Simultaneous Measurement of Top Quark Mass and Jet Energy Scale Using Template Fits at the CMS Experiment

Sebastian Naumann-Emme
Jun 2011

Thesis Type:

In this thesis, pairs of top quarks produced in proton-proton collisions at a center-of-mass energy of 7 TeV and decaying in the muon+jets channel tt → (b μ ν) (b q q') are analyzed using data that were recorded by the CMS detector in the year 2010 and correspond to an integrated luminosity of $35.9\mbox{ pb}^{-1}$. A sample of 78 events is selected by requiring exactly one isolated muon and at least four jets, two of them being identified as jets from the decay of b quarks. Given these selection criteria, the expected fraction of tt events is 94%. The trijet mass, M3, and the dijet mass, M2, are reconstructed, taking into account the b-tagging information. M3 and M2 are estimators of the masses of hadronically decaying top quarks and the corresponding W bosons, respectively. Templates for M2 and for the event-wise mass difference ΔM32 = M3 - M2 are parametrized as linear functions of the top quark mass, mt, and the jet energy scale (JES). Based on the precise knowledge of the W boson mass, M2 provides a strong handle on the energy scale of jets from light quarks. The reconstructed M2 and ΔM32 in data are compared to the template functions from simulation in a combined likelihood fit. The overall JES in the selected sample is found to be 1.048 ± 0.040 (stat) ± 0.015 (syst) relative to the simulated JES and the measured mt is 167.8 ± 7.1 (stat+JES) ± 3.1 (syst) GeV. This is one of the first measurements of mt at the Large Hadron Collider. Furthermore, the JES measurement is an important input for the commissioning of the CMS experiment for the upcoming measurements with more data in the near future.

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