Pseudorapidity dependence of long-range two-particle correlations in $p\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV

Two-particle correlations in $p\mathrm{Pb}$ collisions at a nucleon-nucleon center-of-mass energy of $5.02\text{TeV}$ are studied as a function of the pseudorapidity separation ($\mathrm{\Delta }\eta$) of the particle pair at small relative azimuthal angle ($\left|\mathrm{\Delta }\varphi \right|<\pi /3$). The correlations are decomposed into a jet component that dominates the short-range correlations ($\left|\mathrm{\Delta }\eta \right|<1$), and a component that persists at large $\mathrm{\Delta }\eta$ and may originate from collective behavior of the produced system. The events are classified in terms of the multiplicity of the produced particles. Finite azimuthal anisotropies are observed in high-multiplicity events. The second and third Fourier components of the particle-pair azimuthal correlations, $V_2$ and $V_3$, are extracted after subtraction of the jet component. The single-particle anisotropy parameters $v_2$ and $v_3$ are normalized by their laboratory frame midrapidity value and are studied as a function of ${\eta }_{\text{c.m.}}$. The normalized $v_2$ distribution is found to be asymmetric about ${\eta }_{\text{c.m.}}=0$, with smaller values observed at forward pseudorapidity, corresponding to the direction of the proton beam, while no significant pseudorapidity dependence is observed for the normalized $v_3$ distribution within the statistical uncertainties.

Figure 1

Efficiency-corrected 2D associated yields with Pb-side trigger particle ($-2.4<{\eta }_{\text{lab}}^{\text{trig}}<-2.0$, left panels) and $p$-side trigger particle ($2.0<{\eta }_{\text{lab}}^{\text{trig}}<2.4$, right panels) in low-multiplicity ($2\le N_{\text{trk}}^{\text{offline}}<20$, upper panels) and high-multiplicity ($220\le N_{\text{trk}}^{\text{offline}}<260$, lower panels) are shown for $p\mathrm{Pb}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=5.02\text{TeV}$. The associated and trigger particle $p_{\mathrm{T}}$ ranges are both $0.3<p_{\mathrm{T}}<3\mathrm{GeV}/\mathrm{c}$.

Figure 2

Examples of the distribution of the associated yields after ZYAM subtraction for both low-multiplicity ($2\le N_{\text{trk}}^{\text{offline}}<20$, blue triangles) and high-multiplicity ($220\le N_{\text{trk}}^{\text{offline}}<260$, red circles) are shown for $p\mathrm{Pb}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=5.02\text{TeV}$. The results for Pb-side (left panels) and $p$-side (right panels) trigger particles are both shown; small $\mathrm{\Delta }\eta$ in the upper panels and large $\left|\mathrm{\Delta }\eta \right|$ in the lower panels. The trigger and associated particle $p_{\mathrm{T}}$ ranges are both $0.3<p_{\mathrm{T}}<3\mathrm{GeV}/\mathrm{c}$.

Figure 3

The near-side ($\left|\mathrm{\Delta }\varphi \right|<\pi /3$) correlated yield after ZYAM subtraction in low-multiplicity $2\le N_{\text{trk}}^{\text{offline}}<20$ (upper panels) and high-multiplicity $220\le N_{\text{trk}}^{\text{offline}}<260$ (lower panels) are shown for $p\mathrm{Pb}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=5.02\text{TeV}$. The trigger and associated particle $p_{\mathrm{T}}$ ranges are both $0.3<p_{\mathrm{T}}<3\mathrm{GeV}/\mathrm{c}$. The trigger particles are restricted to the Pb-side ($-2.4<{\eta }_{\text{lab}}^{\text{trig}}<-2.0$, left panels) and the $p$-side ($2.0<{\eta }_{\text{lab}}^{\text{trig}}<2.4$, right panels), respectively. Fit results using Eq. (2) (black solid curves) are superimposed; the red dashed curve and the blue open points are the two fit components, jet and ridge, respectively.

Figure 4

Fourier coefficients, $V_2$ (upper) and $V_3$ (lower), of two-particle azimuthal correlations in high-multiplicity collisions ($220\le N_{\text{trk}}^{\text{offline}}<260$) with (circles) and without (triangles) subtraction of low-multiplicity data, as a function of ${\eta }_{\text{lab}}$. Left panel shows data for Pb-side trigger particles and the right panel for the $p$ side. Statistical uncertainties are mostly smaller than point size; systematic uncertainties are $3.9\%$ and $10\%$ for $V_2$ and $V_3$ without low-multiplicity subtraction, $5.8\%$ and $15\%$ for $V_2$ and $V_3$ with low-multiplicity subtraction, respectively. The systematic uncertainties are shown by the shaded bands.

Figure 5

Self-normalized anisotropy parameters, $v_2\left({\eta }_{\text{lab}}\right)/v_2({\eta }_{\text{lab}}=0)$ (top panel) and $v_3\left({\eta }_{\text{lab}}\right)/v_3({\eta }_{\text{lab}}=0)$ (bottom panel), as a function of ${\eta }_{\text{lab}}$. Data points (curves) are results with (without) low-multiplicity data subtraction; filled circles and solid lines are from the Pb-side trigger. Open circles and dashed lines are from the $p$-side trigger. The bands show systematic uncertainties of $\pm 5.7\%$ and $\pm 14\%$ for $v_2\left({\eta }_{\text{lab}}\right)/v_2({\eta }_{\text{lab}}=0)$ and $v_3\left({\eta }_{\text{lab}}\right)/v_3({\eta }_{\text{lab}}=0)$, respectively. The systematic uncertainties in $v_n\left({\eta }_{\text{lab}}\right)/v_n({\eta }_{\text{lab}}=0)$ without subtraction are similar. Error bars indicate statistical uncertainties only.

Figure 6

$v_2\left({\eta }_{\text{c.m.}}\right)/v_2(-{\eta }_{\text{c.m.}})$, as a function of ${\eta }_{\text{c.m.}}$ in the center-of-mass frame. The data points are results from $V_n^{\text{sub}}$ with low-multiplicity data subtracted. The bands show the systematic uncertainty of $\pm 5.7\%$. Error bars indicate statistical uncertainties only.

Figure 7

Self-normalized $v_2\left({\eta }_{\text{c.m.}}\right)/v_2({\eta }_{\text{c.m.}}=-0.465)$ distribution with low-multiplicity subtraction from Pb-side (filled circles) and $p$-side (open circles) triggers, and $\langle p_{\mathrm{T}}\rangle \left({\eta }_{\text{c.m.}}\right)/\langle p_{\mathrm{T}}\rangle ({\eta }_{\text{c.m.}}=-0.465)$ of $0<p_{\mathrm{T}}<6\mathrm{GeV}/\mathrm{c}$ range from minimum-bias events (solid line) and $0.3<p_{\mathrm{T}}<3\mathrm{GeV}/\mathrm{c}$ range from high-multiplicity ($220\le N_{\text{trk}}^{\text{offline}}<260$) events (dotted line) as functions of ${\eta }_{\text{c.m.}}$. Dashed curve is the hydrodynamic prediction for $\langle p_{\mathrm{T}}\rangle \left({\eta }_{\text{c.m.}}\right)/\langle p_{\mathrm{T}}\rangle ({\eta }_{\text{c.m.}}=-0.465)$ distribution.