Jet Particology & Search for New Massive Particles

When we collide protons at the Large Hadron Collider (LHC) of CERN, the European Organization for Nuclear Research, what actually collides are the quarks and gluons, collectively partons, that make up the protons. The behavior of partons is governed by the rules of Quantum Chromodynamics (QCD), the theory of the strong force. QCD has a peculiar property called confinement, which prevents partons from moving freely and forces the formation of composite particles called hadrons. Confinement has dramatic consequences for particle physics. Whenever we try to separate a quark from a proton, what results is a burst of hadrons that we call a jet. As the LHC collides only hadrons, jets are present in virtually every collision event. Jets are thus building blocks of the majority of the LHC physics analyses, and a thorough understanding of their behavior is very important for achieving precise physics results. By virtue of the particle-flow event reconstruction used in the Compact Muon Solenoid (CMS) experiment, for the first time we can actually take a good look inside the jets and directly measure what the jets are made of, what kind of particles carry most of the energy etc. We call these in-depth jet studies jet particology. In the first half of this thesis I present a series of pioneering studies of jets at the particle level, including measurements of jet energy composition, jet response and mitigation of the effects of pileup, the unwanted noise caused by simultaneous proton-proton collisions. The studies on the one hand demonstrate the excellent performance of the CMS jet reconstruction, detector simulation and the particle-flow algorithm, but on the other hand show where further studies are needed and open a series of questions and ideas for future jet particology research. In the second half of the thesis we apply our understanding of jets to searches for new massive elementary particles that are predicted by theoretical models that try to explain the shortcoming of the current theoretical understanding. For maximal discovery potential, our strategy is to look for everything that interacts with quarks and gluons in an analysis called the dijet resonance search. We study the 13 teraelectronvolt (TeV) proton-proton collisions of the LHC and look for events with back-to-back jets, which is the experimental signature of a heavy particle decaying to partons. This search reaches to the highest energies ever achieved in a collider and we observe events with up to 8 TeV dijet masses. Unfortunately we do not find evidence of production of new massive particles, and thus we exclude a series of theory predictions up to 8 TeV resonance masses.