Search for the standard model Higgs boson produced in association with a W or a Z boson and decaying to bottom quarks

TL;DR

This CMS study targets the Higgs boson decaying to b b-bar produced in association with W or Z bosons, using six leptonic channels across 7 and 8 TeV LHC data. A boosted, multivariate strategy with channel-specific BDTs is employed, supported by data-driven background control regions and rigorous systematic uncertainties. The combined analysis yields an observed 95% CL limit of 1.89×SM for mH = 125 GeV, with a 2.1σ local excess compatible with SM Higgs production and a best-fit signal strength mu = 1.0 ± 0.5, providing the first LHC indication of H→bb decays. Complementary cross-checks in the dijet-mass spectrum and a diboson signal extraction validate the methodology and support consistency with SM predictions for Higgs couplings to fermions and vector bosons.

Abstract

A search for the standard model Higgs boson (H) decaying to b b-bar when produced in association with a weak vector boson (V) is reported for the following channels: W(mu nu)H, W(e nu)H, W(tau nu)H, Z(mu mu)H, Z(e e)H, and Z(nu nu)H. The search is performed in data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at sqrt(s) = 7 TeV and up to 18.9 inverse femtobarns at sqrt(s) = 8 TeV, recorded by the CMS experiment at the LHC. An excess of events is observed above the expected background with a local significance of 2.1 standard deviations for a Higgs boson mass of 125 GeV, consistent with the expectation from the production of the standard model Higgs boson. The signal strength corresponding to this excess, relative to that of the standard model Higgs boson, is 1.0 +/- 0.5.

Figures (15)

• Figure 1: Dijet invariant mass distribution for simulated samples of ${Z}(\ell\ell){H}\xspace({b}{b})$ events ($m_{H} = 125\,\text{Ge\spaceV}\xspace$), before (red) and after (blue) the energy correction from the regression procedure is applied. A Bukin function Verkerke:2003ir is fit to the distribution and the fitted width of the core of the distribution is displayed on the figure.
• Figure 2: Examples of distributions for variables in the simulated samples and in data for different control regions and for different channels after applying the data/MC scale factors in Table \ref{['tab:SFs2012']}. Top left: Dijet $p_{\mathrm{T}}$ distribution in the ${Z}$+jets control region for the ${Z}(\mathrm{e}\mathrm{e}){H}\xspace$ channel. Top right: $p_{\mathrm{T}}$ distribution in the ${t}\overline{{t}}\xspace$ control region for the $\mathrm{W}({\mu}\nu){H}\xspace$ channel. Bottom left: CSV$_{\text{min}}$ distribution for the $\mathrm{W}$+HF high-boost control region for the ${Z}(\nu\nu){H}\xspace$ channel. Bottom right: $E_{\mathrm{T}}^{\text{miss}}$ distribution for the ${Z}$+HF high-boost control region for the ${Z}(\nu\nu){H}\xspace$ channel. The bottom inset in each figure shows the ratio of the number of events observed in data to that of the Monte Carlo prediction for signal and backgrounds.
• Figure 3: Examples of distributions of the event BDT discriminant output in the simulated samples and in data for different control regions and for different channels after applying the data/MC scale factors in Table \ref{['tab:SFs2012']}. Top left: $\mathrm{W}$+jets control region for the $\mathrm{W}(\mathrm{e}\nu){H}\xspace$ channel. Top right: ${t}\overline{{t}}\xspace$ control region for the ${Z}({\mu}{\mu}){H}\xspace$ channel. Bottom left: $\mathrm{W}$+HF high-boost control region for the ${Z}(\nu\nu){H}\xspace$ channel. Bottom right: ${Z}$+HF high-boost control region for the ${Z}(\nu\nu){H}\xspace$ channel. The bottom inset in each figure shows the ratio of the number of events observed in data to that of the Monte Carlo prediction for signal and backgrounds.
• Figure 4: Post-fit BDT output distributions for ${Z}(\nu\nu){H}\xspace$ in the high-boost region for 8$\,\text{Te\spaceV}$ data (points with error bars), all backgrounds, and signal, after all selection criteria have been applied. The event BDT discriminant values for events in the four different subsets are rescaled and offset to assemble a single BDT output variable. This leads to the four equally-sized partitions shown in the top panel. The partitions correspond, starting from the left, to the event subsets enriched in ${t}\overline{{t}}$, ${V}$+jets, diboson, and ${{V}\xspace}{H}\xspace$ production. The bottom panel shows the right-most, ${{V}\xspace}{H}\xspace$-enriched, partition in more detail. The bottom inset in each figure shows the ratio of the number of events observed in data to that of the Monte Carlo prediction for signal and backgrounds.
• Figure 5: Combination of all channels into a single distribution. Events are sorted in bins of similar expected signal-to-background ratio, as given by the value of the output of their corresponding BDT discriminant (trained with a Higgs boson mass hypothesis of 125$\,\text{Ge\spaceV}$). The two bottom insets show the ratio of the data to the background-only prediction (above) and to the predicted sum of background and SM Higgs boson signal with a mass of 125$\,\text{Ge\spaceV}$ (below).