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| CMS-PAS-B2G-24-023 | ||
| Search for heavy neutral leptons in top quark decays with same-sign dilepton final states in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 2026-05-18 | ||
| Abstract: The first search for a Majorana$ \text{-} $type heavy neutral lepton (N) produced in top quark decays with the CMS detector is reported. In type$ \text{-} $I seesaw models, where the N mixes with the first two generations of neutrinos, these heavy leptons lead to novel top quark decay modes that violate lepton number conservation. We therefore search for top quark pair production in proton$ \text{-} $proton collisions, followed by a top quark decay to a bottom quark and a W boson, decaying further into a charged lepton (electron or muon), and N. Here, the Majorana N decays to a W boson and a charged lepton with the same sign as the charged lepton produced with N. The experimental signature of this process is characterized by the presence of same$ \text{-} $sign and same$ \text{-} $flavor lepton pairs and jets. We use proton$ \text{-} $proton collision data corresponding to an integrated luminosity of 138 $ \text{fb}^{-1} $ at a center$ \text{-} $of$ \text{-} $mass energy of 13 TeV, collected during 2016--2018 with the CMS detector at the LHC. The search is performed for N masses between 20 and 100 GeV. Depending on the mass of N, the observed (expected) upper limits at the 95% CL on the branching ratio of the top quark decay to N, charged lepton, and bottom quark vary between 1.1 $ \times10^{-5} $ (6.8 $ \times10^{-6} $) and 4.7 $ \times10^{-5} $ (3.0 $ \times 10^{-5} $) in the dielectron final state and between 3.8 $ \times10^{-6} $ (3.4 $ \times 10^{-6} $) and 2.4 $ \times10^{-5} $ (1.3 $ \times 10^{-5} $) in the dimuon final state. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Representative Feynman diagrams of $ \mathrm{t} \overline{\mathrm{t}} $ production (upper) and SM top quark decays (lower left) and BSM decays involving $ \mathrm{N} $s (lower right), where $ \ell $ represents e or $ \mu $, and $ \text{f},\text{f}^\prime $ represent SM fermions. |
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Figure 1-a:
Representative Feynman diagrams of $ \mathrm{t} \overline{\mathrm{t}} $ production (upper) and SM top quark decays (lower left) and BSM decays involving $ \mathrm{N} $s (lower right), where $ \ell $ represents e or $ \mu $, and $ \text{f},\text{f}^\prime $ represent SM fermions. |
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Figure 1-b:
Representative Feynman diagrams of $ \mathrm{t} \overline{\mathrm{t}} $ production (upper) and SM top quark decays (lower left) and BSM decays involving $ \mathrm{N} $s (lower right), where $ \ell $ represents e or $ \mu $, and $ \text{f},\text{f}^\prime $ represent SM fermions. |
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Figure 1-c:
Representative Feynman diagrams of $ \mathrm{t} \overline{\mathrm{t}} $ production (upper) and SM top quark decays (lower left) and BSM decays involving $ \mathrm{N} $s (lower right), where $ \ell $ represents e or $ \mu $, and $ \text{f},\text{f}^\prime $ represent SM fermions. |
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Figure 1-d:
Representative Feynman diagrams of $ \mathrm{t} \overline{\mathrm{t}} $ production (upper) and SM top quark decays (lower left) and BSM decays involving $ \mathrm{N} $s (lower right), where $ \ell $ represents e or $ \mu $, and $ \text{f},\text{f}^\prime $ represent SM fermions. |
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Figure 2:
Pre$ \text{-} $fit distributions of the $ \Delta R $ between two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. Within the two rows for each variable, the top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 2-a:
Pre$ \text{-} $fit distributions of the $ \Delta R $ between two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. Within the two rows for each variable, the top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 2-b:
Pre$ \text{-} $fit distributions of the $ \Delta R $ between two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. Within the two rows for each variable, the top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 2-c:
Pre$ \text{-} $fit distributions of the $ \Delta R $ between two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. Within the two rows for each variable, the top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 2-d:
Pre$ \text{-} $fit distributions of the $ \Delta R $ between two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. Within the two rows for each variable, the top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 3:
Pre$ \text{-} $fit distributions of theinvariant mass of two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The region between two magenta lines depicts what is removed from $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement in the dielectron channel. The blue line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation and DY simulation combined without $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement. In order to match the $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement to the bin edges, the plots on the left have the bin size of 10 GeV, except for the [70,81.2] and [101.2,110] GeV bins. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 3-a:
Pre$ \text{-} $fit distributions of theinvariant mass of two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The region between two magenta lines depicts what is removed from $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement in the dielectron channel. The blue line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation and DY simulation combined without $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement. In order to match the $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement to the bin edges, the plots on the left have the bin size of 10 GeV, except for the [70,81.2] and [101.2,110] GeV bins. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 3-b:
Pre$ \text{-} $fit distributions of theinvariant mass of two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The region between two magenta lines depicts what is removed from $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement in the dielectron channel. The blue line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation and DY simulation combined without $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement. In order to match the $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement to the bin edges, the plots on the left have the bin size of 10 GeV, except for the [70,81.2] and [101.2,110] GeV bins. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 3-c:
Pre$ \text{-} $fit distributions of theinvariant mass of two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The region between two magenta lines depicts what is removed from $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement in the dielectron channel. The blue line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation and DY simulation combined without $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement. In order to match the $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement to the bin edges, the plots on the left have the bin size of 10 GeV, except for the [70,81.2] and [101.2,110] GeV bins. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 3-d:
Pre$ \text{-} $fit distributions of theinvariant mass of two leptons are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The region between two magenta lines depicts what is removed from $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement in the dielectron channel. The blue line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation and DY simulation combined without $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement. In order to match the $ |m_{\mathrm{e}\mathrm{e}}-91.2| > $ 10 GeV requirement to the bin edges, the plots on the left have the bin size of 10 GeV, except for the [70,81.2] and [101.2,110] GeV bins. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 4:
Pre$ \text{-} $fit distributions of the leading lepton transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 4-a:
Pre$ \text{-} $fit distributions of the leading lepton transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 4-b:
Pre$ \text{-} $fit distributions of the leading lepton transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 4-c:
Pre$ \text{-} $fit distributions of the leading lepton transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 4-d:
Pre$ \text{-} $fit distributions of the leading lepton transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 5:
Pre$ \text{-} $fit distributions of themissing transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 5-a:
Pre$ \text{-} $fit distributions of themissing transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 5-b:
Pre$ \text{-} $fit distributions of themissing transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 5-c:
Pre$ \text{-} $fit distributions of themissing transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 5-d:
Pre$ \text{-} $fit distributions of themissing transverse momentum are shown in dielectron (left) and dimuon (right) events as examples of the input variables for the training. The top row and the bottom row correspond to the $ m_{\mathrm{N}} < m_{\mathrm{W}} $ selection and the $ m_{\mathrm{N}} > m_{\mathrm{W}} $ selection, respectively, which differ in the required number of untagged jets. The cyan line is the $ \mathrm{t} \overline{\mathrm{t}} $ simulation used for the training background. Signal distributions are normalized to the background rate. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 6:
Comparison of BDTG classifier distributions between actual distribution of $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation passing tight ID and expected distribution from $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation dielectron (left) and dimuon (right) events in the sideband region and misidentification rate of QCD simulation with BDT model for $ m_{\mathrm{N}}= $ 50 GeV (top) and for $ m_{\mathrm{N}}= $ 100 GeV (bottom). Error bands include 10$ % $ (15$ % $) uncertainty for dielectron (dimuon) events. |
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Figure 6-a:
Comparison of BDTG classifier distributions between actual distribution of $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation passing tight ID and expected distribution from $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation dielectron (left) and dimuon (right) events in the sideband region and misidentification rate of QCD simulation with BDT model for $ m_{\mathrm{N}}= $ 50 GeV (top) and for $ m_{\mathrm{N}}= $ 100 GeV (bottom). Error bands include 10$ % $ (15$ % $) uncertainty for dielectron (dimuon) events. |
png pdf |
Figure 6-b:
Comparison of BDTG classifier distributions between actual distribution of $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation passing tight ID and expected distribution from $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation dielectron (left) and dimuon (right) events in the sideband region and misidentification rate of QCD simulation with BDT model for $ m_{\mathrm{N}}= $ 50 GeV (top) and for $ m_{\mathrm{N}}= $ 100 GeV (bottom). Error bands include 10$ % $ (15$ % $) uncertainty for dielectron (dimuon) events. |
png pdf |
Figure 6-c:
Comparison of BDTG classifier distributions between actual distribution of $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation passing tight ID and expected distribution from $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation dielectron (left) and dimuon (right) events in the sideband region and misidentification rate of QCD simulation with BDT model for $ m_{\mathrm{N}}= $ 50 GeV (top) and for $ m_{\mathrm{N}}= $ 100 GeV (bottom). Error bands include 10$ % $ (15$ % $) uncertainty for dielectron (dimuon) events. |
png pdf |
Figure 6-d:
Comparison of BDTG classifier distributions between actual distribution of $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation passing tight ID and expected distribution from $ {\mathrm{t}\overline{\mathrm{t}}} $ simulation dielectron (left) and dimuon (right) events in the sideband region and misidentification rate of QCD simulation with BDT model for $ m_{\mathrm{N}}= $ 50 GeV (top) and for $ m_{\mathrm{N}}= $ 100 GeV (bottom). Error bands include 10$ % $ (15$ % $) uncertainty for dielectron (dimuon) events. |
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Figure 7:
Validation of nonprompt background in the trilepton validation region. Plots of $ p_{\mathrm{T}} $ of the electron (left) or muon (right) not from the Z boson decay are shown. The last bin includes the overflow bins. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 7-a:
Validation of nonprompt background in the trilepton validation region. Plots of $ p_{\mathrm{T}} $ of the electron (left) or muon (right) not from the Z boson decay are shown. The last bin includes the overflow bins. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 7-b:
Validation of nonprompt background in the trilepton validation region. Plots of $ p_{\mathrm{T}} $ of the electron (left) or muon (right) not from the Z boson decay are shown. The last bin includes the overflow bins. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 8:
The invariant mass of SS electron pair in charge mismeasurement validation region. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 9:
The invariant mass of three leptons are shown for the conversion electron (left) events and for the conversion muon (right) events for the conversion background validation. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 9-a:
The invariant mass of three leptons are shown for the conversion electron (left) events and for the conversion muon (right) events for the conversion background validation. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 9-b:
The invariant mass of three leptons are shown for the conversion electron (left) events and for the conversion muon (right) events for the conversion background validation. The background uncertainty band shows the total uncertainty including systematics. |
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Figure 10:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 10-a:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 10-b:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 10-c:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 10-d:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 10-e:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 10-f:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 20 (top), 50 (middle), and 75 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 11:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 85 (top) and 100 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 11-a:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 85 (top) and 100 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 11-b:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 85 (top) and 100 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 11-c:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 85 (top) and 100 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 11-d:
Comparison of the observed (points) and predicted (filled histograms) post$ \text{-} $fit BDTG score distributions in the dielectron (left) and dimuon (right) channels. The BDTG score distributions correspond to classifiers trained with signal mass hypotheses of 85 (top) and 100 (bottom) GeV. The expected signal distributions extracted with the same classifier are overlaid on the figures, assuming a $ \mathcal{B}(\mathrm{t}\to\mathrm{b}\ell\mathrm{N}) $ value of 2.4 $ \times10^{-4} $. The background uncertainty band shows the total uncertainty including systematics. |
png pdf |
Figure 12:
The expected and observed upper limits at 95$ % $ CL on the branching fraction, $ \mathcal{B} $ (t}\rightarrow\textb\ell\text{N), at the $ m_{\mathrm{N}} $ values considered in the search. The left and right figures correspond to the electron and muon mixing scenarios. The uncertainty includes statistical and systematic uncertainties. |
png pdf |
Figure 12-a:
The expected and observed upper limits at 95$ % $ CL on the branching fraction, $ \mathcal{B} $ (t}\rightarrow\textb\ell\text{N), at the $ m_{\mathrm{N}} $ values considered in the search. The left and right figures correspond to the electron and muon mixing scenarios. The uncertainty includes statistical and systematic uncertainties. |
png pdf |
Figure 12-b:
The expected and observed upper limits at 95$ % $ CL on the branching fraction, $ \mathcal{B} $ (t}\rightarrow\textb\ell\text{N), at the $ m_{\mathrm{N}} $ values considered in the search. The left and right figures correspond to the electron and muon mixing scenarios. The uncertainty includes statistical and systematic uncertainties. |
png pdf |
Figure 13:
The expected and observed upper limits at 95$ % $ CL on the model parameters, $ |\mathrm{V}_{\mathrm{e}\mathrm{N}}|^{2} $ and $ |\mathrm{V}_{\mu\mathrm{N}}|^{2} $, at the $ m_{\mathrm{N}} $ values considered in the search. The left and right figures correspond to the electron and muon mixing scenarios. For comparison, observed limits from DY channels are overlaid on the figure [9,10]. |
png pdf |
Figure 13-a:
The expected and observed upper limits at 95$ % $ CL on the model parameters, $ |\mathrm{V}_{\mathrm{e}\mathrm{N}}|^{2} $ and $ |\mathrm{V}_{\mu\mathrm{N}}|^{2} $, at the $ m_{\mathrm{N}} $ values considered in the search. The left and right figures correspond to the electron and muon mixing scenarios. For comparison, observed limits from DY channels are overlaid on the figure [9,10]. |
png pdf |
Figure 13-b:
The expected and observed upper limits at 95$ % $ CL on the model parameters, $ |\mathrm{V}_{\mathrm{e}\mathrm{N}}|^{2} $ and $ |\mathrm{V}_{\mu\mathrm{N}}|^{2} $, at the $ m_{\mathrm{N}} $ values considered in the search. The left and right figures correspond to the electron and muon mixing scenarios. For comparison, observed limits from DY channels are overlaid on the figure [9,10]. |
| Tables | |
png pdf |
Table 1:
Summary of trigger $ p_{\mathrm{T}} $ thresholds and additional selection requirements for dielectron and dimuon channels. |
png pdf |
Table 2:
Input variables used for the training of classifiers. The usage in the low and high mass scenarios ($ m_{\mathrm{N}} < m_{\mathrm{W}} $ and $ m_{\mathrm{N}} > m_{\mathrm{W}} $) is shown in the second and third columns. Subscript "a" indices for leptons, and subscript "b" indexes for jets. $ \text{j}_{L} $ denotes the non$ \text{-} $b$ \text{-} $tagged jet. The check marks below $ m_{\mathrm{N}} $ conditions indicate the corresponding input variable is used for the training. |
png pdf |
Table 3:
Summary of event selection criteria for the SR and background validation regions. |
png pdf |
Table 4:
Summary of systematic uncertainties |
| Summary |
| This note presents the first CMS search for heavy neutral leptons ($ \mathrm{N} $) produced in top quark decays using same$ \text{-} $sign dilepton final states in proton$ \text{-} $proton collisions at $ \sqrt{s}= $ 13 TeV, corresponding to an integrated luminosity of 138 $ \text{fb}^{-1} $ collected with the CMS detector. The analysis targeted top quark decays via a $ \mathrm{N} $, predicted by type$ \text{-} $I seesaw models, exploring $ \mathrm{N} $ masses between 20 and 100 GeV. Events are selected using a same$ \text{-} $sign requirement for dileptons and the presence of jets, and a multivariate classifier to enhance the sensitivity. The result of this analysis shows the sensitivity to the branching fraction of the top quark decaying to $ \mathrm{N} $ of order $ 10^{-6} $ to $ 10^{-5} $. Converting into model parameters of the type$ \text{-} $I seesaw model, the mixing angle between $ \mathrm{N} $ and the standard model neutrino, the search shows that this channel is sensitive to those parameters at the order of $ 10^{-4} $ and 0.1 for the mass of $ \mathrm{N} $ values lower and higher than the W boson mass. |
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