ATLAS measurements of the ridge in $pp$ and $p$+Pb collisions

ATLAS measurements of correlations between particle pairs in relative azimuthal angle ($\Delta\phi$) and pseudorapidity separation ($\Delta\eta$) in $pp$ collisions at $\sqrt{s}$=5.02 and 13~TeV, and in $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=5.02~TeV are presented. Prior measurements have shown that in $pp$ collisions with a large multiplicity of produced particles, a long-range structure (the ``ridge'') develops along $\Delta\eta$ at $\Delta\phi\sim$0. However, due to the presence of the large away-side jet, the full $\Delta\phi$ dependence of the long-range correlation could not be studied. In this analysis, a template fitting procedure is implemented to determine the contributions from dijets to the correlations, using low-multiplicity events, and to extract the genuine long-range correlation. The long-range correlations are shown to be present even in events with a small multiplicity of produced particles, implying that the long-range correlations are not unique to rare high multiplicity events. The properties of the correlation in $pp$ and $p$+Pb collisions shown to be remarkably similar.