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CMS-PAS-HIN-18-010

Observation of prompt $\mathrm{J}/\psi$ meson elliptic flow in high-multiplicity pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV

CMS Collaboration

May 2018

Abstract: The measurement of the elliptic flow of prompt $\mathrm{J}/\psi$ mesons in high-multiplicity pPb collisions is reported using a data sample collected by the CMS experiment, at a nucleon-nucleon center-of-mass energy ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV. The $\mathrm{J}/\psi$ mesons are reconstructed using the dimuon channel with both muons detected in a forward rapidity region with either $-2.86 < y_{\rm cm} < -1.86$ or $ 0.94 < y_{\rm cm} < 1.94$. Significant and positive $v_2$ values are measured for the prompt $\mathrm{J}/\psi$ meson, as extracted from long-range two-particle correlations with charged hadrons, over the transverse momentum range of 2 $ < p_{\mathrm{T}} < $ 8 GeV. This measurement provides direct evidence for charm quark collectivity in a small collision system. By comparing the prompt $\mathrm{J}/\psi$ results with previous CMS measurements of elliptic flow for open charm mesons ($\mathrm{D}^{0}$) and strange hadrons, constraints can be obtained on the collective dynamics of charm quarks produced in high-multiplicity events arising from these small system collisions.

Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 791 (2019) 172. The superseded preliminary plots can be found here.

Figures
png pdf	Figure 1: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the $ p_{\mathrm{T}} $ interval 6.0-8.0 GeV for pPb events with 185 $ \leq N_{\mathrm {trk}}^{\mathrm {offline}} < $ 250.
png pdf	Figure 2: The $v_2$ results of the prompt $ \mathrm{J}/\psi $ meson as a function of $ p_{\mathrm{T}} $ in the multiplicity range 185 $ \leq N_{\mathrm {trk}}^{\mathrm {offline}} < $ 250 of pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV. Data for ${\mathrm {K^0_S}}$ and ${\mathrm {D}^{0}}$ mesons from previous CMS measurements [43] are also shown for comparison. The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties.
png pdf	Figure 3: Top: the $v_{2}^\text {sub}$ results for prompt $ \mathrm{J}/\psi $ meson, as well as for ${\mathrm {K^0_S}}$ and prompt ${\mathrm {D}^{0}} $ mesons, as a function of $ p_{\mathrm{T}} $ for pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV with 185 $ \leq N_{\mathrm {trk}}^{\mathrm {offline}} < $ 250. Bottom: the $n_\mathrm {q}$-scaled $v_{2}^\text {sub}$ results. The ${\mathrm {K^0_S}}$ and ${\mathrm {D}^{0}}$ $v_2$ data are taken from Ref. [43]. The error bars correspond to statistical uncertainties, while the shaded areas denote the systematic uncertainties.

Summary

In summary, the elliptic flow harmonic, $v_2$, for prompt $ \mathrm{J}/\psi $ mesons in high-multiplicity proton-lead (pPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV is presented as a function of transverse momentum, ${p_{\mathrm{T}}}$. Significant positive $v_2$ values are observed for prompt $\mathrm{J}/\psi$ mesons at forward rapidity ($-2.86 < y_{\rm cm} < -1.86$ or $ 0.94 < y_{\rm cm} < 1.94$) over a wide ${p_{\mathrm{T}}}$ range (2 $ < {p_{\mathrm{T}}} < $ 8 GeV). This observation provides unambiguous evidence for charm quark collectivity in high-multiplicity pPb collisions, similar to that first observed for light flavor hadrons. The observed ordering of $v_2$ among light flavor, open and hidden heavy flavor mesons at intermediate and high ${p_{\mathrm{T}}}$ regions (e.g., above 4 GeV) add support to the earlier conclusion that heavy quarks exhibit weaker collective behavior than light quarks or gluons in small collision systems, unlike what is found in A-A collisions. The new prompt $ \mathrm{J}/\psi $ meson results, together with previous results for light and open heavy flavor hadrons, provide novel insights into the dynamics of the heavy quarks produced in small hadronic system collisions that lead to high final-state multiplicities.

Additional Figures
png pdf	Additional Figure 1: 1D long-range ($ | \Delta \eta | > $ 1) correlation function for $ \mathrm{J}/\psi $ in the $ p_{{\mathrm{T}}} $ interval 6.0-8.0 GeV for pPb events with 185 $ \leq N_{\mathrm{trk}}^{\text{offline}} < $ 250. $ \mathrm{J}/\psi $ invariant mass is required to be within 2.94-3.24 GeV. The line shows the Fourier fits.
png pdf	Additional Figure 2: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the $ p_{{\mathrm{T}}} $ interval 0.2-1.8 GeV for pPb events with 185 $ \leq N_{\mathrm{trk}}^{\text{offline}} < $ 250.
png pdf	Additional Figure 3: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the $ p_{{\mathrm{T}}} $ interval 1.8-3.0 GeV for pPb events with 185 $ \leq N_{\mathrm{trk}}^{\text{offline}} < $ 250.
png pdf	Additional Figure 4: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the $ p_{{\mathrm{T}}} $ interval 3.0-4.5 GeV for pPb events with 185 $ \leq N_{\mathrm{trk}}^{\text{offline}} < $ 250.
png pdf	Additional Figure 5: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the $ p_{{\mathrm{T}}} $ interval 4.5-6.0 GeV for pPb events with 185 $ \leq N_{\mathrm{trk}}^{\text{offline}} < $ 250.
png pdf	Additional Figure 6: Example of the simultaneous fit to the invariant mass spectrum and $v_{2}^{S+B}(m_{\rm inv})$ in the $ p_{{\mathrm{T}}} $ interval 8.0-10.0 GeV for pPb events with 185 $ \leq N_{\mathrm{trk}}^{\text{offline}} < $ 250.
png pdf	Additional Figure 7: The ${\ell _{\mathrm{J}/\psi}^{3D}} $distributions for prompt and non-prompt $ \mathrm{J}/\psi $ in 8.16 TeV PYTHIA simulations.
png pdf	Additional Figure 8: The $v_2$ results for prompt $ \mathrm{J}/\psi $ compared to results for inclusive $ \mathrm{J}/\psi $ from ALICE in Ref. [44].

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