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CMS-EXO-19-007 ; CERN-EP-2019-159
Search for dark photons in decays of Higgs bosons produced in association with Z bosons in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 10 (2019) 139
Abstract: A search is presented for a Higgs boson that is produced in association with a Z boson and that decays to an undetected particle together with an isolated photon. The search is performed by the CMS Collaboration at the Large Hadron Collider using a data set corresponding to an integrated luminosity of 137 fb$^{-1}$ recorded at a center-of-mass energy of 13 TeV. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for associated Higgs and Z boson production and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a mass of 125 GeV, assuming the standard model production cross section, this corresponds to an observed (expected) upper limit on this branching fraction of 4.6 (3.6)% at 95% confidence level. These are the first limits on Higgs boson decays to final states that include an undetected massless dark photon.
Figures & Tables Summary Additional Figures & Tables References CMS Publications
Figures

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Figure 1:
A Feynman diagram for the production of the $\mathrm{Z} (\to \ell \ell) \mathrm{H} (\to \gamma \gamma _\mathrm {D})$ final state.

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Figure 2:
The $ {m_{\mathrm {T}}} $ distributions for the e$\mu $, WZ, and ZZ control regions after the simultaneous fit to data in the signal and control regions. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 2-a:
The $ {m_{\mathrm {T}}} $ distributions for the e$\mu $ control region after the simultaneous fit to data in the signal and control regions. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 2-b:
The $ {m_{\mathrm {T}}} $ distributions for the WZ control region after the simultaneous fit to data in the signal and control regions. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 2-c:
The $ {m_{\mathrm {T}}} $ distributions for the ZZ control region after the simultaneous fit to data in the signal and control regions. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 3:
The $ {m_{\mathrm {T}}} $ distributions in the signal region for two $m_{\mathrm{H}}$ values for events with $ {{| \eta ^\gamma |}} <$ 1 (left) and $ {{| \eta ^\gamma |}} > $ 1 (right), after the fit to data. The signal size corresponds to 0.1$ \sigma _{\mathrm{Z} \mathrm{H}}$ for both values of $m_{\mathrm{H}}$ shown. The signal processes are stacked on top of all backgrounds. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 3-a:
The $ {m_{\mathrm {T}}} $ distributions in the signal region for two $m_{\mathrm{H}}$ values for events with $ {{| \eta ^\gamma |}} <$ 1, after the fit to data. The signal size corresponds to 0.1$ \sigma _{\mathrm{Z} \mathrm{H}}$ for both values of $m_{\mathrm{H}}$ shown. The signal processes are stacked on top of all backgrounds. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 3-b:
The $ {m_{\mathrm {T}}} $ distributions in the signal region for two $m_{\mathrm{H}}$ values for events with $ {{| \eta ^\gamma |}} > $ 1, after the fit to data. The signal size corresponds to 0.1$ \sigma _{\mathrm{Z} \mathrm{H}}$ for both values of $m_{\mathrm{H}}$ shown. The signal processes are stacked on top of all backgrounds. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. Vertical bars represent data statistical uncertainties, while horizontal bars represent the bin widths.

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Figure 4:
Expected and observed upper limits at 95% CL on the product of $\sigma _{\mathrm{Z} \mathrm{H}}$ and $\mathcal {B}(\mathrm{H} \to \text {invisible}+\gamma)$ as a function of $m_{\mathrm{H}}$. The dot-dashed line shows the predicted signal corresponding to 0.1$ \sigma _{\mathrm{Z} \mathrm{H}}$.
Tables

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Table 1:
Summary of the selection criteria and the main background processes.

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Table 2:
Observed yields, background estimates after the fit to data, and signal predictions after the event selection. The signal size corresponds to 0.1$ \sigma _{\mathrm{Z} \mathrm{H}}$ for all three $m_{\mathrm{H}}$ values shown. The combined statistical and systematic uncertainties are reported. The values in parentheses for the signal processes correspond to the products of acceptance and selection efficiency for $\mathrm{Z} \to \ell \ell $ events.
Summary
A search is presented for a Higgs boson produced in association with a Z boson and decaying to an undetected particle together with an isolated photon. The analysis is based on a data set recorded by the CMS experiment in 2016-18 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 137 fb$^{-1}$ . No significant excess of events above the expectation from standard model backgrounds is found. The results are used to place limits on the product of the cross section for associated ZH production and the branching fraction for such decays of the Higgs boson, in the context of a theoretical model where the undetected particle is a massless dark photon. The observed and expected upper limits at 95% confidence level at $m_{\mathrm{H}} = $ 125 GeV on $\mathcal{B}(\mathrm{H} \to \text{invisible}+\gamma)$, assuming standard model ZH associated production, are 4.6 and 3.6%, respectively. Allowing for deviations from standard model ZH production, the product of $\sigma_{\mathrm{Z}\mathrm{H}}$ and $\mathcal{B}(\mathrm{H} \to \text{invisible}+\gamma)$ is excluded above $\sim$40 to $\sim$4 fb, for $m_{\mathrm{H}}$ ranging from 125 to 300 GeV. These are the first limits on Higgs boson decays to final states that include an undetected massless dark photon.
Additional Figures

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Additional Figure 1:
Muon identification and isolation efficiency as a function of $ {| \eta |}$ and $ {p_{\mathrm {T}}} $.

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Additional Figure 2:
Electron identification and isolation efficiency as a function of $ {| \eta |}$ and $ {p_{\mathrm {T}}} $.

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Additional Figure 3:
Photon identification and isolation efficiency as a function of $ {| \eta |}$ and $ {p_{\mathrm {T}}} $.

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Additional Figure 4:
Covariance matrix for all bins used in the analysis. There are 45 bins in total, 15 for every data-taking year. For every year, the first bin corresponds to events in the $ {\mathrm {e}}\mu $ control region, the following five bins correspond to events with $ {{| \eta ^\gamma |}} < $ 1 in the signal region, the next five bins correspond to events with $ {{| \eta ^\gamma |}} > $ 1 in the signal region, the next two bins correspond to events in the WZ control region, and finally the last two bins correspond to events in the ZZ control region.
Additional Tables

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Additional Table 1:
Expected yields for different processes after several selection stages. The preselection requires two leptons and at least one photon with $ {p_{\mathrm {T}}} $ larger than 25, 20, and 25 GeV, respectively; in addition the dilepton $ {p_{\mathrm {T}}} $ must be larger than 60 GeV, and the $ {{p_{\mathrm {T}}} ^\text {miss}} $ larger than 70 GeV. The signal prediction corresponds to $\mathcal {B}({\mathrm {H}} \to \text {invisible}+\gamma) = $ 10% assuming the standard model ZH cross section at $m_{{\mathrm {H}}} = $ 125 GeV.
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Compact Muon Solenoid
LHC, CERN