Observation of prompt $\mathrm{J}\hspace{-.08em}/\hspace{-.14em}\psi$ meson elliptic flow in high-multiplicity pPb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 8.16~\mathrm{TeV}$

The measurement of the elliptic flow of prompt $\mathrm{J}\hspace{-.08em}/\hspace{-.14em}\psi$ 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 $\sqrt{s_{_{\mathrm{NN}}}} = 8.16~\mathrm{TeV}$. The $\mathrm{J}\hspace{-.08em}/\hspace{-.14em}\psi$ 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 $\mathrm{J}\hspace{-.08em}/\hspace{-.14em}\psi$ meson, as extracted from long-range two-particle correlations with charged hadrons, over the transverse momentum range of $2 < p_{\mathrm{T}} < 8~\mathrm{GeV}$. This measurement provides direct evidence for charm quark collectivity in a small collision system. By comparing the prompt $\mathrm{J}\hspace{-.08em}/\hspace{-.14em}\psi$ results with previous CMS measurements of elliptic flow for open charm mesons ($D^{0}$) 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.