The azimuthal anisotropy of particles associated with jets (jet particles) at midrapidity is measured for the first time in p-Pb and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV down to transverse momentum ($p_{\rm T}$) of 0.5 GeV/$c$ and 2 GeV/$c$, respectively, with ALICE. The results obtained in p-Pb collisions are based on a novel three-particle correlation technique. The azimuthal anisotropy coefficient $v_2$ in high-multiplicity p-Pb collisions is positive, with a significance reaching 6.8$\sigma$ at low $p_{\rm T}$, and its magnitude is smaller than in semicentral Pb-Pb collisions. In contrast to the measurements in Pb-Pb collisions, the $v_2$ coefficient is also found independent of $p_{\rm T}$ within uncertainties. Comparisons with the inclusive charged-particle $v_2$ and with AMPT calculations are discussed. The predictions suggest that parton interactions play an important role in generating a non-zero jet-particle $v_2$ in p-Pb collisions, even though they overestimate the reported measurement. These observations shed new insights on the understanding of the origin of the collective behaviour of jet particles in small systems such as p-Pb collisions, and provide significant stringent new constraints to models.
JHEP 08 (2024) 234
HEP Data
e-Print: arXiv:2212.12609 | PDF | inSPIRE
CERN-EP-2022-290
Figure group
Figure 1
Top: raw associated yield per trigger particle $Y$ (left) as a function of $\Delta \eta$ and $\Delta \varphi$ for pairs of charged particles measured in $\vert \eta \vert < 0.8$ with $3 < p_{\rm T}^{\rm trig} < 5$ GeV/$c$ and $1 < p_{\rm T}^{\rm assoc} < p_{\rm T}^{\rm trig}$ GeV/$c$ in high-multiplicity (0-10%) p-Pb collisions at $\snn = 5.02$ TeV, and fit of the distribution (right). Bottom: extracted jet-particle (left) and background (right) yields. |     |
Figure 2
Top: raw associated yield per trigger particle $Y$ (left) as a function of $\Delta \eta$ and $\Delta \varphi$ for pairs of charged particles measured in $\vert \eta \vert < 0.8$ with $5 < p_{\rm T}^{\rm trig} < 6$ GeV/$c$ and $2 < p_{\rm T}^{\rm assoc} < p_{\rm T}^{\rm trig}$ GeV/$c$ in high-multiplicity (0-10%) Pb-Pb collisions at $\snn = 5.02$ TeV, and fit of the distribution (right). Bottom: extracted jet-particle (left) and background (right) yields. |     |
Figure 3
Top: example of the associated yield per-trigger particle $Y$ in TPC$-$FMD1,2 correlations as a function of $\Delta \eta^\prime$ and $\Delta \varphi^\prime$ for $\vert \eta \vert < 0.8$ with $3 < p_{\rm T}^{\rm trig} < 5$ GeV/$c$, $1 < p_{\rm T}^{\rm assoc} < p_{\rm T}^{\rm trig}$ GeV/$c$, $-0.9 < \Delta\eta < -0.7$ and $3.7 < \Delta\varphi < 4.2$ in high-multiplicity (left) and low-multiplicity (right) p-Pb collisions at $\snn = 5.02$ TeV. Bottom: associated per-trigger yield after the subtraction of the scaled correlation distribution in low-multiplicity collisions(left)and fit of the distribution projected onto $\Delta\varphi^\prime$ with Eq. (3). |     |
Figure 4
Left: $v_2(\Delta \varphi, \Delta \eta)$ distributions of charged particles measured at midrapidity in high-multiplicity (0-10%) p-Pb collisions at $\snn = 5.02$ TeV (top) and semicentral (20-60%) Pb-Pb collisions at $\snn = 5.02$ TeV (bottom). Right: fits fo the two corresponding distributions in p-Pb (top) and Pb-Pb (bottom) collisions. The $p_{\rm T}^{\rm trig}$ and $p_{\rm T}^{\rm assoc}$ intervals are mentioned in the legend. |     |
Figure 5
Jet-particle $v_2$ as a function of the trigger-particle $\pt$ for several $p_{\rm T}^{\rm assoc}$ intervals in 0-10% p-Pb collisions at $\snn = 5.02$ TeV, compared with the inclusive charged-particle $v_2$. The values of the jet-particle $v_2$ are horizontally shifted around the centre of the bin for better visibility. The statistical uncertainties, shown as vertical bars, are determined using the sub-sample technique. The systematic uncertainties are represented as filled boxes. Horizontal bars indicate the bin width. |  |
Figure 6
Jet-particle $v_2$ as a function of the trigger-particle $\pt$ for several $p_{\rm T}^{\rm assoc}$ intervals in 20-60% Pb-Pb collisions at $\snn = 5.02$ TeV, compared with the inclusive charged-particle $v_2$. The values of the jet-particle $v_2$ are horizontally shifted around the centre of the bin for better visibility. The statistical uncertainties, shown as vertical bars, are determined using the sub-sample technique. The systematic uncertainties are represented as filled boxes. Horizontal bars indicate the bin width. The published $v_2$ of reconstructed jets measured in 30-50% Pb-Pb collisions is also shown . |  |
Figure 7
Comparison of the $\pt$-differential jet-particle and inclusive charged-particle $v_2$ measured in high-multiplicity p-Pb collisions at $\snn = 5.02$ TeV with the same measurements performed in semicentral Pb-Pb collisions. The published ALICE $v_2$ of reconstructed jets measured in 30-50% Pb-Pb collisions is also shown. The results obtained in Pb-Pb collisions are scaled by a factor 0.6. See the text for the details. |  |
