The recent discovery of a new boson X by the two experiments (ATLAS and CMS) of the Large Hadron Collider (LHC) is a major breakthrough in the understanding of the fundamental interactions. It could help to elucidate the nature of the spontaneous electroweak symmetry breaking (EWSB) mechanism presumed responsible for the appearance of the mass of the elementary particles in the early moments of the Universe. The decay channels with charged leptons in the final state offer the best opportunities for an exhaustive study of the properties of the new boson. The X-> ZZ*-> 4l channel (where l is an electron or a muon) is the « golden » mode for the discovery. It allows in stand-alone for a precise determination of the mass mX. It moreover can provide a determination of the spin-parity state (S^CP) by exploiting the kinematics of disintegration in the center of mass of the resonance. The X -> 2 taus channel is essential to establish the existence of a direct coupling to leptons. The coupling to tau leptons is the only lepton coupling eventually accessible at the LHC in the timescale of this project. It moreover can provide a good sensitivity to SP, through the study of the polarization of the tau leptons, while offering some sensitivity to boson mass. The X boson production modes can be distinguished experimentally in a rather clean manner in both of the X-> ZZ* -> 4l and X -> 2 taus channels, by the presence of additional leptons or jets. In particular, the distinction between the production by gluon fusion (through virtual loops of top quarks), by associated production with a vector boson W or Z, or by vector boson fusion (ZZH and WWH) is particularly important for the constraint on the couplings. The aim of this very challenging project is to develop and deploy innovative and powerful analysis techniques to provide the best measurements of the properties of the new boson, via leptonic final states, at the LHC with higher energy and luminosity. It consists of providing for the first time at the LHC an analysis chain with new lepton reconstruction techniques incorporated in a global description of the events, interleaved with a full interpretation via event weighting based on a novel Matrix Element approach at next-to-leading order. It will benefit from the unique expertise in Europe at the Laboratoire Leprince Ringuet (LLR) of the Ecole Polytechnique, and with partners from the Fakultet elektrotehnike strojarstva i brodogradnje (FESB) in Split (Croatia), on leptons (reconstruction, identification and isolation of charged leptons), as well as on the X-> ZZ*->4l and X ->2 taus analysis in the CMS experiment at the LHC.