Measurement of long-range pseudorapidity correlations and azimuthal harmonics in $\sqrt{s_{\mathrm{NN}}}$=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 $\mathrm{nb}^{-1}$ of $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV measured in the ATLAS detector at the LHC. Significant long-range ``ridge-like'' correlations are observed for pairs with small relative azimuthal angle ($|\Delta\phi|<\pi/3$) and back-to-back pairs ($|\Delta\phi|>2\pi/3$) over the transverse momentum range $p_{\rm T}<12$ 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 correlation is Fourier decomposed to obtain the harmonics $v_n$ as a function of $p_{\rm T}$ and event activity. The extracted $v_n$ values for $n=2$ to 5 are found to decrease with $n$. The $v_2$ and $v_3$ values are found to be positive in the measured $p_{\rm T}$ range. The $v_1$ is also measured as a function of $p_{\rm T}$ and is observed to change sign around $p_{\rm T}\approx 1.5$-2.0 GeV and then increase to about 0.1 for $p_{\rm T}>4$ GeV. The $v_2(p_{\rm T})$, $v_3(p_{\rm T})$ and $v_4(p_{\rm T})$ are compared to the $v_n$ coefficients in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} =2.76$ TeV with similar event multiplicity. Reasonable agreement is observed after accounting for the difference in their average $p_{\rm T}$.