Search for Evidence of the Type-III Seesaw Mechanism in Multilepton Final States in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search for a signal consistent with the type-III seesaw mechanism in events with three or more electrons or muons is presented. The data sample consists of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ collected by the CMS experiment at the LHC in 2016 and corresponds to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$. Selection criteria based on the number of leptons and the invariant mass of oppositely charged lepton pairs are used to distinguish the signal from the standard model background. The observations are consistent with the expectations from standard model processes. The results are used to place limits on the production of heavy fermions of the type-III seesaw model as a function of the branching ratio to each lepton flavor. In the scenario of equal branching fractions to each lepton flavor, heavy fermions with masses below 840 GeV are excluded. This is the most sensitive probe to date of the type-III seesaw mechanism.

Figure 1

The $L_T+p_T^{\text{miss}}$ distribution for events with three leptons and one OSSF pair with mass above $Z$ (upper) and the $L_T+p_T^{\text{miss}}$ distribution for events with four or more leptons and one OSSF pair (lower). The total SM background is shown as a stacked histogram of all contributing processes. The predictions for signal models (sum of all production and decay modes) with $m_{\mathrm{\Sigma }}=700$ (solid line) and $m_{\mathrm{\Sigma }}=380\mathrm{GeV}$ (dashed line) are also shown. The lower panels show the ratio of observed to expected events. The hatched gray band in the upper panel, and the dark and light gray bands in the lower panel represent the total, statistical, and systematic uncertainties in the expected background, respectively.

Figure 2

The 95% confidence level upper limits on the cross section for production of heavy fermion pairs (${\mathrm{\Sigma }}^0{\mathrm{\Sigma }}^{+}$, ${\mathrm{\Sigma }}^0{\mathrm{\Sigma }}^{-}$, and ${\mathrm{\Sigma }}^{+}{\mathrm{\Sigma }}^{-}$). Also shown is the theoretical prediction for the cross section of the $\mathrm{\Sigma }$ pair production via the type-III seesaw mechanism, with its uncertainty. In the flavor-democratic scenario ($B_e=B_{\mu }=B_{\tau }$), heavy fermion pair production is excluded for masses below 840 GeV.

Figure 3

The 95% confidence level expected (upper) and observed (lower) limits on the heavy fermion mass $m_{\mathrm{\Sigma }}$ (GeV) for each combination of branching fractions to the individual lepton flavors. The color scale represents the mass exclusion limit obtained at each point. The branching fraction to $\mu$ can be obtained from $B_{\mu }=1-[B_e+B_{\tau }]$. The highlighted box indicates the limit on $m_{\mathrm{\Sigma }}$ for the flavor-democratic scenario ($B_e=B_{\mu }=B_{\tau }$).