CMSPASHIG17003  
Search for the associated production of a Higgs boson with a top quark pair in final states with a $\tau$ lepton at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
March 2017  
Abstract: Results of a search for the standard model Higgs boson produced in association with a top quark pair in final states with $\tau$ leptons are presented. The analyzed dataset corresponds to an integrated luminosity of 35.9 fb$^{1}$ which has been recorded in protonproton collisions at $\sqrt{s} = $ 13 TeV centerofmass energy by the CMS experiment during LHC Run2. The sensitivity of the search is improved by using matrix element and machine learning methods to separate the signal from backgrounds. The measured signal rate amounts to 0.72$^{+0.62}_{0.53}$ times the production rate expected in the standard model, with an observed (expected) significance of 1.4$\sigma$ (1.8$\sigma$). An upper limit on the signal rate of 2.0 the standard model production rate is set at the 95% confidence level.  
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; 
Figures  
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Figure 1:
A typical Feynman diagram for ttH production with subsequent decay of the H boson to a pair of $\tau$ leptons. 
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Figure 2:
Distributions in the discriminating observables used for the signal extraction in the 1${\ell }$+2$ {\tau _{\textrm {h}}} $ (top left) and 3${\ell }$+1$ {\tau _{\textrm {h}}} $ (top right) categories and in the "nomissingjet'' (bottom left) and "missingjet'' (bottom right) subcategories of the 2$\ell$ss+1$\tau_{\mathrm{h}}$ category, compared to the SM expectation for the ttH signal and for background processes. The distributions expected for the ttH signal and for the backgrounds are shown for the values of nuisance parameters obtained from the maximum likelihood fit. The lowest bin of the MEM discriminant in the "missingjet'' subcategory collects events for which the kinematics of the reconstructed objects is not compatible with the ttH, $ { {\mathrm {H}} }\to \tau \tau $ signal hypothesis. 
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Figure 2a:
Distribution in the discriminating observable used for the signal extraction in the 1${\ell }$+2$ {\tau _{\textrm {h}}} $ category, compared to the SM expectation for the ttH signal and for background processes. The distributions expected for the ttH signal and for the backgrounds are shown for the values of nuisance parameters obtained from the maximum likelihood fit. 
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Figure 2b:
Distribution in the discriminating observable used for the signal extraction in the 3${\ell }$+1$ {\tau _{\textrm {h}}} $ category, compared to the SM expectation for the ttH signal and for background processes. The distributions expected for the ttH signal and for the backgrounds are shown for the values of nuisance parameters obtained from the maximum likelihood fit. 
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Figure 2c:
Distribution in the discriminating observable used for the signal extraction in the "nomissingjet'' subcategory of the 2$\ell$ss+1$\tau_{\mathrm{h}}$ category, compared to the SM expectation for the ttH signal and for background processes. The distributions expected for the ttH signal and for the backgrounds are shown for the values of nuisance parameters obtained from the maximum likelihood fit. 
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Figure 2d:
Distribution in the discriminating observable used for the signal extraction in the "missingjet'' subcategory of the 2$\ell$ss+1$\tau_{\mathrm{h}}$ category, compared to the SM expectation for the ttH signal and for background processes. The distributions expected for the ttH signal and for the backgrounds are shown for the values of nuisance parameters obtained from the maximum likelihood fit. The lowest bin of the MEM discriminant collects events for which the kinematics of the reconstructed objects is not compatible with the ttH, $ { {\mathrm {H}} }\to \tau \tau $ signal hypothesis. 
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Figure 3:
Signal rates $\mu $, in units of the SM ttH production rate, measured in each of the categories 2$\ell$ss+1$\tau_{\mathrm{h}}$, 3${\ell }$+1$ {\tau _{\textrm {h}}} $, and 1${\ell }$+2$ {\tau _{\textrm {h}}} $ individually and for the combination of all three categories. 
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Figure 4:
95% CL upper limits on the ttH signal rate, obtained in each of the categories 2$\ell$ss+1$\tau_{\mathrm{h}}$, 3${\ell }$+1$ {\tau _{\textrm {h}}} $, and 1${\ell }$+2$ {\tau _{\textrm {h}}} $ individually and for the combination of all three event categories. The expected limits are computed for the backgroundonly ($\mu =$ 0) hypothesis. 
Tables  
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Table 1:
Number of events events selected in the 2$\ell$ss+1$\tau_{\mathrm{h}}$, 3${\ell }$+1$ {\tau _{\textrm {h}}} $, and 1${\ell }$+2$ {\tau _{\textrm {h}}} $ categories compared to the SM expectation for the ttH signal and background processes. The event yield in the 1${\ell }$+2$ {\tau _{\textrm {h}}} $ category is given in the signallike region MVA $ >$ 0.2 of the output of the BDT that is used for the signal extraction. The event yields expected for the ttH signal and for the backgrounds are shown for the values of nuisance parameters obtained from the maximum likelihood fit. Quoted uncertainties represent the combination of statistical and systematic uncertainties. 
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Table 2:
Signal rates $\mu $, in units of the SM ttH production rate, measured and expected in each of the categories 2$\ell$ss+1$\tau_{\mathrm{h}}$, 3${\ell }$+1$ {\tau _{\textrm {h}}} $, and 1${\ell }$+2$ {\tau _{\textrm {h}}} $ individually and for the combination of all three categories. 
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Table 3:
95% CL upper limits on the ttH signal rate, in units of the SM ttH production rate, obtained in each of the categories 2$\ell$ss+1$\tau_{\mathrm{h}}$, 3${\ell }$+1$ {\tau _{\textrm {h}}} $, and 1${\ell }$+2$ {\tau _{\textrm {h}}} $ individually and for the combination of all three event categories. The observed limit is compared to the limits expected for the backgroundonly hypothesis ($\mu =$ 0) and for the case that a ttH signal of SM production rate is present in the data ($\mu =$ 1). The limits expected in the case $\mu =$ 1 are computed using an Asimov dataset, while the limits expected in the case $\mu =$ 0 are computed for the values of nuisance parameters obtained from a maximum likelihood fit of the backgroundonly hypothesis to the data. The $\pm$1$ \sigma $ uncertainty intervals of the limits expected in case of the backgroundonly hypothesis are also given in the table. 
Summary 
A search for the associated production of a Higgs boson with top quark pair in final states with a $\tau$ lepton has been presented. The analyzed dataset corresponds to 35.9 fb$^{1}$ of pp collision data recorded in 2016 by the CMS experiment at $\sqrt{s}=$ 13 TeV. The analysis is performed in three event categories: 1$\ell$+2$\tau_{\mathrm{h}}$, 2$\ell$ss+1$\tau_{\mathrm{h}}$ and 3$\ell$+1$\tau_{\mathrm{h}}$. The sensitivity of the analysis is enhanced by using multivariate analysis techniques, based on the matrix element method and boosted decision trees. The results of the analysis are in agreement with the SM expectation. The measured signal rate amounts to 0.72$^{+0.62}_{0.53}$ times the SM ttH production rate, with an observed (expected) significance of 1.4$\sigma$ (1.8$\sigma$). An upper limit on the signal rate of 2.0 times the SM ttH production rate at 95% CL is set. 
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