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| CMS-PAS-EXO-21-013 | ||
| Search for long-lived heavy neutral leptons with lepton flavour conserving or violating decays to a jet and an electron, muon, or tau lepton | ||
| CMS Collaboration | ||
| 18 March 2023 | ||
| Abstract: A search for long-lived heavy neutral leptons (HNLs) is presented, which considers coupling scenarios involving all three lepton generations. Events containing two leptons (electron or muon) and jets are analysed in a data sample of proton-proton collisions corresponding to an integrated luminosity of 138 fb$^{-1}$. A novel jet tagger, based on a deep neural network, has been developed to identify jets from a HNL decay using various features of the jet and its constituent particles. The network can be used as a powerful discriminant to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data in sideband regions. No excess of events in data over the expected background is observed. Limits on the HNL production cross section are derived as a function of the HNL mass and the three coupling strengths $ V_{l\textrm{N}} $ to each lepton generation $ l $. The best limit on the coupling strength is obtained for pure muon coupling scenarios excluding values of $ |V_{\mu\textrm{N}}|^{2} > $ 5 (4) $\times$ 10$^{-7} $ for Dirac (Majorana) HNLs with a mass of 10 GeV at 95% CL. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, JHEP 03 (2024) 105. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Born-level Feynman diagram for Dirac HNL production and decay via charged currents. Corresponding diagrams exist also for Dirac anti-HNL and Majorana HNL productions and decays. |
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Figure 2:
Distributions of $ m_{ll\textrm{j}^{\star}} $ for events with (left) opposite-sign (OS) and (right) same-sign (SS) leptons in the signal region. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 2-a:
Distributions of $ m_{ll\textrm{j}^{\star}} $ for events with (left) opposite-sign (OS) and (right) same-sign (SS) leptons in the signal region. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 2-b:
Distributions of $ m_{ll\textrm{j}^{\star}} $ for events with (left) opposite-sign (OS) and (right) same-sign (SS) leptons in the signal region. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 3:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 3-a:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 3-b:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 3-c:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 3-d:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 3-e:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
png pdf |
Figure 3-f:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted jets: (top row) control region; (middle row) signal region with opposite-sign (OS) leptons; (bottom row) signal region with same-sign (SS) leptons. A representative signal scenario for Majorana HNL production with equal coupling to all lepton generations is overlaid with its expected cross section scaled up as indicated. The hatched band shows the total experimental systematic uncertainty on the simulated samples. |
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Figure 4:
Observed number of events and predicted number of background events per category for (left) resolved and (right) boosted categories. Two representative signal scenarios for Majorana HNL production with equal coupling to all lepton generations are overlaid. The bottom panel shows the ratio of the data over the predicted background. The hatched band shows the total systematic uncertainty on the predicted background. |
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Figure 4-a:
Observed number of events and predicted number of background events per category for (left) resolved and (right) boosted categories. Two representative signal scenarios for Majorana HNL production with equal coupling to all lepton generations are overlaid. The bottom panel shows the ratio of the data over the predicted background. The hatched band shows the total systematic uncertainty on the predicted background. |
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Figure 4-b:
Observed number of events and predicted number of background events per category for (left) resolved and (right) boosted categories. Two representative signal scenarios for Majorana HNL production with equal coupling to all lepton generations are overlaid. The bottom panel shows the ratio of the data over the predicted background. The hatched band shows the total systematic uncertainty on the predicted background. |
png pdf |
Figure 5:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 5-a:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 5-b:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 5-c:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 5-d:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 5-e:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 5-f:
Expected and observed 95% CL limits on Dirac HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6-a:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6-b:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6-c:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6-d:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6-e:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 6-f:
Expected and observed 95% CL limits on Majorana HNL production for various coupling scenarios as a function of the HNL mass and coupling strengths: (top row) pure electron and muon couplings; (middle row) mixed electron-muon and electron-muon-tau couplings; (bottom row) mixed electron-tau and muon-tau couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 7:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as a function of the relative coupling to the three lepton generations considering a fixed proper lifetime of (top row) 0.1 mm and (bottom row) 1 mm. The limits are determined within 2 $ < m_{\textrm{N}} < $ 20 GeV. |
png pdf |
Figure 7-a:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as a function of the relative coupling to the three lepton generations considering a fixed proper lifetime of (top row) 0.1 mm and (bottom row) 1 mm. The limits are determined within 2 $ < m_{\textrm{N}} < $ 20 GeV. |
png pdf |
Figure 7-b:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as a function of the relative coupling to the three lepton generations considering a fixed proper lifetime of (top row) 0.1 mm and (bottom row) 1 mm. The limits are determined within 2 $ < m_{\textrm{N}} < $ 20 GeV. |
png pdf |
Figure 7-c:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as a function of the relative coupling to the three lepton generations considering a fixed proper lifetime of (top row) 0.1 mm and (bottom row) 1 mm. The limits are determined within 2 $ < m_{\textrm{N}} < $ 20 GeV. |
png pdf |
Figure 7-d:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as a function of the relative coupling to the three lepton generations considering a fixed proper lifetime of (top row) 0.1 mm and (bottom row) 1 mm. The limits are determined within 2 $ < m_{\textrm{N}} < $ 20 GeV. |
png pdf |
Figure 8:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper lifetime as a function of the relative coupling to the three lepton generations considering a fixed mass of (top row) 4.5 GeV and (bottom row) 8.0 GeV. |
png pdf |
Figure 8-a:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper lifetime as a function of the relative coupling to the three lepton generations considering a fixed mass of (top row) 4.5 GeV and (bottom row) 8.0 GeV. |
png pdf |
Figure 8-b:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper lifetime as a function of the relative coupling to the three lepton generations considering a fixed mass of (top row) 4.5 GeV and (bottom row) 8.0 GeV. |
png pdf |
Figure 8-c:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper lifetime as a function of the relative coupling to the three lepton generations considering a fixed mass of (top row) 4.5 GeV and (bottom row) 8.0 GeV. |
png pdf |
Figure 8-d:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper lifetime as a function of the relative coupling to the three lepton generations considering a fixed mass of (top row) 4.5 GeV and (bottom row) 8.0 GeV. |
| Summary |
| A search for long-lived heavy neutral leptons (HNLs) is presented, which are predicted in extensions to the standard model of particle physics through the seesaw mechanism. Dirac or Majorana HNLs can mix with the three standard model lepton generations in a nontrivial way, resulting in lepton flavour number violation. Hence, a complete set of coupling scenarios involving all three lepton generations is considered. A data sample of proton-proton collision events, corresponding to an integrated luminosity of 138 fb$ ^{-1} $, is analyzed. Events containing two leptons (electron or muon) and jets are selected and categorized according to the lepton flavour and electrical charge, the displacement of the lower-momentum lepton, and whether the lepton is clustered in a nearby jet. Jets originating from the decay of a long-lived HNL are identified through a deep neural network using various features of the jet and its constituent particles. The network training relies on simulated event samples that cover a broad range of HNL masses and lifetimes, and thus acts as a powerful discriminant for jets that are spatially separated from the luminous region, even in the absence of reconstructed secondary vertices. Contributions from background processes are determined from data in sideband regions. No excess of events in the data over the expected background is observed. Upper limits on the HNL production cross section are determined as a function of the HNL mass and the three coupling strengths to each lepton generation. Results are given for Dirac and Majorana HNL production considering various coupling combinations. The best limit on the coupling strength is obtained for pure muon coupling scenarios excluding values of $ |V_{\mu\textrm{N}}|^{2} > $ 5 (4) $\times$ 10$^{-7} $ for Dirac (Majorana) HNLs with a mass of 10 GeV at 95% CL. |
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