Measurement of long-range pseudorapidity correlations and azimuthal harmonics in $\sqrt{s_{\mathrm{N}\mathrm{N}}}$ = 5.02 TeV proton-lead collisions with the ATLAS detector

Measurements of two-particle correlation functions and the first five azimuthal harmonics, $v_1$ to $v_5$, are presented, using 28 ${\mathrm{n}\mathrm{b}}^{-1}$ of $p$+Pb collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{\mathrm{N}\mathrm{N}}}=5.02$ TeV measured with the ATLAS detector at the LHC. Significant long-range ``ridge-like’’ correlations are observed for pairs with small relative azimuthal angle ($\mathrm{\mid }\mathrm{\Delta }\varphi \mathrm{\mid }<\pi \mathrm{/}3$) and back-to-back pairs ($\mathrm{\mid }\mathrm{\Delta }\varphi \mathrm{\mid }>2\pi \mathrm{/}3$) over the transverse momentum range $0.4<p_{\mathrm{T}}<12$ GeV and in different intervals of event activity. The event activity is defined by either the number of reconstructed tracks or the total transverse energy on the Pb-fragmentation side. The azimuthal structure of such long-range correlations is Fourier decomposed to obtain the harmonics $v_n$ as a function of $p_{\mathrm{T}}$ and event activity. The extracted $v_n$ values for $n=2$ to $5$ decrease with $n$. The $v_2$ and $v_3$ values are found to be positive in the measured $p_{\mathrm{T}}$ range. The $v_1$ is also measured as a function of $p_{\rm T}$andisobservedtochangesignaround$p_{\rm T}\approx 1.5-2.0$ GeV and then increase to about 0.1 for $p_{\mathrm{T}}>4$ GeV. The $v_2(p_{\mathrm{T}})$, $v_3(p_{\mathrm{T}})$ and $v_4(p_{\mathrm{T}})$ are compared to the $v_n$ coefficients in Pb+Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=2.76$ TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average $p_{\mathrm{T}}$ of particles produced in the two collision systems.