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| CMS-EXO-21-013 ; CERN-EP-2023-256 | ||
| Search for long-lived heavy neutral leptons with lepton flavour conserving or violating decays to a jet and a charged lepton | ||
| CMS Collaboration | ||
| 12 December 2023 | ||
| JHEP 03 (2024) 105 | ||
| Abstract: A search for long-lived heavy neutral leptons (HNLs) is presented, which considers the hadronic final state and coupling scenarios involving all three lepton generations in the 2--20 GeV HNL mass range for the first time. Events comprising two leptons (electrons or muons) and jets are analyzed in a data sample of proton-proton collisions, recorded with the CMS experiment at the CERN LHC at a centre-of-mass energy of 13 TeV, 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 an HNL decay using various features of the jet and its constituent particles. The network output can be used as a powerful discriminating tool to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data. No excess of events in data over the expected background is observed. Upper limits on the HNL production cross section are derived as functions of the HNL mass and the three coupling strengths $ V_{\ell\mathrm{N}} $ to each lepton generation $\ell$ and presented as exclusion limits in the coupling-mass plane, as lower limits on the HNL lifetime, and on the HNL mass. In this search, the most stringent limit on the coupling strength is obtained for pure muon coupling scenarios; values of $ |V_{\mu\mathrm{N}}|^{2} > $ 5 (4) $\times$ 10$^{-7} $ are excluded for Dirac (Majorana) HNLs with a mass of 10 GeV at a confidence level of 95% that correspond to proper decay lengths of 17 (10) mm. | ||
| Links: e-print arXiv:2312.07484 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Born-level Feynman diagram for Dirac HNL production and decay via a charged-current interaction. Corresponding diagrams exist also for Dirac anti-HNL and Majorana HNL production and decay. In this search, the HNL decay products, encapsulated by the boxes, can be found collimated within a single jet $ \text{j}^{\star} $. |
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Figure 2:
Distributions of $ m_{\ell\ell\text{j}^{\star}} $ for events with (left) OS leptons and (right) 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 2-a:
Distributions of $ m_{\ell\ell\text{j}^{\star}} $ for events with (left) OS leptons and (right) 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 2-b:
Distributions of $ m_{\ell\ell\text{j}^{\star}} $ for events with (left) OS leptons and (right) 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3-a:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3-b:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3-c:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3-d:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3-e:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 3-f:
Distributions of the displaced jet tagging score for (left) resolved and (right) boosted categories: (upper row) control region; (middle row) signal region with OS leptons; (lower row) signal region with 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 in parentheses. The hatched band shows the total experimental systematic uncertainty in the simulated background prediction including the uncertainty from the finite sample size. The bottom panel shows the ratio of data over the prediction. |
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Figure 4:
Schematic of the orthogonal regions in the ($ m_{\ell\ell\text{j}^{\star}} $,$ P_{\mathrm{q},\ell}(j^{\star}) $) plane, that are used to determine the background in region D from data through an ABCD method. The threshold $ P_\text{opt.} $ depends on the category. Regions with $ P_{\mathrm{q},\ell}(j^{\star}) < $ 0.1 (0.2) for resolved (boosted) events are not considered. The three VRs that are subsets of regions A and C are indicated in lighter shades. The figure is not to scale. |
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Figure 5:
Observed number of events and predicted number of background events per category for (left) resolved and (right) boosted categories. The bin label denotes the flavour of the prompt ($ \ell_{1} $) and displaced ($ \ell_{2} $) lepton as $ \ell_{1} \ell_{2} $. Two representative signal scenarios for Majorana HNL production with equal coupling to all lepton generations are overlaid. The lower panels show the ratio of the data to the predicted background. The hatched band shows the total systematic uncertainty in the predicted background. |
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Figure 5-a:
Observed number of events and predicted number of background events per category for (left) resolved and (right) boosted categories. The bin label denotes the flavour of the prompt ($ \ell_{1} $) and displaced ($ \ell_{2} $) lepton as $ \ell_{1} \ell_{2} $. Two representative signal scenarios for Majorana HNL production with equal coupling to all lepton generations are overlaid. The lower panels show the ratio of the data to the predicted background. The hatched band shows the total systematic uncertainty in the predicted background. |
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Figure 5-b:
Observed number of events and predicted number of background events per category for (left) resolved and (right) boosted categories. The bin label denotes the flavour of the prompt ($ \ell_{1} $) and displaced ($ \ell_{2} $) lepton as $ \ell_{1} \ell_{2} $. Two representative signal scenarios for Majorana HNL production with equal coupling to all lepton generations are overlaid. The lower panels show the ratio of the data to the predicted background. The hatched band shows the total systematic uncertainty in the predicted background. |
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Figure 6:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
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Figure 6-a:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
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Figure 6-b:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
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Figure 6-c:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
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Figure 6-d:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
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Figure 6-e:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
png pdf |
Figure 6-f:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production as functions of the HNL mass and coupling strengths for (upper row) pure electron, (middle row) pure muon, and (lower row) pure $ \tau $ lepton coupling scenarios. For the last coupling case no limits are derived for $ m_{\mathrm{N}} \lesssim $ 3 GeV because of the tau mass. The dashed-dotted line shows the result of an orthogonal CMS analysis targeting HNL decays to leptons. |
png pdf |
Figure 7:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 7-a:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
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Figure 7-b:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
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Figure 7-c:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 7-d:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 7-e:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
png pdf |
Figure 7-f:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for various coupling scenarios as functions of the HNL mass and coupling strengths: (upper row) mixed e-$\mu $ couplings; (middle row) mixed e-$\tau $ couplings; (lower row) mixed $ \mu$-$\tau $ couplings. The relative ratios of the coupling per lepton generation are indicated in the plots. |
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Figure 8:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for the democratic couplings to all lepton generation scenario as functions of the HNL mass and coupling strengths. |
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Figure 8-a:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for the democratic couplings to all lepton generation scenario as functions of the HNL mass and coupling strengths. |
png pdf |
Figure 8-b:
Expected and observed 95% CL limits on (left) Majorana and (right) Dirac HNL production for the democratic couplings to all lepton generation scenario as functions of the HNL mass and coupling strengths. |
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Figure 9:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as functions of the relative couplings to the three lepton generations considering a fixed proper decay length of (upper row) 0.1 mm and (lower row) 1 mm. The limits are determined for $ m_{\mathrm{N}} > $ 3 GeV. |
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Figure 9-a:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as functions of the relative couplings to the three lepton generations considering a fixed proper decay length of (upper row) 0.1 mm and (lower row) 1 mm. The limits are determined for $ m_{\mathrm{N}} > $ 3 GeV. |
png pdf |
Figure 9-b:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as functions of the relative couplings to the three lepton generations considering a fixed proper decay length of (upper row) 0.1 mm and (lower row) 1 mm. The limits are determined for $ m_{\mathrm{N}} > $ 3 GeV. |
png pdf |
Figure 9-c:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as functions of the relative couplings to the three lepton generations considering a fixed proper decay length of (upper row) 0.1 mm and (lower row) 1 mm. The limits are determined for $ m_{\mathrm{N}} > $ 3 GeV. |
png pdf |
Figure 9-d:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL mass as functions of the relative couplings to the three lepton generations considering a fixed proper decay length of (upper row) 0.1 mm and (lower row) 1 mm. The limits are determined for $ m_{\mathrm{N}} > $ 3 GeV. |
png pdf |
Figure 10:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper decay length as functions of the relative couplings to the three lepton generations considering a fixed mass of (upper row) 4.5 GeV and (lower row) 8.0 GeV. |
png pdf |
Figure 10-a:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper decay length as functions of the relative couplings to the three lepton generations considering a fixed mass of (upper row) 4.5 GeV and (lower row) 8.0 GeV. |
png pdf |
Figure 10-b:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper decay length as functions of the relative couplings to the three lepton generations considering a fixed mass of (upper row) 4.5 GeV and (lower row) 8.0 GeV. |
png pdf |
Figure 10-c:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper decay length as functions of the relative couplings to the three lepton generations considering a fixed mass of (upper row) 4.5 GeV and (lower row) 8.0 GeV. |
png pdf |
Figure 10-d:
Observed 95% CL lower limits on the (left column) Majorana and (right column) Dirac HNL proper decay length as functions of the relative couplings to the three lepton generations considering a fixed mass of (upper row) 4.5 GeV and (lower row) 8.0 GeV. |
| Summary |
| A search is presented for long-lived heavy neutral leptons (HNLs), which are predicted in extensions of the standard model of particle physics through a 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 novelty of this analysis is the simultaneous sensitivity to prompt and displaced scenarios through the use of a dedicated displaced jet tagger that does not explicitly rely on the presence of secondary vertices. A data sample of proton-proton collision events recorded with the CMS experiment at the CERN LHC at a centre-of-mass energy of 13 TeV is analyzed, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Events containing two leptons (electrons or muons) 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 overlaps with 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 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 functions of the HNL mass and the three coupling strengths to each lepton generation. Exclusion limits are presented in the coupling-mass plane, as lower limits on the HNL lifetime, and on the HNL mass. Results are provided for Dirac and Majorana HNL production considering various coupling combinations. The most stringent limit on the coupling strength is obtained for pure muon coupling scenarios, with values of $ |V_{\mu\mathrm{N}}|^{2} > $ 5 (4) $\times$ 10$^{-7} $ excluded for Dirac (Majorana) HNLs with a mass of 10 GeV at a confidence level of 95% that correspond to proper decay lengths of 17 (10) mm. This analysis is the first HNL search at the LHC that targets long-lived and hadronically decaying HNLs in the 2-20 GeV mass range, with inclusive coupling to all three lepton generations. The results show comparable sensitivity to an orthogonal analysis that targets long-lived HNLs decaying to leptons. |
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