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| CMS-PAS-EXO-22-017 | ||
| Search for long-lived heavy neutral leptons decaying in the CMS muon detectors in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 17 July 2023 | ||
| Abstract: A search for heavy neutral leptons (HNLs) decaying in the CMS muon system is presented. The data sample consists of 137 fb$^{-1}$ of proton-proton collisions at $ \sqrt{s}= $ 13 TeV, recorded at the LHC in 2016-2018. Decay products of long-lived HNLs could interact with the shielding materials in the CMS muon system and create hadronic and electromagnetic showers detected by the muon chambers. This distinctive signature provides a unique handle to search for HNLs with masses below 10 GeV and proper decay lengths of the order of meters. The signature is sensitive to HNL couplings to all three generations of leptons. Candidate events are required to contain a prompt electron or muon and a shower in the muon chambers. No significant deviations from the standard model background expectation are observed, and the most stringent limits to date are found for HNLs in the mass range of 2.1-3.0 (1.9-3.3) GeV, reaching squared mixing parameter values as low as 8.9 $ \times$ 10$^{-6} $ (4.6 $ \times$ 10$^{-6} $) in the electron (muon) channel. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to PRD. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Feynman diagrams for the production of a Majorana (left) and a Dirac (right) HNL via a W boson decay and through its mixing with an SM neutrino. The prompt lepton from the W boson serves as a clean signature for triggering, while the decay products of the HNL are reconstructed as a cluster of muon detector hits. |
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Figure 1-a:
Feynman diagrams for the production of a Majorana (left) and a Dirac (right) HNL via a W boson decay and through its mixing with an SM neutrino. The prompt lepton from the W boson serves as a clean signature for triggering, while the decay products of the HNL are reconstructed as a cluster of muon detector hits. |
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Figure 1-b:
Feynman diagrams for the production of a Majorana (left) and a Dirac (right) HNL via a W boson decay and through its mixing with an SM neutrino. The prompt lepton from the W boson serves as a clean signature for triggering, while the decay products of the HNL are reconstructed as a cluster of muon detector hits. |
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Figure 2:
Definition of the ABCD plane. The size of the blue boxes illustrates the relative amount of expected events in each of the bins, with bin B and C having the majority of the event yields. Bin D is the signal region (SR). |
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Figure 3:
The expected and observed number of events in the signal region (bin D) of different event categories. Signal yields of a 2 GeV Majorana HNL with the mean proper decay length of 1 m are also overlayed on top of the expected background. |
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Figure 4:
Expected and observed upper 95% CL limits on Majorana HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 5.2 $ \times$ 10$^{-4} $ (3.5 $ \times$ 10$^{-2} $) for electron/muon (tau)-type HNL as the signal acceptance to the muon system approaches zero due to the short lifetime of the HNL. |
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Figure 4-a:
Expected and observed upper 95% CL limits on Majorana HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 5.2 $ \times$ 10$^{-4} $ (3.5 $ \times$ 10$^{-2} $) for electron/muon (tau)-type HNL as the signal acceptance to the muon system approaches zero due to the short lifetime of the HNL. |
png pdf |
Figure 4-b:
Expected and observed upper 95% CL limits on Majorana HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 5.2 $ \times$ 10$^{-4} $ (3.5 $ \times$ 10$^{-2} $) for electron/muon (tau)-type HNL as the signal acceptance to the muon system approaches zero due to the short lifetime of the HNL. |
png pdf |
Figure 4-c:
Expected and observed upper 95% CL limits on Majorana HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 5.2 $ \times$ 10$^{-4} $ (3.5 $ \times$ 10$^{-2} $) for electron/muon (tau)-type HNL as the signal acceptance to the muon system approaches zero due to the short lifetime of the HNL. |
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Figure 5:
Expected and observed upper 95% CL limits on Dirac HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 1.3 $ \times$ 10$^{-3} $ (1.2 $ \times$ 10$^{-1} $) for electron/muon (tau)-type HNL as the signal acceptance in the muon system approaches zero due to the short lifetime of the HNL. |
png pdf |
Figure 5-a:
Expected and observed upper 95% CL limits on Dirac HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 1.3 $ \times$ 10$^{-3} $ (1.2 $ \times$ 10$^{-1} $) for electron/muon (tau)-type HNL as the signal acceptance in the muon system approaches zero due to the short lifetime of the HNL. |
png pdf |
Figure 5-b:
Expected and observed upper 95% CL limits on Dirac HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 1.3 $ \times$ 10$^{-3} $ (1.2 $ \times$ 10$^{-1} $) for electron/muon (tau)-type HNL as the signal acceptance in the muon system approaches zero due to the short lifetime of the HNL. |
png pdf |
Figure 5-c:
Expected and observed upper 95% CL limits on Dirac HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling (top left), pure muon coupling (top right) and pure tau coupling. The tau-coupling limit is obtained by combining the results from the electron channel and muon channel. No limit is set below $ m_{\mathrm{N}} $ of 2 (1.5) GeV and above $ |V_{{\mathrm{N}} \ell}|^2 $ of 1.3 $ \times$ 10$^{-3} $ (1.2 $ \times$ 10$^{-1} $) for electron/muon (tau)-type HNL as the signal acceptance in the muon system approaches zero due to the short lifetime of the HNL. |
png pdf |
Figure 6:
Observed upper 95% CL limits on the Majorana (top) and Dirac (bottom) HNL mass (left) and mean proper decay length (right) as a function of relative coupling to the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV respectively. |
png pdf |
Figure 6-a:
Observed upper 95% CL limits on the Majorana (top) and Dirac (bottom) HNL mass (left) and mean proper decay length (right) as a function of relative coupling to the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV respectively. |
png pdf |
Figure 6-b:
Observed upper 95% CL limits on the Majorana (top) and Dirac (bottom) HNL mass (left) and mean proper decay length (right) as a function of relative coupling to the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV respectively. |
png pdf |
Figure 6-c:
Observed upper 95% CL limits on the Majorana (top) and Dirac (bottom) HNL mass (left) and mean proper decay length (right) as a function of relative coupling to the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV respectively. |
png pdf |
Figure 6-d:
Observed upper 95% CL limits on the Majorana (top) and Dirac (bottom) HNL mass (left) and mean proper decay length (right) as a function of relative coupling to the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV respectively. |
| Tables | |
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Table 1:
Validation of the ABCD method in the OOT and in-time validation regions. |
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Table 2:
The event yields in the bins A, B, and C are shown in each of the event categories considered in the search, as well as the prefit prediction for the ABCD background in the signal enhanced bin D. |
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Table 3:
Summary of the $ \mathrm{Z}\to\mu\mu $ background estimate in different categories. The first three columns shows estimates in the $ \mathrm{Z}\to\mu\mu $ enriched control region, and the last two columns show the transfer factors $ \zeta $ to predict the $ \mathrm{Z}\to\mu\mu $ background in the signal region. |
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Table 4:
Summary of systematic uncertainties affecting the signal yield prediction. For DT clusters, the systematics uncertainties due to jet and muon vetos are found to be negligible, and are omitted. |
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Table 5:
Summary of the most stringent observed limits of $ |V_{{\mathrm{N}} \ell}|^2 $ for Majorana and Dirac type HNL in this search. |
| Summary |
| A search for long-lived Dirac or Majorana heavy neutral leptons (HNL) has been performed using proton-proton collision data at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 137 fb$ ^{-1} $, using events with one prompt electron or muon and a muon detector shower (MDS) resulting from the HNL decay occurring in the CMS muon detector. The presence of the MDS signature along with the associated vetos and identification criteria suppresses the standard model background by a factor exceeding $ 10^7 $. No significant excess over the standard model backgrounds is observed. The results are interpreted as 95% confidence level limits on the HNL mixing parameters $ V_{{\mathrm{N}} \mathrm{e}} $, $ V_{{\mathrm{N}} \mu} $, and $ V_{{\mathrm{N}} \tau} $. We set the most stringent limits to date for HNLs in the mass range of 2.1-3.0 (1.9-3.3) GeV, reaching squared mixing parameter values as low as 8.6 $ \times$ 10$^{-6} $ (4.6 $ \times$ 10$^{-6} $) in the electron (muon) channel. |
| Additional Figures | |
png pdf |
Additional Figure 1:
Expected and observed upper 95% confidence level (CL) limits on Majorana HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure muon coupling. Results from previous CMS searches [70,30], as well as other experiments, including ATLAS [27], BEBC [71], Belle [72], CHARM [69], DELPHI [25] and NuTeV [73], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments. |
png pdf |
Additional Figure 2:
Expected and observed upper 95% confidence level (CL) limits on Dirac HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure muon coupling. Results from previous CMS searches [70,30], as well as other experiments, including ATLAS [27], BEBC [71], Belle [72], CHARM [69], DELPHI [25] and NuTeV [73], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments. |
png pdf |
Additional Figure 3:
Expected and observed upper 95% confidence level (CL) limits on Majorana HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling. Results from previous CMS searches [70,30], as well as other experiments, including ATLAS [27], BEBC [71], Belle [72], CHARM [69] and DELPHI [25], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments. |
png pdf |
Additional Figure 4:
Expected and observed upper 95% confidence level (CL) limits on Dirac HNL production as a function of the HNL mass ($ m_{\mathrm{N}} $) and coupling strengths on pure electron coupling. Results from previous CMS searches [70,30], as well as other experiments, including ATLAS [27], BEBC [71], Belle [72], CHARM [69] and DELPHI [25], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments. |
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