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| CMS-HIN-18-010 ; CERN-EP-2018-256 | ||
| 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 | ||
| 3 October 2018 | ||
| Phys. Lett. B 791 (2019) 172 | ||
| Abstract: A measurement of the elliptic flow ($v_2$) of prompt $ \mathrm{J}/\psi$ mesons in high-multiplicity pPb collisions is reported using data collected by the CMS experiment at a nucleon-nucleon center-of-mass energy ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV. Prompt $ \mathrm{J}/\psi$ mesons decaying into two muons are reconstructed in the rapidity region in the nucleon-nucleon center-of-mass frame ($y_\mathrm{cm}$), corresponding to either $-2.86 < y_\mathrm{cm} < -1.86$ or $ 0.94 < y_\mathrm{cm} < 1.94$. The average $v_2$ result from the two rapidity ranges is reported over the transverse momentum (${p_{\mathrm{T}}}$) range from 0.2 to 10 GeV. Positive $v_2$ values are observed for the prompt $ \mathrm{J}/\psi$ meson, as extracted from long-range two-particle correlations with charged hadrons, for 2 $ < {p_{\mathrm{T}}} < $ 8 GeV. The prompt $ \mathrm{J}/\psi$ results are compared with previous CMS measurements of elliptic flow for open charm mesons (${\mathrm{D^0}}$) and strange hadrons. From these measurements, constraints can be obtained on the collective dynamics of charm quarks produced in high-multiplicity events arising from small systems. | ||
| Links: e-print arXiv:1810.01473 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Example of fits to the invariant mass spectrum (left) and the $v_{2}^{S+B}(m_\text {inv})$ distribution (right) 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$ mesons at forward rapidities ($-2.86 < y_\mathrm {cm} < -1.86$ or $ 0.94 < y_\mathrm {cm} < 1.94$), as a function of ${p_{\mathrm {T}}}$ in the multiplicity range 185 $\leq {N_\mathrm {trk}^\mathrm {offline}} < $ 250 for pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV. Data for ${\mathrm {K^0_S}}$ and $\Lambda $ hadrons, and prompt ${\mathrm {D^0}}$ mesons at midrapidity ($-1.46 < y_\mathrm {cm} < 0.54$) 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. |
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Figure 3:
Upper: the $v_{2}^\text {sub}$ values for prompt $ \mathrm{J}/\psi$ mesons at forward rapidities ($-2.86 < y_\mathrm {cm} < -1.86$ or $ 0.94 < y_\mathrm {cm} < 1.94$), as well as for ${\mathrm {K^0_S}}$ and $\Lambda $ hadrons, and prompt ${\mathrm {D^0}}$ mesons at midrapidity ($-1.46 < y_\mathrm {cm} < 0.54$), 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. Lower: the $n_ {\mathrm {q}}$-normalized $v_{2}^\text {sub}$ results. The ${\mathrm {K^0_S}}$, $\Lambda $ and ${\mathrm {D^0}}$ $v_{2}^\text {sub}$ 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 (${p_{\mathrm{T}}}$). Positive $v_2$ values are observed for prompt $ \mathrm{J}/\psi$ mesons at forward rapidity ($-2.86 < y_\mathrm{cm} < -1.86$ or $0.94 < y_\mathrm{cm} < 1.94$) over a wide ${p_{\mathrm{T}}}$ range (2 $ < {p_{\mathrm{T}}} < $ 8 GeV). This observation provides 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 hadrons at intermediate and high-${p_{\mathrm{T}}}$ regions (e.g., above 4 GeV) adds support to the earlier conclusion that heavy quarks exhibit weaker collective behavior than light quarks or gluons in small systems, unlike what is found in AA collisions. For particle transverse kinetic energy per constituent quark values less than 1 GeV, the $v_2$ of prompt $ \mathrm{J}/\psi$ mesons is consistent with prompt ${\mathrm{D^0}}$, ${\mathrm{K^0_S}}$ and $\Lambda$ hadrons, within current uncertainties. A model calculation based on final-state interactions between charm quarks and a QGP medium in pPb collisions significantly underestimates the measured prompt $ \mathrm{J}/\psi$ $v_2$ signal. The new prompt $ \mathrm{J}/\psi$ meson results, together with previous results for light-flavor and open heavy-flavor hadrons, provide novel insights into the dynamics of the heavy quarks produced in small systems that lead to high final-state multiplicities. |
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