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| CMS-PAS-EXO-20-009 | ||
| Search for long-lived heavy neutral leptons with displaced vertices in pp collisions at $\sqrt{s}=$ 13 TeV with the CMS detector | ||
| CMS Collaboration | ||
| July 2021 | ||
| Abstract: A search for heavy neutral leptons (HNLs), the hypothetical right-handed Dirac or Majorana neutrinos, is performed in final states with three charged leptons (electrons or muons) using proton-proton collision data collected by the CMS experiment at a center-of-mass energy of $\sqrt{s}=$ 13 TeV. The data correspond to an integrated luminosity of 137 fb$^{-1}$. The HNLs can be produced at the LHC through mixing with standard model (SM) neutrinos. For small values of the HNL mass ($ < $ 20 GeV) and of the HNL-SM neutrino mixing parameter squared (10$^{-7}$-10$^{-2}$), the decay length of these particles can be large enough to produce a resolved secondary vertex in the CMS silicon tracker. The study aims at identifying two leptons that form a displaced vertex with respect to the primary proton-proton collision vertex, while the third lepton is assumed to emerge from the primary vertex. No significant deviations from the SM expectations are observed, and constraints are obtained on the HNL mass and coupling strength parameters, extending the exclusion limits from previous searches in the mass range 1-18 GeV and mixing parameter values in the range of 10$^{-7}$-10$^{-5}$. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Typical diagrams for the production of an HNL (N) through its mixing with a SM neutrino, leading to final states with three charged leptons and a neutrino. The HNL decay is mediated by either a W* (top row) or a Z* (bottom row) boson. In the diagrams on the left, N is assumed to be a Majorana neutrino, thus $\ell$ and $\ell'$ in the W*-mediated diagram (top) can have the same electric charge, with lepton-number violation (LNV). In the diagrams on the right the N decay conserves lepton number (LNC) and can be either a Majorana or a Dirac particle. Therefore $\ell$ and $\ell'$ in the W*-mediated diagram (top right) have always opposite charge. The present study only considers the former case, thus $\ell$ and $\ell'$ (or $\nu$-$\ell'$) always belong to the same lepton generation, and lepton flavor is conserved. In the case of the HNL decay mediated by a Z boson, $\ell$ and $\ell'$ are required to be of the same flavor. |
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Figure 1-a:
Typical diagrams for the production of an HNL (N) through its mixing with a SM neutrino, leading to final states with three charged leptons and a neutrino. The HNL decay is mediated by either a W* (top row) or a Z* (bottom row) boson. In the diagrams on the left, N is assumed to be a Majorana neutrino, thus $\ell$ and $\ell'$ in the W*-mediated diagram (top) can have the same electric charge, with lepton-number violation (LNV). In the diagrams on the right the N decay conserves lepton number (LNC) and can be either a Majorana or a Dirac particle. Therefore $\ell$ and $\ell'$ in the W*-mediated diagram (top right) have always opposite charge. The present study only considers the former case, thus $\ell$ and $\ell'$ (or $\nu$-$\ell'$) always belong to the same lepton generation, and lepton flavor is conserved. In the case of the HNL decay mediated by a Z boson, $\ell$ and $\ell'$ are required to be of the same flavor. |
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Figure 1-b:
Typical diagrams for the production of an HNL (N) through its mixing with a SM neutrino, leading to final states with three charged leptons and a neutrino. The HNL decay is mediated by either a W* (top row) or a Z* (bottom row) boson. In the diagrams on the left, N is assumed to be a Majorana neutrino, thus $\ell$ and $\ell'$ in the W*-mediated diagram (top) can have the same electric charge, with lepton-number violation (LNV). In the diagrams on the right the N decay conserves lepton number (LNC) and can be either a Majorana or a Dirac particle. Therefore $\ell$ and $\ell'$ in the W*-mediated diagram (top right) have always opposite charge. The present study only considers the former case, thus $\ell$ and $\ell'$ (or $\nu$-$\ell'$) always belong to the same lepton generation, and lepton flavor is conserved. In the case of the HNL decay mediated by a Z boson, $\ell$ and $\ell'$ are required to be of the same flavor. |
png pdf |
Figure 1-c:
Typical diagrams for the production of an HNL (N) through its mixing with a SM neutrino, leading to final states with three charged leptons and a neutrino. The HNL decay is mediated by either a W* (top row) or a Z* (bottom row) boson. In the diagrams on the left, N is assumed to be a Majorana neutrino, thus $\ell$ and $\ell'$ in the W*-mediated diagram (top) can have the same electric charge, with lepton-number violation (LNV). In the diagrams on the right the N decay conserves lepton number (LNC) and can be either a Majorana or a Dirac particle. Therefore $\ell$ and $\ell'$ in the W*-mediated diagram (top right) have always opposite charge. The present study only considers the former case, thus $\ell$ and $\ell'$ (or $\nu$-$\ell'$) always belong to the same lepton generation, and lepton flavor is conserved. In the case of the HNL decay mediated by a Z boson, $\ell$ and $\ell'$ are required to be of the same flavor. |
png pdf |
Figure 1-d:
Typical diagrams for the production of an HNL (N) through its mixing with a SM neutrino, leading to final states with three charged leptons and a neutrino. The HNL decay is mediated by either a W* (top row) or a Z* (bottom row) boson. In the diagrams on the left, N is assumed to be a Majorana neutrino, thus $\ell$ and $\ell'$ in the W*-mediated diagram (top) can have the same electric charge, with lepton-number violation (LNV). In the diagrams on the right the N decay conserves lepton number (LNC) and can be either a Majorana or a Dirac particle. Therefore $\ell$ and $\ell'$ in the W*-mediated diagram (top right) have always opposite charge. The present study only considers the former case, thus $\ell$ and $\ell'$ (or $\nu$-$\ell'$) always belong to the same lepton generation, and lepton flavor is conserved. In the case of the HNL decay mediated by a Z boson, $\ell$ and $\ell'$ are required to be of the same flavor. |
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Figure 2:
Comparison between data and lepton background predictions in validation control region for final states with at least two muons (top) and at least two electrons (bottom). |
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Figure 2-a:
Comparison between data and lepton background predictions in validation control region for final states with at least two muons (top) and at least two electrons (bottom). |
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Figure 2-b:
Comparison between data and lepton background predictions in validation control region for final states with at least two muons (top) and at least two electrons (bottom). |
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Figure 3:
Comparison between the observed number of events in data and simulation for converted photons. Events are selected in the final states with three (top left) or four (top right, bottom) leptons, with one (or two) of the leptons identified as nonprompt electron(s). The distributions are shown for the nonprompt electron displacement (top left) and reconstructed invariant mass of four leptons (top right). Additionally, the distance between the reconstructed primary and fitted secondary vertex is presented (bottom). The simulated events correspond to the processes with external conversions, Z$ \gamma ^{(*)}$ ; internally converted photons, Z ($\gamma ^{\ast}$); and other processes with the production of vector bosons and top quarks. |
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Figure 3-a:
Comparison between the observed number of events in data and simulation for converted photons. Events are selected in the final states with three (top left) or four (top right, bottom) leptons, with one (or two) of the leptons identified as nonprompt electron(s). The distributions are shown for the nonprompt electron displacement (top left) and reconstructed invariant mass of four leptons (top right). Additionally, the distance between the reconstructed primary and fitted secondary vertex is presented (bottom). The simulated events correspond to the processes with external conversions, Z$ \gamma ^{(*)}$ ; internally converted photons, Z ($\gamma ^{\ast}$); and other processes with the production of vector bosons and top quarks. |
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Figure 3-b:
Comparison between the observed number of events in data and simulation for converted photons. Events are selected in the final states with three (top left) or four (top right, bottom) leptons, with one (or two) of the leptons identified as nonprompt electron(s). The distributions are shown for the nonprompt electron displacement (top left) and reconstructed invariant mass of four leptons (top right). Additionally, the distance between the reconstructed primary and fitted secondary vertex is presented (bottom). The simulated events correspond to the processes with external conversions, Z$ \gamma ^{(*)}$ ; internally converted photons, Z ($\gamma ^{\ast}$); and other processes with the production of vector bosons and top quarks. |
png pdf |
Figure 3-c:
Comparison between the observed number of events in data and simulation for converted photons. Events are selected in the final states with three (top left) or four (top right, bottom) leptons, with one (or two) of the leptons identified as nonprompt electron(s). The distributions are shown for the nonprompt electron displacement (top left) and reconstructed invariant mass of four leptons (top right). Additionally, the distance between the reconstructed primary and fitted secondary vertex is presented (bottom). The simulated events correspond to the processes with external conversions, Z$ \gamma ^{(*)}$ ; internally converted photons, Z ($\gamma ^{\ast}$); and other processes with the production of vector bosons and top quarks. |
png pdf |
Figure 4:
The invariant mass distribution of the ${\mathrm{K^0_S}}$ candidates reconstructed using the $\pi^{\pm} \pi^{\mp} $ tracks for various displacement regions. The fitted ${\mathrm{K^0_S}}$ mass in data in each region is also shown. The bottom plot shows the ${\mathrm{K^0_S}}$ yields after subtracting the background in data and in simulation, as well as their ratio, as a function of radial distance of the ${\mathrm{K^0_S}}$ vertex to the primary vertex. |
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Figure 4-a:
The invariant mass distribution of the ${\mathrm{K^0_S}}$ candidates reconstructed using the $\pi^{\pm} \pi^{\mp} $ tracks for various displacement regions. The fitted ${\mathrm{K^0_S}}$ mass in data in each region is also shown. The bottom plot shows the ${\mathrm{K^0_S}}$ yields after subtracting the background in data and in simulation, as well as their ratio, as a function of radial distance of the ${\mathrm{K^0_S}}$ vertex to the primary vertex. |
png pdf |
Figure 4-b:
The invariant mass distribution of the ${\mathrm{K^0_S}}$ candidates reconstructed using the $\pi^{\pm} \pi^{\mp} $ tracks for various displacement regions. The fitted ${\mathrm{K^0_S}}$ mass in data in each region is also shown. The bottom plot shows the ${\mathrm{K^0_S}}$ yields after subtracting the background in data and in simulation, as well as their ratio, as a function of radial distance of the ${\mathrm{K^0_S}}$ vertex to the primary vertex. |
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Figure 4-c:
The invariant mass distribution of the ${\mathrm{K^0_S}}$ candidates reconstructed using the $\pi^{\pm} \pi^{\mp} $ tracks for various displacement regions. The fitted ${\mathrm{K^0_S}}$ mass in data in each region is also shown. The bottom plot shows the ${\mathrm{K^0_S}}$ yields after subtracting the background in data and in simulation, as well as their ratio, as a function of radial distance of the ${\mathrm{K^0_S}}$ vertex to the primary vertex. |
png pdf |
Figure 4-d:
The invariant mass distribution of the ${\mathrm{K^0_S}}$ candidates reconstructed using the $\pi^{\pm} \pi^{\mp} $ tracks for various displacement regions. The fitted ${\mathrm{K^0_S}}$ mass in data in each region is also shown. The bottom plot shows the ${\mathrm{K^0_S}}$ yields after subtracting the background in data and in simulation, as well as their ratio, as a function of radial distance of the ${\mathrm{K^0_S}}$ vertex to the primary vertex. |
png pdf |
Figure 4-e:
The invariant mass distribution of the ${\mathrm{K^0_S}}$ candidates reconstructed using the $\pi^{\pm} \pi^{\mp} $ tracks for various displacement regions. The fitted ${\mathrm{K^0_S}}$ mass in data in each region is also shown. The bottom plot shows the ${\mathrm{K^0_S}}$ yields after subtracting the background in data and in simulation, as well as their ratio, as a function of radial distance of the ${\mathrm{K^0_S}}$ vertex to the primary vertex. |
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Figure 5:
Comparison between the number of observed events in data and their background predictions (shaded histograms, stacked) for the ${\Delta _{\mathrm {2D}}}$ variable in ${\mathrm{e} \mathrm{e} \mathrm{X}}$ (left) and ${\mu \mu \mathrm{X}}$ (right) final states. Predictions for signal events are shown for several benchmark hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). |
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Figure 5-a:
Comparison between the number of observed events in data and their background predictions (shaded histograms, stacked) for the ${\Delta _{\mathrm {2D}}}$ variable in ${\mathrm{e} \mathrm{e} \mathrm{X}}$ (left) and ${\mu \mu \mathrm{X}}$ (right) final states. Predictions for signal events are shown for several benchmark hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). |
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Figure 5-b:
Comparison between the number of observed events in data and their background predictions (shaded histograms, stacked) for the ${\Delta _{\mathrm {2D}}}$ variable in ${\mathrm{e} \mathrm{e} \mathrm{X}}$ (left) and ${\mu \mu \mathrm{X}}$ (right) final states. Predictions for signal events are shown for several benchmark hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). |
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Figure 6:
Comparison between the number of observed events in data and their background predictions (shaded histograms, stacked) for the ${m({\ell _2} {\ell _3})}$ variable in ${\mathrm{e} \mathrm{e} \mathrm{X}}$ (left) and ${\mu \mu \mathrm{X}}$ (right) final states. Predictions for signal events are shown for several benchmark hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). |
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Figure 6-a:
Comparison between the number of observed events in data and their background predictions (shaded histograms, stacked) for the ${m({\ell _2} {\ell _3})}$ variable in ${\mathrm{e} \mathrm{e} \mathrm{X}}$ (left) and ${\mu \mu \mathrm{X}}$ (right) final states. Predictions for signal events are shown for several benchmark hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). |
png pdf |
Figure 6-b:
Comparison between the number of observed events in data and their background predictions (shaded histograms, stacked) for the ${m({\ell _2} {\ell _3})}$ variable in ${\mathrm{e} \mathrm{e} \mathrm{X}}$ (left) and ${\mu \mu \mathrm{X}}$ (right) final states. Predictions for signal events are shown for several benchmark hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). |
png pdf |
Figure 7:
Predicted and observed yields in the different search regions for (top) ${\mathrm{e} \mathrm{e} \mathrm{X}}$ and (bottom) ${\mu \mu \mathrm{X}}$ final states, compared to one HNL signal scenario with nonzero ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ parameter, respectively. The predictions for signal process are obtained assuming Majorana HNL. Predictions for signal events are shown for several benchmark hypotheses: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). The uncertainty band assigned to the background prediction includes statistical and systematic contributions. |
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Figure 7-a:
Predicted and observed yields in the different search regions for (top) ${\mathrm{e} \mathrm{e} \mathrm{X}}$ and (bottom) ${\mu \mu \mathrm{X}}$ final states, compared to one HNL signal scenario with nonzero ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ parameter, respectively. The predictions for signal process are obtained assuming Majorana HNL. Predictions for signal events are shown for several benchmark hypotheses: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). The uncertainty band assigned to the background prediction includes statistical and systematic contributions. |
png pdf |
Figure 7-b:
Predicted and observed yields in the different search regions for (top) ${\mathrm{e} \mathrm{e} \mathrm{X}}$ and (bottom) ${\mu \mu \mathrm{X}}$ final states, compared to one HNL signal scenario with nonzero ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ parameter, respectively. The predictions for signal process are obtained assuming Majorana HNL. Predictions for signal events are shown for several benchmark hypotheses: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \ell}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). The uncertainty band assigned to the background prediction includes statistical and systematic contributions. |
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Figure 8:
Limits on ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ (left) and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ (right) as a function of ${m_{\mathrm{N}}}$ for a Majorana HNL. Results from the Delphi [62] and the CMS [22,23] Collaborations are shown for reference. |
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Figure 8-a:
Limits on ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ (left) and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ (right) as a function of ${m_{\mathrm{N}}}$ for a Majorana HNL. Results from the Delphi [62] and the CMS [22,23] Collaborations are shown for reference. |
png pdf |
Figure 8-b:
Limits on ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ (left) and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ (right) as a function of ${m_{\mathrm{N}}}$ for a Majorana HNL. Results from the Delphi [62] and the CMS [22,23] Collaborations are shown for reference. |
png pdf |
Figure 9:
Limits on ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ (left) and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ (right) as a function of ${m_{\mathrm{N}}}$ for a Dirac HNL. Results from the Delphi [62] and the CMS [22,23] Collaborations are shown for reference. |
png pdf |
Figure 9-a:
Limits on ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ (left) and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ (right) as a function of ${m_{\mathrm{N}}}$ for a Dirac HNL. Results from the Delphi [62] and the CMS [22,23] Collaborations are shown for reference. |
png pdf |
Figure 9-b:
Limits on ${{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2}$ (left) and ${{| {V_{{\mathrm{N}} \mu}} |}^2}$ (right) as a function of ${m_{\mathrm{N}}}$ for a Dirac HNL. Results from the Delphi [62] and the CMS [22,23] Collaborations are shown for reference. |
| Tables | |
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Table 1:
Baseline selection criteria (left) and dilepton invariant mass vetoes (right) applied in the analysis. |
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Table 2:
Definition of kinematic regions in terms of dilepton invariant mass ${m({\ell _2} {\ell _3})}$ and SV displacement ${\Delta _{\mathrm {2D}}}$. |
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Table 3:
Number of predicted and observed events in the ${\mathrm{e} \mathrm{e} \mathrm{X}}$ final states. The quoted uncertainties include statistical and systematic uncertainties. |
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Table 4:
Number of predicted and observed events in the ${\mu \mu \mathrm{X}}$ final states. The quoted uncertainties include statistical and systematic uncertainties. |
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Table 5:
Number of predicted signal events in the ${\mathrm{e} \mathrm{e} \mathrm{X}}$ final states. The results are presented for several benchmark signal hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \mathrm{e}}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). The quoted uncertainties include statistical and systematic uncertainties. |
png pdf |
Table 6:
Number of predicted signal events in the ${\mu \mu \mathrm{X}}$ final states. The results are presented for several benchmark signal hypotheses for Majorana HNL: $ {m_{\mathrm{N}}} = $ 2 GeV and $ {{| {V_{{\mathrm{N}} \mu}} |}^2} = $ 0.8 $\times$ 10$^{-4}$ (HNL2), $ {m_{\mathrm{N}}} = $ 6 GeV and $ {{| {V_{{\mathrm{N}} \mu}} |}^2} = $ 1.3 $\times$ 10$^{-6}$ (HNL6), $ {m_{\mathrm{N}}} = $ 12 GeV and $ {{| {V_{{\mathrm{N}} \mu}} |}^2} = $ 1.0 $\times$ 10$^{-6}$ (HNL12). The quoted uncertainties include statistical and systematic uncertainties. |
| Summary |
| A study on the search for heavy neutral leptons (HNLs) in final states with three leptons is performed using data from the CMS detector corresponding to an integrated luminosity of 137 fb$^{-1}$. The analysis uses dedicated methods to identify displaced leptons associated with HNL decays. Several novel methods were developed to apply data-driven estimates for the relevant background processes representing one of the important challenges in this type of search at the LHC. No significant deviation from the standard model predictions is observed in data, and 95% confidence level limits are set on HNL masses and coupling strengths of these particles to standard model neutrinos. These results represent the world best limits to date on this type of processes in the explored parameter space of the HNL production at the LHC. |
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