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| CMS-EXO-21-011 ; CERN-EP-2024-161 | ||
| Search for long-lived heavy neutral leptons in proton-proton collision events with a lepton-jet pair associated with a secondary vertex at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 15 July 2024 | ||
| Submitted to J. High Energy Phys. | ||
| Abstract: A search for long-lived heavy neutral leptons (HNLs) using proton-proton collision data corresponding to an integrated luminosity of 138 fb$ ^{-1} $ collected at $ \sqrt{s} = $ 13 TeV with the CMS detector at the CERN LHC is presented. Events are selected with a charged lepton originating from the primary vertex associated with the proton-proton interaction, as well as a second charged lepton and a hadronic jet associated with a secondary vertex that corresponds to the semileptonic decay of a long-lived HNL. No excess of events above the standard model expectation is observed. Exclusion limits at 95% confidence level are evaluated for HNLs that mix with electron and/or muon neutrinos. Limits are presented in the mass range of 1-16.5 GeV, with excluded square mixing parameter values reaching as low as 2 $ \times $ 10$^{-7} $. For masses above 11 GeV, the presented limits exceed all previous results in the semileptonic decay channel, and for some of the considered scenarios are the strongest to date. | ||
| Links: e-print arXiv:2407.10717 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Examples of LO Feynman diagrams for production and decay of an HNL (indicated with the symbol N) resulting in a final state with two charged leptons and two quarks. In the left diagram, the HNL is a Dirac particle and thus the two charged leptons must have opposite charge. In the right diagram, the HNL is a Majorana particle and the two charged leptons can have the same charge. |
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Figure 1-a:
Examples of LO Feynman diagrams for production and decay of an HNL (indicated with the symbol N) resulting in a final state with two charged leptons and two quarks. In the left diagram, the HNL is a Dirac particle and thus the two charged leptons must have opposite charge. In the right diagram, the HNL is a Majorana particle and the two charged leptons can have the same charge. |
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Figure 1-b:
Examples of LO Feynman diagrams for production and decay of an HNL (indicated with the symbol N) resulting in a final state with two charged leptons and two quarks. In the left diagram, the HNL is a Dirac particle and thus the two charged leptons must have opposite charge. In the right diagram, the HNL is a Majorana particle and the two charged leptons can have the same charge. |
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Figure 2:
Selection efficiencies of nonprompt electrons (upper) and muons (lower), evaluated in simulated HNL signal events as functions of the generated lepton $ p_{\mathrm{T}} $ (left) and transverse displacement of the generated SV (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 2-a:
Selection efficiencies of nonprompt electrons (upper) and muons (lower), evaluated in simulated HNL signal events as functions of the generated lepton $ p_{\mathrm{T}} $ (left) and transverse displacement of the generated SV (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 2-b:
Selection efficiencies of nonprompt electrons (upper) and muons (lower), evaluated in simulated HNL signal events as functions of the generated lepton $ p_{\mathrm{T}} $ (left) and transverse displacement of the generated SV (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 2-c:
Selection efficiencies of nonprompt electrons (upper) and muons (lower), evaluated in simulated HNL signal events as functions of the generated lepton $ p_{\mathrm{T}} $ (left) and transverse displacement of the generated SV (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 2-d:
Selection efficiencies of nonprompt electrons (upper) and muons (lower), evaluated in simulated HNL signal events as functions of the generated lepton $ p_{\mathrm{T}} $ (left) and transverse displacement of the generated SV (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
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Figure 3:
The SV reconstruction efficiency in simulated HNL signal events as a function of the SV displacement for vertices with a nonprompt electron (left) or muon (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 3-a:
The SV reconstruction efficiency in simulated HNL signal events as a function of the SV displacement for vertices with a nonprompt electron (left) or muon (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 3-b:
The SV reconstruction efficiency in simulated HNL signal events as a function of the SV displacement for vertices with a nonprompt electron (left) or muon (right). Three HNL signal scenarios are shown, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3, corresponding to $ c\tau_{\mathrm{N}} = $ 23 mm), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 1.6 $\times$ 10$^{-6} $ (HNL5, corresponding to $ c\tau_{\mathrm{N}} = $ 92 mm), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2= $ 5.7 $\times$ 10$^{-7} $ (HNL10, corresponding to $ c\tau_{\mathrm{N}} = $ 7 mm). The error bars in the plot represent statistical uncertainties. |
png pdf |
Figure 4:
The $ m(\ell_1,\text{SV}) $ distribution of predicted events yields after applying the selection summarized in Table 1, for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mu\mu $ (upper right), $ \mathrm{e}\mu $ (lower left), and $ \mu\mathrm{e} $ (lower right) categories. The filled histograms display the predicted background yields. The lines show the predicted yields for three HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-5} $ (HNL5), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. The last bins include the overflow. |
png pdf |
Figure 4-a:
The $ m(\ell_1,\text{SV}) $ distribution of predicted events yields after applying the selection summarized in Table 1, for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mu\mu $ (upper right), $ \mathrm{e}\mu $ (lower left), and $ \mu\mathrm{e} $ (lower right) categories. The filled histograms display the predicted background yields. The lines show the predicted yields for three HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-5} $ (HNL5), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. The last bins include the overflow. |
png pdf |
Figure 4-b:
The $ m(\ell_1,\text{SV}) $ distribution of predicted events yields after applying the selection summarized in Table 1, for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mu\mu $ (upper right), $ \mathrm{e}\mu $ (lower left), and $ \mu\mathrm{e} $ (lower right) categories. The filled histograms display the predicted background yields. The lines show the predicted yields for three HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-5} $ (HNL5), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. The last bins include the overflow. |
png pdf |
Figure 4-c:
The $ m(\ell_1,\text{SV}) $ distribution of predicted events yields after applying the selection summarized in Table 1, for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mu\mu $ (upper right), $ \mathrm{e}\mu $ (lower left), and $ \mu\mathrm{e} $ (lower right) categories. The filled histograms display the predicted background yields. The lines show the predicted yields for three HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-5} $ (HNL5), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. The last bins include the overflow. |
png pdf |
Figure 4-d:
The $ m(\ell_1,\text{SV}) $ distribution of predicted events yields after applying the selection summarized in Table 1, for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mu\mu $ (upper right), $ \mathrm{e}\mu $ (lower left), and $ \mu\mathrm{e} $ (lower right) categories. The filled histograms display the predicted background yields. The lines show the predicted yields for three HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-5} $ (HNL5), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. The last bins include the overflow. |
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Figure 5:
The PFN score distribution of predicted events yields after applying the selection summarized in Table 1, for the combined $ \mathrm{e}\mathrm{e} $ and $ \mu\mathrm{e} $ (left) or $ \mu\mu $ and $ \mathrm{e}\mu $ (right) categories, using the low-mass (upper) or high-mass (lower) PFNs. The filled histograms display the predicted background yields, where the QCD multijet background displays huge statistical fluctuations for PFN scores above 0.2. The lines show the predicted yields for four HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5), $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-6} $ (HNL6), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. |
png pdf |
Figure 5-a:
The PFN score distribution of predicted events yields after applying the selection summarized in Table 1, for the combined $ \mathrm{e}\mathrm{e} $ and $ \mu\mathrm{e} $ (left) or $ \mu\mu $ and $ \mathrm{e}\mu $ (right) categories, using the low-mass (upper) or high-mass (lower) PFNs. The filled histograms display the predicted background yields, where the QCD multijet background displays huge statistical fluctuations for PFN scores above 0.2. The lines show the predicted yields for four HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5), $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-6} $ (HNL6), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. |
png pdf |
Figure 5-b:
The PFN score distribution of predicted events yields after applying the selection summarized in Table 1, for the combined $ \mathrm{e}\mathrm{e} $ and $ \mu\mathrm{e} $ (left) or $ \mu\mu $ and $ \mathrm{e}\mu $ (right) categories, using the low-mass (upper) or high-mass (lower) PFNs. The filled histograms display the predicted background yields, where the QCD multijet background displays huge statistical fluctuations for PFN scores above 0.2. The lines show the predicted yields for four HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5), $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-6} $ (HNL6), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. |
png pdf |
Figure 5-c:
The PFN score distribution of predicted events yields after applying the selection summarized in Table 1, for the combined $ \mathrm{e}\mathrm{e} $ and $ \mu\mathrm{e} $ (left) or $ \mu\mu $ and $ \mathrm{e}\mu $ (right) categories, using the low-mass (upper) or high-mass (lower) PFNs. The filled histograms display the predicted background yields, where the QCD multijet background displays huge statistical fluctuations for PFN scores above 0.2. The lines show the predicted yields for four HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5), $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-6} $ (HNL6), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. |
png pdf |
Figure 5-d:
The PFN score distribution of predicted events yields after applying the selection summarized in Table 1, for the combined $ \mathrm{e}\mathrm{e} $ and $ \mu\mathrm{e} $ (left) or $ \mu\mu $ and $ \mathrm{e}\mu $ (right) categories, using the low-mass (upper) or high-mass (lower) PFNs. The filled histograms display the predicted background yields, where the QCD multijet background displays huge statistical fluctuations for PFN scores above 0.2. The lines show the predicted yields for four HNL signal scenarios, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5), $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-6} $ (HNL6), and $ m_{\mathrm{N}} = $ 10 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The HNL signal yield is normalized to the total background yield. |
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Figure 6:
Predicted and observed event yields in the PFN score distribution for $ \mathrm{K^0_S}\to\pi^{+}\pi^{-} $ decays in $ \mathrm{Z}\to\mu^{+}\mu^{-} $ events, using the low-mass PFNs for the electron (left) and muon (right) channels, where the $ \pi^{\pm} $ with higher $ p_{\mathrm{T}} $ is treated as the lepton. The prediction is scaled to match the overall data yield. The lower panels show the data-to-prediction ratio. |
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Figure 6-a:
Predicted and observed event yields in the PFN score distribution for $ \mathrm{K^0_S}\to\pi^{+}\pi^{-} $ decays in $ \mathrm{Z}\to\mu^{+}\mu^{-} $ events, using the low-mass PFNs for the electron (left) and muon (right) channels, where the $ \pi^{\pm} $ with higher $ p_{\mathrm{T}} $ is treated as the lepton. The prediction is scaled to match the overall data yield. The lower panels show the data-to-prediction ratio. |
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Figure 6-b:
Predicted and observed event yields in the PFN score distribution for $ \mathrm{K^0_S}\to\pi^{+}\pi^{-} $ decays in $ \mathrm{Z}\to\mu^{+}\mu^{-} $ events, using the low-mass PFNs for the electron (left) and muon (right) channels, where the $ \pi^{\pm} $ with higher $ p_{\mathrm{T}} $ is treated as the lepton. The prediction is scaled to match the overall data yield. The lower panels show the data-to-prediction ratio. |
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Figure 7:
Illustration of the target and sideband region definitions for the ABCD method applied to the SR, in terms of $ N $ (jets), $ m(\ell_1,\text{SV}) $, and the PFN score. |
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Figure 8:
The PFN scores in data in the VR, shown for the electron (left) and muon (right) trainings with the low-mass (upper) and high-mass (lower) samples. The distributions are normalized to unity to facilitate a shape comparison. Statistical uncertainties are indicated with error bars and shaded areas. The lower panels show the ratio of the VR-AB to the VR-CD yields. |
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Figure 8-a:
The PFN scores in data in the VR, shown for the electron (left) and muon (right) trainings with the low-mass (upper) and high-mass (lower) samples. The distributions are normalized to unity to facilitate a shape comparison. Statistical uncertainties are indicated with error bars and shaded areas. The lower panels show the ratio of the VR-AB to the VR-CD yields. |
png pdf |
Figure 8-b:
The PFN scores in data in the VR, shown for the electron (left) and muon (right) trainings with the low-mass (upper) and high-mass (lower) samples. The distributions are normalized to unity to facilitate a shape comparison. Statistical uncertainties are indicated with error bars and shaded areas. The lower panels show the ratio of the VR-AB to the VR-CD yields. |
png pdf |
Figure 8-c:
The PFN scores in data in the VR, shown for the electron (left) and muon (right) trainings with the low-mass (upper) and high-mass (lower) samples. The distributions are normalized to unity to facilitate a shape comparison. Statistical uncertainties are indicated with error bars and shaded areas. The lower panels show the ratio of the VR-AB to the VR-CD yields. |
png pdf |
Figure 8-d:
The PFN scores in data in the VR, shown for the electron (left) and muon (right) trainings with the low-mass (upper) and high-mass (lower) samples. The distributions are normalized to unity to facilitate a shape comparison. Statistical uncertainties are indicated with error bars and shaded areas. The lower panels show the ratio of the VR-AB to the VR-CD yields. |
png pdf |
Figure 9:
Predicted and observed event yields in the CR for the low-mass PFNs in the SS (upper) and OS (lower) categories, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed bands represent the total uncertainty in the background prediction. The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 9-a:
Predicted and observed event yields in the CR for the low-mass PFNs in the SS (upper) and OS (lower) categories, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed bands represent the total uncertainty in the background prediction. The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 9-b:
Predicted and observed event yields in the CR for the low-mass PFNs in the SS (upper) and OS (lower) categories, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed bands represent the total uncertainty in the background prediction. The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 10:
Predicted and observed event yields in the CR for the high-mass PFNs in the SS (upper) and OS (lower) categories, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed bands represent the total uncertainty in the background prediction. The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 10-a:
Predicted and observed event yields in the CR for the high-mass PFNs in the SS (upper) and OS (lower) categories, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed bands represent the total uncertainty in the background prediction. The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 10-b:
Predicted and observed event yields in the CR for the high-mass PFNs in the SS (upper) and OS (lower) categories, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed bands represent the total uncertainty in the background prediction. The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 11:
Predicted and observed SR event yields for the SS (upper) and OS (lower) categories of the low-mass PFNs, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed band represents the total systematic and statistical uncertainty in the background prediction. Signal predictions are shown for two HNL production hypotheses, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), and $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5). The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical, main systematic, and DY scale factor contributions. |
png pdf |
Figure 11-a:
Predicted and observed SR event yields for the SS (upper) and OS (lower) categories of the low-mass PFNs, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed band represents the total systematic and statistical uncertainty in the background prediction. Signal predictions are shown for two HNL production hypotheses, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), and $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5). The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical, main systematic, and DY scale factor contributions. |
png pdf |
Figure 11-b:
Predicted and observed SR event yields for the SS (upper) and OS (lower) categories of the low-mass PFNs, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed band represents the total systematic and statistical uncertainty in the background prediction. Signal predictions are shown for two HNL production hypotheses, with $ m_{\mathrm{N}} = $ 3 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 9.9 $\times$ 10$^{-5} $ (HNL3), and $ m_{\mathrm{N}} = $ 5 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 1.6 $\times$ 10$^{-6} $ (HNL5). The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical, main systematic, and DY scale factor contributions. |
png pdf |
Figure 12:
Predicted and observed SR event yields for the SS (upper) and OS (lower) categories of the high-mass PFNs, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed band represents the total systematic and statistical uncertainty in the background prediction. Signal predictions are shown for two HNL production hypotheses, with $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-7} $ (HNL6), and $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 12-a:
Predicted and observed SR event yields for the SS (upper) and OS (lower) categories of the high-mass PFNs, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed band represents the total systematic and statistical uncertainty in the background prediction. Signal predictions are shown for two HNL production hypotheses, with $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-7} $ (HNL6), and $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 12-b:
Predicted and observed SR event yields for the SS (upper) and OS (lower) categories of the high-mass PFNs, binned by flavor channel, $ m_{\text{SV}} $ (as specified below the flavor channel), and $ \Delta_{\mathrm{2D}} $. The hashed band represents the total systematic and statistical uncertainty in the background prediction. Signal predictions are shown for two HNL production hypotheses, with $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 2.0 $\times$ 10$^{-7} $ (HNL6), and $ m_{\mathrm{N}} = $ 6 GeV and $ |V_{\ell{\mathrm{N}} }|^2=$ 5.7 $\times$ 10$^{-7} $ (HNL10). The lower panels show the data-to-prediction ratio and the background prediction uncertainty is split into statistical and systematic contributions. |
png pdf |
Figure 13:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
png pdf |
Figure 13-a:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
png pdf |
Figure 13-b:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
png pdf |
Figure 13-c:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
png pdf |
Figure 13-d:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
png pdf |
Figure 13-e:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
png pdf |
Figure 13-f:
Exclusion limits at 95% CL on $ |V_{\mathrm{e}{\mathrm{N}} }|^2 $ (upper row), $ |V_{\mu{\mathrm{N}} }|^2 $ (middle row), and $ |V_{\mathrm{e}{\mathrm{N}} }V_{\mu{\mathrm{N}} }|^2/(|V_{\mathrm{e}{\mathrm{N}} }|^2+|V_{\mu{\mathrm{N}} }|^2) $ (lower row) as functions of $ m_{\mathrm{N}} $ for a Majorana (left) and Dirac (right) HNL. The solid (dashed) black curve indicates the observed (expected) exclusion, where the parameter combinations inside the curve are excluded. |
| Tables | |
png pdf |
Table 1:
Selection criteria for electrons and muons. Numbers in parentheses indicate values applied in the 2017-2018 data sets, when different from those for 2016. |
png pdf |
Table 2:
Summary of the event selection criteria. |
png pdf |
Table 3:
Definition of target and sideband regions used in the ABCD background estimation method for the signal (SR), validation (VR), and control (CR) regions. The threshold value $ x $ is chosen between 0.97 and 0.998 separately for each event category, as described in the text. |
png pdf |
Table 4:
Summary of systematic uncertainty sources in the signal and background predictions. |
png pdf |
Table 5:
Comparison of lowest and highest $ |V_{\ell{\mathrm{N}} }|^2 $ values excluded at 95% CL for Majorana and Dirac HNLs with different coupling scenarios. For each scenario, the $ m_{\mathrm{N}} $ value where the lowest $ |V_{\ell{\mathrm{N}} }|^2 $ value is excluded is shown. |
| Summary |
| A search for long-lived heavy neutral leptons (HNLs) has been presented using proton-proton collision events with one prompt lepton and a system of a nonprompt lepton and a jet associated with a secondary vertex. The data set corresponds to 138 fb$ ^{-1} $ and was collected by the CMS experiment at the LHC in 2016-2018. A dedicated machine-learning method is developed and utilized to identify the secondary vertex associated with the HNL decay. No excess of events above the standard model background prediction obtained from control samples in data is found. Exclusion limits at 95% confidence level are evaluated for different HNL coupling scenarios as functions of the HNL mass and the mixing parameter with standard model neutrinos. The obtained exclusion limits cover HNL masses from 1 to 16.5 GeV and squared mixing parameters as low as 2 $ \times $ 10$^{-7} $, depending on the scenario. These results exceed previous experimental constraints derived in the single-lepton decay channel in the mass range 11-16.5 GeV. For some of the considered coupling scenarios and mass ranges, the presented limits are the strongest to date. |
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