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 $^{ +1.6}_{ -0.9}$.
Categories: hep-ex