Search for New Physics Using Jets in Proton-Proton Collisions

In this thesis a search for New Physics in events with two jets is presented. These events are produced in proton-proton collisions at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) close to the lake Geneva in Switzerland. These collisions have been recorded with the ATLAS experiment, a large multi-purpose particle detector that is placed at one of four points in the LHC where the proton beams are brought to collision. The theory to describe the processes of particle physics, the Standard Model, was able to describe all measurements made so far. Apart from this amazing agreement, the Standard Model does not give an answer to some fundamental questions. How can gravity be described within the Standard Model? What is the mysterious form of matter that has not been seen, but 26 % of the universe are made of? Why are neutrinos massive? Models answering some of these questions lead to the prediction of additional particles, that have not be seen before. A search for new particles might be able to narrow down the number of models, that could describe nature. The LHC collides protons at high energies, at which the underlying fundamental interaction is seen between partons and thus particles with a color charge. %the largest production cross-sections are found for Quantum Chromodynamics processes. The simplest and most frequent final-state consists of two partons, namely a quark-quark, quark-gluon or gluon-gluon pair and their respective anti-particles. When partons at high energies leave the confined state of the proton a spray of particles is produced, which can be seen as so-called jets in the detector. Thus the search for New Physics in a final state with two jets is promising to probe the process with large cross-section and a very large mass range. This thesis describes a search for new particles showing up as narrow resonances in the dijet mass spectrum. An integrated luminosity of 20.3 fb$^{-1}$ recorded at a center of mass energy of 8 TeV by the ATLAS detector has been used. This energy was the highest energy ever reached in a laboratory at this time. The data used in this analysis have been combined from events recorded by twelve different triggers. This extended the range of invariant masses probed from 253 GeV up to 4.1 TeV. The background estimation of the spectrum in this vast range was performed by fitting a functional form to data. A comparison between data and estimated background showed no significant excesses. Hence 95 % C.L. upper limits on the cross section times acceptance have been set for seven different models. The production of a Quantum Black Hole could be excluded for dijet masses of up to 5.75 TeV, while an excited quark model was excluded up to 3.90 TeV. Additionally exclusion limits for signals of a Gaussian shape have been set. A more realistic model-independent limit was set for a Breit-Wigner shape convoluted with parton density effects, parton-shower effects and the detector resolution.