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

Measurement of two-particle correlation functions and the first five azimuthal harmonics, $v_{1} - v_{5}$, are presented, using 31 $nb^{−1}$ of p+Pb collisions at $\sqrt{s_{N,N}}$ = 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 $p_T$ < 12 GeV and in different intervals of event activity. The event activity is defined either by $N_{ch}^{rec}$, the number reconstructed tracks for |$\eta$| < 2.5, or by $E_{T}^{Pb}$, the total transverse energy over 3.2 < $\eta$ < 4.9 in 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 $p_T$ and event activity. The extracted $v_n$ values for n = 2− 5 are found to decrease with n. The $v_2$ and $v_3$ values are found to be positive in the measured $p_T$ range. The $v_1$ is also measured as a function of $p_T$ and is observed to change sign around $p_T$ ≈ 1.5–2.0 GeV and then increase to about 0.1 for $p_T$ > 4 GeV. The $v_2$($p_T$), $v_3$($p_T$) and $v_4$($p_T$) are compared to the $v_n$ coefficients in Pb+Pb collisions at $\sqrt{s_{N,N}}$ = 2.76 TeV with similar $N_{ch}$. The $v_n$($p_T$) shows similar shape in the two collision systems, after rescaling the $p_T$-axis of the Pb+Pb data by a factor of 1.25 to account for the difference in their $\langle p_{T} \rangle$, as predicted by a hydrodynamic model.