Measurement of Quarkonium Polarization to Probe QCD at the LHC

This thesis discusses in detail the measurement of the polarizations of all S-wave vector quarkonium states. It also describes an original phenomenological approach within the theoretical framework of non-relativistic quantum chromodynamics (NRQCD), which leads to a coherent picture of quarkonium production cross-sections and polarizations within a simple model, dominated by one single color-octet production mechanism. Quantum chromodynamics (QCD) is an integral part of the standard model of particle physics. While hard scattering processes are well understood, the realm of non-perturbative QCD remains elusive, and with it the understanding of how quarks bind into hadrons. Heavy quarkonium states constitute an ideal laboratory to study these effects. However, until quite recently, experimental and phenomenological efforts have not resulted in a satisfactory overall picture of quarkonium production cross sections and polarizations. The CMS detector is ideally suited to studying quarkonium production in LHC proton-proton collisions. Surprisingly, no significant polarization was found in any of the studied states. The most favored approach to model quarkonium production is non-relativistic QCD (NRQCD). These findings provide new insights into the dynamics of heavy quarkonium production at the LHC, an important step towards a satisfactory understanding of hadron formation within the standard model.