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| CMS-PAS-HIG-22-002 | ||
| Search for the lepton flavor violating decay of a Higgs boson in the e$ \mu $ final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 3 March 2023 | ||
| Abstract: A search for the lepton-flavor violating decay of a Higgs boson or other exotic resonances with a mass from 110-160 GeV to an $ \mathrm{e}^{\pm}\mu^{\mp} $ pair is presented. The search is performed with a proton-proton collision data set at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. No significant excess is observed for the standard model Higgs boson with a mass at around 125 GeV. The observed (expected) upper limit on the branching fraction for the standard model Higgs boson is determined to be 4.4 (4.7) $\times$ 10$^{-5} $ at 95% confidence level, the most stringent limit set thus far from direct searches. An excess of events over the expected background is observed at an electron-muon invariant mass of approximately 146 GeV with a global (local) significance of 2.8 (3.8) standard deviations. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to PRD. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
The $ m_{\mathrm{e}\mu} $ distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 0.2% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. |
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Figure 1-a:
The $ m_{\mathrm{e}\mu} $ distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 0.2% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. |
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Figure 1-b:
The $ m_{\mathrm{e}\mu} $ distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 0.2% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. |
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Figure 2:
The ggH and VBF BDT discriminant distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ for each BDT trained in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 1.0% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. Vertical lines in the plots illustrate boundaries of the subcategories: ggH cat 0--3 and VBF cat 0--1, as defined in Section 6.2. Events in the shaded region of the VBF category with a VBF BDT discriminant less than 0.78 are discarded since their sensitivity is an order of magnitude lower than other subcategories. |
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Figure 2-a:
The ggH and VBF BDT discriminant distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ for each BDT trained in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 1.0% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. Vertical lines in the plots illustrate boundaries of the subcategories: ggH cat 0--3 and VBF cat 0--1, as defined in Section 6.2. Events in the shaded region of the VBF category with a VBF BDT discriminant less than 0.78 are discarded since their sensitivity is an order of magnitude lower than other subcategories. |
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Figure 2-b:
The ggH and VBF BDT discriminant distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ for each BDT trained in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 1.0% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. Vertical lines in the plots illustrate boundaries of the subcategories: ggH cat 0--3 and VBF cat 0--1, as defined in Section 6.2. Events in the shaded region of the VBF category with a VBF BDT discriminant less than 0.78 are discarded since their sensitivity is an order of magnitude lower than other subcategories. |
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Figure 3:
The $ p_{\mathrm{T}}^\text{miss} $ distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 1.0% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. |
png pdf |
Figure 3-a:
The $ p_{\mathrm{T}}^\text{miss} $ distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 1.0% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. |
png pdf |
Figure 3-b:
The $ p_{\mathrm{T}}^\text{miss} $ distributions of the data, simulated backgrounds and signals of $ \mathrm{H}(125)\to\mathrm{e}\mu $ in the ggH (upper) and the VBF categories (lower). The $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu)=$ 1.0% is assumed for the signal for illustration. The lower panel in each plot shows the ratio of the data to the total estimated background. The uncertainty band corresponds to the background uncertainties, adding in quadrature the statistical and the SM cross section uncertainties. |
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Figure 4:
Example fits of the signal models to the simulated $ \mathrm{H}(125)\to\mathrm{e}\mu $ signal in the analysis categories ggH cat 0 and ggH cat 3 (left), as well as VBF cat 0 and VBF cat 1 (right), summing events from both the ggH and VBF production modes. The smallest symmetric $ m_{\mathrm{e}\mu} $ interval that contains 68% of the signal events, $ \sigma_{\text{eff}} $, is shown in the legends for each signal as an illustration of the signal resolution. The signal resolution in general improves with the signal purity of the analysis categories. |
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Figure 4-a:
Example fits of the signal models to the simulated $ \mathrm{H}(125)\to\mathrm{e}\mu $ signal in the analysis categories ggH cat 0 and ggH cat 3 (left), as well as VBF cat 0 and VBF cat 1 (right), summing events from both the ggH and VBF production modes. The smallest symmetric $ m_{\mathrm{e}\mu} $ interval that contains 68% of the signal events, $ \sigma_{\text{eff}} $, is shown in the legends for each signal as an illustration of the signal resolution. The signal resolution in general improves with the signal purity of the analysis categories. |
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Figure 4-b:
Example fits of the signal models to the simulated $ \mathrm{H}(125)\to\mathrm{e}\mu $ signal in the analysis categories ggH cat 0 and ggH cat 3 (left), as well as VBF cat 0 and VBF cat 1 (right), summing events from both the ggH and VBF production modes. The smallest symmetric $ m_{\mathrm{e}\mu} $ interval that contains 68% of the signal events, $ \sigma_{\text{eff}} $, is shown in the legends for each signal as an illustration of the signal resolution. The signal resolution in general improves with the signal purity of the analysis categories. |
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Figure 5:
Observed (expected) 95% CL upper limits on $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu) $ for each individual analysis category and for the combination of all analysis categories. |
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Figure 6:
Constraints on the lepton flavor violating Yukawa couplings, $ |Y_{\mathrm{e}\mu}| $ and $ |Y_{\mu\mathrm{e}}| $. The observed (expected) limit in black (red) line is derived from the limit on $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu) $ in this analysis. The green (yellow) band indicates the one (two) standard deviation uncertainty in the expected limit. The hashed region is excluded by this direct search. Other shaded regions represent indirect constraints derived from the null searches for $ \mu\to3\mathrm{e} $ (gray) [86], $ \mu\to\mathrm{e} $ conversion (light blue) [87], and $ \mu\to\mathrm{e}\gamma $ (dark green) [30]. The flavor-diagonal Yukawa couplings, $ |Y_{\mathrm{e}\mathrm{e}}| $ and $ |Y_{\mu\mu}| $, are assumed to be at their SM values in the calculation of these indirect limits. The purple line is the theoretical naturalness limit of $ |Y_{\mathrm{e}\mu}Y_{\mu\mathrm{e}}|\leq{m_{\mathrm{e}}}m_{\mu}/v^2 $, where $ v $ is the vacuum expectation value of the Higgs field. Dotted lines represent the corresponding constraints on $ |Y_{\mathrm{e}\mu}| $ and $ |Y_{\mu\mathrm{e}}| $ at upper limits on $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu) $ at 10$^{-5}$, 10$^{-6}$, 10$^{-7}$, and 10$^{-8} $, respectively. |
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Figure 7:
Observed (expected) 95% CL upper limits on $ \sigma(\mathrm{p}\mathrm{p} \to \mathrm{X} \to \mathrm{e} \mu) $ as functions of the hypothesised exotic Higgs boson mass. |
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Figure 8:
The $ m_{\mathrm{e}\mu} $ distribution of the observed data is shown, with the S+B fit at $ m_{\mathrm{X}}= $ 146 GeV in red solid line, and the background component of the fit in red dotted line. Events and fit in each category are weighted by S/(S+B). The one and two standard deviation uncertainty bands of the background component are shown in green and yellow. The lower panel shows the residuals after subtracting the background component of the fit from data. |
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Figure 9:
Observed global (blue) and local p-values (black, dotted) against the background-only hypothesis are shown as a function of the hypothesised exotic Higgs boson mass. |
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Figure 10:
Observed (expected) 95% CL upper limits on $ \sigma(\mathrm{p}\mathrm{p} \to \mathrm{X}(146) \to \mathrm{e} \mu) $ for each individual analysis category and for the combination of all analysis categories. |
| Tables | |
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Table 1:
Range of the ggH (VBF) BDT discriminant to define the ggH (VBF) subcategories, and the corresponding expected background ($ B $), and signal yield of $ \mathrm{H}(125)\to\mathrm{e}\mu $ at $ \mathcal{B}=$ 10$^{-4} $ (S). The yields are estimated by the number of MC events within a $ m_{\mathrm{e}\mu} $ interval of 125 $ \mathrm{GeV}\pm\sigma_{eff} $, where $ \sigma_{eff} $ is the smallest symmetric interval that contains 68% of the signal events in each category. The fraction contributions of the expected signal yields from the ggH and VBF production mode are listed. An estimate of the expected significance in each category by $ S/\sqrt{B} $ is also listed. |
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Table 2:
Systematic uncertainties in the expected signal yields from different sources for the ggH and VBF production modes. All the uncertainties are treated as correlated among categories. |
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Table 3:
Observed and expected 95% CL upper limits on $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu) $ at for each individual analysis category and for the combination of all analysis categories. |
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Table 4:
Observed (expected) 95% CL upper limits, best fit, and local significance in unit of standard deviation ($ \sigma $) of $ \sigma(\mathrm{p}\mathrm{p} \to \mathrm{X}(146) \to \mathrm{e} \mu) $ for each individual analysis category and for the combination of all analysis categories. |
| Summary |
| Searches for the lepton flavor violation decay of the standard model Higgs boson with a mass of 125 GeV (H(125)) and for a beyond the standard model Higgs boson (X) with a mass ($ m_{\mathrm{X}} $) from 110-160 GeV have been performed in the $ \mathrm{e} \mu $ final state in data collected by the CMS experiment. The data corresponds to an integrated luminosity of 138 fb$ ^{-1} $ of proton-proton collisions at a center-of-mass energy of 13 TeV. The results are extracted through a maximum likelihood fit to the electron-muon invariant mass distribution from 100-170 GeV. The observed (expected) upper limit on the branching fraction of the H(125) decay $ \mathcal{B}(\mathrm{H}(125)\to\mathrm{e}\mu) $ is found to be 4.4 (4.7) $\times$ 10$^{-5} $ at 95% confidence level, which is the most stringent direct limit set thus far. An excess of events over the expected background is observed with a global (local) significance of 2.8 (3.8) standard deviations at an invariant mass of the $ \mathrm{e} \mu $ final state of around 146 GeV. This is the first result of a direct search for $ \mathrm{X}\to\mathrm{e}\mu $, with $ m_{\mathrm{X}} $ below twice the W boson mass, as motivated by the largely unconstrained Type-III two-Higgs-doublet-model parameter space [34]. The observed significance of the excess in this analysis at around $ m_{\mathrm{e}\mu}\approx $ 146 GeV is insufficient to draw any conclusions on the naure of this excess. Data from the ongoing Run 3 of the LHC will help clarifying the nature of this excess in the future. |
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