CMS-EXO-17-006

CMS-EXO-17-006 ; CERN-EP-2017-194
Search for evidence of the type-III seesaw mechanism in multilepton final states in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
25 August 2017
Phys. Rev. Lett. 119 (2017) 221802
Abstract: 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 TeV collected by the CMS experiment at the LHC in 2016 and corresponds to an integrated luminosity of 35.9 fb$^{-1}$. Selection criteria based on the number of leptons and the invariant mass of opposite-sign 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.
Links: e-print arXiv:1708.07962 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures & Tables	References	CMS Publications

Figures
png pdf	Figure 1: The $ {L_\mathrm {T}} + { {p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with three leptons and one OSSF pair with mass above-Z (left) and the $ {L_\mathrm {T}} + { {p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with four or more leptons and one OSSF pair (right). 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_{\Sigma } = $ 700 GeV (solid line) and $m_{\Sigma } = $ 380 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.
png pdf	Figure 1-a: The $ {L_\mathrm {T}} + { {p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with three leptons and one OSSF pair with mass above-Z. 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_{\Sigma } = $ 700 GeV (solid line) and $m_{\Sigma } = $ 380 GeV (dashed line) are also shown. The lower panel shows 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.
png pdf	Figure 1-b: The $ {L_\mathrm {T}} + { {p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with four or more leptons and one OSSF pair. 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_{\Sigma } = $ 700 GeV (solid line) and $m_{\Sigma } = $ 380 GeV (dashed line) are also shown. The lower panel shows 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.
png pdf	Figure 2: The 95% confidence level upper limits on the cross section for production of heavy fermion pairs ($\Sigma^{0} \Sigma^{+} $, $\Sigma^{0} \Sigma^{-} $, and $\Sigma^{+} \Sigma^{-} $). Also shown is the theoretical prediction for the cross section of the $ \Sigma $ pair production via the type III seesaw mechanism, with its uncertainty. In the flavor-democratic scenario ($B_\mathrm{ e } =B_\mu =B_\tau $), heavy fermion pair production is excluded for masses below 840 GeV.
png pdf	Figure 3: The 95% confidence level expected (left) and observed (right) limits on the heavy fermion mass ${m_\Sigma }$ (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 $B_{\mu }=1- [B_{\mathrm{ e } } + B_{\tau }]$. The highlighted box indicates the limit on $m_{\Sigma }$ for the flavor-democratic scenario ($B_\mathrm{ e } =B_\mu =B_\tau $).
png pdf	Figure 3-a: The 95% confidence level expected limits on the heavy fermion mass ${m_\Sigma }$ (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 $B_{\mu }=1- [B_{\mathrm{ e } } + B_{\tau }]$. The highlighted box indicates the limit on $m_{\Sigma }$ for the flavor-democratic scenario ($B_\mathrm{ e } =B_\mu =B_\tau $).
png pdf	Figure 3-b: The 95% confidence level observed limits on the heavy fermion mass ${m_\Sigma }$ (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 $B_{\mu }=1- [B_{\mathrm{ e } } + B_{\tau }]$. The highlighted box indicates the limit on $m_{\Sigma }$ for the flavor-democratic scenario ($B_\mathrm{ e } =B_\mu =B_\tau $).

Tables
png pdf	Table 1: The signal regions summarized in terms of the number of leptons, the presence of an OSSF pair, and the kinematic variable used for signal discrimination. Each region is divided into eight bins in the kinematic variable, leading to 48 independent signal regions. Additional criteria based on $ { {p_{\mathrm {T}}} ^\text {miss}} $ ensure that signal and control regions do not overlap.

Summary

In summary, a search has been performed for type-III seesaw heavy fermions in multilepton final states using 35.9 fb$^{-1}$ of pp collision data at $ \sqrt{s} = $ 13 TeV, collected using the CMS detector. No significant discrepancies are observed between the data and the standard model prediction. Assuming degenerate heavy fermion masses, previously unexplored regions of the signal model as a function of the branching ratio of heavy fermions to each lepton flavor are excluded. In the lepton-flavor democratic scenario ($B_\mathrm{ e } =B_\mu =B_\tau $), heavy fermions with mass below 840 GeV are excluded at 95% confidence level. In the $\tau$-phobic case ($B_\mathrm{ e } =B_\mu =B_\tau $) the mass limits range from 900 GeV in the pure electron scenario to 930 GeV in the pure muon scenario. In the e/$\mu$-phobic case ($B_\tau =$ 1, $ B_\mathrm{e} + B_\mu =$ 0) the mass limit is 390 GeV. These are the strongest constraints to date on the mass of heavy fermions associated with the type-III seesaw mechanism.

Additional Figures
png pdf	Additional Figure 1: The $ {M_\mathrm {T}} $ distribution for events with three leptons, one OSSF pair with mass on-Z, and $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 100 GeV. 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_{\Sigma} = $ 700 GeV (solid line) and $m_{\Sigma} = $ 380 GeV (dashed line) are also shown. The lower panel shows 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.
png pdf	Additional Figure 2: The $ {L_\mathrm {T}} + {{p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with three leptons, one OSSF pair with mass below-Z, and $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 50 GeV. 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_{\Sigma} = $ 700 GeV (solid line) and $m_{\Sigma} = $ 380 GeV (dashed line) are also shown. The lower panel shows 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.
png pdf	Additional Figure 3: The $ {L_\mathrm {T}} + {{p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with three leptons and no OSSF pair. 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_{\Sigma} = $ 700 GeV (solid line) and $m_{\Sigma} = $ 380 GeV (dashed line) are also shown. The lower panel shows 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.
png pdf	Additional Figure 4: The $ {L_\mathrm {T}} + {{p_{\mathrm {T}}} ^\text {miss}} $ distribution for events with four or more leptons and two or more OSSF pairs. If one OSSF pair is on-Z, then $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 50 GeV is required. 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_{\Sigma} = $ 700 GeV (solid line) and $m_{\Sigma} = $ 380 GeV (dashed line) are also shown. The lower panel shows 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.

Additional Tables
png pdf	Additional Table 1: Observed and expected number of events with 3 and 4 or more leptons in the 48 signal regions, classified by the number of leptons, the presence and the mass of an OSSF pair, and $ {L_\mathrm {T}} + {{p_{\mathrm {T}}} ^\text {miss}} $ or $ {M_\mathrm {T}} $. The uncertainties quoted on the expected number of background events include statistical and systematic uncertainties.

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