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Radiating top quarks
Gosselink, M.
Publication date 2010
Link to publication
Citation for published version (APA):
Gosselink, M. (2010). Radiating top quarks.
http://www.nikhef.nl/pub/services/biblio/theses_pdf/thesis_M_Gosselink.pdf
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Summary
This thesis describes the preparatory studies carried out in the light of a top quark cross section measurement and a future t¯tH search with the ATLAS experiment. It starts with an overview of top quark physics, Monte Carlo generators, and the ATLAS experiment. The analysis work described in subsequent chapters (except for the last), is based on Monte Carlo simulation of proton-proton collisions at a centre-of-mass energy √s of 14 TeV.
First, a comparison is made of the properties between the t¯tH production signal and its two dominant backgrounds from t¯t and W + jets. For this purpose, the fractions of the protons’ momenta carried by the interacting particles are studied. It is found that, although these momentum fractions help distinguish between the three processes, it does not give a satisfactory level of improvement for event selection with respect the more conventional jet selection criteria.
Secondly, a measurement of the top quark pair production rate is presented. It can be performed with a relatively small amount of collision data at an early stage. The cross section is extracted from t¯t pairs decaying semi-leptonically with either an electron or a muon. A special feature of this measurement is that it does not rely on tagging jets from b-quarks. It was expected that this more advanced tool would only be available at a later stage, after extensive calibration with collision data. For an integrated luminosity of 100 pb−1 the precision of the measurement is estimated to be:
∆σ/σ = ± 7% (stat) ± 15% (syst) ± 3% (pdf) ± 5% (lumi)
where the dominant systematic uncertainty is the shape of the fit function used to extract the t¯t signal.
Another large uncertainty in this measurement, which is also of importance for t¯tH searches, is the amount of radiation from the top quarks. Different state-of-the-art Monte Carlo techniques are compared to assess this uncertainty. The accuracy of the methods correspond to leading order (AcerMC), next-to-leading order (MC@NLO), and tree-level matrix elements matched with the parton shower (Alpgen). It is shown that there
Summary
are significant differences in the predictions for the kinematical jet distributions that affect the jet multiplicity of an event. This leads to a 10% (5%) systematic uncertainty in muon (electron) channel of the t¯t cross section measurement.
Collisions involving a W± or Z boson may look similar to those with t¯t(H) and therefore form a background. Predictions for the production of W± and Z bosons at a centre-of-mass energy√s of 10 TeV are made using a Monte Carlo simulation based on radiation from dipole antennae (Ariadne). Unlike conventional parton shower genera-tors, radiation is considered to originate from two particles with opposite (colour-)charge instead of a single splitting particle. Although the approach predicts significant more radiation in the forward direction of the detector, the total amount of background for the top quark measurement is predicted to be 50% − 75% less than with Alpgen. It is shown that this is partly attributable to differences in the parton showering and partly due to differences in the underlying event modelling. Another remarkable feature is the prediction of a 10% larger W± to Z boson cross section ratio for events with high jet multiplicities.
