Measurement of Quarkonium Polarization to Probe QCD at the LHC Introduction Quarkonium Physics Experimental Setup Data Analysis Discussion of Results Conclusions. This thesis discusses in detail the measurement of the polarizations of all S wave vector quarkonium states in LHC proton proton collisions with the CMS detector. Heavy quarkonium states constitute an ideal laboratory to study non perturbative effects of quantum chromodynamics and to understand how quarks bind into hadrons. The experimental results are interpreted through an original phenomenological approach 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. 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 of particle physics.