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

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.

 

Phys. Lett. B 726 (2013) 164–177
HEP Data
e-Print: arXiv:1307.3237 | PDF | inSPIRE
CERN-PH-EP-2013-115

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%.
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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]