Abstract : The H1 experiment, situated on the HERA collider in Hamburg, enables studies of positron-proton interactions with a spatial resolution 10 times better than that obtained in previous fixed target experiments. This thesis describes the analysis of interactions having large transverse energies and the interpretation of the results following composite models. This kind of models predicts that fermions, considered as elementary particles at present, are in fact bound states of constituants.
Any observation of quarks or leptons in an excited state would be the sign of a new structure of matter. In the analysis, topologies with leptons and/or transverse missing momentum in the final state have been considered. The event selection was performed on data accumulated by H1 from 1994 to 1997. This corresponds to an integrated luminosity of about 40 pb-1. In order to realise this study, we had to eliminate the non-physical background composed of cosmic muons and halo muons (producted in the interactions of the proton beam with the residual gas or the beam pipe wall) which pollute the data. We developed some selection criteria which allow the rejection of this kind of event by topological recognition.
The comparison between the number of candidates from the data and the standard model estimation shows no evidence for excited states of fermions. Upper limits on the product of the production cross section and the branching ratio and on the characteristic couplings f/Lambda are derived for masses Mf up to 275 GeV/c2. Results obtained extend the limits to new channels and improve by factor 10 those derived in a previous analysis. In the particuliar choice f/Lambda = 1/Mf, we exclude at the 95% confidence level, excited electrons with masses between 25 and 221 GeV/c2 excited neutrinos with masses between 50 and 112 GeV/c2 and excited quarks between 50 and 194 GeV/c2.