CERN Accelerating science

The tree-level diagram for the production of a heavy Majorana neutrino ($N$) in the mTISM model. Lepton flavour is denoted by $\alpha$ and $\beta$. Lepton flavour is assumed to be conserved, such that $\alpha = \beta$. The $W$ boson produced from the $N$ decay is on-shell and, in this case, decays hadronically.

The tree-level diagrams for the production of a heavy Majorana neutrino ($N$) in the LRSM model, in which heavy gauge bosons $W_{\text{R}}$ and $\ZR$ are also incorporated. Lepton flavour is denoted by $\alpha$ and $\beta$. Lepton flavour is assumed to be conserved, such that $\alpha = \beta$. The $W_{\text{R}}$ boson produced from the $N$ decay is off-shell and, in this case, decays hadronically.

The tree-level diagrams for the production of a heavy Majorana neutrino ($N$) in the LRSM model, in which heavy gauge bosons $W_{\text{R}}$ and $\ZR$ are also incorporated. Lepton flavour is denoted by $\alpha$ and $\beta$. Lepton flavour is assumed to be conserved, such that $\alpha = \beta$. The $W_{\text{R}}$ boson produced from the $N$ decay is off-shell and, in this case, decays hadronically.

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Distribution of (a) the number of jets and (b) the leading jet $\pt$ in events containing any combination of exactly three or four leptons. The events must contain one lepton with $\pt > 25$~GeV and all other leptons must satisfy $\pt > 20$~GeV. The contribution labelled `Other' is from processes described in section~\ref{sec-prompt} (with MC samples described in section \ref{sec-MC}), other than the contributions from $WZ$ and $ZZ$, which are labelled separately. The shaded bands indicate the experimental uncertainties on the total expected background, including all contributions described in section \ref{sec:systMC}, but not including any uncertainty on the $WZ$~and $ZZ$~cross-sections. The lower plots show the ratio of data to the total expected background.

Distribution of the number of jets in a validation region consisting of events containing exactly two same-sign muons with one muon satisfying $\pt > 25 \GeV$ and the second satisfying $10 \GeV < \pt < 20 \GeV$. The shaded bands indicate the total uncertainty, including all contributions described in section \ref{sys}, on the total expected background and the lower plots show the ratio of data to the total expected background.

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The $\met$ (top) and lepton $\pt$ (bottom) distributions for the $ee$ (left) and $\mu\mu$ (right) channels in a validation region consisting of events with exactly two same-sign leptons and no jets. The shaded bands indicate the total uncertainty, including all contributions described in section \ref{sys}, on the total expected background and the lower plots show the ratio of data to the total expected background.

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The distribution of the transverse momentum $\pt$ of the leading jet (top) and the distribution of the number of jets (bottom) for the $ee$ (left) and $\mu\mu$ (right) channels in a validation region consisting of events with exactly two same-sign leptons. The shaded bands indicate the total uncertainty, including all contributions described in section \ref{sys}, on the total expected background and the lower plots show the ratio of data to the total expected background.

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Invariant mass of the two highest-$\pt$ jets ($m_{jj}$) in events satisfying the mTISM signal region criteria (excluding the $m_{jj}$ criteria) for (a) $ee$ and (b) $\mu\mu$ events. Events satisfying all selection criteria are in the region indicated by the arrows. The expected mTISM signal distribution for $m_N = 120 \GeV$ is shown by the dashed (blue) histogram. The values of the mixing parameter $|V_{\ell N}|^2$ are chosen such that the signal distribution is clearly visible, $|V_{e N}|^2=0.2$~for (a) and $|V_{\mu N}|^2=0.02$~for (b). The shaded bands indicate the total uncertainty, including all contributions described in section \ref{sys}, on the total expected background and the lower plots show the ratio of data to the total expected background.

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Observed and expected $95\%$~confidence level upper limits on the cross-section times branching ratio for the production of mTISM heavy Majorana neutrinos as a function of the heavy neutrino mass for (a) the $ee$~channel and (c) the $\mu\mu$~channel. The limits on the mixing between the heavy Majorana neutrinos and the SM neutrinos are shown in (b) and (d). Values larger than the solid black line are excluded by this analysis.

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Invariant mass of two leading leptons and up to two leading jets after applying additional $W_{\text{R}}$ selection criteria (two same-sign leptons, at least one jet, $m_{\ell\ell} > 110 \GeV$ and $m_{\ell \ell j(j)} > 400 \GeV$), for the $ee$-channel (a) and $\mu\mu$-channel (b). A finely binned LRSM signal sample is represented by the dashed (blue) histogram corresponding to $\mWR = 2600 \GeV$ and $\mN = 1950 \GeV$. The shaded bands indicate the total uncertainty, including all contributions described in section \ref{sys}, on the total expected background and the lower plots show the ratio of data to the total expected background.

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Invariant mass of two leading leptons and up to four leading jets after applying additional $\ZR$ selection criteria (two same-sign leptons, at least two jets, $m_{\ell\ell} > 110 \GeV$ and $m_{\ell \ell jj(jj)} > 200 \GeV$), for the $ee$-channel (a) and $\mu\mu$-channel (b). A finely binned LRSM signal sample is represented by the dashed (blue) histogram corresponding to $\mZR = 2200 \GeV$ and $\mN = 550 \GeV$. The shaded bands indicate the total uncertainty, including all contributions described in section \ref{sys}, on the total expected background and the lower plots show the ratio of data to the total expected background.

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Observed and expected exclusion contour at $95\%$~confidence level as a function of the mass of a heavy Majorana neutrino and of a $\WR$ (left) or $\ZR$~boson (right) within the LRSM. The limits in (a) and (b) show the scenario where the heavy neutrino has electron flavour and those in (c) and (d) show the scenario where it has muon flavour. The limits in (e) and (f) show the case of two degenerate neutrinos, one has electron flavour, and the other muon flavour (no mixing between lepton flavours is assumed).

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