Measurements of azimuthal anisotropies at forward and backward rapidity with muons in high-multiplicity p-Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV

The study of the azimuthal anisotropy of inclusive muons produced in p-Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV, using the ALICE detector at the LHC is reported. The measurement of the second-order Fourier coefficient of the particle azimuthal distribution, $v_2$, is performed as a function of transverse momentum $p_{\rm T}$ in the 0-20% high-multiplicity interval at both forward ($2.03 <~ y_{\rm CMS} <~ 3.53$) and backward ($-4.46 <~ y_{\rm CMS} <~ -2.96$) rapidities over a wide $p_{\rm T}$ range, $0.5 <~ p_{\rm T} <~ 10$ GeV/$c$, in which a dominant contribution of muons from heavy-flavour hadron decays is expected at $p_{\rm T} > 2$ GeV/$c$. The $v_2$ coefficient of inclusive muons is extracted using two different techniques, namely two-particle cumulants, used for the first time for heavy-flavour measurements, and forward-central two-particle correlations. Both techniques give compatible results. A positive $v_2$ is measured at both forward and backward rapidities with a significance larger than $4.7\sigma$ and $7.6\sigma$, respectively, in the interval $2 <~ p_{\rm T} <~ 6$ GeV/$c$. Comparisons with previous measurements in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, and with AMPT and CGC-based theoretical calculations are discussed. The findings impose new constraints on the theoretical interpretations of the origin of the collective behaviour in small collision systems.

 

Submitted to: PLB
e-Print: arXiv:2210.08980 | PDF | inSPIRE
CERN-EP-2022-209
Figure group

Figure 1

Left: associated yield per trigger particle as a function of $\Delta \eta$ and $\Delta \varphi$ between inclusive muons with $2 < \pT < 2.5$ \GeVc and SPD tracklets, $Y_{\rm sub}$, in high-multiplicity (\cent{0}{20}) p--Pb collisions at $\sNN = 8.16$ \TeV after the subtraction of the unscaled (top) and scaled (bottom) muon--tracklet correlation in low-multiplicity (\cent{60}{90}) events Right: fit of the corresponding correlation distributions in $-5 < \Delta\eta < -1.5$ projected onto $\Delta\varphi$ with a Fourier decomposition, see Eq. (\ref{eq:fourier}) The three first harmonics are also presented.

Figure 2

Comparison of the two-particle second-order $\pT$-differential muon cumulant before and after the subtraction of the $\pT$-differential cumulant in low-multiplicity collisions and remaining long-range jet correlations at forward (left) and backward (right) rapidities in high-multiplicity p--Pb collisions at $\sNN = 8.16$ \TeV The comparison is presented with the V0M estimator. Only statistical uncertainties are shown.

Figure 3

Inclusive muon $v_2^{\rm \mu}$ as a function of $\pT$ at forward (left) and backward (right) rapidities in high-multiplicity p--Pb collisions at $\sNN =$ 8.16 TeV. The event activity is estimated with the V0M estimator. Open and full symbols refer to the measurements with two-particle correlations and two-particle cumulants, respectively.

Figure 4

Inclusive muon $v_2^{\rm \mu}$ as a function of $\pT$ at forward (left) and backward (right) rapidity in high-multiplicity p--Pb collisions at $\sNN = 8.16$ \TeV, extracted with two-particle cumulants (top) and two-particle correlations (bottom) The results are obtained with three different estimators of the event activity: V0M, CL1 and ZN.

Figure 5

Comparison of the $\pT$-differential $v_2\lbrace {\rm 2PC} \rbrace$ of inclusive muons at forward (left) and backward (right) rapidity in high-multiplicity p--Pb collisions at $\sNN = 8.16$ \TeV, extracted with the two-particle correlation method with previous measurements performed in p--Pb collisions at \fivenn for inclusive muons  and electrons from heavy-flavour hadron decays .

Figure 6

Comparison of the $\pT$-differential $v_2^{\rm \mu} \lbrace {\rm 2PC} \rbrace$ of inclusive muons at forward (left) and backward (right) rapidities in high-multiplicity p--Pb collisions at $\sNN = 8.16$ \TeV with AMPT calculations  The predictions are shown for muons from charm-hadron decays and beauty-hadron decays, separately, muons from charged-pion and kaon decays, and for muons from the combination of the various sources.

Figure 7

Left: comparison of the $\pT$-differential $v_2^{\rm \mu} \lbrace {\rm 2P} \rbrace$ of inclusive muons at forward rapidity in high-multiplicity p--Pb collisions at $\sNN = 8.16$ \TeV with CGC-based calculations  The predictions are shown for muons from charm-hadron decays and beauty-hadron decays, separately, muons from charged-pion and kaon decays, and for muons from the combination of the various sources Right: comparison of the $\pT$-differential $v_2$ of muons from heavy-flavour hadron decays as obtained with CGC and AMPT calculations with the measured inclusive muon $v_2$.