Measurements of four-lepton production in pp collisions at √s=8 TeV with the ATLAS Detector

The observation of a new particle consistent with the Standard Model (SM) Higgs boson reported by the ATLAS and CMS Collaborations, is a long-sought achievement of modern physics and represents a new starting point for investigating all the fundamental open questions in particle physics and cosmology. The study of four–lepton production in ATLAS at LHC provides a clean experimental environment for investigating the properties of the Higgs boson, for searching for exotic particles, and for precision tests of the SM gauge symmetry. The inclusive four–lepton line shape measurement (4l, where l = e or μ) in proton-proton collisions at 8 TeV with the ATLAS experiment at LHC, is presented. The measurement is performed in four final states: e+e−e+e− (4e), e+e−μ+μ− (2e2μ), μ+μ−e+e− (2μ2e) and μ+ μ− μ+ μ− (4μ), where the first lepton pair is defined to be the one with invariant mass closest to the Z boson mass. The analysis considers all four–leptons production modes in the invariant mass range between 80 GeV and 1 TeV: off–shell H∗/Z∗Z∗ → 4l, single resonant Z → 4l, on–shell Higgs production H → 4l, ZZ∗ → 4l, as well as on–shell diboson process ZZ → 4l. At low 4l invariant mass, the single resonant process Z → 4l provides a cross–check of the detector response to the 4l topology from Higgs decays in four leptons. At intermediate invariant mass, the 4l cross section measurement includes the on–shell Higgs boson and on–shell Z Z processes. The high 4l mass region is sensitive to beyond SM physics and it has been used for constraining the total width of the Higgs boson. Fiducial and total cross sections measurements are presented separately for the investigated 4l topologies (4e, 2e2μ, 2μ2e and 4μ) as well as for their combination. Moreover, the combined four–lepton fiducial cross section is measured differentially as a function of the four–lepton invariant mass (m(4l)) and transverse momentum (pT (4l)). As additional analysis result, a first study of the 4l production induced by gluon–gluon scattering (gg → H∗/Z(∗)Z(∗) → 4l) for m(4l) ≥ 180 GeV is presented. The data are compared to the best theoretical knowledge available for describing the 4l–lineshape in the invariant mass region between 80 GeV and 1 TeV. The data are found to agree with the theoretical predictions confirming the validity of the Standard Model and its scalar sector in a wide kinematic regime.