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Elliptic flow of $\Upsilon$ mesons in PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV with the CMS detector


Quantum Chromodynamics (QCD) describes all the interactions of quarks and gluons, including the various phases of QCD matter such as the quark-gluon plasma (QGP) which is thought to exist at very high temperatures and baryon densities, and dominated the very early universe.

Heavy-ion collisions are one way in which the QGP can be created and studied in a laboratory environment. The Large Hadron Collider (LHC) is capable of colliding lead nuclei with a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=5.02$ TeV. The CMS detector accurately and efficiently measures the energy and momenta of particles exiting these collisions, even in extremely high-occupancy environments, and provides a rich set of data which can be analyzed to learn more about the thermodynamic properties of the QGP, particularly through the measurement of muons.

Recent studies indicate that the QGP generated in heavy-ion collisions exhibits liquid-like collective behavior, expressed as an elliptic flow of the colliding medium generated by internal pressure gradients. The QGP flows with the lowest viscosity ever observed, making it a near perfect fluid. More studies are warranted to nail down the properties of this unusual form of matter.

The second-order Fourier coefficients ($v_2$) characterizing the elliptic flow of $\Upsilon$s in \PbPb\ collisions at $\sqrt{s_{NN}} = 5.02$ TeV at CMS, are reported. The $\Upsilon$s are reconstructed through their dimuon decay channel in the rapidity range $|y|<2.4$. The suppression of quarkonia in the presence of a quark-gluon plasma is expected to result in a very small $v_2$ signature. The $v_2$ is estimated using the event-plane method, validated by a closure test, and is found to be consistent with zero.

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