CMSPASTOP22011  
Search for lepton flavour violation in top quark interactions with an uptype quark, a muon, and a $ \tau $ lepton  
CMS Collaboration  
5 August 2024  
Abstract: We present a search for chargedlepton flavour violation (CLFV) in the top quark (t) sector using 138 fb$ ^{1} $ of protonproton collision data collected with the CMS experiment at a centreofmass energy of 13 TeV. The analysis focuses on events containing a single muon ($ \mu $) and a hadronically decaying $ \tau $ lepton. Machine learning multiclass classification techniques are used to distinguish signal from standard model background events. The CLFV signal consists of the production of a single top quark via a CLFV interaction or top quark pair production followed by a CLFV decay. The results of this search are consistent with the standard model expectations. The upper limits at 95% confidence level on the branching fraction $ \mathcal{B} $ for CLFV top quark decays to an up (u) or a charm (c) quark, a muon and a $ \tau $ lepton are $ \mathcal{B}(\mathrm{t} \to \mu\tau\mathrm{u}) < $ 0.04, 0.078, and 0.118 $ \times$ 10$^{6} $, and $ \mathcal{B}(\mathrm{t} \to \mu\tau\mathrm{c}) < $ 0.81, 1.71, and 2.05 $ \times$ 10$^{6} $ for scalar, vector, and tensorlike operators, respectively.  
Links: CDS record (PDF) ; CADI line (restricted) ; 
Figures  
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Figure 1:
Example Feynman diagrams at leading order for the CLFV single production of a top quark (left and centre) and top quark pair production followed by a CLFV decay (right) 
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Figure 1a:
Example Feynman diagrams at leading order for the CLFV single production of a top quark (left and centre) and top quark pair production followed by a CLFV decay (right) 
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Figure 1b:
Example Feynman diagrams at leading order for the CLFV single production of a top quark (left and centre) and top quark pair production followed by a CLFV decay (right) 
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Figure 1c:
Example Feynman diagrams at leading order for the CLFV single production of a top quark (left and centre) and top quark pair production followed by a CLFV decay (right) 
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Figure 2:
Distributions in $ p_{\mathrm{T}} $ of the muon (top left), $ \tau_\mathrm{h} $ (top right), $ p_{\mathrm{T}} $leading jet (bottom left), and $ p_{\mathrm{T}} $trailing jet (bottom right) after all selection steps. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 2a:
Distributions in $ p_{\mathrm{T}} $ of the muon (top left), $ \tau_\mathrm{h} $ (top right), $ p_{\mathrm{T}} $leading jet (bottom left), and $ p_{\mathrm{T}} $trailing jet (bottom right) after all selection steps. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 2b:
Distributions in $ p_{\mathrm{T}} $ of the muon (top left), $ \tau_\mathrm{h} $ (top right), $ p_{\mathrm{T}} $leading jet (bottom left), and $ p_{\mathrm{T}} $trailing jet (bottom right) after all selection steps. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 2c:
Distributions in $ p_{\mathrm{T}} $ of the muon (top left), $ \tau_\mathrm{h} $ (top right), $ p_{\mathrm{T}} $leading jet (bottom left), and $ p_{\mathrm{T}} $trailing jet (bottom right) after all selection steps. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 2d:
Distributions in $ p_{\mathrm{T}} $ of the muon (top left), $ \tau_\mathrm{h} $ (top right), $ p_{\mathrm{T}} $leading jet (bottom left), and $ p_{\mathrm{T}} $trailing jet (bottom right) after all selection steps. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 3:
Distributions in the reconstructed top quark mass (top left), W boson mass (top right), and minimum $ \chi^2 $ (bottom) from the top quark reconstruction. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 3a:
Distributions in the reconstructed top quark mass (top left), W boson mass (top right), and minimum $ \chi^2 $ (bottom) from the top quark reconstruction. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
png pdf 
Figure 3b:
Distributions in the reconstructed top quark mass (top left), W boson mass (top right), and minimum $ \chi^2 $ (bottom) from the top quark reconstruction. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
png pdf 
Figure 3c:
Distributions in the reconstructed top quark mass (top left), W boson mass (top right), and minimum $ \chi^2 $ (bottom) from the top quark reconstruction. The solid lines show the signal distributions, individually for each type of operator and interaction. The signals are normalized to the total number of events in data for visibility. The last bin in each histogram contains the overflow. The shaded band displays the total uncertainty in the predicted background, consisting of statistical and systematic uncertainties. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 4:
Combined distributions in the DNN score after the profile likelihood fit for all datataking periods for the vector operators with $ \mathrm{t}\mathrm{u}\mu\tau $ (left) and $ \mathrm{t}\mathrm{c}\mu\tau $ (right) couplings. The signal distributions are normalized to the total number of events observed in the data. The last bin of each histogram contains the overflow. The hatched bands represent the total postfit uncertainties in the background predictions, including statistical and systematic sources. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 4a:
Combined distributions in the DNN score after the profile likelihood fit for all datataking periods for the vector operators with $ \mathrm{t}\mathrm{u}\mu\tau $ (left) and $ \mathrm{t}\mathrm{c}\mu\tau $ (right) couplings. The signal distributions are normalized to the total number of events observed in the data. The last bin of each histogram contains the overflow. The hatched bands represent the total postfit uncertainties in the background predictions, including statistical and systematic sources. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 4b:
Combined distributions in the DNN score after the profile likelihood fit for all datataking periods for the vector operators with $ \mathrm{t}\mathrm{u}\mu\tau $ (left) and $ \mathrm{t}\mathrm{c}\mu\tau $ (right) couplings. The signal distributions are normalized to the total number of events observed in the data. The last bin of each histogram contains the overflow. The hatched bands represent the total postfit uncertainties in the background predictions, including statistical and systematic sources. The panels below the distributions show the ratio of data to the background prediction. 
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Figure 5:
Exclusion contours for the observed and expected upper limits and central probability intervals containing 68% of the expected upper limits for the branching fractions (left) and Wilson coefficients (right) corresponding to the $ \mathrm{t}\mathrm{u}\mu\tau $ and $ \mathrm{t}\mathrm{c}\mu\tau $ couplings for scalar, vector and tensor Lorentz structures. 
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Figure 5a:
Exclusion contours for the observed and expected upper limits and central probability intervals containing 68% of the expected upper limits for the branching fractions (left) and Wilson coefficients (right) corresponding to the $ \mathrm{t}\mathrm{u}\mu\tau $ and $ \mathrm{t}\mathrm{c}\mu\tau $ couplings for scalar, vector and tensor Lorentz structures. 
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Figure 5b:
Exclusion contours for the observed and expected upper limits and central probability intervals containing 68% of the expected upper limits for the branching fractions (left) and Wilson coefficients (right) corresponding to the $ \mathrm{t}\mathrm{u}\mu\tau $ and $ \mathrm{t}\mathrm{c}\mu\tau $ couplings for scalar, vector and tensor Lorentz structures. 
Tables  
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Table 1:
The EFT operators considered in this analysis and their definition. The $ \varepsilon $ is a fully antisymmetric twodimensional matrix, $ \gamma^{\mu} $ are the Dirac matrices, and $ \sigma^{\mu\nu}=\frac{i}{2}[\gamma^{\mu},\gamma^{\nu}] $. Lefthanded doublets of leptons and quarks are denoted by $ \ell_i $ and $ \mathrm{q}_k $, respectively, where the indices $ i $ and $ k $ denote the lepton and quark flavours. Righthanded lepton and quark singlets are denoted by $ \mathrm{e}_i $ and $ \mathrm{u}_k $, respectively. The operator $ O_{\ell\mathrm{q}}^{1} $ represents the lefthanded fermion interaction $ O_{\ell\mathrm{q}} $. 
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Table 2:
Predicted cross sections for CLFV signal processes. Operators with different Lorentz structures are considered with $ C_a / \Lambda^2 = $ 1 TeV$^{2} $. The results for ST CLFV are at LO accuracy and the ones for TT CLFV are at NNLO+NNLL accuracy for the $ \mathrm{t} \overline{\mathrm{t}} $ production with LO accuracy for the CLFV decay. 
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Table 3:
Estimated event yields including the background corrections from the ABCD method discussed in Sec 5. The numbers shown correspond to observed events before the maximumlikelihood fit described in Section 8. Only statistical uncertainties are shown, related to the size of the data sets. 
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Table 4:
Input features of the DNN. The angular distance $ \Delta R_{ij} $ between two objects $ i $ and $ j $ is defined as $ \Delta R_{ij} = \sqrt{\Delta\eta_{ij}^2 + \Delta\phi_{ij}^2} $, where $ \Delta\eta_{ij} $ and $ \Delta\phi_{ij} $ are the differences in pseudorapidity and azimuthal angle, respectively. 
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Table 5:
The 95% CL observed and expected upper limits on CLFV cross sections, Wilson coefficients $ C_{\mathrm{t}\mathrm{q}\mu\tau} $, and branching fractions for different types of interactions and Lorentz structures. The expected upper limits are shown in brackets behind the observed limits. The central probability intervals containing 68% of the expected upper limits are given in square brackets below the upper limits. 
Summary 
A search for chargedlepton flavour violation (CLFV) in the top quark sector has been presented. The search uses data corresponding to an integrated luminosity of 138 fb$ ^{1} $ collected by the CMS experiment during 20162018 in protonproton (pp) collisions at a centreofmass energy of 13 TeV. Interactions of a top quark with a muon, a tau lepton, and an uptype quark u or c are considered, where the scale of new physics responsible for CLFV is assumed to be larger than the energy scale of pp collisions at the LHC. The signal extraction is performed using measured distributions in a multiclass discriminator obtained with a deep neural network. No significant deviation is observed from the standard model background prediction and upper limits on the signal cross sections are set at 95% confidence level (CL). The limits are interpreted in terms of CLFV branching fractions ($ \mathcal{B} $) of the top quark, resulting in $ \mathcal{B}(\mathrm{t} \to \mu\tau\mathrm{u}) < $ 0.04, 0.078, and 0.118 $ \times$ 10$^{6} $, and $ \mathcal{B}(\mathrm{t} \to \mu\tau\mathrm{c}) < $ 0.81, 1.71, and 2.05 $ \times$ 10$^{6} $ at 95% CL for scalar, vector, and tensorlike operators, respectively. This search complements previous CMS results involving $ \mathrm{e}\mu $ CLFV interactions [12,13] and results in more stringent upper limits on Wilson coefficients in an effective field theory by approximately a factor of two compared to the latest experimental results involving $ \mu\tau $ CLFV interactions [11]. 
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