Cid Vidal, XabierMombaecher, TitusRodríguez Fernández, Emilio Xosé2026-01-272026-01-272025https://hdl.handle.net/10347/45445This thesis contributes to the search for Long-Lived Particles beyond the Standard Model in the LHCb and CODEX-b experiments. It presents an analysis of the rare Bmeson decays into multi-muon final states predicted by non-minimal Composite Higgs models. Experimental data from LHCb corresponding to Run 2 are used to set limits in both the prompt and displaced regimes for Bs and Bu decay chains, with several reconstruction techniques explored to maximize the sensitivity reach. The work also describes contributions to the early development of the future CODEX-b detector, including the installation of the CODEX-beta demonstrator during Run 3 and the creation of a fast-simulation framework.engAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-nd/4.0/LHCbCODEX-bLLPsMultimuons220703 Física atómicadoctoral thesisopen access