Search for heavy lepton partners of neutrinos in the context of type-III Seesaw Mechanism at CMS

The Seesaw Mechanism was introduced to explain why the masses of neutrinos are many orders of magnitude smaller than the other lepton masses. Considering neutrinos as Majorana particles, ``natural'' Yukawa couplings yield neutrinos with very small masses, along with heavy partners. Such particles may be observable at the LHC experiments. CMS searched for a fermionic triplet (type-III Seesaw Mechanism) by selecting events with three isolated leptons, jets and missing transverse energy in the final state. Backgrounds are due to events with leptons from electroweak processes either leptons coming from secondary vertices or ``fake leptons''. The estimate of fake leptons is a crucial point of the analysis. Results obtained with data collected in 2012, corresponding to 19.7~$\mbox{fb}^{-1}$ and $\sqrt{s}$= 8 TeV, show no evidence of signal, and so we have set lower limits on the masses of the fermionic triplet.