Expanding the Reach of Heavy Neutrino Searches at the LHC

Abstract: The observation of neutrino oscillations establishes that neutrinos ($\nu_{\ell}$) have non-zero mass and provides one of the more compelling arguments for physics beyond the standard model (SM) of particle physics. We present a feasibility study to search for hypothetical Majorana neutrinos ($N_{\ell}$) with TeV scale masses, predicted by extensions of the SM to explain the small but non-zero $\nu_{\ell}$ mass, using vector boson fusion (VBF) processes at the 13 TeV LHC. In the context of the minimal Type-I seesaw mechanism (mTISM), the VBF $\ell N_{\ell}$ production cross-section surpasses that of the Drell-Yan process at approximately $m_{N_{\ell}} = 1.4$ TeV. We consider $\mu N_{\mu}$ and $\tau N_{\tau}$ production through VBF processes (e.g. $qq'\to \tau N_{\tau}qq'$), with subsequent $N_{\mu}$ and $N_{\tau}$ decays to $\mu jj$ and $\tau jj$, as benchmark cases to show the effectiveness of the VBF topology for $N_{\ell}$ seaches at the 13 TeV LHC. The requirement of a dilepton pair combined with four jets, two of which are identified as VBF jets with large separation in pseudorapidity and a TeV scale dijet mass, is effective at reducing the SM background. This criteria may provide expected exclusion bounds, at 95\% confidence level, of $m_{N_{\ell}} < 1.7$ ($2.4$) TeV, assuming $100$ (1000) fb$^{-1}$ of 13 TeV data from the LHC and mixing $|V_{\ell N_{\ell}}|^{2} = 1$. The use of the VBF topology to search for $m_{N_{\ell}}$ increases the discovery reach at the LHC, with expected significances greater than 5$\sigma$ (3$\sigma$) for $N_{\ell}$ masses up to 1.7 (2.05) TeV using 1000 fb$^{-1}$ of 13 TeV data from the LHC.