Observation of $H \to b\bar{b}$ decays and $VH$ production with the ATLAS detector

A search for the decay of the Standard Model Higgs boson into a $b\bar{b}$ pair when produced in association with a $W$ or $Z$ boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 79.8 fb$^{-1}$, were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination is performed with the results from other searches for the Higgs boson in the $b\bar{b}$ decay mode, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into $b$-quarks. The measured ratio of the branching fraction for a Higgs boson decaying to $b\bar{b}$ to the Standard Model expectation is $1.01 \pm 0.12 \mathrm{(stat.)} ^{+0.16}_{-0.15} \mathrm{(syst.)}$. Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson, yields an observed (expected) significance of 5.3 (4.8) standard deviations.