Probing the seesaw mechanism at CERN LHC

We have recently proposed a simple SU(5) theory with an adjoint fermionic multiplet on top of the usual minimal spectrum. This leads to the hybrid scenario of both type I and type III seesaw and it predicts the existence of the fermionic SU(2) triplet between 100 GeV and 1 TeV for a conventional grand unified theory scale of about ${10}^{16}\mathrm{GeV}$, with main decays into $W$ ($Z$) and leptons, correlated through Dirac Yukawa couplings, and lifetimes shorter than about ${10}^{-12}\sec $. These decays are lepton number violating and they offer an exciting signature of $\Delta L=2$ dilepton events together with 4 jets at future $pp$ ($p\overline{p}$) colliders. Increasing the triplet mass endangers the proton stability and so the seesaw mechanism could be directly testable at LHC.

Figure 1

The maximum value of the effective triplet mass $m_3$ as a function of the unification scale $M_{\mathrm{GUT}}$ from the two-loop analysis.

Figure 2

The region that gives unification at $M_{\mathrm{GUT}}={10}^{15.5}\mathrm{GeV}$.