Flavour-dependent jet energy corrections and top quark mass

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Perustieteiden korkeakoulu | Master's thesis
Engineering physics
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Master’s Programme in Engineering Physics
53 + 40
The top quark is the heaviest elementary particle in the standard model of particle physics. Its substantial mass indicates it may introduce significant corrections to the Higgs potential. Assuming no new physics below the Planck scale and that the interpretation of theory and measurements is right, these corrections and current experimental results imply that the electroweak vacuum lies in a metastable state. It is therefore important to examine how sound the measurements are. The most accurate single measurements of the top quark mass m_t performed by the CMS and ATLAS collaborations at the LHC differ from a similarly precise result of the D0 experiment at the Tevatron by approximately 2.5 GeV. This amounts to almost 3 standard deviations, so that the D0 result lifts the world average top mass value to approximately 173.3 GeV. Since the top quark decays almost exclusively into a bottom quark b and a W boson, many top mass measurement channels depend on the reconstruction of b-jets, sprays of particles originating from bottom quarks. The very high accuracy of the D0 is based on a unique and very precise calibration of the flavour-dependent jet energy scale corrections. In particular, the D0 correction factor for b-jets is notably different from those used by the ATLAS, CDF and CMS collaborations. An accurate reproduction of the D0 b-jet energy scale corrections is performed using standalone Monte Carlo simulations. We investigate the sensitivity of the b-jet correction to various assumptions, suggest improvements to the method and discuss the results’ implications to top mass measurements.
Alava, Mikko
Thesis advisor
Voutilainen, Mikko
Siikonen, Hannu
top quark, flavour-dependent correction, jet, D0 experiment, energy calibration, standard model
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