Exotic colored fermions and lepton number violation at the LHC

Majorana neutrino mass models with a scale of lepton number violation of order tera-electron-volts potentially lead to signals at the LHC. Here, we consider an extension of the standard model with a colored octet fermion and a scalar leptoquark. This model generates neutrino masses at two-loop order. We make a detailed Monte Carlo study of the lepton number violating signal at the LHC in this model, including a simulation of standard model backgrounds. Our forecast predicts that the LHC with $300/\mathrm{fb}$ should be able to probe this model up to color-octet fermion masses in the range of (2.6–2.7) TeV, depending on the lepton flavor of the final state.

Figure 1

Two-loop neutrino mass diagram for the model considered in this work.

Figure 2

Pair production of the color-octet fermion $\mathrm{\Psi }$, followed by three-body decays, at the LHC.

Figure 3

Signal (solid lines) and SM background (stacked histograms) $H_T^{ll}$ distributions after the preselection cuts described in the text are applied. Signal samples for the $ee$ (left), $\mu \mu$ (right), and $e\mu$ (both plots) scenarios are shown. Only a few signal mass points are shown for the sake of clarity.

Figure 4

Signal (solid lines) and SM background (stacked histograms) $H_T^{e\mu }$ distributions after the preselection cuts described in the text are applied. Only a few signal mass points are shown for the sake of clarity.

Figure 5

Minimum $\mathrm{Br}(\mathrm{\Psi }\to \ell jj)$ for discovery (solid lines) and $2\sigma$-limits (dashed lines) as a function of $m_{\mathrm{\Psi }}$, for two values of the $H_T$ cut. Left for $\ell =e$ (in the scenario where the scalar leptoquark couples to electrons only) and right for $\ell =\mu$ (in the scenario where the scalar leptoquark couples to muons only). The red dashed line does not reach $\mathrm{Br}=1$ because we restrict the curves to regions with more than one signal event.

Figure 6

Minimum $\mathrm{Br}(\mathrm{\Psi }\to ejj)=\mathrm{Br}(\mathrm{\Psi }\to \mu jj)$ for discovery (solid lines) and $2\sigma$-limits (dashed lines) as a function of $m_{\mathrm{\Psi }}$, for two values of the $H_T$ cut, in the scenario where the scalar leptoquark couples to electrons and muons with the same strength.