Measurement of W-Helicity Fractions in $t\bar{t}$ decays and Search for Exotic Dihiggs Production in the $b\bar{b}WW^*$ Decay Channel Using the ATLAS Detector

Two analyses using the ATLAS detector at the Large Hadron Collider are presented. The first analysis uses 20.2 fb$^{-1}$ of data collected at a center of mass energy of $\sqrt{s}=8$ TeV to produce a measurement of the helicity fractions of $W$ bosons produced in the semileptonic decays of top quark pairs. The helicity fractions are measured using both the leptonically and hadronically decaying $W$ bosons. The fractions measured using the leptonic $W$ are the most precise values obtained to date, and the fractions obtained using the $W$ decaying to quarks represent the first direct measurement using the hadronic $W$ decays in top quark pairs. The fitted fractions of longitudinal, left-handed, and right-handed polarization states using the leptonic $W$ are $F_{0}$ = $0.709 \pm 0.019$, $F_{L}$ = $0.299 \pm 0.015$, and $F_{R}$ = $−0.008 \pm 0.014$ respectively. These results are in agreement with the Standard Model prediction of $F_{0}$ = 0.687 $\pm$ 0.005, $F_{L}$ = 0.311 $\pm$ 0.005, and $F_{R}$ = 0.0017 $\pm$ 0.0001. The second analysis uses 36.1 fb$^{-1}$ of data collected at a center of mass energy of $\sqrt{s}=13$ TeV and presents a search for Higgs boson pair production where one Higgs boson decays via $h\rightarrow b\bar{b}$ and the other Higgs boson via $h\rightarrow WW^*\rightarrow\ell\nu q\bar{q}$ (where $\ell$ is either an electron or a muon). This is the first search using the $b\bar{b}\ell\nu q\bar{q}$ final state to search for double Higgs production. Upper limits are placed on the cross-section of non-resonant and resonant Higgs pair production with resonant masses ranging from 500 to 3000 GeV. The most stringent observed (expected) limit is found for a resonant mass of 2500 GeV at 0.23 (0.45) pb. The largest deviation between the observed and expected limits is found at a resonance mass of 1800 GeV and has a local significance of 1.87 $\sigma$.