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| CMS-EXO-22-017 ; CERN-EP-2024-022 | ||
| Search for long-lived heavy neutral leptons decaying in the CMS muon detectors in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 28 February 2024 | ||
| Accepted for publication in Phys. Rev. D | ||
| Abstract: A search for heavy neutral leptons (HNLs) decaying in the CMS muon system is presented. A data sample is used corresponding to an integrated luminosity of 138 fb$ ^{-1} $ of proton-proton collisions at $ \sqrt{s}= $ 13 TeV, recorded at the CERN 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 in the muon chambers. This distinctive signature provides a unique handle to search for HNLs with masses below 4 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 originating from a vertex on the beam axis and a displaced shower in the muon chambers. No significant deviations from the standard model background expectation are observed. In the electron (muon) channel, the most stringent limits to date are set for HNLs in the mass range of 2.1-3.0 (1.9-3.3) GeV, reaching mixing matrix element squared values as low as 8.6 (4.6) $ \times $ 10$^{-6} $. | ||
| Links: e-print arXiv:2402.18658 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; Physics Briefing ; CADI line (restricted) ; | ||
| Figures | |
png pdf |
Figure 1:
Feynman diagrams for the production of a Majorana HNL $ N_M $ (left) and a Dirac HNL $ \overline{N_D} $ (right) via a W$^{-}$ boson decay and through its mixing with an SM neutrino of the same flavor. The prompt lepton from the W$^{-}$ boson serves as a clean signature for triggering, whereas the decay products of the HNL are reconstructed as a cluster of muon detector hits. |
png pdf |
Figure 1-a:
Feynman diagrams for the production of a Majorana HNL $ N_M $ (left) and a Dirac HNL $ \overline{N_D} $ (right) via a W$^{-}$ boson decay and through its mixing with an SM neutrino of the same flavor. The prompt lepton from the W$^{-}$ boson serves as a clean signature for triggering, whereas the decay products of the HNL are reconstructed as a cluster of muon detector hits. |
png pdf |
Figure 1-b:
Feynman diagrams for the production of a Majorana HNL $ N_M $ (left) and a Dirac HNL $ \overline{N_D} $ (right) via a W$^{-}$ boson decay and through its mixing with an SM neutrino of the same flavor. The prompt lepton from the W$^{-}$ boson serves as a clean signature for triggering, whereas the decay products of the HNL are reconstructed as a cluster of muon detector hits. |
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Figure 2:
Distribution of $ N_{\text{hits}} $ (upper) and $ \Delta\phi_\text{lep} $ (lower) for DT clusters (left) and CSC clusters (right). Signal distributions of a Majorana HNL with $ m_{\mathrm{N}} = $ 2 GeV and $ c\tau_{0}= $ 1 m are compared with the OOT background distributions selected with $ t_\mathrm{cluster}^\mathrm{DT} $ matched to bunch crossings earlier than the PV for DT clusters and $ t_\mathrm{cluster}^\mathrm{CSC} < - $ 12.5 ns for CSC clusters. The distributions are normalized to unit area. The shapes of the distributions shown are similar for the electron and muon channels. |
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Figure 3:
Definition of the ABCD plane. The area of the blue squares illustrates the relative amount of expected events in each of the bins, with bins B and C having the majority of the event yields. Bin D is the signal region. |
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Figure 4:
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 added to the expected background. |
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Figure 5:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 5-a:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 5-b:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 5-c:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 5-d:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 5-e:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 5-f:
Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The tau neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure. |
png pdf |
Figure 6:
The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with 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:
The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with 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:
The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with 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:
The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with 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:
The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with 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. The predictions of the method for the signal bin (last column) are consistent with the observed number of events, shown in the second-to-last column. |
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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-enriched 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 show the estimates in the $ \mathrm{Z}\to\mu\mu $ enriched control region of the total background and its $ \mathrm{Z}\to\mu\mu $ and non-muon-induced components. The fourth column shows the transfer factors $ \zeta $ used to predict the $ \mathrm{Z}\to\mu\mu $ background in the signal region, shown in the fifth column. |
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Table 4:
Summary of systematic uncertainties affecting the signal yield prediction. For DT clusters, the systematic uncertainties due to jet and muon vetoes are found to be negligible and are omitted. The uncertainties are reported relative to their impact on the predicted signal yield. |
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Table 5:
Excluded ranges of $ |V_{{\mathrm{N}} \ell}|^2 $ for Majorana and Dirac type HNLs at select HNL masses. The chosen HNL masses are those at which the excluded values of $ |V_{{\mathrm{N}} \ell}|^2 $ have the smallest magnitude. |
| Summary |
| A search for long-lived Dirac or Majorana heavy neutral leptons (HNLs) has been performed using proton-proton collision data at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The search targets events with one prompt electron or muon and a muon detector shower (MDS) that would result from HNL decays occurring in the CMS muon detector. The presence of the MDS signature along with the associated vetoes and identification criteria suppresses the standard model background by a factor exceeding 10$^{7} $, while maintaining typical signal efficiencies of 25-35%. No significant excess over the standard model background is observed. The results are interpreted as 95% confidence level limits on the HNL mixing matrix elements squared $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $, $ |V_{{\mathrm{N}} \mu}|^2 $, and $ |V_{{\mathrm{N}} \tau}|^2 $. We also present limits on the HNL mass and mean proper decay length as a function of the mixing matrix element squared fractions to the three lepton generations. The most stringent limits to date for HNLs in the mass range of 2.1-3.0 (1.9-3.3) GeV are set, reaching squared mixing matrix element values as low as 8.6 (4.6) $ \times $ 10$^{-6} $ in the electron (muon) channel. |
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