Search for Higgs boson pair production in the $b\bar{b} b\bar{b}$ final state from $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

TL;DR

This ATLAS study searches for Higgs-boson pair production in the fully hadronic $b\bar{b}b\bar{b}$ final state using 19.5 fb$^{-1}$ of 8 TeV pp collisions. It employs two complementary hh reconstruction strategies: a resolved method with four $b$-tagged small-$R$ jets and a boosted method using large-$R$ jets with internal track-jet flavor tagging, allowing sensitivity across a broad mass range. No significant excess is found, and the analysis sets stringent limits on both non-resonant SM hh production ($\sigma<202$ fb) and resonant scenarios such as bulk RS gravitons and CP-even Higgs bosons in 2HDMs, with particular mass ranges excluded at 95% CL. The combination of the two analyses yields robust constraints over $500\text{ GeV}$ to $2\text{ TeV}$, highlighting the power of hadronic hh searches and jet-substructure techniques for probing new physics in the Higgs sector.

Abstract

A search for Higgs boson pair production $pp \to hh$ is performed with 19.5 fb$^{-1}$ of proton--proton collision data at $\sqrt{s}=$ 8 TeV, which were recorded by the ATLAS detector at the Large Hadron Collider in 2012. The decay products of each Higgs boson are reconstructed as a high-momentum $b\bar{b}$ system with either a pair of small-radius jets or a single large-radius jet, the latter exploiting jet substructure techniques and associated $b$-tagged track-jets. No evidence for resonant or non-resonant Higgs boson pair production is observed. The data are interpreted in the context of the Randall--Sundrum model with a warped extra dimension as well as the two-Higgs-doublet model. An upper limit on the cross-section for $pp \to G^{*}_{\mathrm{KK}} \to hh \to b\bar{b} b\bar{b}$ of 3.2 (2.3) fb is set for a Kaluza--Klein graviton $G^{*}_{\mathrm{KK}}$ mass of 1.0 (1.5) TeV, at the 95\% confidence level. The search for non-resonant Standard Model $hh$ production sets an observed 95\% confidence level upper limit on the production cross-section $σ(pp \to hh \to b\bar{b}b\bar{b})$ of 202 fb, compared to a SM prediction of $σ(pp \to hh \to b\bar{b}b\bar{b}) = 3.6 \pm 0.5$ fb.

Figures (18)

• Figure 1: Acceptance times reconstruction and selection efficiency as a function of graviton mass for the bulk RS model, for the resolved and boosted analyses. The shapes of the curves are driven by the separation between the $b$-quarks from the Higgs boson decays and the impact on jet clustering. The requirements are defined in Sects. \ref{['sec:resolvedSel']} and \ref{['sec:BoostedSel']}.
• Figure 2: The selection efficiency as a function of resonance mass at each stage of the event selection for (a) $G^{*}_{\mathrm{KK}}$$\space\to\space hh\space\to\space b\bar{b}b\bar{b}$ events and (b) $H\space\to\space hh\space\to\space b\bar{b}b\bar{b}$ events in the resolved analysis.
• Figure 3: The distribution of the subleading dijet mass, $m_{\mathrm{2j}}^{\mathrm{subl}}$, vs the leading dijet mass, $m_{\mathrm{2j}}^{\mathrm{lead}}$, for the 2-tag data sample used to model the multijet background in the resolved analysis. The signal region is the area surrounded by the inner black contour line, centred on $m_{\mathrm{2j}}^{\mathrm{lead}}\xspace=124$, $m_{\mathrm{2j}}^{\mathrm{subl}}\xspace=115$. The control region is the area inside the outer black contour line, excluding the signal region. The sideband region is the area outside the outer contour line.
• Figure 4: The four-jet mass, $m_{\mathrm{4j}}$, distribution in the control region (points) for the resolved analysis, compared to the predicted background (histograms). The small filled blocks represent the statistical uncertainty in the total background estimate. The bottom panel shows the first-order polynomial fit to the data-to-background ratio of the $m_{\mathrm{4j}}$ distribution that is used to fix the multijet shape systematic uncertainty. The dashed lines show the $\pm 1\sigma$ uncertainties in the two fitted parameters.
• Figure 5: The individual impact of the systematic uncertainties considered in the resolved analysis on the expected $\sigma\left(pp\space\to\space G^{*}_{\mathrm{KK}}\xspace\space\to\space hh\space\to\space b\bar{b}b\bar{b}\right)$ 95% confidence level exclusion limit, as a function of graviton mass. The calculation of the expected limit is described in Sect. \ref{['sec:results']}. Only the mass-dependent uncertainties are shown. The impact is the ratio of the limit calculated using all systematic uncertainties sources to the limit calculated using all systematic uncertainty sources excluding those under investigation.