Long-range correlations in proton-lead collisions at $\sqrt{s_{NN}} = 5.02$ TeV from ATLAS

Two-particle correlations in relative azimuth $\Delta\phi$ and relative pseudorapidity $\Delta\eta$ are studied in $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ATLAS detector at LHC. The correlations are studied as a function of charged particle $p_{\mathrm T}$ and the collision $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ summed over $3.1 < \eta < 4.9$ in the direction of the Pb beam. After subtracting the known sources of correlations such as dijets, resonances and momentum conservation, estimated using events with low $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$, the resulting correlations exhibit a $\Delta\phi$ modulation that is flat in $\Delta\eta$ out to $|\Delta\eta|=5$. The modulation is comparable in magnitude to similar modulations observed in heavy ion collisions, and can be described by a $1+2c_2\cos2\Delta\phi+2c_3\cos3\Delta\phi$ function over $0.5<p_{\mathrm T}<7$ GeV in broad ranges of $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$. The correlation analysis is repeated for event classes defined by the number of reconstructed charged particles $N_{\mathrm{ch}}^{\mathrm{rec}}$. This analysis gives nearly the same result as the analysis based on $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ for the long-range correlation at the near-side ($\Delta\phi\sim0$), but leads to biases in the long-range correlations at the away-side ($\Delta\phi\sim\pi$). HIJING simulation suggests that this bias is mainly associated with the contributions from dijets which are correlated strongly with the $N_{\mathrm{ch}}^{\mathrm{rec}}$.