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| CMS-TOP-19-006 ; CERN-EP-2021-248 | ||
| Search for charged-lepton flavor violation in top quark production and decay in pp collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 19 January 2022 | ||
| JHEP 06 (2022) 082 | ||
| Abstract: Results are presented from a search for charged-lepton flavor violating (CLFV) interactions in top quark production and decay in pp collisions at a center-of-mass energy of 13 TeV. The events are required to contain one oppositely charged electron-muon pair in the final state, along with at least one jet identified as originating from a bottom quark. The data correspond to an integrated luminosity of 138 fb$^{-1}$, collected by the CMS experiment at the LHC. This analysis includes both the production (q $\to$ e$\mu$t) and decay (t $\to$ e$ \mu$q) modes of the top quark through CLFV interactions, with q referring to an u or c quark. These interactions are parametrized using an effective field theory approach. With no significant excess over the standard model expectation, the results are interpreted in terms of vector-, scalar-, and tensor-like CLFV four-fermion effective interactions. Finally, observed exclusion limits are set at 95% confidence levels on the respective branching fractions of a top quark to an e$\mu$ pair and an up (charm) quark of 0.13 $\times $ 10$^{-6}$ (1.31 $\times $ 10$^{-6}$), 0.07 $\times $ 10$^{-6}$ (0.89 $\times $ 10$^{-6}$), and 0.25 $\times $ 10$^{-6}$ (2.59 $\times $ 10$^{-6}$) for vector, scalar, and tensor CLFV interactions, respectively. | ||
| Links: e-print arXiv:2201.07859 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Feynman diagrams for single top quark production (left and middle) and top quark decays in SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ events (right) via CLFV interactions. The CLFV vertex is marked as a filled circle. |
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Figure 2:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
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Figure 2-a:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
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Figure 2-b:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 2-c:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 2-d:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 2-e:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 2-f:
The distributions of the leading lepton ${p_{\mathrm {T}}}$ (upper row), $\Delta R(\mathrm{e}, \mu)$ (middle row), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 3:
The distributions of the leading jet ${p_{\mathrm {T}}}$ (upper row) and the number of jets (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 3-a:
The distributions of the leading jet ${p_{\mathrm {T}}}$ (upper row) and the number of jets (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 3-b:
The distributions of the leading jet ${p_{\mathrm {T}}}$ (upper row) and the number of jets (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 3-c:
The distributions of the leading jet ${p_{\mathrm {T}}}$ (upper row) and the number of jets (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 3-d:
The distributions of the leading jet ${p_{\mathrm {T}}}$ (upper row) and the number of jets (lower row) are shown for data (points) and simulation (histograms). Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Overflow events are added to the last bin. Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 4:
The BDT output distributions for data (points) and backgrounds (histograms) with the ratio of data to the total background yield, before (middle panel) and after (lower panel) the fit. Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 4-a:
The BDT output distributions for data (points) and backgrounds (histograms) with the ratio of data to the total background yield, before (middle panel) and after (lower panel) the fit. Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 4-b:
The BDT output distributions for data (points) and backgrounds (histograms) with the ratio of data to the total background yield, before (middle panel) and after (lower panel) the fit. Events with one or more b-tagged jets are shown in the left and right column, respectively. The hatched bands indicate the total uncertainty (statistical and systematic taken in quadrature) for the SM background predictions (cf. Section 6). Examples of the predicted signal contribution for the vector type CLFV interactions via $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ and $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ vertices are shown, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. The signal production- and decay-mode contributions are summed. The $\mathrm{e} \mu \mathrm{t} \mathrm{c} $ signal cross section is scaled up by a factor of 10 for improved visualization. |
png pdf |
Figure 5:
The observed 95% CL exclusion limits on the $\mathrm{e} \mu \mathrm{t} \mathrm{c} $; of the $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ Wilson coefficient (left) and $\mathcal {B}(\mathrm{t} \to \mathrm{e} \mu \mathrm{c})$ as a function of $\mathcal {B}(\mathrm{t} \to \mathrm{e} \mu \mathrm{u})$ (right) for the vector-, scalar-, and tensor-like CLFV interactions. The hatched bands indicate the regions containing 68% of the distribution of limits expected under the background-only hypothesis. |
png pdf |
Figure 5-a:
The observed 95% CL exclusion limits on the $\mathrm{e} \mu \mathrm{t} \mathrm{c} $; of the $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ Wilson coefficient (left) and $\mathcal {B}(\mathrm{t} \to \mathrm{e} \mu \mathrm{c})$ as a function of $\mathcal {B}(\mathrm{t} \to \mathrm{e} \mu \mathrm{u})$ (right) for the vector-, scalar-, and tensor-like CLFV interactions. The hatched bands indicate the regions containing 68% of the distribution of limits expected under the background-only hypothesis. |
png pdf |
Figure 5-b:
The observed 95% CL exclusion limits on the $\mathrm{e} \mu \mathrm{t} \mathrm{c} $; of the $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ Wilson coefficient (left) and $\mathcal {B}(\mathrm{t} \to \mathrm{e} \mu \mathrm{c})$ as a function of $\mathcal {B}(\mathrm{t} \to \mathrm{e} \mu \mathrm{u})$ (right) for the vector-, scalar-, and tensor-like CLFV interactions. The hatched bands indicate the regions containing 68% of the distribution of limits expected under the background-only hypothesis. |
| Tables | |
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Table 1:
Theoretical cross sections, in fb, for single top quark production and top quark decays via the vector, scalar, and tensor CLFV interactions, assuming a top quark mass of 172.5 GeV, the top quark decay width 1.33 GeV, $\Lambda = $ 1 TeV and $\text {C}^{\mathrm{e} \mu \mathrm{t} \mathrm{q}}_{x}=$ 1. The uncertainties from the QCD scales and PDF are given ($\sigma ^{+\text {scale}}_{-\text {scale}}$ $\pm$ PDF). |
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Table 2:
The number of expected events from SM ${\mathrm{t} {}\mathrm{\bar{t}}}$, ${\mathrm{t} \mathrm{W}}$, and from the other backgrounds; and the total background expectation and the number of events observed in data collected during 2016-2018, after all selections in signal (1 b tagged) and control ($ > 1$ b tagged) regions. The total uncertainty, including both statistical and unfited systematic components, is quoted in quadrature for the expected backgrounds. The expected signal yields for single top quark production and top quark decays via the vector, scalar, and tensor CLFV interactions are also shown with their MC statistical uncertainties, assuming $\text {C}_x/\Lambda ^2 =$ 1 TeV$ ^{-2}$. |
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Table 3:
Summary of representative systematic uncertainties in selection efficiency for the SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ process and for single top quark production and decays via vector $\mathrm{e} \mu \mathrm{t} \mathrm{u} $ CLFV interactions in the signal plus ${\mathrm{t} {}\mathrm{\bar{t}}}$ control regions. |
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Table 4:
Expected and observed 95% CL upper limits on signal cross sections (production and decay modes), the CLFV Wilson coefficients, and top quark CLFV branching fractions. |
| Summary |
|
A search is reported for charged-lepton flavor violation in top quark production and decay. The analysis is based on pp collisions collected by the CMS detector at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events are selected if they contain an oppositely charged electron-muon pair and at least one b-tagged jet. An effective field theory approach is used for parametrizing top quark lepton flavor violating interactions. The production and decay modes of the top quark through these effective interactions are included in this analysis. A boosted decision tree is used to distinguish signal from background. No significant excess is observed over the expectations from the standard model. Upper limits are set on the strength of the individual vector-, scalar-, and tensor-like four-fermion effective operators. These are converted to limits on the branching fractions of the top quark $\mathcal{B}(\mathrm{t} \to \mathrm{e}\mu \mathrm{q})$, q = u (c) quark, $ < $ 0.13$\times $10$^{-6}$ (1.31$\times $10$^{-6}$), 0.07$\times $10$^{-6}$ (0.89$\times $10$^{-6}$), and 0.25$\times $10$^{-6}$ (2.59$\times $10$^{-6}$) for vector, scalar, and tensor CLFV interactions, respectively. The resulting limits are the most restrictive bounds to date. |
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