Measurements of long-range azimuthal anisotropies and associated Fourier coefficients for $pp$ collisions at $\sqrt{s}=5.02$ and $13$ TeV and $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ATLAS detector

Abstract

ATLAS measurements of two-particle correlations are presented for $\sqrt{s}=5.02$ and $13$ TeV $pp$ collisions and for $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV $p$+Pb collisions at the LHC. The correlation functions are measured as a function of relative azimuthal angle $\Delta \varphi$, and pseudorapidity separation $\Delta \eta$, using charged particles detected within the pseudorapidity interval $|\eta |<2.5$. Azimuthal modulation in the long-range component of the correlation function, with $|\Delta \eta |>2$, is studied using a template fitting procedure to remove a "back-to-back" contribution to the correlation function that primarily arises from hard-scattering processes. In addition to the elliptic, $\cos (2\Delta \varphi )$, modulation observed in a previous measurement, the $pp$ correlation functions exhibit significant $\cos (3\Delta \varphi )$ and $\cos (4\Delta \varphi )$ modulation. The Fourier coefficients $v_{n,n}$ associated with the $\cos (n\Delta \varphi )$ modulation of the correlation functions for $n=$ $2$-$4$ are measured as a function of charged-particle multiplicity and charged-particle transverse momentum. The Fourier coefficients are observed to be compatible with $\cos (n\varphi )$ modulation of per-event single-particle azimuthal angle distributions. The single-particle Fourier coefficients $v_n$ are measured as a function of charged-particle multiplicity, and charged-particle transverse momentum for $n=$ $2$-$4$. The integrated luminosities used in this analysis are, $64$ $\mathrm{n}{\mathrm{b}}^{-1}$ for the $\sqrt{s}=13$ TeV $pp$ data, $170$ $\mathrm{n}{\mathrm{b}}^{-1}$ for the $\sqrt{s}=5.02$ TeV $pp$ data and $28$ $\mathrm{n}{\mathrm{b}}^{-1}$ for the $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV $p$+Pb data.

57 pages in total, author list starting page 41, 19 figures, 4 tables, published in Physical Review C. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2016-01. This version matches the publication