Measurement of the mass and natural width of the Higgs boson in the $H$ to $ZZ$ to $4\ell$ decay channel with the ATLAS detector

This thesis presents a measurement of the mass, natural width, and signal strength, defined as the yield normalized to the Standard Model prediction, of the Higgs boson in the $H\rightarrow ZZ^{(*)} \rightarrow 4\ell$ decay channel using an approach which utilizes event-by-event detector response information. The measurement is performed on $p$-$p$ collision data recorded by the ATLAS experiment at the CERN Large Hadron Collider. The data corresponds to an integrated luminosity of $25 \ fb^{-1}$ with center-of-mass energies of $7 \ \mathrm{TeV}$ and $8 \ \mathrm{TeV}$. The measured mass of the Higgs boson is $m_{H}= 124.57^{+0.48}_{-0.43} \ \mathrm{GeV}$. The signal strength was estimated at $\mu=1.76^{+0.46}_{-0.37}$. Finally, the natural width of the Higgs was determined to be $<2.6 \ \mathrm{GeV}$ with $95\%$ confidence. The event-by-event approach used in this analysis involves the parameterization of the behavior of single leptons in the ATLAS detector and the convolution of a mass response with the Higgs truth distribution to derive the reconstruction level signal model.