Hunting for Heavy Majorana Neutrinos with Lepton Number Violating Signatures at LHC

The neutrinophilic two-Higgs-doublet model ($ u$2HDM) provides a natural way to generate tiny neutrino mass from interactions with the new doublet scalar $Phi_ u$ ($H^pm,~H,~A$) and singlet neutrinos $N_R$ of TeV scale. In this paper, we perform detailed simulations for the lepton number violating (LNV) signatures at LHC arising from cascade decays of the new scalars and neutrinos with the mass order $m_{N_R}<m_{Phi_ u}$. Under constraints from lepton flavor violating processes and direct collider searches, their decay properties are explored and lead to three types of LNV signatures: $2ell^pm 4j+cancel{E}_T$, $3ell^pm 4j+cancel{E}_T$, and $3ell^pmell^mp 4j$. We find that the same-sign trilepton signature $3ell^pm4j+cancel{E}_T$ is quite unique and is the most promising discovery channel at the high-luminosity LHC. Our analysis also yields the $95%$ C.L. exclusion limits in the plane of the $Phi_ u$ and $N_R$ masses at 13 (14) TeV LHC with an integrated luminosity of 100~(3000)/fb.