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| CMS-PAS-EXO-24-024 | ||
| Search for a right-handed W boson decaying into a tau-flavored heavy neutral lepton in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 2026-07-31 | ||
| Abstract: A search is presented for a right-handed W boson ($ \mathrm{W}_\mathrm{R} $) and a heavy neutral lepton (N) in the decay chain $ \mathrm{W}_\mathrm{R}\to\tau\mathrm{N} $, followed by $ \mathrm{N}\to\tau\mathrm{qq} $. The $ \tau $ lepton originating from the N decay is assumed to decay leptonically, producing an electron or muon, while the other one is assumed to decay hadronically. The search explores two regions of parameter space: one where the decay products of the heavy neutral lepton are merged into a single large-radius jet, and the other where they are well separated in angular distance. The search is performed with the CMS experiment at the CERN LHC using the proton-proton collision data set collected during 2016-2018 at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of 138 fb$ ^{-1} $. No excess of events above the standard model expectation is observed. For an N mass of 0.1 TeV, $ \mathrm{W}_\mathrm{R} $ boson masses up to 3.07 TeV are excluded at 95% confidence level; assuming an N mass of half the $ \mathrm{W}_\mathrm{R} $ boson mass, $ \mathrm{W}_\mathrm{R} $ boson masses up to 3.47 TeV are excluded. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Feynman diagram for the production of a heavy neutral lepton via the decay of a $ \mathrm{W_R} $ boson. |
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Figure 2:
Prefit $ \text{LSF}_3 $ distributions of the leading AK8 jet for events passing the boosted preselection in the electron (left) and muon (right) channels. The stacked histograms show the prefit prediction, comprising the nonprompt $ \tau_\mathrm{h} $ background estimated from data and the remaining processes estimated from simulation (``MC background''). The data are shown as points and the hatched band represents the total prefit uncertainty. The lower panels show the ratio of the data to the total background prediction. The signal distribution for the mass point $ (m_{\mathrm{W_R}},m_{\mathrm{N}})=(4.0,0.2) \text{TeV} $, normalized to the nominal signal cross section of the model, is overlaid for comparison. Prediction is normalized to data. |
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Figure 2-a:
Prefit $ \text{LSF}_3 $ distributions of the leading AK8 jet for events passing the boosted preselection in the electron (left) and muon (right) channels. The stacked histograms show the prefit prediction, comprising the nonprompt $ \tau_\mathrm{h} $ background estimated from data and the remaining processes estimated from simulation (``MC background''). The data are shown as points and the hatched band represents the total prefit uncertainty. The lower panels show the ratio of the data to the total background prediction. The signal distribution for the mass point $ (m_{\mathrm{W_R}},m_{\mathrm{N}})=(4.0,0.2) \text{TeV} $, normalized to the nominal signal cross section of the model, is overlaid for comparison. Prediction is normalized to data. |
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Figure 2-b:
Prefit $ \text{LSF}_3 $ distributions of the leading AK8 jet for events passing the boosted preselection in the electron (left) and muon (right) channels. The stacked histograms show the prefit prediction, comprising the nonprompt $ \tau_\mathrm{h} $ background estimated from data and the remaining processes estimated from simulation (``MC background''). The data are shown as points and the hatched band represents the total prefit uncertainty. The lower panels show the ratio of the data to the total background prediction. The signal distribution for the mass point $ (m_{\mathrm{W_R}},m_{\mathrm{N}})=(4.0,0.2) \text{TeV} $, normalized to the nominal signal cross section of the model, is overlaid for comparison. Prediction is normalized to data. |
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Figure 3:
Schematic diagram showing the definition of the FF measurement and application regions with respect to the signal region. |
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Figure 4:
Postfit distributions of the fit observables in the nonprompt control region and the resolved and boosted signal regions, where $ m_\text{eff} $ is defined as $ m_{\tau\ell\text{jj}} $ ($ m_{\tau\text{J}} $) for events passing the resolved (boosted) preselection and the nonprompt CR selection. The stacked histograms show the postfit prediction, comprising the nonprompt $ \tau_\mathrm{h} $ background estimated from data and the remaining processes estimated from simulation (``MC background''). The data are shown as points and the hatched band represents the total postfit uncertainty. The lower panel shows the ratio of the data to the total background prediction. The signal distribution for the mass point $ (m_{\mathrm{W_R}},m_{\mathrm{N}})=(2.0,0.2) \text{TeV} $, normalized to the nominal signal cross section of the model, is overlaid for comparison. |
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Figure 5:
Upper limits at 95% CL on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction, obtained by combining the resolved and boosted categories. The left (right) limit is the result for the $ m_{\mathrm{N}} = $ 0.1 TeV ($ m_{\mathrm{N}} = m_{\mathrm{W_R}}/ $ 2) mass point, which corresponds to the boosted (resolved) $ \mathrm{W_R} $ boson decay topology. The observed (median expected) limit is shown as the solid (dashed) black line, and the green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected limit. The red line represents the theory prediction. |
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Figure 5-a:
Upper limits at 95% CL on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction, obtained by combining the resolved and boosted categories. The left (right) limit is the result for the $ m_{\mathrm{N}} = $ 0.1 TeV ($ m_{\mathrm{N}} = m_{\mathrm{W_R}}/ $ 2) mass point, which corresponds to the boosted (resolved) $ \mathrm{W_R} $ boson decay topology. The observed (median expected) limit is shown as the solid (dashed) black line, and the green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected limit. The red line represents the theory prediction. |
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Figure 5-b:
Upper limits at 95% CL on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction, obtained by combining the resolved and boosted categories. The left (right) limit is the result for the $ m_{\mathrm{N}} = $ 0.1 TeV ($ m_{\mathrm{N}} = m_{\mathrm{W_R}}/ $ 2) mass point, which corresponds to the boosted (resolved) $ \mathrm{W_R} $ boson decay topology. The observed (median expected) limit is shown as the solid (dashed) black line, and the green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected limit. The red line represents the theory prediction. |
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Figure 6:
Upper limits at 95% CL on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction, divided by the theory prediction (color scale), in the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}/m_{\mathrm{W_R}}) $ plane (left) and in the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}) $ plane with $ m_{\mathrm{N}} < $ 1 TeV (right) to emphasize the boosted regime. The observed exclusion contours are shown for the combined (blue) channel; the combined median expected contour is shown as the solid black line, with its 68 and 95% (${\pm}$1 and ${\pm}$2 standard deviations) bands depicted as dotted and dashed black lines, respectively. The median expected contours for the resolved and boosted categories are shown in magenta and green, respectively. The values at intermediate masses are obtained by linear interpolation between the simulated hypotheses using a Delaunay triangulation of the mass grid. |
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Figure 6-a:
Upper limits at 95% CL on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction, divided by the theory prediction (color scale), in the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}/m_{\mathrm{W_R}}) $ plane (left) and in the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}) $ plane with $ m_{\mathrm{N}} < $ 1 TeV (right) to emphasize the boosted regime. The observed exclusion contours are shown for the combined (blue) channel; the combined median expected contour is shown as the solid black line, with its 68 and 95% (${\pm}$1 and ${\pm}$2 standard deviations) bands depicted as dotted and dashed black lines, respectively. The median expected contours for the resolved and boosted categories are shown in magenta and green, respectively. The values at intermediate masses are obtained by linear interpolation between the simulated hypotheses using a Delaunay triangulation of the mass grid. |
png pdf |
Figure 6-b:
Upper limits at 95% CL on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction, divided by the theory prediction (color scale), in the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}/m_{\mathrm{W_R}}) $ plane (left) and in the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}) $ plane with $ m_{\mathrm{N}} < $ 1 TeV (right) to emphasize the boosted regime. The observed exclusion contours are shown for the combined (blue) channel; the combined median expected contour is shown as the solid black line, with its 68 and 95% (${\pm}$1 and ${\pm}$2 standard deviations) bands depicted as dotted and dashed black lines, respectively. The median expected contours for the resolved and boosted categories are shown in magenta and green, respectively. The values at intermediate masses are obtained by linear interpolation between the simulated hypotheses using a Delaunay triangulation of the mass grid. |
| Tables | |
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Table 1:
Summary of the systematic uncertainties considered in this analysis. For each source, the type, magnitude, affected processes, and correlation across data-taking eras are listed. The magnitude refers to the variation applied to each source. Sources of type ``norm.'' are included as a normalization-only effect on the signal and background yields, while those of type ``shape'' affect the bin-by-bin distribution of the final discriminant template used in the fit. |
| Summary |
| A search for right-handed W bosons ($ \mathrm{W_R} $) and heavy neutral leptons ($ \mathrm{N} $) in the left-right symmetric extension of the standard model has been presented. The analysis is based on proton-proton collision data collected at $ \sqrt{s} = $ 13 TeV with the CMS detector, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The final state of interest consists of two $ \tau $ leptons, one decaying hadronically and the other leptonically, and two quarks, and events are categorized into two regions: the resolved region, in which all four objects are well separated, and the boosted region, where the heavy neutral lepton decay is identified using jet substructure techniques applied to large-radius jets. The inclusion of the boosted region significantly enhances the search sensitivity for $ \mathrm{N} $ masses $ m_{\mathrm{N}} < $ 0.5 TeV. Upper limits are set on the product of the $ \mathrm{W_R} $ boson production cross section and the branching fraction for the decay chain into two $ \tau $ leptons and two quarks, assuming couplings identical to those in the standard model. For $ m_{\mathrm{N}} = m_{\mathrm{W_R}}/ $ 2, where $ m_{\mathrm{W_R}} $ is the $ \mathrm{W_R} $ boson mass, the observed (expected) lower limit on $ m_{\mathrm{W_R}} $ at 95% confidence level is 3.47 (3.48) TeV. By introducing a dedicated boosted category, this analysis sets the most stringent limits in the $ \tau $ channel at low $ m_{\mathrm{N}} $. For example, at $ m_{\mathrm{N}}=0.2 m_{\mathrm{W_R}} $ the observed lower limit on $ m_{\mathrm{W_R}} $ increases from 2.75 TeV, obtained by the previous CMS search in the fully hadronic final state with two hadronically decaying $ \tau $ leptons and two jets [9], to 3.35 TeV. For $ m_{\mathrm{N}} = $ 100 GeV, the corresponding observed (expected) limit is 3.07 (2.93) TeV, a region of parameter space for which no experimental limits had previously been set in the $ \tau $ channel. At larger $ m_{\mathrm{N}} $, the fully hadronic di-$ \tau $ search retains greater sensitivity, making the two $ \tau $-channel results complementary across the $ (m_{\mathrm{W_R}},m_{\mathrm{N}}) $ plane. These results are also complementary to the most stringent existing bounds on $ \mathrm{W_R} $ bosons, which come from CMS searches in the same-flavor dilepton channels using the same 138 fb$ ^{-1} $ data set, where $ m_{\mathrm{W_R}} $ is excluded up to 4.7 and 5.0 TeV in the electron and muon channels, respectively [57]. The $ \tau $ lepton channel probes a complementary and theoretically motivated region of the parameter space of the left-right symmetric model, given that right-handed neutrino masses and mixings are flavor dependent and constraints from the leptonic channels do not directly translate to the $ \tau $ lepton sector. No equivalent limits in the $ \tau $ lepton channel have been reported by the ATLAS Collaboration. Exploiting the full $ \sqrt{s}= $ 13 TeV data set of 138 fb$ ^{-1} $ in the final state with one electron or muon (\ell), one hadronically decaying $ \tau $ lepton, and two jets, and introducing a dedicated boosted signal region targeting $ \mathrm{W_R} $ boson decay topologies in which the $ \mathrm{N} $ decay products are reconstructed as a single large-radius jet, this analysis therefore provides the most stringent constraints to date on $ \mathrm{W_R} $ bosons decaying through $ \tau $-flavored heavy neutral leptons, and the first such limits at low $ m_{\mathrm{N}} $, where the boosted category contributes significantly to the sensitivity. \newpage |
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