Measurement of Quarkonia Polarization to probe QCD at the LHC

We propose to measure the polarization properties of heavy quarkonia decays to + - pairs, using data collected in the CMS detector at the LHC. Polarization parameters will be studied as a function of quarkonia rapidity and transverse momentum. Special attention will be paid to any associated activity in the events, like the presence of hadronic jets. In the first thesis, we consider the charmonium states ` and c, for which the data collection of 2010 is being continued in the ongoing 2011 data taking. In the second thesis we plan to investigate the polarizations of the bottomonium states (2S) and the (3S). Because of the smaller rates for these channels, also future 2012 data will have to be included. With the two proposed analyses, and based on the use of the recently developed formalism to measure polarization parameters in an unambiguous way, we expect to probe the production mechanisms of heavy quarkonia in the framework of the theory of Strong Interactions, Quantum Chromodynamics (QCD).

Similar to the hydrogen atom, which is a bound state of an electron and a proton, the quarkonium is a bound system of a heavy quark and its antiquark. Atoms are bound through the electromagnetic interaction which is described by the well understood theory of quantum electrodynamics (QED). Quarkonia on the other hand are bound through the strong interaction which is described by quantum chromodynamics (QCD). Unfortunately, in most applications the full QCD theory cannot provide quantitative descriptions.Quarkonia are produced copiously at energies as high as the ones reached at the LHC at CERN where protons are collided. Therefore, quarkonia are easily accessible for experimental studies. As the binding energy of the quarkonium is small in comparison to the mass of the heavy (anti)quark, experimental results on quarkonium production may be described by approximations to the full theory.One example is the angular distribution of the quarkonium decaying to two muons which can give insight into the properties of the quark-antiquark system before they are bound to a quarkonium. The experimental findings on the angular decay distribution before the start of this FWF project were confusing and ambiguous. This project funded two Ph.D. students who were employed at the Institute of High Energy Physics in Vienna for three years. The Ph.D. students were able to deliver the first consistent results on the angular distribution of quarkonia by developing new analysis methods that made optimal use of the data. The measurement of the angular distribution extends up to high quarkonium transverse momentum values, a region where a non-relativistic QCD inspired model (NRQCD) is believed to be especially useful. The results provided in the frame of this FWF project have contributed to the understanding of quarkonium production.