Search for Heavy Resonances in the Dimuon Channel with the DØ Detector
- Univ. of Amsterdam (Netherlands)
High-energy particle physics is often described as the quest for a fundamental theory. One can think of one theory as being more "fundamental" than a second theory if the first explains some or all of the predictions of the second in terms of processes at smaller distance scales and higher energies. Another sense in which a theory can be thought of as being more fundamental than another, is if the first theory "explains" the second; that is, if some arbitrary aspect of the second theory is a special case of a more general principle in the first theory. The Standard Model of particle physics is the most fundamental experimentally tested theory in either sense. It was developed in its final form in the 1960's and 1970's and continues to successfully predict the outcomes of high-energy physics experiments; the research presented in this thesis offers no exception to this trend ( unfortunately). However, the Standard Model only makes sense as an effective theory; that is, a theory that is not actually fundamental, but which is a low-energy limit of some underlying theory, without being dependent on the details of that underlying theory. Moreover, one important aspect of the Standard Model so far remains unexplained, namely electroweak symmetry breaking. Phase transitions occur in many places in physics, and can often be explained as a low-energy aspect of some more fundamental process. However, there is no such underlying process for electroweak symmetry breaking within the Standard Model; the introduction of the scalar Higgs field does not, in itself, explain electroweak symmetry breaking, it merely parameterizes it. Therefore, it is perhaps not surprising that the (most obvious) problem with the Standard Model, the '·gauge hierarchy problem", occurs in relation with the Higgs field. This problem, the "unnaturalness" of the Standard Model, will be discussed (briefly) in the next chapter. It so happens that the unnaturalness of the Standard Model strongly suggests the presence of new physics at an energy scale that is very close to the energies achieved with the Tevatron collider (and presumably well within the reach of the next collider, the LHC). Therefore, there is a general expectation that new physics is "just around the corner". Many, many models for new physics exist, and it is not possible to devise an optimal search strategy for all of them at once. Therefore (and possibly to reduce bias), it is preferable to search for new physics in a model-independent way. While the analysis presented in this thesis is certainly not model-independent, it uses the fact that many, often very different models share the prediction of a new mass resonance. Specifically, we search for such resonances in the dimuon channel. Since no new resonance was in fact observed, the various models only serve to parameterize the degree to which nothing was found. In chapter 2, an outline of the Standard Model is given, and some extensions to the Standard Model are described. Chapter 3 describes the accelerator complex and the D0 detector. Chapter 4 describes the simulation of both Standard Model and new physics processes and the detector simulation. Chapter 5 describes the data sample, the reconstruction algorithms, the event selection and the kinematic fit. A sample of high-PT dimuon events is obtained by applying a number of cuts, and the simulation is tuned to match the efficiencies and track resolution measured on Z→ μ+μ- data. The integrated luminosity times the trigger efficiency of the final sample is determined by comparing it to the predicted (and well-tested) Z cross section. The mass resolution is improved by a kinematic fit which uses the transverse energy balance in the event to constrain the muon track PT for each event in the final selection. In chapter 6, a final cut on the (fitted) invariant mass of the selected events is chosen in such a way that it maximizes the sensitivity to the presence of new physics for specific models and resonance masses. A counting experiment is performed for several values of the mass of a hypothetical new particle. Since no excess over the predicted background is observed, Bayesian credibility limits on the production cross-section times branching ratio of the hypothetical new particles were calculated, as well as limits on the parameters of new physics models. The uncertainties on the theory and detector simulation are taken into account. An event display of the highest reconstructed mass event is shown as well. Finally, chapter 7 gives a conclusion, some prospects of improving the result and a brief outlook.
- Research Organization:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- DOE Contract Number:
- AC02-07CH11359
- OSTI ID:
- 1151591
- Report Number(s):
- FERMILAB-THESIS-2009-34
- Country of Publication:
- United States
- Language:
- English
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