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New massive long-lived particles (LLP) are predicted by multiple beyond the Standard Model theories. This thesis presents the search of such long-lived particles decaying leptonically in the eμν final state. The search is performed using 2.31 fb−1 of proton-proton collisions collected by the LHCb detector at a center-of-mass energy of s=13 TeV, seeking for displaced vertices of electrons and muons of opposite charges.LLPs with masses between 7 to 50 GeV/c2, and lifetime between 2 and 50 ps, are explored in this search.
Three kinds of LLP production modes are considered: the direct pair production of LLPs from quark interactions, the pair production from a Standard Model like Higgs boson with a mass of 125 GeV/c2, and charged current production from an on-shell W boson with an additional lepton.No evidence of these long-lived states has been observed, upper limits at 95% CL on the production cross-section times branching ratio have been set on the different production mode.