CMS-PAS-EXO-20-006

CMS-PAS-EXO-20-006
Search for Z' bosons decaying to pairs of heavy Majorana neutrinos in proton-proton collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
March 2022

Abstract: A search for pair production of heavy Majorana neutrinos ( ${\mathrm{N}}_{\ell}$ ) from the decays of Z' bosons is performed using the CMS detector at the LHC. The data were collected in proton-proton collisions at a center-of-mass energy of $\sqrt{s} =$ 13 TeV, with an integrated luminosity of 138 fb $^{-1}$ . The signature of this search is two same-flavor leptons (e or $\mu$ ) and at least two jets. We do not observe any significant excess of events beyond the expected background. Upper limits at the 95% CL are set on the cross section times branching ratio for the pair production of ${\mathrm{N}}_{\ell}$ as a function of ${\mathrm{N}}_{\ell}$ and Z' boson masses ( ${m}_{{\mathrm{N}}_{\ell}}$ and ${m}_\mathrm{{Z}^{\prime}}$ , respectively) for ${m}_\mathrm{{Z}^{\prime}}$ from 400 GeV to 4.4 TeV and ${m}_{{\mathrm{N}}_{\ell}}$ from 100 GeV to $\frac{{m}_\mathrm{{Z}^{\prime}}}{2}$ . In the theoretical framework of a left-right symmetric model, exclusion bounds on the ${m}_{{\mathrm{N}}_{\ell}}$ - ${m}_\mathrm{{Z}^{\prime}}$ plane are presented in both the electron and muon channels.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Pair production of ${{\mathrm {N}}_{\ell}}$ via Z' boson is shown in the diagram, where $\ell$ = e and $\mu$ . The ${{\mathrm {N}}_{\ell}}$ decays into a lepton and two quarks. ${{\mathrm {W}}_{\text {R}}^{\pm}}$ is offshell since we assume ${{\mathrm {W}}_{\text {R}}^{\pm}}$ is heavier than ${{\mathrm {N}}_{\ell}}$ . Both opposite- and same-sign dileptons from decays of heavy neutrino pair are allowed due to the Majorana nature.
png pdf	Figure 2: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 2-a: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 2-b: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 2-c: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 2-d: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 2-e: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 2-f: The reconstructed mass of the Z' boson in CR1, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Pre(Post)fit results are shown left (right).
png pdf	Figure 3: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 3-a: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 3-b: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 3-c: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 3-d: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 3-e: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 3-f: The reconstructed mass of the Z' boson in CR2, which are flavor sidebands of SR1 (top), SR2 (middle), and SR3 (bottom). Dielectron (Dimuon) channel results are shown left (right).
png pdf	Figure 4: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 4-a: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 4-b: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 4-c: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 4-d: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 4-e: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 4-f: Distributions of ${{M}_\mathrm {{Z}^{\prime}}^{\text {Reco}}}$ at SR1 (top), SR2 (middle) and SR3 (bottom) are presented. Left (right) plot is from dielectron (dimuon) channel. Signal samples with $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 200 GeV) and $({{m}_\mathrm {{Z}^{\prime}}}, {{m}_{{{\mathrm {N}}_{\ell}}}}) =$ (4000 GeV, 1200 GeV) are plotted together with best-fit shapes.
png pdf	Figure 5: The observed and expected limits on the cross section times branching ratio of Z' is shown for the case of ${{m}_{{{\mathrm {N}}_{\ell}}}} = {{m}_\mathrm {{Z}^{\prime}}}$ /4 (top) and ${{m}_{{{\mathrm {N}}_{\ell}}}} =$ 100 GeV (bottom) at dielectron (left) and dimuon (right) channels. The green and yellow bands indicate the 68% and 95% CL bands for the expected limit.
png pdf	Figure 5-a: The observed and expected limits on the cross section times branching ratio of Z' is shown for the case of ${{m}_{{{\mathrm {N}}_{\ell}}}} = {{m}_\mathrm {{Z}^{\prime}}}$ /4 (top) and ${{m}_{{{\mathrm {N}}_{\ell}}}} =$ 100 GeV (bottom) at dielectron (left) and dimuon (right) channels. The green and yellow bands indicate the 68% and 95% CL bands for the expected limit.
png pdf	Figure 5-b: The observed and expected limits on the cross section times branching ratio of Z' is shown for the case of ${{m}_{{{\mathrm {N}}_{\ell}}}} = {{m}_\mathrm {{Z}^{\prime}}}$ /4 (top) and ${{m}_{{{\mathrm {N}}_{\ell}}}} =$ 100 GeV (bottom) at dielectron (left) and dimuon (right) channels. The green and yellow bands indicate the 68% and 95% CL bands for the expected limit.
png pdf	Figure 5-c: The observed and expected limits on the cross section times branching ratio of Z' is shown for the case of ${{m}_{{{\mathrm {N}}_{\ell}}}} = {{m}_\mathrm {{Z}^{\prime}}}$ /4 (top) and ${{m}_{{{\mathrm {N}}_{\ell}}}} =$ 100 GeV (bottom) at dielectron (left) and dimuon (right) channels. The green and yellow bands indicate the 68% and 95% CL bands for the expected limit.
png pdf	Figure 5-d: The observed and expected limits on the cross section times branching ratio of Z' is shown for the case of ${{m}_{{{\mathrm {N}}_{\ell}}}} = {{m}_\mathrm {{Z}^{\prime}}}$ /4 (top) and ${{m}_{{{\mathrm {N}}_{\ell}}}} =$ 100 GeV (bottom) at dielectron (left) and dimuon (right) channels. The green and yellow bands indicate the 68% and 95% CL bands for the expected limit.
png pdf	Figure 6: Observed and expected exclusion regions at 95% CL in the 2D phase space of ${{m}_\mathrm {{Z}^{\prime}}}$ VS ${{\mathrm {N}}_{\ell}}$ at dielectron (left) and dimuon (right) channels.
png pdf	Figure 6-a: Observed and expected exclusion regions at 95% CL in the 2D phase space of ${{m}_\mathrm {{Z}^{\prime}}}$ VS ${{\mathrm {N}}_{\ell}}$ at dielectron (left) and dimuon (right) channels.
png pdf	Figure 6-b: Observed and expected exclusion regions at 95% CL in the 2D phase space of ${{m}_\mathrm {{Z}^{\prime}}}$ VS ${{\mathrm {N}}_{\ell}}$ at dielectron (left) and dimuon (right) channels.

Tables
png pdf	Table 1: Events selections based on multiplicities of objects (AK8 jets, tight leptons, and AK4 jets).
png pdf	Table 2: The list of systematic uncertainties and their impacts on the total number of events in signal regions.

Summary

A search for the pair production of heavy Majorana neutrinos via the decay of a Z' boson, in a final state with two same-flavor leptons and at least two reconstructed jets has been performed in proton-proton collisions at a center-of-mass energy of 13 TeV, using data corresponding to an integrated luminosity 138 fb

$^{-1}$ . We do not observe any significant excess above the expected background. We set upper limits on the signal cross section times branching ratio in the context of an LRSM scenario [38]. Exclusion regions in the dielectron and dimuon channels are set with 95% CL. These are the first results for the search of this process using the Run 2 data set of the LHC, and are the most restrictive direct limits in the phase space of

${{m}_\mathrm{{Z}^{\prime}}}$ vs

${{m}_{{{\mathrm{N}}_{\ell}} }}$ to date.

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