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 $Δϕ$, and pseudorapidity separation $Δη$, using charged particles detected within the pseudorapidity interval $|η|{<}2.5$. Azimuthal modulation in the long-range component of the correlation function, with $|Δη|{>}2$, 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, $\cos{(2Δϕ)}$, modulation observed in a previous measurement, the $pp$ correlation functions exhibit significant $\cos{(3Δϕ)}$ and $\cos{(4Δϕ)}$ modulation. The Fourier coefficients $v_{n,n}$ associated with the $\cos{(nΔϕ)}$ 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 $\cos{(nϕ)}$ 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, $64$ $\mathrm{nb^{-1}}$ for the $\sqrt{s}=13$ TeV $pp$ data, $170$ $\mathrm{nb^{-1}}$ for the $\sqrt{s}=5.02$ TeV $pp$ data and $28$ $\mathrm{nb^{-1}}$ for the $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV $p$+Pb data.