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CMS-EXO-20-002 ; CERN-EP-2021-228
Search for a right-handed W boson and a heavy neutrino in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 04 (2022) 047
Abstract: A search is presented for a right-handed W boson (W$_{R}$) and a heavy neutrino (N), in a final state consisting of two same-flavor leptons (ee or $\mu\mu$) and two quarks. The search is performed with the CMS experiment at the CERN LHC using a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138 fb$^{-1}$ . The search covers two regions of phase space, one where the decay products of the heavy neutrino are merged into a single large-area jet, and one where the decay products are well separated. The expected signal is characterized by an excess in the invariant mass distribution of the final-state objects. No significant excess over the standard model background expectations is observed. The observations are interpreted as upper limits on the product of W$_{R}$ production cross sections and branching fractions assuming that couplings are identical to those of the standard model W boson. For N masses ${m_{{\mathrm{N}} }}$ equal to half the W$_{R}$ mass ${m_{\mathrm{W}_{R}}}$ (${m_{{\mathrm{N}} }} = $ 0.2 TeV), ${m_{\mathrm{W}_{R}}}$ is excluded at 95% confidence level up to 4.7 (4.8) and 5.0 (5.4) TeV for the electron and muon channels, respectively. This analysis provides the most stringent limits on the W$_{R}$ mass to date.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Feynman diagram for the production of a heavy neutrino via the decay of a W$_{R}$ boson.

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Figure 2:
A schematic diagram of the analysis region. The minimum values of the dilepton mass in the SR and the flavor CR is 400 (200) GeV for the resolved (boosted) region. The backgrounds from tW and ${\mathrm{t} {}\mathrm{\bar{t}}}$ production are estimated from the flavor CR (green), where different flavor (DF) leptons are required. The DY background is estimated from the DY CR (blue).

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Figure 3:
The ${m_{\ell \ell {\mathrm {j}} {\mathrm {j}}}}$ (${m_{\ell {\mathrm {J}}}}$) distributions in the resolved (boosted) DY CRs are shown in the upper (lower) row. Results in the ee ($\mu \mu $) channels are shown in the left (right) plots. The hatched uncertainty bands on the simulated background histograms include statistical and systematic components.

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Figure 3-a:
The ${m_{\ell \ell {\mathrm {j}} {\mathrm {j}}}}$ distribution in the resolved DY CRs in the ee channel is shown. The hatched uncertainty bands on the simulated background histograms include statistical and systematic components.

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Figure 3-b:
The ${m_{\ell \ell {\mathrm {j}} {\mathrm {j}}}}$ distribution in the resolved DY CRs in the $\mu \mu $ channel is shown. The hatched uncertainty bands on the simulated background histograms include statistical and systematic components.

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Figure 3-c:
The ${m_{\ell {\mathrm {J}}}}$ distribution in the boosted DY CRs in the ee channel is shown. The hatched uncertainty bands on the simulated background histograms include statistical and systematic components.

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Figure 3-d:
The ${m_{\ell {\mathrm {J}}}}$ distribution in the boosted DY CRs in the $\mu \mu $ channel is shown. The hatched uncertainty bands on the simulated background histograms include statistical and systematic components.

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Figure 4:
The reconstructed mass of the W$_{R}$ boson in the resolved (upper), boosted with e-jet (lower left), and boosted with $\mu $-jet (lower right) flavor CR.

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Figure 4-a:
The reconstructed mass of the W$_{R}$ boson in the resolved flavor CR.

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Figure 4-b:
The reconstructed mass of the W$_{R}$ boson in the boosted with e-jet flavor CR.

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Figure 4-c:
The reconstructed mass of the W$_{R}$ boson in the boosted with $\mu $-jet flavor CR.

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Figure 5:
The post-fit ${m_{\ell \ell {\mathrm {j}} {\mathrm {j}}}}$ (${m_{\ell {\mathrm {J}}}}$) distributions in the resolved (boosted) SR are shown in the upper (lower) plot. Results for the dielectron (dimuon) channel are shown on the left (right). Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. The simulated signal distribution is scaled up by a factor of five to enhance visibility.

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Figure 5-a:
The post-fit ${m_{\ell \ell {\mathrm {j}} {\mathrm {j}}}}$ distribution in the resolved SR for the dielectron channel is shown. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. The simulated signal distribution is scaled up by a factor of five to enhance visibility.

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Figure 5-b:
The post-fit ${m_{\ell \ell {\mathrm {j}} {\mathrm {j}}}}$ distribution in the resolved SR for the dimuon channel is shown. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. The simulated signal distribution is scaled up by a factor of five to enhance visibility.

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Figure 5-c:
The post-fit ${m_{\ell {\mathrm {J}}}}$ distribution in the boosted SR for the dielectron channel is shown. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. The simulated signal distribution is scaled up by a factor of five to enhance visibility.

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Figure 5-d:
The post-fit ${m_{\ell {\mathrm {J}}}}$ distribution in the boosted SR for the dimuon channel is shown. Statistical and systematic uncertainties in the expected background yields are represented by the hatched band. The simulated signal distribution is scaled up by a factor of five to enhance visibility.

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Figure 6:
The expected (black dashed line) and the observed (black solid line) 95% CL upper limits on the product of the cross section for W$_{R}$ production and the branching fractions for the electron channel (left) and muon channel (right) from the combination of the resolved and boosted categories. The plots in the upper (lower) row are the results for the $ {m_{{\mathrm {N}}}} = {m_{\mathrm{W}_{R}}} /2$ ($ {m_{{\mathrm {N}}}} = $ 0.2 TeV) mass point. The green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected upper limits. The red solid lines represent the values expected from the theory [17].

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Figure 6-a:
The expected (black dashed line) and the observed (black solid line) 95% CL upper limits on the product of the cross section for W$_{R}$ production and the branching fractions for the electron channel from the combination of the resolved and boosted categories. The plot is the result for the $ {m_{{\mathrm {N}}}} = {m_{\mathrm{W}_{R}}} /2$ mass point. The green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected upper limits. The red solid lines represent the values expected from the theory [17].

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Figure 6-b:
The expected (black dashed line) and the observed (black solid line) 95% CL upper limits on the product of the cross section for W$_{R}$ production and the branching fractions for the muon channel from the combination of the resolved and boosted categories. The plot is the result for the $ {m_{{\mathrm {N}}}} = {m_{\mathrm{W}_{R}}} /2$ mass point. The green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected upper limits. The red solid lines represent the values expected from the theory [17].

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Figure 6-c:
The expected (black dashed line) and the observed (black solid line) 95% CL upper limits on the product of the cross section for W$_{R}$ production and the branching fractions for the electron channel from the combination of the resolved and boosted categories. The plot is the result for the $ {m_{{\mathrm {N}}}} = $ 0.2 TeV mass point. The green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected upper limits. The red solid lines represent the values expected from the theory [17].

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Figure 6-d:
The expected (black dashed line) and the observed (black solid line) 95% CL upper limits on the product of the cross section for W$_{R}$ production and the branching fractions for the muon channel from the combination of the resolved and boosted categories. The plot is the result for the $ {m_{{\mathrm {N}}}} = $ 0.2 TeV mass point. The green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected upper limits. The red solid lines represent the values expected from the theory [17].

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Figure 7:
The observed 95% CL upper limits on the product of the production cross sections and the branching fractions of a right-handed W$_{R}$ boson divided by the theory expectation for a coupling constant ${g_{\mathrm {R}}}$ equal to the SM coupling of the W$_{R}$ boson (${g_{\mathrm {L}}}$), for the electron channel (left) and muon channel (right). The observed exclusion regions are shown for the resolved (solid green), boosted (solid blue), and combined (solid black) channels, together with the expected exclusion region for the combined result (dotted black). The dash-dotted lines represent the 68% coverage of the boundaries of the expected exclusion regions. The observed exclusion regions obtained in the previous search performed by the CMS Collaboration [12] are bounded by the magenta lines. The biggest improvement can be seen in the $ {m_{{\mathrm {N}}}} < $ 0.5 TeV region, where the new boosted category greatly improves the sensitivity over the previous result.

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Figure 7-a:
The observed 95% CL upper limits on the product of the production cross sections and the branching fractions of a right-handed W$_{R}$ boson divided by the theory expectation for a coupling constant ${g_{\mathrm {R}}}$ equal to the SM coupling of the W$_{R}$ boson (${g_{\mathrm {L}}}$), for the electron channel. The observed exclusion regions are shown for the resolved (solid green), boosted (solid blue), and combined (solid black) channels, together with the expected exclusion region for the combined result (dotted black). The dash-dotted lines represent the 68% coverage of the boundaries of the expected exclusion regions. The observed exclusion regions obtained in the previous search performed by the CMS Collaboration [12] are bounded by the magenta lines. The biggest improvement can be seen in the $ {m_{{\mathrm {N}}}} < $ 0.5 TeV region, where the new boosted category greatly improves the sensitivity over the previous result.

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Figure 7-b:
The observed 95% CL upper limits on the product of the production cross sections and the branching fractions of a right-handed W$_{R}$ boson divided by the theory expectation for a coupling constant ${g_{\mathrm {R}}}$ equal to the SM coupling of the W$_{R}$ boson (${g_{\mathrm {L}}}$), for the muon channel. The observed exclusion regions are shown for the resolved (solid green), boosted (solid blue), and combined (solid black) channels, together with the expected exclusion region for the combined result (dotted black). The dash-dotted lines represent the 68% coverage of the boundaries of the expected exclusion regions. The observed exclusion regions obtained in the previous search performed by the CMS Collaboration [12] are bounded by the magenta lines. The biggest improvement can be seen in the $ {m_{{\mathrm {N}}}} < $ 0.5 TeV region, where the new boosted category greatly improves the sensitivity over the previous result.
Tables

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Table 1:
Summary of the relative uncertainties in the total yields of the signal and background (bkgd) predictions. The uncertainties are given for the resolved (boosted) SR. The values for the signal correspond to $ {m_{\mathrm{W}_{R}}} = $ 5 TeV. The range given for each systematic uncertainty source covers the variation across the years and the year-to-year treatment is shown as either fully correlated (C) or uncorrelated (U). The EW1, EW2, and EW3 entries are the uncertainties from the $\mathcal {O}(\alpha ^{2})$ Sudakov terms, the NLO portion of the nNLO EW uncertainty, and the uncertainty in the Sudakov approximation at nNLO, respectively.
Summary
A search for right-handed bosons (W$_{R}$) and heavy right-handed neutrinos (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 by the CMS detector, corresponding to an integrated luminosity of 138 fb$^{-1}$. The final state consists of events with two same-flavor leptons (ee or $\mu\mu$) and two quarks, and is identified through two regions: the resolved region, where all four objects are well separated, and the boosted region, where the heavy neutrino decay is identified using jet substructure techniques applied to large-area jets. The addition of the boosted region greatly improves the search sensitivity in the region where ${m_{{\mathrm{N}} }} < $ 0.5 TeV. No significant excess over the standard model background expectations is observed in the invariant mass distributions. Upper limits are set on the products of the W$_{R}$ and N production cross sections and their branching fraction to two leptons and two quarks assuming that couplings are identical to those of the standard model. For N masses ${m_{{\mathrm{N}} }}$ equal to half the W$_{R}$ mass ${m_{\mathrm{W}_{R}}}$ (${m_{{\mathrm{N}} }} = $ 0.2 TeV), ${m_{\mathrm{W}_{R}}}$ is excluded at 95% confidence level up to 4.7 (4.8) and 5.0 (5.4) TeV for the electron and muon channels, respectively. This analysis provides the most stringent limits on the W$_{R}$ mass to date.
Additional Figures

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Additional Figure 1:
The $\mathrm {LSF_{3}}$ distributions of the leading AK8 jet. Data and simulated background from three data periods are compared. Also plotted are the distributions for the simulated signals with $ {m_{{\mathrm {W_R}}}} = $ 5 TeV. The simulated signal distributions are scaled up by a factor of 30 to enhance visibility. The events are required to pass the full electron channel boosted event selection except for the $\mathrm {LSF_{3}}$ requirement.

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Additional Figure 2:
The $\mathrm {LSF_{3}}$ distributions of the leading AK8 jet. Data and simulated background from three data periods are compared. Also plotted are the distributions for the simulated signals with $ {m_{{\mathrm {W_R}}}} = $ 5 TeV. The simulated signal distributions are scaled up by a factor of 30 to enhance visibility. The events are required to pass the full muon channel boosted event selection except for the $\mathrm {LSF_{3}}$ requirement.

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Additional Figure 3:
The expected (black dashed line) and the observed (black solid line) 95% CL upper limits on the product of the cross section for W$_R$ production and the branching fractions for the electron channel from the combination of the resolved and boosted categories. The plot is the result for the $ {m_{{\mathrm {N}}}} = $ 0.8 TeV mass point. The green (inner) and yellow (outer) bands indicate the 68 and 95% coverage of the expected upper limits. The red solid line represents the values expected from the theory.
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Compact Muon Solenoid
LHC, CERN