CMS-PAS-FTR-22-003

CMS-PAS-FTR-22-003
Seesaw Model Searches Using Multilepton Final States at the HL-LHC
CMS Collaboration
March 2022

Abstract: A simulation-based sensitivity study is performed for physics beyond the standard model (BSM) in the context of neutrino mass mechanisms. The simulated samples correspond to an integrated luminosity of 3000 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} =$ 14 TeV with the Phase-2 CMS detector tuned to the high-luminosity LHC scenario. Type-I and II seesaw models, which can explain the nonzero mass of neutrinos in a most elegant way, are the targeted BSM models. The postulated BSM particles for all 2 models are assumed to be heavy in the study, with masses ranging between 200 and 3000 GeV. The signature used in this search consists of 3 or 4 leptons in any flavor combination of electrons and muons. With a common strategy, the analysis is expected to exclude the mass of BSM fermion singlet N up to 2020 (2440) GeV in the type-I seesaw model for cases where N mixes only with e ($\mu$) type neutrinos. For the type-II seesaw model, mass of BSM scalar triplet $\Delta$ is excluded up to 1280 (1300) GeV, for normal (inverted) neutrino mass structure.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: W mediated (left) and W$\gamma $ vector boson fusion (right) processes in type-I seesaw model.
png pdf	Figure 1-a: W mediated (left) and W$\gamma $ vector boson fusion (right) processes in type-I seesaw model.
png pdf	Figure 1-b: W mediated (left) and W$\gamma $ vector boson fusion (right) processes in type-I seesaw model.
png pdf	Figure 2: W mediated (left) and Z mediated (left) processes in type-II seesaw model.
png pdf	Figure 2-a: W mediated (left) and Z mediated (left) processes in type-II seesaw model.
png pdf	Figure 2-b: W mediated (left) and Z mediated (left) processes in type-II seesaw model.
png pdf	Figure 3: Maximum $m_{\ell \ell}$(right) and $L_{\mathrm{T}}$ (left) after applying the baseline object selection for all lepton multiplicities and flavors combined. Yields from background processes are plotted as filled histograms. Signal yields are plotted as solid lines for BSM particle mass of 500 GeV and are scaled by 0.1 or 0.05 for type-I or II respectively. Color scheme are shown in the legends.
png pdf	Figure 3-a: Maximum $m_{\ell \ell}$(right) and $L_{\mathrm{T}}$ (left) after applying the baseline object selection for all lepton multiplicities and flavors combined. Yields from background processes are plotted as filled histograms. Signal yields are plotted as solid lines for BSM particle mass of 500 GeV and are scaled by 0.1 or 0.05 for type-I or II respectively. Color scheme are shown in the legends.
png pdf	Figure 3-b: Maximum $m_{\ell \ell}$(right) and $L_{\mathrm{T}}$ (left) after applying the baseline object selection for all lepton multiplicities and flavors combined. Yields from background processes are plotted as filled histograms. Signal yields are plotted as solid lines for BSM particle mass of 500 GeV and are scaled by 0.1 or 0.05 for type-I or II respectively. Color scheme are shown in the legends.
png pdf	Figure 4: Signal and background distributions in the 10 most sensitive bins for a type-I seesaw model with only N$\mu $ mixing scenario (top) and a type-II seesaw model with normal hierarchy SM neutrino mass structure (bottom). Yields from background processes are categorized and plotted as the filled histograms and the total background uncertainties are plotted as the shaded areas. For each plot the signal distribution is plotted with a light green solid line.
png	Figure 4-a: Signal and background distributions in the 10 most sensitive bins for a type-I seesaw model with only N$\mu $ mixing scenario (top) and a type-II seesaw model with normal hierarchy SM neutrino mass structure (bottom). Yields from background processes are categorized and plotted as the filled histograms and the total background uncertainties are plotted as the shaded areas. For each plot the signal distribution is plotted with a light green solid line.
png	Figure 4-b: Signal and background distributions in the 10 most sensitive bins for a type-I seesaw model with only N$\mu $ mixing scenario (top) and a type-II seesaw model with normal hierarchy SM neutrino mass structure (bottom). Yields from background processes are categorized and plotted as the filled histograms and the total background uncertainties are plotted as the shaded areas. For each plot the signal distribution is plotted with a light green solid line.
png pdf	Figure 5: Type-I seesaw model exclusion region at 95% CL in the cross section vs. $m_{\mathrm{N}}$ plane. Left e only, right $\mu $ only mixing scenarios. The dash-dotted black line with green and yellow bands shows the expected limit with its 1- and 2-s.d. uncertainties. The theoretical cross section is indicated by the dashed red line. Both offshell W boson and W$\gamma $ vector boson fusion channels are included in the production cross section.
png	Figure 5-a: Type-I seesaw model exclusion region at 95% CL in the cross section vs. $m_{\mathrm{N}}$ plane. Left e only, right $\mu $ only mixing scenarios. The dash-dotted black line with green and yellow bands shows the expected limit with its 1- and 2-s.d. uncertainties. The theoretical cross section is indicated by the dashed red line. Both offshell W boson and W$\gamma $ vector boson fusion channels are included in the production cross section.
png	Figure 5-b: Type-I seesaw model exclusion region at 95% CL in the cross section vs. $m_{\mathrm{N}}$ plane. Left e only, right $\mu $ only mixing scenarios. The dash-dotted black line with green and yellow bands shows the expected limit with its 1- and 2-s.d. uncertainties. The theoretical cross section is indicated by the dashed red line. Both offshell W boson and W$\gamma $ vector boson fusion channels are included in the production cross section.
png pdf	Figure 6: Type-II seesaw model exclusion region at 95% CL in the cross section vs. $m_{\Delta}$ plane. Left normal hierarchy and right inverted hierarchy scenarios. Both are assuming mass degeneracy, $m_\Delta = m_{\Delta ^{++/-}} = m_{\Delta ^{+/-}}$. The dash-dotted black line with green and yellow bands shows the expected limit with its 1- and 2-s.d. uncertainties. The theoretical cross section is indicated by the dashed red line.
png	Figure 6-a: Type-II seesaw model exclusion region at 95% CL in the cross section vs. $m_{\Delta}$ plane. Left normal hierarchy and right inverted hierarchy scenarios. Both are assuming mass degeneracy, $m_\Delta = m_{\Delta ^{++/-}} = m_{\Delta ^{+/-}}$. The dash-dotted black line with green and yellow bands shows the expected limit with its 1- and 2-s.d. uncertainties. The theoretical cross section is indicated by the dashed red line.
png	Figure 6-b: Type-II seesaw model exclusion region at 95% CL in the cross section vs. $m_{\Delta}$ plane. Left normal hierarchy and right inverted hierarchy scenarios. Both are assuming mass degeneracy, $m_\Delta = m_{\Delta ^{++/-}} = m_{\Delta ^{+/-}}$. The dash-dotted black line with green and yellow bands shows the expected limit with its 1- and 2-s.d. uncertainties. The theoretical cross section is indicated by the dashed red line.

Summary

An inclusive study for neutrino mass models, namely, type-I, and II seesaw models, was presented for at the high-luminosity LHC with 3000 fb$^{-1}$ of integrated luminosity assuming Phase-2 CMS detector conditions. The masses of the BSM particles, assumed to be degenerate for type-II, were explored in the range starting from 200 GeV up to 3000 GeV and 2000 GeV for type-I and type-II respectively. The analysis is expected to exclude N masses up to 2020 (2440) GeV in the type-I seesaw model for cases where N mixes only with e ($\mu$) type neutrinos. For the type-II seesaw model, mass of $\Delta$ is excluded up to 1280 (1300) GeV, for normal (inverted) neutrino mass structure.

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