Precise measurement of the top-quark mass at the CMS experiment using the ideogram method

Author: 
Markus Seidel
Date: 
Jun 2015

Thesis Type:

The mass of the top quark is measured using a sample of $t\bar{t}$ candidate events with one electron or muon and at least four jets in the final state, collected by CMS in proton-proton collisions at $\sqrt{s}=8\,\text{TeV}$ at the LHC. The candidate events are selected from data corresponding to an integrated luminosity of 19.7 fb$^{−1}$.
For each event the top-quark mass is reconstructed from a kinematic fit of the decay products to a $t\bar{t}$ hypothesis. In order to minimize the uncertainties from jet energy corrections, the top-quark mass is determined simultaneously with a jet energy scale factor (JSF), constrained by the known mass of the $W$ boson decaying to quark-antiquark pairs. A joint likelihood fit taking into account multiple interpretations per event -- the ideogram method -- is used.
From the simultaneous fit, a top-quark mass of $172.15\pm0.19\,\text{(stat.+JSF)}\pm0.61\,\text{(syst.)}\,\text{GeV}$ is obtained. Using an additional constraint from the determination of the jet energy scale in $\gamma/Z$+jet events yields $m_{t}=172.38\pm0.16\,\text{(stat.+JSF)}\pm0.49\,\text{(syst.)}\,\text{GeV}$.
The results are discussed in the context of different event generator implementations. Possible kinematic biases are studied by performing the measurement in different regions of the phase space.

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