Speaker
Description
ATLAS measurements of two-particle correlations in $\Delta\phi$ and $\Delta\eta$ are presented for $pp$ collisions at 2.76, 5.02 and 13~TeV, and for $p$+Pb collisions at 5.02 and 8.16~TeV. A template fitting procedure is used to subtract the dijet contribution and to extract the genuine long-range ridge correlations. In all collision systems, the ridge correlations are shown to be present even in events with a low multiplicity of produced particles, implying that the long-range correlations are not unique to rare high-multiplicity events. The measured correlations in $pp$ collisions are shown to exhibit only a weak energy dependence and are found to be remarkably similar to those observed in $p$+Pb collisions. The measurements are extended to correlations where one of the particles comes from decays of heavy-flavor particles. Significant long-range correlations are observed for particle pairs containing the heavy-flavor muon with similar features as the inclusive hadron pairs. This presentation also includes a new, detailed study of ridge properties in collisions with hard processes, characterized by large four-momentum transfer. This may help answering the question whether the ridge arises from hard or semi-hard processes, or if it is the result of mechanisms unrelated to the initial hardness scale.
