a wide range of phase space can significantly constrain the current knowledge of nuclear parton distribution functions (nPDFs), which are substantially less well understood than the corresponding PDFs in protons and which have only recently begun to be treated in a spatially-dependent way. An accurate knowledge of nPDFs is crucial for a definitive control of perturbative processes in a cold nuclear environment, since high-$p_\mathrm{T}$ probes are used to quantitatively investigate the hot QCD matter created in ultrarelativistic nucleus-nucleus collisions. Furthermore, jets from low Bjorken-$x$ partons can probe the transition from the dilute to saturated nuclear regimes. Jet production is investigated in $d$+Au collisions at $\sqrt{s} = 200$ GeV with the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC), and in $p$+Pb collisions at $\sqrt{s} = 5.02$ TeV with the ATLAS detector at the Large Hadron Collider (LHC). The measurements shown here utilize $\int\mathcal{L}dt = 23$ nb$^{-1}$ and $0.2$ pb$^{-1}$ of 200 GeV $d$+Au and $pp$ data, respectively, recorded in 2007-8 at RHIC and $\int\mathcal{L}dt = 31$ nb$^{-1}$ and $4.1$ pb$^{-1}$ of $5.02$ TeV $p$+Pb and $2.76$ TeV $pp$ data, respectively, recorded in 2013 at the LHC. Jets are reconstructed using the $\sigma=0.3$ Gaussian filter and $R=0.4, 0.6$ anti-$k_\mathrm{T}$ algorithms. Inclusive, centrality-dependent jet yields within $\left|\eta\right| < 0.35$ and $10 \mathrm{GeV} < p_\mathrm{T} < 40$ GeV in 200 GeV $d$+Au and $pp$ collisions are presented. The jet yield in $d$+Au collisions relative to the geometric expectation is found to be slightly suppressed ($\approx0.9$) in central events and moderately enhanced ($\approx1.3$) in peripheral events, with no modification when averaged over all $d$+Au events. Separately, inclusive, centrality-dependent jet yields within $\left|y^*\right| < 4.4$ and $25 \mathrm{GeV} < p_\mathrm{T} < 800$ GeV in 5.02 TeV $p$+Pb and 2.76 TeV $pp$ collisions are presented. The event centrality in $p$+Pb collisions is determined by the sum of the transverse energy in the Pb-going forward calorimeter, $\Sigma{E}_\mathrm{T}^\mathrm{Pb}$, and the mean number of participating nucleons $N_\mathrm{part}$ is estimated using the Glauber and Glauber-Gribov models of semiclassical $p$+A collisions. The jet yield in $p$+Pb collisions relative to the geometric expectation is found to be suppressed in central events and enhanced in peripheral events. The modifications are found to be stronger at higher-$p_\mathrm{T}$ and at more forward (downstream proton) rapidities. Furthermore, it is observed that for each centrality selection, the modification at all rapidities is consistent with a simple function of the total jet energy only. The implications of the results are discussed, including a comparison of the modifications between RHIC and LHC energies.