Measurement of the Higgs Boson Mass in Decays into Four Leptons with the ATLAS Detector

This master thesis presents a measurement of the mass of the observed Higgs boson candidate in the decay channel $H \rightarrow ZZ^* \rightarrow \ell^+\ell^-\ell^{'+}\ell^{'-}$, with $\ell,\ell' = e$ or $\mu$. The result is based on the full 2011 and 2012 proton-proton collision dataset recorded with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $4.5~\mathrm{fb}^{-1}$ and $20.3~\mathrm{fb}^{-1}$ at a center-of-mass energy of $\sqrt{s} = 7~\mathrm{TeV}$ and $8~\mathrm{TeV}$, respectively. The mass is measured to be $m_H = 124.58 ^{+0.53}_{-0.47}{\rm{~(stat)~}}\pm0.06{\rm{~(syst)~}} ~\mathrm{GeV}$ using an analytical parametrization of the expected mass distribution on an event-by-event basis.