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CMS-PAS-HIG-17-001
Search for lepton flavour violating decays of the Higgs boson to $\mu\tau$ and $\textrm{e}\tau$ in proton-proton collisions at $\sqrt{s}= $ 13 TeV
Abstract: A search for lepton flavor violating decays of the 125 GeV Higgs boson in the $\mu\tau$ and $\textrm{e}\tau$ decay modes is presented. The search is based on a dataset of 35.9 fb$^{-1}$ of proton-proton collisions collected with the CMS detector in 2016, at a center-of-mass energy of 13 TeV. The tau leptons are reconstructed in the leptonic and hadronic decay modes. No significant excess over the standard model background expectation is observed. The observed (expected) upper limits on the branching fraction of the Higgs boson are found to be $\mathcal{B}(\textrm{H}\rightarrow\mu\tau)< $ 0.25(0.25)% and $\mathcal{B}(\textrm{H}\rightarrow\textrm{e}\tau)< $ 0.61(0.37)% at 95% confidence level. These results are used to derive upper limits on the off-diagonal $\mu\tau$ and $\textrm{e}\tau$ Yukawa couplings, $\sqrt{|{Y_{\mu\tau}}|^{2}+|{Y_{\tau\mu}}|^{2}}<1.43\times 10^{-3}$ and $\sqrt{|{Y_{\textrm{e}\tau}}|^{2}+|{Y_{\tau\textrm{e}}}|^{2}}<2.26\times 10^{-3}$ at 95% CL.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
$ {M_{col}} $ in different control regions defined in the text. The distributions are pre-fit and include both statistical and systematic uncertainties.

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Figure 1-a:
$ {M_{col}} $ in the like-sign lepton control region. The distribution is pre-fit and includes both statistical and systematic uncertainties.

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Figure 1-b:
$ {M_{col}} $ in the W+jets enriched control region. The distribution is pre-fit and includes both statistical and systematic uncertainties.

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Figure 1-c:
$ {M_{col}} $ in the $\mathrm{t \bar{t}}$ enriched control region. The distribution is pre-fit and includes both statistical and systematic uncertainties.

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Figure 2:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis, in the different channels and categories compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in each plot shows the fractional difference between the observed data and the fitted background. The left column of plots corresponds to the $\mathrm{ H } \to \mu {\tau _{h}} $ categories, from 0-jets (first row) to 2-jets VBF (fourth row). The right one to their $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ counterparts.

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Figure 2-a:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 0-jet category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-b:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 0-jet category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-c:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 1-jet category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-d:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 1-jet category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-e:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 2-jets gg category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-f:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 2-jets gg category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-g:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 2-jets VBF category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 2-h:
Distribution of the collinear mass $ {M_{col}} $ for the $\mathrm{ H } \rightarrow \mu \tau $ process in $ {M_{col}} $-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 2-jets VBF category compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the overlayed simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=5%$. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis, in the different channels and categories compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in each plot shows the fractional difference between the observed data and the fitted background. The left column of plots corresponds to the $\mathrm{ H } \to \mu {\tau _{h}} $ categories, from 0-jets (first row) to 2-jets VBF (fourth row). The right one to their $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ counterparts.

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Figure 3-a:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 0-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-b:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 0-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-c:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 1-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-d:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 1-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-e:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 2-jets gg category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-f:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 2-jets gg category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-g:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu {\tau _{h}} $ 2-jets VBF category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 3-h:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mu \tau $ process in the BDT-fit analysis in the $\mathrm{ H } \to \mu \tau _{\mathrm{ e } }$ 2-jets VBF category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mu \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 4:
Observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ for each individual category and combined. Left: $ {M_{col}} $-fit analysis. Right: BDT-fit analysis.

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Figure 4-a:
Observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ for each individual category and combined: $ {M_{col}} $-fit analysis.

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Figure 4-b:
Observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ for each individual category and combined: BDT-fit analysis.

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Figure 5:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the different channels and categories compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in each plot shows the fractional difference between the observed data and the fitted background. The left column of plots correspond to the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ categories, from 0-jets (first row) to 2 jets VBF (fourth row). The right one to their $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ counterparts.

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Figure 5-a:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 0-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-b:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 0-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-c:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 1-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-d:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 1-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-e:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 2-jets gg category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-f:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 2-jets gg category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-g:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 2-jets VBF category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 5-h:
Distribution of the collinear mass $M_\text {col}$ for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process in the $ {M_{col}} $-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 2-jets VBF category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=5%$. The lower panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the different channels and categories compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in each plot shows the fractional difference between the observed data and the fitted background. The left column of plots corresponds to the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ categories, from 0-jets (first row) to 2-jets VBF (fourth row). The right one to their $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ counterparts.

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Figure 6-a:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 0-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-b:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 0-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-c:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 1-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-d:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 1-jet category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-e:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 2-jets gg category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-f:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 2-jets gg category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-g:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } {\tau _{h}} $ 2-jets VBF category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 6-h:
Distribution of the BDT output for the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process for the BDT-fit analysis, in the $\mathrm{ H } \to \mathrm{ e } \tau _{\mu }$ 2-jets VBF category, compared to the signal and background estimation. The background is normalized to the best-fit values from the signal plus background fit while the simulated signal corresponds to $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )=$ 5%. The bottom panel in the plot shows the fractional difference between the observed data and the fitted background.

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Figure 7:
Observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ for each individual category and combined. Left: $ {M_{col}} $-fit analysis. Right: BDT-fit analysis.

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Figure 7-a:
Observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ for each individual category and combined: $ {M_{col}} $-fit analysis.

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Figure 7-b:
Observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ for each individual category and combined: BDT-fit analysis.

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Figure 8:
Constraints on the flavour violating Yukawa couplings, $|Y_{\mu \tau }|,|Y_{\tau \mu }|$ and $|Y_{\mathrm{ e } \tau }|,|Y_{\tau \mathrm{ e } }|$, from the BDT result. The expected (red solid line) and observed (black solid line) limits are derived from the limit on $B(\mathrm{ H } \to \mu \tau )$ and $B(\mathrm{ H } \to \mathrm{ e } \tau )$ from the present analysis. The flavour diagonal Yukawa couplings are approximated by their SM values. The green (yellow) band indicates the range that is expected to contain 68% (95%) of all observed limit excursions from the expected limit. The shaded regions are derived constraints from null searches for $\tau \to 3\mu $ or $\tau \to 3\mathrm{ e } $ (dark green) and $\tau \to \mu \gamma $ or $\tau \to \mathrm{ e } \gamma $ (lighter green).The purple diagonal line is the theoretical naturalness limit $Y_{ij}Y_{ji} \leq m_im_j/v^2$.

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Figure 8-a:
Constraints on the flavour violating Yukawa couplings, $|Y_{\mu \tau }|,|Y_{\tau \mu }|$, from the BDT result. The expected (red solid line) and observed (black solid line) limits are derived from the limit on $B(\mathrm{ H } \to \mu \tau )$ from the present analysis. The flavour diagonal Yukawa couplings are approximated by their SM values. The green (yellow) band indicates the range that is expected to contain 68% (95%) of all observed limit excursions from the expected limit. The shaded regions are derived constraints from null searches for $\tau \to 3\mu $ (dark green) and $\tau \to \mu \gamma $ (lighter green).The purple diagonal line is the theoretical naturalness limit $Y_{ij}Y_{ji} \leq m_im_j/v^2$.

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Figure 8-b:
Constraints on the flavour violating Yukawa couplings, $|Y_{\mathrm{ e } \tau }|,|Y_{\tau \mathrm{ e } }|$, from the BDT result. The expected (red solid line) and observed (black solid line) limits are derived from the limit on $B(\mathrm{ H } \to \mathrm{ e } \tau )$ from the present analysis. The flavour diagonal Yukawa couplings are approximated by their SM values. The green (yellow) band indicates the range that is expected to contain 68% (95%) of all observed limit excursions from the expected limit. The shaded regions are derived constraints from null searches for $\tau \to 3\mathrm{ e } $ (dark green) and $\tau \to \mathrm{ e } \gamma $ (lighter green).The purple diagonal line is the theoretical naturalness limit $Y_{ij}Y_{ji} \leq m_im_j/v^2$.
Tables

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Table 1:
Definition of the samples used to estimate the misidentified lepton ($\ell $) background. They are defined by the charge of the two leptons and by the isolation requirements on each. The definition of not-isolated differs in each channel.

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Table 2:
The systematic uncertainties in the expected event yield. All uncertainties are treated as correlated between the categories, except those which have two values. In this case the first value is the correlated uncertainty and the second value is the uncorrelated uncertainty for each individual category. Anticorrelations arise due to migration of events between the categories and are expressed as negative numbers.

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Table 3:
The expected and observed upper limits at 95% CL, and best fit branching fractions in percent for the different jet categories for the $\mathrm{ H } \rightarrow \mu \tau $ process obtained with the $ {M_{col}} $-fit analysis.

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Table 4:
The expected and observed upper limits at 95% CL, and the best fit branching fractions in percent for each individual jet category, and combined, in the $\mathrm{ H } \rightarrow \mu \tau $ process obtained with the BDT-fit analysis.

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Table 5:
The expected and observed upper limits at 95% CL and best fit branching fractions in percent for each individual jet category, and combined, in the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process obtained with the $ {M_{col}} $-fit analysis.

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Table 6:
The observed and expected upper limits at 95% CL and the best fit branching fractions in percent for the different jet categories in the $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ process obtained with the BDT-fit analysis.

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Table 7:
The observed and expected upper limits at the 95% CL and the best fit branching fractions in percent for the $\mathrm{ H } \rightarrow \mu \tau $ and $\mathrm{ H } \rightarrow \mathrm{ e } \tau $ processes, with the different selections.

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Table 8:
95% CL upper limit on the Yukawa couplings
Summary
This article presents the search for LFV decays of the Higgs boson in the $\mu\tau$ and $\textrm{e}\tau$ final states, with the 2016 data collected by the CMS detector. The dataset analyzed corresponds to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collision data recorded at $ \sqrt{s} = $ 13 TeV. The results are extracted by a fit to the output of a BDT trained to discriminate the signal from backgrounds. The results are cross-checked with alternate analysis that fits the $ {M_{col}} $ distribution after applying selection criteria on kinematic variables. No evidence is found for LFV Higgs boson decays. The observed (expected) limits on the branching fraction of the Higgs boson to $\mu\tau$ and to $\mathrm{ e }\tau$ are found to be less than 0.25(0.25)% and 0.61(0.37)%, respectively, at 95% confidence level, and constitute a significant improvement with respect to the previously obtained limits by CMS and ATLAS using 20 fb$^{-1}$ of 8 TeV proton-proton collision data. Upper limits on the off-diagonal $\mu\tau$ and $\mathrm{ e }\tau$ Yukawa couplings are derived from these constraints on the branching ratios, and found to be $\sqrt{ | {Y_{\mu\tau}} | ^{2}+ | {Y_{\tau\mu}} | ^{2}}<1.43\times 10^{-3}$ and $\sqrt{ | {Y_{\mathrm{ e }\tau}}| ^{2}+ | {Y_{\tau\mathrm{ e }}} | ^{2}}<2.26\times 10^{-3}$ at 95% CL.
Additional Figures

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Additional Figure 1:
Constraints on the flavour violating Yukawa couplings, $|Y_{\mu \tau }|,|Y_{\tau \mu }|$ and $|Y_{\mathrm{ e } \tau }|,|Y_{\tau \mathrm{ e } }|$, from the $ {M_{col}} $-fit result. The expected (red solid line) and observed (black solid line) limits are derived from the limit on $B(\mathrm{ H } \to \mu \tau )$ and $B(\mathrm{ H } \to \mathrm{ e } \tau )$ from the present analysis. The flavour diagonal Yukawa couplings are approximated by their SM values. The green (yellow) band indicates the range that is expected to contain 68% (95%) of all observed limit excursions from the expected limit. The shaded regions are derived constraints from null searches for $\tau \to 3\mu $ or $\tau \to 3\mathrm{ e } $ (dark green) and $\tau \to \mu \gamma $ or $\tau \to \mathrm{ e } \gamma $ (lighter green). The purple diagonal line is the theoretical naturalness limit $Y_{ij}Y_{ji} \leq m_im_j/v^2$.

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Additional Figure 1-a:
Constraints on the flavour violating Yukawa coupling, $|Y_{\mu \tau }|,|Y_{\tau \mu }|$, from the $ {M_{col}} $-fit result. The expected (red solid line) and observed (black solid line) limits are derived from the limit on $B(\mathrm{ H } \to \mu \tau )$ from the present analysis. The flavour diagonal Yukawa couplings are approximated by their SM values. The green (yellow) band indicates the range that is expected to contain 68% (95%) of all observed limit excursions from the expected limit. The shaded regions are derived constraints from null searches for $\tau \to 3\mu $ (dark green) and $\tau \to \mu \gamma $ (lighter green). The purple diagonal line is the theoretical naturalness limit $Y_{ij}Y_{ji} \leq m_im_j/v^2$.

png pdf
Additional Figure 1-b:
Constraints on the flavour violating Yukawa coupling, $|Y_{\mathrm{ e } \tau }|,|Y_{\tau \mathrm{ e } }|$, from the $ {M_{col}} $-fit result. The expected (red solid line) and observed (black solid line) limits are derived from the limit on $B(\mathrm{ H } \to \mathrm{ e } \tau )$ from the present analysis. The flavour diagonal Yukawa couplings are approximated by their SM values. The green (yellow) band indicates the range that is expected to contain 68% (95%) of all observed limit excursions from the expected limit. The shaded regions are derived constraints from null searches for $\tau \to 3\mathrm{ e } $ (dark green) and $\tau \to \mu \gamma $ or $\tau \to \mathrm{ e } \gamma $ (lighter green).The purple diagonal line is the theoretical naturalness limit $Y_{ij}Y_{ji} \leq m_im_j/v^2$.

png pdf
Additional Figure 2:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (1/2)

png pdf
Additional Figure 2-a:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 2-b:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 2-c:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 2-d:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 3:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (2/2)

png pdf
Additional Figure 3-a:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 3-b:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 3-c:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 3-d:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 4:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (1/2)

png pdf
Additional Figure 4-a:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 4-b:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 4-c:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (1/2)

png pdf
Additional Figure 4-d:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 5:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (2/2)

png pdf
Additional Figure 5-a:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 5-b:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 5-c:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (2/2)

png pdf
Additional Figure 5-d:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 6:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (1/2).

png pdf
Additional Figure 6-a:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 6-b:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 6-c:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 6-d:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 7:
Variables used as input in the BDT analysis of the $ {\mathrm{ H } \to \mathrm{ e } \tau _{h}} $ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (2/2).

png pdf
Additional Figure 7-a:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 7-b:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 7-c:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 7-d:
Ons of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{h}}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 8:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (1/2).

png pdf
Additional Figure 8-a:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 8-b:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 8-c:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 8-d:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 9:
Variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties. (2/2).

png pdf
Additional Figure 9-a:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 9-b:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 9-c:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 9-d:
One of the variables used as input in the BDT analysis of the ${\mathrm{ H } \to \mathrm{ e } \tau _{\mu }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 10:
$ {M_{col}} $ and BDT output distributions from ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control region (at least 1 b-jet in the event) after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties.

png pdf
Additional Figure 10-a:
$ {M_{col}} $ output distribution from ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control region (at least 1 b-jet in the event) after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 10-b:
BDT output distribution from ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control region (at least 1 b-jet in the event) after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 11:
$ {M_{col}} $ and BDT output distributions from $\mathrm{ Z } \to \tau \tau $ control region after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties.

png pdf
Additional Figure 11-a:
$ {M_{col}} $ distribution from $\mathrm{ Z } \to \tau \tau $ control region after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 11-b:
BDT output distribution from $\mathrm{ Z } \to \tau \tau $ control region after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 12:
$ {M_{col}} $ and BDT output distributions from QCD multijet control region after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distributions are shown before fitting, and include both statistical and systematics uncertainties.

png pdf
Additional Figure 12-a:
$ {M_{col}} $ distribution from QCD multijet control region after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 12-b:
BDT output distribution from QCD multijet control region after loose selection for the ${\mathrm{ H } \to \mu \tau _{\mathrm{ e } }}$ channel. The distribution is shown before fitting, and includes both statistical and systematics uncertainties.

png pdf
Additional Figure 13:
Comparison of observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ (left) and best fit values on $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ (right) of the 8 TeV publications by ATLAS and CMS, the preliminary result by CMS at 13 TeV using the 2015 2.3 fb$^{-1}$ data set, and the result from the BDT analysis of the 2016 13 TeV data set.

png pdf
Additional Figure 13-a:
Comparison of observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ of the 8 TeV publications by ATLAS and CMS, the preliminary result by CMS at 13 TeV using the 2015 2.3 fb$^{-1}$ data set, and the result from the BDT analysis of the 2016 13 TeV data set.

png pdf
Additional Figure 13-b:
Best fit values on $\mathcal {B}(\mathrm{ H } \to \mu \tau )$ of the 8 TeV publications by ATLAS and CMS, the preliminary result by CMS at 13 TeV using the 2015 2.3 fb$^{-1}$ data set, and the result from the BDT analysis of the 2016 13 TeV data set.

png pdf
Additional Figure 14:
Comparison of observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ (left) and best fit values on $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ (right) of the 8 TeV publications by ATLAS and CMS and the result from the BDT analysis of the 2016 13 TeV data set.

png pdf
Additional Figure 14-a:
Comparison of observed and expected 95% CL upper limits on the $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ of the 8 TeV publications by ATLAS and CMS and the result from the BDT analysis of the 2016 13 TeV data set.

png pdf
Additional Figure 14-b:
Best fit values on $\mathcal {B}(\mathrm{ H } \to \mathrm{ e } \tau )$ of the 8 TeV publications by ATLAS and CMS and the result from the BDT analysis of the 2016 13 TeV data set.
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Compact Muon Solenoid
LHC, CERN