Evidence of Higgs Boson Production through Vector Boson Fusion

Abstract

The discovery of the Higgs boson in 2012 provided confirmation of theproposed mechanism for preserving the electroweak $SU(2) \times U(1)$gauge symmetry of the Standard Model of particle physics. It alsoheralded in a new era of precision Higgs physics. This thesis presents ameasurement of the rate at which the Higgs boson is produced by vectorboson fusion in the $WW^{(\ast)}\rightarrow\,\ell\nu\ell\nu$ decay channel. With gauge boson couplingsin both the production and decay vertices, a VBF measurement in thischannel is a powerful probe of the $VVH$ vertex strength. Using$4.5$~fb$^{-1}$ and $20.3$~fb$^{-1}$ of $pp$ collision data collectedat respective center-of-mass energies of 7 and $8 \tev$ in the ATLASdetector, measurements of the statistical significance and the signalstrength are carried out in the Higgs mass range $100 \leq m_H \leq200 \gev$. These measurements are enhanced with a boosted decisiontree that exploits the correlations between eight kinematic inputs inorder to separate signal and background processes. At the benchmarkHiggs mass of $125.36 \gev$, the significance of the data assuming thebackground-only hypothesis to be true has been observed to be$3.2\sigma$ ($2.7\sigma$ expected), constituting evidence of VBF Higgs bosonproduction. The measured signal strength (ratio of observed cross section timesbranching ratio to that predicted by the SM) is$1.27^{+0.53}_{-0.45}$. The inclusive cross section timesbranching ratio is found to be $0.51^{+0.22}_{-0.17}$~pb at $\sqrts =8 \tev$, consistent with the SM prediction of $0.34$~pb. Nosignificant deviations from the SM predictions for VBF Higgs bosonproduction are observed.