Analysis method for the search for neutrinoless double beta decay in the NEMO3 experiment: study of the background and first results

The NEMO3 detector, installed in the Frejus Underground Laboratory, is dedicated to the study of neutrinoless double beta decay: the observation of this process would sign the massive and Majorana nature of neutrino. The experiment consists in very thin central source foils (the total mass is equal to 10 kg), a tracking detector made of drift cells operating in Geiger mode, a calorimeter made of plastic scintillators associated to photomultipliers, a coil producing a 30 gauss magnetic field and two shields, dedicated to the reduction of the γ-ray and neutron fluxes. In the first part, I describe the implications of several mechanisms, related to trilinear R-parity violation, on double beta decay. The second part is dedicated to a detailed study of the tracking detector of the experiment: after a description of the different working tests, I present the determination of the characteristics of the tracking reconstruction (transverse and longitudinal resolution, by Geiger cell and precision on vertex determination, charge recognition). The last part corresponds to the analysis of the data taken by the experiment. On the one hand, an upper limit on the Tl208 activity of the sources has been determined: it is lower than 68 mBq/kg, at 90% of confidence level. On the other hand, I have developed and tested on these data a method in order to analyse the neutrinoless double beta decay signal; this method is based on a maximum of likelihood using all the available information. Using this method, I could determine a first and very preliminary upper limit on the effective mass of the neutrino. (author)