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Search for Higgs bosons decaying into new spin-0 or spin-1 particles in four-lepton final states with the ATLAS detector with 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV

The collaboration
CERN-EP-2021-193, 2021.

Abstract (data abstract)
Searches are conducted for new spin-0 or spin-1 bosons using events where a Higgs boson with mass $125$ GeV decays to four leptons ($\ell =$ $e$, $\mu$). This decay is presumed to occur via an intermediate state which contains two on-shell, promptly decaying bosons $H \rightarrow XX/ZX \rightarrow 4\ell$, where the new boson $X$ has a mass between 1 and 60 GeV. The search uses $pp$ collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 139 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s}=13$ TeV. The data are found to be consistent with Standard Model expectations. Limits are set on fiducial cross sections and on the branching ratio of the Higgs boson to decay to $XX/ZX$. Limits are also set on mixing parameters relevant in extensions of the Standard Model containing a dark sector where $X$ is interpreted to be a dark boson.

• #### $\langle m_{\ell\ell}\rangle$ distribution in the HM SR

Data from Figure 3a

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• #### $m_{34}$ distribution in the $ZX$ signal region

Data from Figure 9

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Distribution of $m_{34}$ for data and background events in the mass range $115 \,\text{GeV} < m_{4\ell} < 130 \,\text{GeV}$ after...

• #### Low and High Mass Efficiencies, $4\mu$ final state

Model-independent per-channel efficiencies calculated in the fiducial volumes of the Low and High Mass channels for the $4\mu$ final state.

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Data from Figure 11a

• #### High Mass Efficiency, $2e2\mu$ final state

Data from Figure 11a

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Model-independent per-channel efficiencies calculated in the fiducial volume of the High Mass channel for the $2e2\mu$ final state.

• #### High Mass Efficiency, $4e$ final state

Data from Figure 11a

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Model-independent per-channel efficiencies calculated in the fiducial volume of the High Mass channel for the $4e$ final state.

• #### Low and High Mass Acceptance, $aa / ss \to 4\mu$

Data from Figure 11b

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Model-dependent per-channel fiducial region acceptances for the $H\to aa\to 4\mu$ process. The regions of 2 GeV to 4.4 GeV, and...

• #### High Mass Acceptance, $Z_dZ_d \to 4\mu$

Data from Figure 11b

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Model-dependent per-channel fiducial region acceptances for the $H\to Z_dZ_d\to 4\ell$ process. The regions of 2 GeV to 4.4 GeV, and...

• #### High Mass Acceptance, $Z_dZ_d \to 2e2\mu$

Data from Figure 11b

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Model-dependent per-channel fiducial region acceptances for the $H\to Z_dZ_d\to 2e2\mu$ process.

• #### High Mass Acceptance, $Z_dZ_d \to 4e$

Data from Figure 11b

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Model-dependent per-channel fiducial region acceptances for the $H\to Z_dZ_d\to 4e$ process.

• #### Fiducial $H\to XX \to 4\mu$ limits

Data from Figure 12a

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Upper limits at 95% CL on fiducial cross-sections for the $H\rightarrow XX \rightarrow 4\mu$ process. The step change at $m_X$...

• #### Fiducial $H\to XX \to 4e$ limits

Data from Figure 12b

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Upper limits at 95% CL on fiducial cross-sections for the $H\rightarrow XX \rightarrow 4e$ process.

• #### Fiducial $H\to XX \to 2e2\mu$ limits

Data from Figure 12c

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Upper limits at 95% CL on fiducial cross-sections for the $H\rightarrow XX \rightarrow 2e2\mu$ process.

• #### $gg \to H\to Z_dZ_d\to 4\ell$ limits

Data from Figure 13

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Observed and expected upper limits at 95% CL for the cross section of the $H\to Z_dZ_d\to 4\ell$ process, assuming SM...

• #### $gg \to H\to Z_dZ_d\to 4\mu$ limits

Data from Figure 14a

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Observed and expected upper limits at 95% CL for the cross sections of the $H\to Z_dZ_d\to 4\mu$ processes, assuming SM...

• #### $gg \to H\to aa \to 4\mu$ limits

Data from Figure 14b

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Observed and expected upper limits at 95% CL for the cross sections of the $H\to aa\to 4\mu$ processes, assuming SM...

• #### ZX Efficiency; $4\mu, 2e2\mu$ final states

Data from Figure 15a

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Model-independent efficiencies for the $H\to ZX$ process for the $4\mu$ and $2e2\mu$ final states combined calculated in the $ZX$ fiducial...

• #### ZX Efficiency; $4\mu, 2e2\mu, 2\mu2e, 4e$ final states

Data from Figure 15a

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Model-independent efficiencies for the $H\to ZX$ process for the $4\mu$, $2e2\mu$, $2\mu 2e$, and $4e$ final states combined calculated in...

• #### ZX Efficiency; $2\mu2e, 4e$ final states

Data from Figure 15a

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Model-independent efficiencies for the $H\to ZX$ process for the $2\mu 2e$, and $4e$ final states combined calculated in the $ZX$...

• #### ZX Acceptance; $4\mu, 2e2\mu$ final states

Data from Figure 15b

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Model-dependent per-channel fiducial region acceptances for the $H\to ZZ_d\to 4\ell$ process for the $4\mu$, and $2e2\mu$ final states.

• #### ZX Acceptance; $4\mu, 2e2\mu, 2\mu2e, 4e$ final states

Data from Figure 15b

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Model-dependent per-channel fiducial region acceptances for the $H\to ZZ_d\to 4\ell$ process for the $4\mu$, $2e2\mu$, $2\mu 2e$, and $4e$ final...

• #### ZX Acceptance; $2\mu2e, 4e$ final states

Data from Figure 15b

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Model-dependent per-channel fiducial region acceptances for the $H\to ZZ_d\to 4\ell$ process for the $2\mu 2e$, and $4e$ final states.

• #### Fiducial $H\to ZX \to 2\ell2e$ limits

Data from Figure 16, $ZX \to 2\ell2e$ channel

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Per-channel upper limit at 95% CL on the fiducial cross-section for the $H\rightarrow ZX \to 2\ell2e$ process.

• #### Fiducial $H\to ZX \to 2\ell2\mu$ limits

Data from Figure 16, $ZX \to 2\ell2\mu$ channel

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Per-channel upper limit at 95% CL on the fiducial cross-section for the $H\rightarrow ZX \to 2\ell2\mu$ process.

• #### $gg \to H\to ZZ_d \to 4\ell$ limits

Data from Figure 17a

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Observed and expected upper limits at 95% CL for the cross sections of the $H\to ZZ_d\to 4\ell$ process, assuming SM...

• #### $gg \to H\to Za \to 4\mu$ limits

Data from Figure 17b

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Observed and expected upper limits at 95% CL for the cross sections of the $H\to Za\to 2\ell2\mu$ process, assuming SM...

• #### Limits on the $H \to Z_dZ_d$ branching ratio

Data from Figure 18a

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95% CL upper limits on the cross section times the model-dependent branching ratio divided by the SM Higgs boson production...

• #### Limits on the $H \to aa$ branching ratio

Data from Figure 18b

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95% CL upper limits on the cross section times the model-dependent branching ratio divided by the SM Higgs boson production...

• #### Limits on the effective Higgs mixing parameter

Data from Figure 19

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Upper limit at 95% CL on the effective Higgs mixing parameter $\kappa^\prime=\kappa \, m_H^2 / |m_H^2-m_S^2|$. The regions of 2...

• #### Limits on the kinematic mixing parameter

Data from Figure 20a

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Upper limit at 95% CL on the $Z_d$ kinetic mixing parameter $\epsilon$, assuming the SM Higgs boson production cross section.

• #### Limits on the mass mixing parameter

Data from Figure 20b

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Upper limit at 95% CL on the $Z$-$Z_d$ mass mixing parameter $\delta^2 \times \textrm{BR}(Z_d\to \ell\ell)$, assuming the SM Higgs boson...