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

Abstract

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 a nucleon-nucleon center-of-mass energy of \(\sqrt{s_{\mathrm{NN}}}=5.02\) TeV measured with the ATLAS detector at the LHC. Significant long-range "ridge-like" correlations are observed for pairs with small relative azimuthal angle \(|\Delta\phi|<\pi/3\) and back-to-back pairs \(|\Delta\phi|>2\pi/3\) over the transverse momentum range \(0.4<p_{\rm T}<12\) 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 correlations 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 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 multiplicities. Reasonable agreement is observed after accounting for the difference in the average \(p_{\rm T}\) of particles produced in the two collision systems.