Search for the Higgs Boson Decay to Bottom and Charm Quarks

A search for the Standard Model (SM) Higgs boson, produced in association with a $Z$ boson, in decays to bottom or charm quarks is presented. The SM Higgs boson mass is assumed to be $125$ GeV. These searches analyse a data set of $36.1$ fb$^{-1}$ of proton-proton collisions at a collision energy of $\sqrt{s}=13$ TeV delivered by the Large Hadron Collider and recorded by the ATLAS experiment. The SM $Z(H\to b\bar{b})$ search targets $Z\to\nu\bar{\nu}$ and $Z\to\ell^{-}\ell^{+}$ decays. To enhance the sensitivity of this search a multivariate $b$-jet energy correction is developed, which improves the $b$-jet energy resolution and subsequently the resolution of the invariant di-$b$-jet mass. For the SM $Z(H\to b\bar{b})$ search an excess of events over the expected background from other SM processes is found with an observed (expected) significance of 2.9 (2.8) standard deviations. The ratio of the measured event yield for the $Z(H\to b\bar{b})$ process to the SM expectation is $\mu=1.15^{+0.46}_{-0.41}$. The search for the SM $Z(H\to c\bar{c})$ decay targets only $Z\to\ell^{-}\ell^{+}$ decays. The observed upper limit on the cross section for $ZH$ production times the branching ratio for the $H\to c\bar{c}$ decay relative to its SM expectation is $\mu<110$ at a $95\%$ confidence level. In addition, a search for a new boson $A$ that decays to $Z(H\to b\bar{b})$ is presented. This search uses a sub-set of the data set for the SM Higgs boson searches and probes $A$ boson masses from $220$ GeV to $2$ TeV. No significant excess over the background-only hypothesis is observed in data.