Quark Gluon Plasma and Cold Nuclear Matter modification of $\Upsilon$ states at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with the CMS Detector

Quantum chromodynamics describes the phases of strongly-interacting matter and their boundaries, including the deconfined quark-gluon plasma (QGP) phase reached in the high energy density regime. Properties of the QGP are studied using ultrarelativistic collisions of fully-ionized heavy nuclei, which also exhibit (cold) nuclear matter properties unrelated to the plasma. An indicator of the QGP temperature is the modification of quarkoniumproduction in collisions between two heavy ions relative to collisions between two protons. The modification in collisions between a heavy ion and a proton, where the QGP is typically not produced but nuclear matter is abundant, provides an essential baseline. Production cross sections of $\Upsilon$ (1S), $\Upsilon$ (2S), and $\Upsilon$ (3S) mesons decaying into μ$^{+}$μ$^{−}$ in proton-lead (pPb) collisions are measured using data collected by the CMS experiment at √sNN = 5.02 TeV. Nuclear modification factors R$_{pPb}$ for all three $\Upsilon$ states, obtained using measured proton-proton (pp) cross sections at the same collision energy, show that $\Upsilon$ states are suppressed in pPb collisions compared to pp collisions. Sequential ordering of the $\Upsilon$ R$_{pPb}$, with $\Upsilon$ (1S) least suppressed and $\Upsilon$ (3S) most suppressed, indicates presence of final-state modification of $\Upsilon$ mesons in pPb collisions. The R$_{pPb}$ of individual $\Upsilon$ states are found to be consistent with constant values when studied as functions of transverse momentum and center-of-mass rapidity. Predictions using the final-state comover interaction model,which incorporates sequential suppression of bottomonia in pPb, are found to be in better agreement with the measured R$_{pPb}$ versus rapidity than predictions using initial-state modification models. Nuclear modification is less pronounced in pPb collisions than in lead-lead collisions, where the additional lead nucleus and QGP effects result in greater $\Upsilon$ suppression. Forward-backward production ratios R$_{FB}$ of $\Upsilon$ states, which help investigate regions of different nuclear matter densities, are found to be consistent with unity and constant with increasing event activity measured both far away from and near to the measured $\Upsilon$.