Long-range angular correlations on the near and away side in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 <  $p_{\rm T}^{\rm assoc}$ < $p_{\rm T}^{\rm trig}$ < 4 GeV/$c$. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and $p_{\rm T}$ bins, and the widths show no significant evolution with event multiplicity or $p_{\rm T}$. These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge.

Figures

Figure 1

 The associated yield per trigger particle in $\Dphi$ and $\Deta$ for pairs of charged particles with 2 < $p_{\mathrm{T, trig}}$ < 4 $\gevc$ and 1 < $p_{\mathrm{T, assoc}}$ < 2 $\gevc$ in p-Pb collisions at $\snn=5.02$ TeV for the 60-100$\%$ (left) and 0-20$\%$ (right) event classes.

Figure 2

 Associated yield per trigger particle as a function of $\Dphi$ averaged over $|\Deta| < 1.8$ for pairs of charged particles with $2< p_{\mathrm{T, trig}}< 4$ $\gevc$ and $1< p_{\mathrm{T, assoc}}< 2$ $\gevc$ in p-Pb collisions at $\snn=5.02$ TeV for different event classes, and in pp collisions at 2.76 and 7 TeV. The yield between the peaks (determined at $\Dphi \approx 1.3$) has been subtracted in each case. Only statistical uncertainties are shown; systematic uncertainties are less than 0.01 (absolute) per bin.

Figure 3

 Left: Associated yield per trigger particle in $\Dphi$ and $\Deta$ for pairs ofcharged particles with $2< p_{\mathrm{T, trig}}< 4$ GeV/$c$ and $1< p_{\mathrm{T, assoc}}< 2$ GeV/$c$ in p-Pb collisions at $\snn=5.02$ TeV for the 0-20$\%$ multiplicity class, after subtraction of the associated yield obtained in the 60-100$\%$ event class. Top right: the associated per-trigger yield after subtraction (as shown on the left) projected onto $\Deta$ averaged over $|\Dphi| < \pi/3$ (black circles), $|\Dphi - \pi| < \pi/3$ (red squares), and the remaining area (blue triangles, $\Dphi < -\pi/3$, $\pi/3 < \Dphi < 2\pi/3$ and $\Dphi > 4\pi/3$). Bottom right: as above but projected onto $\Dphi$ averaged over $0.8 < |\Deta| < 1.8$ on the near side and $|\Deta| < 1.8$ on the away side. Superimposed are fits containing a $\cos(2\Dphi)$ shape alone (black dashed line) and a combination of $\cos(2\Dphi)$and $\cos(3\Dphi)$ shapes (red solid line). The blue horizontal line shows the baseline obtained from the latter fit which is used for the yield calculation. Also shown for comparison is the subtracted associated yield when the same procedure is applied on HIJING shifted to the same baseline. The figure shows only statistical uncertainties. Systematic uncertainties are mostly correlated and affect the baseline. Uncorrelated uncertainties are less than 1$\%$.

Figure 4

 Left: $v_2$ (black closed symbols) and $v_3$ (red open symbols) for different multiplicity classes and overlapping $p_{\mathrm{T, assoc}}$ and $p_{\mathrm{T, trig}}$ intervals. Right: Near-side (black closed symbols) and away-side (red open symbols) ridge yields per unit of $\Deta$ for different $p_{\mathrm{T, trig}}$ and $p_{\mathrm{T, assoc}}$ bins as a function of the multiplicity class. The error bars show statistical and systematic uncertainties added in quadrature. In both panels the points are slightly displaced horizontally for visibility.

Figure 5

 Associated yield per trigger particle as a function of $\Dphi$ averaged over $|\Deta| < 1.8$ for pairs of charged particles with $2< p_{\mathrm{T, trig}}< 4$ $\gevc$ and $1< p_{\mathrm{T, assoc}}< 2$ $\gevc$ in p-Pb collisions at $\snn=5.02$ TeV for different event classes, compared to pp collisions at $\sqrt{s} = 2.76$ and 7 TeV. For the event classes 0-20$\%$, 20-40$\%$ and 40-60$\%$ the long-range contribution on the near-side $1.2< |\Deta|< 1.8$ and $|\Dphi|< \pi/2$ has been subtracted from both the near side and the away side as described in the text. Subsequently, the yield between the peaks (determined at $\Dphi \approx 1.3$) has been subtracted in each case. Only statistical uncertainties are shown; systematic uncertainties are less than 0.01 (absolute) per bin.