CMS-HIG-17-001 ; CERN-EP-2017-292 | ||
Search for lepton flavour violating decays of the Higgs boson to $\mu\tau$ and $\mathrm{e}\tau$ in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
CMS Collaboration | ||
20 December 2017 | ||
JHEP 06 (2018) 001 | ||
Abstract: A search for lepton flavour violating decays of the Higgs boson in the $\mu\tau$ and $\mathrm{e}\tau$ decay modes is presented. The search is based on a data set corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions collected with the CMS detector in 2016, at a centre-of-mass energy of 13 TeV. No significant excess over the standard model expectation is observed. The observed (expected) upper limits on the lepton flavour violating branching fractions of the Higgs boson are $\mathcal{B}(\mathrm{H}\to\mu\tau) < $ 0.25% (0.25%) and $\mathcal{B}(\mathrm{H}\to\mathrm{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 $\mathrm{e}\tau$ Yukawa couplings $\sqrt{|{Y_{\mu\tau}}|^{2}+|{Y_{\tau\mu}}|^{2}} < 1.43\times 10^{-3}$ and $\sqrt{|{Y_{\mathrm{e}\tau}}|^{2}+|{Y_{\tau\mathrm{e}}}|^{2}} < 2.26\times 10^{-3}$ at 95% confidence level. The limits on the lepton flavour violating branching fractions of the Higgs boson and on the associated Yukawa couplings are the most stringent to date. | ||
Links: e-print arXiv:1712.07173 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Distributions of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible in the plots. |
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Figure 1-a:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-b:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-c:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-d:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-e:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-f:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-g:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 1-h:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{\text {h}}} $ channel. The background from SM Higgs boson production is small and not visible. |
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Figure 2:
Distributions of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-a:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-b:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-c:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-d:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-e:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-f:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-g:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 2-h:
Distribution of one of the input variables to the BDT for the $ {{\mathrm {H}} \to {{\mu}} {\tau}_{{\mathrm {e}}}} $ channel. |
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Figure 3:
$ {M_{\text {col}}} $ distribution in $ {\mathrm {t}\overline {\mathrm {t}}} $ enriched (left), like-sign lepton (central), and W+jets enriched (right) control samples defined in the text. The distributions include both statistical and systematic uncertainties. |
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Figure 3-a:
$ {M_{\text {col}}} $ distribution in the $ {\mathrm {t}\overline {\mathrm {t}}} $ enriched control sample defined in the text. The distributions include both statistical and systematic uncertainties. |
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Figure 3-b:
$ {M_{\text {col}}} $ distribution in the like-sign lepton control sample defined in the text. The distributions include both statistical and systematic uncertainties. |
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Figure 3-c:
$ {M_{\text {col}}} $ distribution in the W+jets enriched control sample defined in the text. The distributions include both statistical and systematic uncertainties. |
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Figure 4:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the individual 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} _\mathrm {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 4-a:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm {h}}} $ channel, 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-b:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm { e}}} $ channel, 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-c:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm {h }}} $ channel, 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-d:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm { e}}} $ channel, 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-e:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm {h }}} $ channel, 2-jets ggH 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-f:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm { e}}} $ channel, 2-jets ggH 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-g:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm {h }}} $ channel, 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 4-h:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in the BDT fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau}_{\mathrm { e}}} $ channel, 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 shows the fractional difference between the observed data and the fitted background. |
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Figure 5:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-a:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in the $ {\mathrm {H}} \to {{\mu}} {{\tau} _{\mathrm {h} {\mathrm {e}} } $ channel, 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})=5%$. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-b:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-c:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-d:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-e:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-f:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-g:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 5-h:
Distribution of the collinear mass $ {M_{\text {col}}} $ for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ process in $ {M_{\text {col}}} $ fit analysis, in 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 overlaid simulated signal corresponds to $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})= $ 5%. The bottom panel in each plot shows the ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {{\mu}} {{\tau} _\mathrm {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 6:
Observed and expected 95% CL upper limits on the $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})$ for each individual category and combined. Left: BDT fit analysis. Right: $ {M_{\text {col}}} $ fit analysis. |
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Figure 6-a:
Observed and expected 95% CL upper limits on the $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})$ for each individual category and combined. Left: BDT fit analysis. Right: $ {M_{\text {col}}} $ fit analysis. |
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Figure 6-b:
Observed and expected 95% CL upper limits on the $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})$ for each individual category and combined. Left: BDT fit analysis. Right: $ {M_{\text {col}}} $ fit analysis. |
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Figure 7:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-a:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-b:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-c:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-d:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-e:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-f:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-g:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 7-h:
Distribution of the BDT discriminator for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process for the BDT fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots corresponds to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8-a:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8-b:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8-c:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8-d:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8-e:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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 8-f:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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. |
png pdf |
Figure 8-g:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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. |
png pdf |
Figure 8-h:
Distribution of the collinear mass $M_\text {col}$ for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ process in the $ {M_{\text {col}}} $ fit analysis, in 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 ratio between the observed data and the fitted background. The left column of plots correspond to the $ {\mathrm {H}} \to {\mathrm {e}} {{\tau} _\mathrm {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. |
png pdf |
Figure 9:
Observed and expected 95% CL upper limits on the $\mathcal {B}({\mathrm {H}} \to {\mathrm {e}} {\tau})$ for each individual category and combined. Left: BDT fit analysis. Right: $ {M_{\text {col}}} $ fit analysis. |
png pdf |
Figure 9-a:
Observed and expected 95% CL upper limits on the $\mathcal {B}({\mathrm {H}} \to {\mathrm {e}} {\tau})$ for each individual category and combined. Left: BDT fit analysis. Right: $ {M_{\text {col}}} $ fit analysis. |
png pdf |
Figure 9-b:
Observed and expected 95% CL upper limits on the $\mathcal {B}({\mathrm {H}} \to {\mathrm {e}} {\tau})$ for each individual category and combined. Left: BDT fit analysis. Right: $ {M_{\text {col}}} $ fit analysis. |
png pdf |
Figure 10:
Constraints on the flavour violating Yukawa couplings, $|Y_{{{\mu}} {\tau}}|, |Y_{{\tau} {{\mu}}}|$ (left) and $|Y_{{\mathrm {e}} {\tau}}|, |Y_{{\tau} {\mathrm {e}}}|$ (right), from the BDT result. The expected (red dashed line) and observed (black solid line) limits are derived from the limit on $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})$ and $\mathcal {B}({\mathrm {H}} \to {\mathrm {e}} {\tau})$ 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) [91,92,41] and $ {\tau}\to {{\mu}}\gamma $ or $ {\tau}\to {\mathrm {e}}\gamma $ (lighter green) [92,41]. The green hashed region is derived by the CMS direct search presented in this paper. The blue solid lines are the CMS limits from [44] (left) and [45](right). The purple diagonal line is the theoretical naturalness limit $|Y_{ij}Y_{ji}| \leq m_im_j/v^2$ [41]. |
png pdf |
Figure 10-a:
Constraints on the flavour violating Yukawa couplings, $|Y_{{{\mu}} {\tau}}|, |Y_{{\tau} {{\mu}}}|$ (left) and $|Y_{{\mathrm {e}} {\tau}}|, |Y_{{\tau} {\mathrm {e}}}|$ (right), from the BDT result. The expected (red dashed line) and observed (black solid line) limits are derived from the limit on $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})$ and $\mathcal {B}({\mathrm {H}} \to {\mathrm {e}} {\tau})$ 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) [91,92,41] and $ {\tau}\to {{\mu}}\gamma $ or $ {\tau}\to {\mathrm {e}}\gamma $ (lighter green) [92,41]. The green hashed region is derived by the CMS direct search presented in this paper. The blue solid lines are the CMS limits from [44] (left) and [45](right). The purple diagonal line is the theoretical naturalness limit $|Y_{ij}Y_{ji}| \leq m_im_j/v^2$ [41]. |
png pdf |
Figure 10-b:
Constraints on the flavour violating Yukawa couplings, $|Y_{{{\mu}} {\tau}}|, |Y_{{\tau} {{\mu}}}|$ (left) and $|Y_{{\mathrm {e}} {\tau}}|, |Y_{{\tau} {\mathrm {e}}}|$ (right), from the BDT result. The expected (red dashed line) and observed (black solid line) limits are derived from the limit on $\mathcal {B}({\mathrm {H}} \to {{\mu}} {\tau})$ and $\mathcal {B}({\mathrm {H}} \to {\mathrm {e}} {\tau})$ 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) [91,92,41] and $ {\tau}\to {{\mu}}\gamma $ or $ {\tau}\to {\mathrm {e}}\gamma $ (lighter green) [92,41]. The green hashed region is derived by the CMS direct search presented in this paper. The blue solid lines are the CMS limits from [44] (left) and [45](right). The purple diagonal line is the theoretical naturalness limit $|Y_{ij}Y_{ji}| \leq m_im_j/v^2$ [41]. |
Tables | |
png pdf |
Table 1:
Event selection criteria for the kinematic variables for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ channels. |
png pdf |
Table 2:
Event selection criteria for the kinematic variables for the $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ channels. |
png pdf |
Table 3:
Systematic uncertainties in the expected event yields. All uncertainties are treated as correlated between the categories, except those that have two values separated by the $+$ sign. In this case, the first value is the correlated uncertainty and the second value is the uncorrelated uncertainty for each individual category. Theoretical uncertainties on VBF Higgs boson production [84] are also applied to VH production. Uncertainties on acceptance lead to migration of events between the categories, and can be correlated or anticorrelated between categories. Ranges of uncertainties for the Higgs boson production indicate the variation in size, from negative (anticorrelated) to positive (correlated). |
png pdf |
Table 4:
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}} \to {{\mu}} {\tau}$ process obtained with the BDT fit analysis. |
png pdf |
Table 5:
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}} \to {{\mu}} {\tau}$ process obtained with the $ {M_{\text {col}}} $ fit analysis. |
png pdf |
Table 6:
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}} \to {\mathrm {e}} {\tau}$ process obtained with the BDT fit analysis. |
png pdf |
Table 7:
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}} \to {\mathrm {e}} {\tau}$ process obtained with the $ {M_{\text {col}}} $ fit analysis. |
png pdf |
Table 8:
Summary of the observed and expected upper limits at the 95% CL and the best fit branching fractions in percent for the $ {\mathrm {H}} \to {{\mu}} {\tau}$ and $ {\mathrm {H}} \to {\mathrm {e}} {\tau}$ processes, for the main analysis (BDT fit) and the cross check ($ {M_{\text {col}}} $ fit) method. |
png pdf |
Table 9:
95% CL observed upper limit on the Yukawa couplings, for the main analysis (BDT fit) and the cross check ($ {M_{\text {col}}} $ fit) method. |
Summary |
The search for lepton flavour violating decays of the Higgs boson in the $\mu\tau$ and $\mathrm{e}\tau$ channels, with the 2016 data collected by the CMS detector, is presented in this paper. The data set analysed 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 boosted decision trees discriminator trained to distinguish the signal from backgrounds. The results are cross-checked with an alternate analysis that fits the collinear mass distribution after applying selection criteria on kinematic variables. No evidence is found for lepton flavour violating Higgs boson decays. The observed (expected) limits on the branching fraction of the Higgs boson to $\mu\tau$ and to $\mathrm{e}\tau$ are less than 0.25% (0.25%) and 0.61% (0.37%), respectively, at 95% confidence level. These limits constitute a significant improvement over the previously obtained limits by CMS and ATLAS using 8 TeV proton-proton collision data corresponding to an integrated luminosity of about 20 fb$^{-1}$. Upper limits on the off-diagonal $\mu\tau$ and $\mathrm{e}\tau$ Yukawa couplings are derived from these constraints, $\sqrt{\smash[b]{|{Y_{\mu\tau}}|^{2}+|{Y_{\tau\mu}}}|^{2}} < 1.43 \times 10^{-3}$ and $\sqrt{|{Y_{\mathrm{e}\tau}}|^{2}+|{Y_{\tau\mathrm{e}}}|^{2}} < 2.26\times 10^{-3}$ at 95% confidence level. |
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