Search for the SM Higgs boson in the (W/Z)H channel with $H\to b\bar{b}$ using the ATLAS detector at the LHC

This thesis presents the latest ATLAS results on the search for the Standard Model Higgs boson decaying into a pair of $b$-quarks. The Higgs boson is searched for in the production channel for which the Higgs is produced in association with a $W$ o $Z$ boson in the final state. This includes the leptonic (electron and muon) decay channels for the vector bosons in addition to the channel with $Z$ decaying into a pair of neutrinos. In particular, this document is mostly focused on the $ZH$ ($Z \to\nu\nu$ and $H \to b\bar{b}$) channel. The analysis has considered all the proton-proton collision data collected by the ATLAS experiment at $\sqrt{s} = 7$TeV and $ \sqrt{s}= 8$ TeV. The analysis is based on a characterization of the events according to the vector boson decay, the number of jets and $b$-jets in the final state, and the vector boson transverse momentum. This permits a detail study of the main background processes. Two different approaches are considered for the analysis: using the $m_{bb}$ distribution as main discriminating variable or using a neutral-net multivariate technique. The analysis involves a complex fitting strategy of data and background predictions in control and signal regions that contributes to constrain the background processes, to reduce the systematic uncertainties, and to increase the sensitivity to the Higgs signal. The whole procedure is validated by searching for the known diboson ($W$/$Z$)$Z$ ($Z\to b\bar{b}$) process with similar final state. The diboson signal is clearly observed with a significance of $4.9\sigma$. Its observed strength (compared to that of the SM) is $\mu_{VZ}= 0.74\pm 0.09(stat.)\pm 0.14(syst.)$, in agreement with the SM prediction. The measured output distribution of the multivariate algorithm in each event category are compared with the expected signal and background distributions and an excess of events above the expected background is observed at a Higgs mass $m_H$ of 125GeV. The observed (expected) significance of this excess is 1.4$\sigma$ (2.6$\sigma$) and the ratio of the measured signal yield to the Standard Model expectation is found to be $\mu= 0.51 \pm 0.31(stat) \pm 0.24(syst)$. These results are consistently among channels and in agreement with those obtained with the analysis that uses $m_{bb}$ distributions as the main discriminating variables. Although an excess is observed, its statistical significance is not large enough to claim neither evidence nor a discovery of the process.