Measurement of long-range azimuthal anisotropies in pp and p+Pb collisions with the ATLAS detector

ATLAS measurements of correlations between particle pairs in relative azimuthal angle ($\Delta\phi$) and pseudorapidity separation ($\Delta\eta$), in $pp$ collisions at $\sqrt{s}$=2.76, 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,commonly called 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 previously. In this analysis, a template fitting procedure is implemented to determine the contributions from dijets to the correlations, using low-multiplicity events, and extract the genuine long-range correlation. The Fourier harmonics of the genuine long-range correlation for orders $n$=2--4 are extracted, and their dependence on $p_{\mathrm{T}}$, event-multiplicity and collision energy are studied. It is shown that the second Fourier coefficient, $v_{2,2}$, dominates the long range correlation, with small but significant contributions from $v_{3,3}$ and $v_{4,4}$. The $v_{2,2}$ is shown to factorize into the product of single-particle anisotropies $v_{2}$. A large $v_{2}$ is 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, but are present even in low multiplicity pp collisions. Comparisons to the $v_{n,n}$ and $v_{n}$ obtained when applying the template fitting procedure to $p$+Pb collisions are also presented. These measurements can help determine if the long range correlations in $pp$ and $p$+Pb collisions arise from similar mechanisms or not.