Observation of Associated Near-Side and Away-Side Long-Range Correlations in $\sqrt{s_{NN}}\mathbf{=}5.02\mathrm{TeV}$ Proton-Lead Collisions with the ATLAS Detector

Two-particle correlations in relative azimuthal angle ($\Delta \varphi$) and pseudorapidity ($\Delta \eta$) are measured in $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ $p+\mathrm{Pb}$ collisions using the ATLAS detector at the LHC. The measurements are performed using approximately $1\mu {\mathrm{b}}^{-1}$ of data as a function of transverse momentum ($p_{\mathrm{T}}$) and the transverse energy ($\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$) summed over $3.1<\eta <4.9$ in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range ($2<|\Delta \eta |<5$) “near-side” ($\Delta \varphi \sim 0$) correlation that grows rapidly with increasing $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$. A long-range “away-side” ($\Delta \varphi \sim \pi$) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$, is found to match the near-side correlation in magnitude, shape (in $\Delta \eta$ and $\Delta \varphi$) and $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ dependence. The resultant $\Delta \varphi$ correlation is approximately symmetric about $\pi /2$, and is consistent with a dominant $\cos 2\Delta \varphi$ modulation for all $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ ranges and particle $p_{\mathrm{T}}$.

Figure 1

Distribution of $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ for minimum-bias $p+\mathrm{Pb}$ events. Vertical lines indicate the boundaries of the event activity classes. Shaded bands indicate the larger peripheral and central intervals having $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}<20\mathrm{GeV}$ and $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}>80\mathrm{GeV}$, respectively.

Figure 2

Two-dimensional correlation functions for (a) peripheral events and (b) central events, both with a truncated maximum to suppress the large correlation at $(\Delta \eta ,\Delta \varphi )=(0,0)$; (c) the per-trigger yield $\Delta \varphi$ distribution together with pedestal levels for peripheral ($b_{\mathrm{ZYAM}}^{\mathrm{P}}$) and central ($b_{\mathrm{ZYAM}}^{\mathrm{C}}$) events, and (d) integrated per-trigger yield as function of $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ for pairs in $2<|\Delta \eta |<5$. The shaded boxes represent the systematic uncertainties, and the statistical uncertainties are smaller than the symbols.

Figure 3

Distributions of per-trigger yield in the peripheral and the central event activity classes and their differences (solid symbols), for different ranges of $p_{\mathrm{T}}^a$ and $0.5<p_{\mathrm{T}}^{\mathrm{b}}<4\mathrm{GeV}$, together with functions $a_0+2a_2\cos 2\Delta \varphi$ (solid line) and $a_0+2a_2\cos 2\Delta \varphi +2a_3\cos 3\Delta \varphi$ (dashed line) obtained via a Fourier decomposition (see text). The values for the ZYAM-determined pedestal levels are indicated on each panel for peripheral ($b_{\mathrm{ZYAM}}^{\mathrm{P}}$) and central ($b_{\mathrm{ZYAM}}^{\mathrm{C}}$) $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ bins.

Figure 4

Integrated per-trigger yields, $Y_{\mathrm{int}}$ (see text), vs $p_{\mathrm{T}}^a$ for $0.5<p_{\mathrm{T}}^b<4\mathrm{GeV}$ in peripheral and central events, on the (a) near-side and (b) away-side. The panels (c) and (d) show the difference, $\Delta Y_{\mathrm{int}}$. Panels (e) and (f) show the $p_{\mathrm{T}}$ dependence of $c_n$ and $s_n$ for $n=2,3$, respectively. The error bars and shaded boxes represent the statistical and systematic uncertainties, respectively.