Measurement of quarkonium production to probe QCD at the LHC

The strong nuclear force binds triplets of quarks into nucleons (protons and neutrons) as well as nucleons into atomic nuclei. Thus it is responsible for keeping the world we see together at the innermost level. The strong force is described in a theory called Quantum ChromoDynamics (QCD). However, our current knowledge of QCD is limited. We do not yet clearly understand how the protons and neutrons are formed from the three quarks they consist of. The quarkonium is a simpler system to study how quarks form bound states. It is a bound state consisting of a charm or beauty quark and its antiquark. These quarks are the heaviest quarks that can form bound states. Due to the large quark masses, the formation of the bound state happens slowly, and is therefore accessible to experimental study. Quarkonia are not found in ordinary matter in the world around us, but can be produced in large quantities in the collisions in the Large Hadron Collider (LHC) at CERN (the largest and most powerful particle collider ever built!). In this work, we will study quarkonia using the CMS experiment at the LHC to answer the fundamental question How does the quark and antiquark interact to form a bound state?. This is done by measuring properties of quarkonia and studying their production together with other particles. The proposed studies have never been performed before and together will shed new light on how quarks form bound states.

The strong nuclear force binds triplets of quarks into nucleons (protons and neutrons) as well as nucleons into atomic nuclei. Thus, it is responsible for keeping the world together at the innermost level. The strong force is described in a theory called Quantum ChromoDynamics (QCD). However, our current knowledge of QCD is limited. We do not yet clearly understand how the protons and neutrons are formed from the three quarks they consist of. The simplest system to study how quarks form bound states are quarkonia which consist of a charm or beauty quark and its antiquark. These quarks are the heaviest quarks that can form bound states. Due to the large quark masses, the formation of the bound state happens slowly, and is therefore accessible to experimental study. Quarkonia are not found in ordinary matter in the world around us but can be produced in large quantities in the collisions at the Large Hadron Collider (LHC) at CERN. For this project, which funded a PhD student and a postdoctoral researcher at the Institute of High Energy Physics in Vienna, we used data provided by the LHC and its experiments to study the fundamental question "How do the quark and antiquark interact to form a bound state?". Experimental results from the ATLAS and CMS experiments on quarkonium cross sections and angular distributions were studied in a novel way independently of any input from theory. We found that all quarkonia are produced in the same way, contrary to what is expected from the currently most widely used theory, non-relativistic QCD (NRQCD). A measurement of the angular distribution of certain quarkonium states was suggested to show whether the simple data-driven approach or the more complex theory is correct in describing the production of quarkonia. This measurement was realized for the first time in the context of this project using data taken at the CMS experiment. It surprisingly shows an agreement with the more complex theory. This new insight will have a huge impact on the understanding of quarkonium production and the field of quarkonium physics in general.