Search for exotic long-lived particles with the LHCb detector

This thesis describes a search for exotic long-lived particles using proton- proton collision data collected by the LHCb experiment in 2011 and 2012 at centre-of-mass energies of 7 TeV and 8 TeV and corresponding to integrated luminosities of 0.62 fb$^{-1}$ and 1.38 fb$^{-1}$, respectively, focussing specifically on the improvements in the reconstruction and strategy for the analysis of the 2012 data sample. The long-lived particles are assumed to be pair-produced in the decay of a scalar resonance similar to the standard model Brout-Englert-Higgs boson, but only one is required to be reconstructed. No excess is observed, and limit are placed on the production cross-section of long-lived particles with masses in the range 25-50 GeV/$c^{2}$ and lifetimes in the range 2-500 ps.