Measurement of Quarkonium Polarization to Probe QCD at the LHC

With the first proton-proton collisions in the Large Hadron Collider (LHC) at CERNin 2010, a new era in high energy physics has been initiated. The data collected bythe various experiments open up the possibility to study standard model processes withhigh precision, in new areas of phase space. The LHC provides excellent conditions forstudies of quarkonium production, due to the high quarkonium production rates giventhe high center-of-mass energy and high instantaneous luminosity of the colliding protonbeams. Studies of the production of heavy quarkonium mesons – bound states of a heavyquark and its respective antiquark – are very important to improve our understanding ofhadron formation. Until quite recently, experimental and phenomenological efforts havenot resulted in a satisfactory overall picture of quarkonium production cross sections andquarkonium polarizations.The Compact Muon Solenoid (CMS) detector is ideally suited to study quarkoniumproduction in the experimentally very clean dimuon decay channel, up to considerablyhigher values of transverse momentum than accessible in previous experiments. Thescope of this thesis is to describe in detail the measurements of the polarizations ofthe Υ(nS) bottomonium states and (in less detail) of the ψ(nS) charmonium states,based on a dimuon data sample collected with the CMS detector in proton-protoncollisions at a center-of-mass energy of 7 TeV. Surprisingly, no significant polarizationswere found in any of the studied quarkonium states, in none of the studied referenceframes, nor in a frame-independent analysis. From an experimental point of view, theseresults, together with recent results from other experiments, clarify the confusing pictureoriginating from previous measurements, which were plagued by experimental ambiguitiesand inconsistencies.The currently most favored approach to model and understand quarkonium productionis non-relativistic quantum chromodynamics (NRQCD), a QCD-inspired model whichallows color-octet pre-resonant quark-antiquark states to contribute to quarkonium boundstate formation. The measurements obtained as a result of this work, together withother LHC measurements in the field of quarkonium production, are interpreted withan original phenomenological approach within the theoretical framework of NRQCD,guided by the observation of a few general features of the data, and corroborated bya detailed study of the quarkonium production cross section and polarization observables.This phenomenological analysis leads to a coherent picture of quarkonium productioncross sections and polarizations within a simple model, dominated by one single coloroctet production mechanism. These findings provide new insight in the dynamics ofheavy quarkonium production at the LHC, an important step towards a satisfactoryunderstanding of hadron formation within the standard model.