Long-range angular correlations of $\rm π$, K and p in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

ALICE Collaboration

Angular correlations between unidentified charged trigger particles and various species of charged associated particles (unidentified particles, pions, kaons, protons and antiprotons) are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV in the transverse-momentum range $0.3 < p_{\rm T} < 4$ GeV/$c$. The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range $|\eta_{\rm lab}|<0.8$. Fourier coefficients are extracted from the long-range correlations projected onto the azimuthal angle difference and studied as a function of $p_{\rm T}$ and in intervals of event multiplicity. In high-multiplicity events, the second-order coefficient for protons, $v_2^p$, is observed to be smaller than that for pions, $v_2^\pi$, up to about $p_{\rm T} = 2$ GeV/$c$. To reduce correlations due to jets, the per-trigger yield measured in low-multiplicity events is subtracted from that in high-multiplicity events. A two-ridge structure is obtained for all particle species. The Fourier decomposition of this structure shows that the second-order coefficients for pions and kaons are similar. The $v_2^p$ is found to be smaller at low $p_{\rm T}$ and larger at higher $p_{\rm T}$ than $v_2^pi$, with a crossing occurring at about 2 GeV. This is qualitatively similar to the elliptic-flow pattern observed in heavy-ion collisions. A mass ordering effect at low transverse momenta is consistent with expectations from hydrodynamic model calculations assuming a collectively expanding system.

Figures

Figure 1

Left panel: associated yield per trigger particle as a function of $\Dphi$ and $\Deta$ for $h-\pi$ correlations with $1.5< \pt< 2$ GeV/$c$ in the 0-20% event class.
Right panel: projection of the left panel correlation onto $\Dphi$ averaged over $0.8< |\Deta|< 1.6$ on the near side and $|\Deta|< 1.6$ on the away side. The combined fit and its individual components are superimposed. The figure contains only statistical uncertainty. Systematic uncertainties are mostly correlated and are less than 5%.
[png]   [pdf]   [eps]

Figure 2

The Fourier coefficient $v_2$ for all charged particles, pions, kaons and protons as a function of $\pt$ is shown for the different multiplicity classes extracted for $v_2\{{\rm 2PC}\}$ (symbols) and $v_2\{{\rm SP}\}$ (shaded bands, with a line connecting the central values). The data is plotted at the average-$\pt$ for each considered $\pt$ interval and particle species under study. Error bars and widths of the bands show statistical uncertainties and systematic uncertainties, essentially uncorrelated in $\pt$, added in quadrature.
[png]   [pdf]   [eps]

Figure 3

Top panels: associated yield per trigger particle as a function of $\Dphi$ and $\Deta$ for $h-\pi$ correlations (left) and $h-{\rm p}$ correlations (right) for $1.5< \pt< 2$ GeV/$c$ for the 0-20% event class where the corresponding correlation from the 60-100% event class has been subtracted.
Bottom panels: projection of the top panel correlations to $\Dphi$ averaged over $0.8< |\Deta|< 1.6$ on the near side and $|\Deta|< 1.6$ on the away side. The figure contains only statistical uncertainty. Systematic uncertainties are mostly correlated and are less than 5%.
[png]   [pdf]   [eps]

Figure 4

The Fourier coefficient $v_2\{{\rm 2PC,sub}\}$ for hadrons (black squares), pions (red triangles), kaons (green stars) and protons (blue circles) as a function of $\pt$ from the correlation in the 0-20% multiplicity class after subtraction of the correlation from the 60-100% multiplicity class. The data is plotted at the average-$\pt$ for each considered $\pt$ interval and particle species under study. Error bars show statistical uncertainties while shaded areas denote systematic uncertainties.
[png]   [pdf]   [eps]