Observation of Associated Near-Side and Away-Side Long-Range Correlations in s N N = 5.02 TeV Proton-Lead Collisions with the ATLAS Detector

Two-particle correlations in relative azimuthal angle ($ensuremath{Delta}ensuremath{phi}$) and pseudorapidity ($ensuremath{Delta}ensuremath{eta}$) are measured in $sqrt{{s}_{mathrm{NN}}}=5.02text{ }text{ }mathrm{TeV}$ $p+mathrm{Pb}$ collisions using the ATLAS detector at the LHC. The measurements are performed using approximately $1text{ }text{ }ensuremath{mu}{mathrm{b}}^{ensuremath{-}1}$ of data as a function of transverse momentum (${p}_{mathrm{T}}$) and the transverse energy ($ensuremath{Sigma}{E}_{mathrm{T}}^{mathrm{Pb}}$) summed over $3.1<ensuremath{eta}<4.9$ in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range ($2<|ensuremath{Delta}ensuremath{eta}|<5$) “near-side'' ($ensuremath{Delta}ensuremath{phi}ensuremath{sim}0$) correlation that grows rapidly with increasing $ensuremath{Sigma}{E}_{mathrm{T}}^{mathrm{Pb}}$. A long-range “away-side'' ($ensuremath{Delta}ensuremath{phi}ensuremath{sim}ensuremath{pi}$) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small $ensuremath{Sigma}{E}_{mathrm{T}}^{mathrm{Pb}}$, is found to match the near-side correlation in magnitude, shape (in $ensuremath{Delta}ensuremath{eta}$ and $ensuremath{Delta}ensuremath{phi}$) and $ensuremath{Sigma}{E}_{mathrm{T}}^{mathrm{Pb}}$ dependence. The resultant $ensuremath{Delta}ensuremath{phi}$ correlation is approximately symmetric about $ensuremath{pi}/2$, and is consistent with a dominant $mathrm{cos}2ensuremath{Delta}ensuremath{phi}$ modulation for all $ensuremath{Sigma}{E}_{mathrm{T}}^{mathrm{Pb}}$ ranges and particle ${p}_{mathrm{T}}$.