Search for collectivity with azimuthal J/$ψ$-hadron correlations in high multiplicity p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 and 8.16 TeV

We present a measurement of azimuthal correlations between inclusive J/$\psi$ and charged hadrons in p-Pb collisions recorded with the ALICE detector at the CERN LHC. The J/$\psi$ are reconstructed at forward (2.03 $<~$ y $<~$ 3.53) and backward ($-$4.46 $<~$ y $<~$ $-$2.96) rapidity via their $\mu^+\mu^-$ decay channel, while the charged hadrons are reconstructed at mid-rapidity ($|\eta|$ $<~$ 1.8). The correlations are expressed in terms of associated charged-hadron yields per J/$\psi$ trigger. A rapidity gap of at least 1.5 units between the trigger J/$\psi$ and the associated charged hadrons is required. Possible correlations due to collective effects are assessed by subtracting the associated per-trigger yields in the low-multiplicity collisions from those in the high-multiplicity collisions. After the subtraction, we observe a strong indication of remaining symmetric structures at $\Delta\varphi$ $\approx$ 0 and $\Delta\varphi$ $\approx$ $\pi$, similar to those previously found in two-particle correlations at middle and forward rapidity. The corresponding second-order Fourier coefficient ($v_2$) in the transverse momentum interval between 3 and 6 GeV/$c$ is found to be positive with a significance of about 5$\sigma$. The obtained results are similar to the J/$\psi$ $v_2$ coefficients measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, suggesting a common mechanism at the origin of the J/$\psi$ $v_2$.


Phys. Lett. B 780 (2018) 7-20
HEP Data
e-Print: arXiv:1709.06807 | PDF | inSPIRE

Figure 1

The $M_{\mu\mu}$ distribution in 3 $< $ $p_{\rm T}^{\mu\mu}$ $< $ 6 GeV/$c$ interval fitted with a combination of an CB2 function for the signal and a VWG function for the background, for high-multiplicity (left panel) and low-multiplicity (middle panel) p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV. Right panel illustrates the fit from Eq. (\ref{eq:ext_meth_2}) in the same $p_{\rm T}^{\mu\mu}$ interval. The shaded area corresponds to the signal term in Eq. (\ref{eq:ext_meth_2}).

Figure 2

Associated charged-hadron yields per \jpsi\ trigger in 3 $< $ $p_{\rm T}^{\rm J/\psi}$ $< $ $6$GeV/$c$ in p--Pb collisions at $\sqrt{s_{\rm NN}}$ $=$ 5.02 TeV (left panels) and 8.16 TeV (right panels) The top and the middle panels correspond to the low-multiplicity and the high-multiplicity event classes, respectively. The bottompanels show the yields after the subtraction of the low-multiplicity collision yields from the high-multiplicity collisionones. The solid line represent the fit to the data as described in the text. The dashed, dot-dashed and dottedlines correspond to the individual terms of the fit function defined in Eq. (\ref{eq:V2_fit}) All the yields are normalized to the value in $\Delta\varphi$ $< $ $\pi /6$ in low-multiplicity (40--100\%) event class Only the statistical uncertainties are shown.

Figure 3

$v_2^{\rm J/\psi}\{{\rm 2,sub}\}$ in bins of $p_{\rm T}^{\rm J/\psi}$ for p--Pb (left panels)and Pb--p (right panels) collisions at $\sqrt{s_{\rm NN}}$ $=$ 5.02 TeV (top panels) and 8.16 TeV (bottom panels) The results are compared to the$v_2^{\rm J/\psi}\{{\rm EP}\}$ coefficients measured in central Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ $=$ 5.02TeV in forward rapidity using event plane (EP) based methods . The statisticaland uncorrelated systematic uncertainties are represented by lines and boxes, respectively. The quoted globalsystematic uncertainties correspond to the combined statistical and systematic uncertaintiesof the measured $v_2^{\rm tracklet}\{{\rm 2,sub}\}$ coefficient.

Figure 4

Combined $v_2^{\rm J/\psi}\{{\rm 2,sub}\}$ coefficients in p--Pb and Pb--p collisions compared to the results in central and semi-central Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ $=$ 5.02 TeV and the transport model calculations for semi-central Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ $=$ 5.02 TeV . The solid line corresponds to the contribution from path-length dependent suppression inside the medium. The band shows the resulting $v_2^{\rm J/\psi}$ including also the recombination of thermalized charm quarks and the feed-down from b-hadron decays assuming thermalization of b quarks