Search for the $b\bar{b}$ decay of the Standard Model Higgs boson in associated $(W/Z)H$ production with the ATLAS detector

TL;DR

This ATLAS study searches for H→bb in associated VH production using Run 1 data (7 and 8 TeV) across W→ℓν, Z→ℓℓ, and Z→νν channels. It employs two parallel strategies—a dijet-mass analysis and a multivariate VH discriminant (BDT_VH)—to separate the Higgs signal from dominant backgrounds, with extensive data-driven control regions and background modelling validated by VZ diboson measurements. The combined result at m_H ≈ 125 GeV yields μ ≈ 0.52 with statistical and systematic uncertainties, corresponding to a modest significance (~1.4σ observed) and an upper limit near 1.2× the SM expectation; cross-checks with VZ production corroborate the analysis framework. Overall, the results are consistent with the Standard Model within uncertainties and provide important calibration for Higgs couplings to bottom quarks and the total width through global fits.

Abstract

A search for the $b\bar{b}$ decay of the Standard Model Higgs boson is performed with the ATLAS experiment using the full dataset recorded at the LHC in Run 1. The integrated luminosities used from $pp$ collisions at $\sqrt{s}=7$ and 8 TeV are 4.7 and 20.3 fb$^{-1}$, respectively. The processes considered are associated $(W/Z)H$ production, where $W\to eν/μν$, $Z\to ee/μμ$ and $Z\toνν$. The observed (expected) deviation from the background-only hypothesis corresponds to a significance of 1.4 (2.6) standard deviations and the ratio of the measured signal yield to the Standard Model expectation is found to be $μ= 0.52 \pm 0.32 \mathrm{(stat.)} \pm 0.24 \mathrm{(syst.)}$ for a Higgs boson mass of 125.36 GeV. The analysis procedure is validated by a measurement of the yield of $(W/Z)Z$ production with $Z\to b\bar{b}$ in the same final states as for the Higgs boson search, from which the ratio of the observed signal yield to the Standard Model expectation is found to be $0.74 \pm 0.09 \mathrm{(stat.)} \pm 0.14 \mathrm{(syst.)}$.

Figures (15)

• Figure 1: Event classification as a function of the output of the MV1c $b$-tagging algorithm for the two highest $p_{\mathrm{T}}$ jets. The bin boundaries denote the operating points (MV1c(jet) OP) as defined in section \ref{['sec:reco']}, corresponding to $b$-tagging efficiencies of 100%, 80%, 70%, 50%, i.e., the $b$-jet purity increases from left (bottom) to right (top). The event categories are 0-tag, 1-tag, and TT, MM and LL for 2-tag, as explained in the text.
• Figure 2: Dijet-invariant-mass distribution for the decay products of a Higgs boson with $m_H = 125$ GeV in the 2-lepton MVA selection. The distributions are shown (a) using jets after global sequential calibration (GSC, solid), and after adding muons inside jets (dotted) and after correcting for resolution effects specific to the kinematics of the decay of a Higgs boson with $m_H = 125$ GeV (dash-dotted); (b) using jets after global sequential calibration (GSC, solid), and after adding muons inside jets and applying the kinematic fit (dash-dotted). The distributions are fit to the Bukin function Bukin and the parameter representing the width of the core of the distribution is shown in the figures, as well as the relative improvement in the resolution with respect to jets after the global sequential calibration.
• Figure 3: Top: The dijet-mass distributions for the expected background and signal contributions in the 1-lepton channel and the 2-jet 2-tag TT category for $160 \GeV < p_{\mathrm{T}}^W \leq 200 \GeV$ (a) before and (b) after applying the transformation of the histogram bins. Bottom: The BDT-output distribution for the expected background and signal contributions in the 1-lepton channel and the 2-jet 2-tag TT category for $p_{\mathrm{T}}^W > 120 \GeV$ (c) before and (d) after applying the transformation of the histogram bins. The background contributions after the relevant global fit (of the dijet-mass analysis in (a) and (b) and of the MVA in (c) and (d)) are shown as filled histograms. The Higgs boson signal ($m_H = 125$ GeV) is shown as a filled histogram on top of the fitted backgrounds, as expected from the SM (indicated as $\mu=1.0$), and, unstacked as an unfilled histogram, scaled by the factor indicated in the legend. The dashed histogram shows the total background as expected from the pre-fit MC simulation. The entries in overflow are included in the last bin. The size of the combined statistical and systematic uncertainty on the sum of the signal and fitted background is indicated by the hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel.
• Figure 4: The $p_{\mathrm{T}}^W$ distribution observed in data (points with error bars) and expected (histograms) for the 2-jet 0-tag control region of the 1-muon sub-channel (MVA selection), (a) before and (b) after $\Delta\phi(\mathrm{jet}_1,\mathrm{jet}_2)$ reweighting. The multijet and simulated-background normalisations are provided by the multijet fits. The size of the statistical uncertainty is indicated by the shaded band. The data-to-background ratio is shown in the lower panel.
• Figure 5: The $\Delta\phi(\mathrm{jet}_1,\mathrm{jet}_2)$ distribution observed in data (points with error bars) and expected (histograms) for the 2-jet 0-tag control region of the 1-muon sub-channel (MVA selection), (a) before and (b) after reweighting. All $p_{\mathrm{T}}^W$ intervals are combined. The multijet and simulated-background normalisations are provided by the multijet fits. The size of the statistical uncertainty is indicated by the shaded band. The data-to-background ratio is shown in the lower panel.