Measurements of Higgs boson production and couplings in the four-lepton channel in $pp$ collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector

Abstract: The final ATLAS Run 1 measurements of Higgs boson production and couplings in the decay channel $H\to ZZ^{*}\to\ell^+\ell^-\ell^{'+}\ell^{'-}$, where $\ell,\ell^{'}=e$ or $\mu$, are presented. These measurements were performed using $pp$ collision data corresponding to integrated luminosities of 4.5 fb$^{-1}$ and 20.3 fb$^{-1}$ at center-of-mass energies of 7 TeV and 8 TeV, respectively, recorded with the ATLAS detector at the LHC. The $H\to ZZ^{*}\to 4\ell$ signal is observed with a significance of 8.1 standard deviations, with an expectation of 6.2 standard deviations, at $m_{H}$ = 125.36 GeV, the combined ATLAS measurement of the Higgs boson mass from the $H\to \gamma\gamma$ and $H\to ZZ^{*}\to 4\ell$ channels. The production rate relative to the Standard Model expectation, the signal strength, is measured in four different production categories in the $H\to ZZ^{*}\to 4\ell$ channel. The measured signal strength, at this mass, and with all categories combined, is 1.44 $^{+0.40}_{-0.33}$. The signal strength for Higgs boson production in gluon fusion or in association with $t\bar{t}$ or $b\bar{b}$ pairs is found to be 1.7 $^{+0.5}_{-0.4}$, while the signal strength for vector-boson fusion combined with $WH/ZH$ associated production is found to be 0.3 $^{+1.6}_{-0.9}$.