Search for Higgs Bosons Decaying into Long-Lived Exotic Particles in the LHCb Experiment

The four experiments of the Large Hadron Collider (LHC) at CERN have collected their first data at √s = 7 TeV, hailing an exciting era in particle physics. The LHCb detector is a single-arm forward spectrometer, dedicated to precision measurements of CP violation, as well as to the study of rare b-hadron decays. In this work the excellent capability of the detector to reconstruct decay vertices in the region close to the interaction point with a resolution of few tenths of microns has been exploited. To achieve such a resolution, a precise measurement of the charged particle trajectories is essential. The Inner Tracker is the detector that provides tracking information for the particles flying in the innermost part of LHCb. While preparing this thesis, I contributed to the construction of the Inner Tracker by setting up of an assembly procedure for the twelve detector boxes. This included the preparation and thorough testing of numerous pieces, among them 386 sensor modules. Inner Tracker detector boxes have been installed in the LHCb cavern in summer 2008. After software alignment, the overall precision of the Inner Tracker modules position is on average 19 µm along the relevant direction. The careful box assembly and the quality tests along the procedure allowed to keep the fraction of dead strips below 1%. Several theoretical models beyond the Standard Model predict the existence of exotic long-lived particles that are potentially detectable in accelerator experiments like LHCb. The simulation software have been adapted in order to simulate some of the models producing a relevant set of topologies. The main features of these selected models are shown at generator level. Algorithms and methods have been developed to select events with vertices that are consistent with the decay of exotic long-lived particles. Key features of the candidates for the selected models are shown, leading to the design of on-line (trigger) and offline selections. Finally, we have analysed the LHCb data collected in 2010 to identify secondary vertices which can be associated with the decay of such particles. Pairs of candidates have been combined to reconstruct Higgs boson candidates. The analysis has selected zero candidate, with an efficiency to the signal at the level of 0.2-0.3% for some chosen points in the parameter space of a chosen theoretical model. This allowed to set limits on their production cross-section. The analysis of background MC events is consistent with an expectation of zero event for our integrated luminosity.

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