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This thesis is mainly about the search for exotic heavy particles -Intermediate Mass Magnetic Monopoles, Nuclearites and Q-balls with the SLIM experiment at the Chacaltaya High Altitude Laboratory (5230 m, Bolivia), establishing upper limits (90% CL) in the absence of candidates, which are among the best if not the only one for all three kind of particles. A preliminary study of the background induced by cosmic neutron in CR39 at the SLIM site, using Monte Carlo simulations. The measurement of the elemental abundance of the primary cosmic ray with the CAKE experiment on board of a stratospherical balloon; the charge distribution obtained spans in the range 5≤Z≤31. Both experiments were based on the use of plastic Nuclear Track Detectors, which records the passage of ionizing particles; by using some chemical reagents such passage can be make visible at optical microscopes.

This thesis comes after a strong contribution on the realization of the CMS computing system, which can be seen as a relevant part of the experiment itself. A physics analysis completes the road from Monte Carlo production and analysis tools realization to the final physics study which is the actual goal of the experiment. The topic of physics work of this thesis is the study of tt events fully hadronic decay in the CMS experiment. A multi-jet trigger has been provided to fix a reasonable starting point, reducing the multi-jet sample to the nominal trigger rate. An offline selection has been provided to reduce the S/B ratio. The b-tag is applied to provide a further S/B improvement. The selection is applied to the background sample and to the samples generated at different top quark masses. The top quark mass candidate is reconstructed for all those samples using a kinematic fitter. The resulting distributions are used to build p.d.f.’s, interpolating them with a continuous arbitrary curve. These curves are used to perform the top mass measurement through a likelihood comparison

In the present study we are using multi variate analysis techniques to discriminate signal from background in the fully hadronic decay channel of ttbar events. We give a brief introduction to the role of the Top quark in the standard model and a general description of the CMS Experiment at LHC. We have used the CMS experiment computing and software infrastructure to generate and prepare the data samples used in this analysis. We tested the performance of three different classifiers applied to our data samples and used the selection obtained with the Multi Layer Perceptron classifier to give an estimation of the statistical and systematical uncertainty on the cross section measurement.

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