Études des désintégrations rares dans le charmonium

This thesis presents the study of rare decays of charmonium states formed in the interaction of an antiproton beam with an hydrogen gas jet target. Electromagnetic final states are used to sign the charmonium state formation (e/sup +/e/sup -/, e/sup +/e/sup -/ + CHI, gamma gamma ). The selection of events used a two arms non magnetic spectrometer, with a charged track system, a threshold Cerenkov counter to tag the electron (positron), and an e.m. calorimeter. Energy scan technic have been used to observe the resonant formation through the excitation curves. Parameters of the states (mass, total and partial widths) are extracted from these curves using a statistical analysis. Two types of decays have been studied in this thesis: /sup 1/P/sub 1/ charmonium state decay to the psi (signed by its e/sup +/e/sup -/ decay). In the energy scan around the center of gravity of the P charmonium states, we observe a cluster of 5 events, in a narrow mass range. This cluster correspond to a 2.7 sigma signal. The most probable interpretation of this signal is given by a narrow resonance, with a mass of 3526. MeV. Due to the properties (mass, width and decay) of this signal, this could be interpreted as the /sup 1/P/sub 1/ charmonium state. 2 photons decay of the eta/sub c/ and CHI/sub 2/. 22 gamma gamma events are observed, 15 in the eta/sub c/ region, and 7 in the CHI/sub 2/ region. This sample is interpreted as a direct observation of eta/sub c/ and CHI/sub 2/ decay into gamma gamma . Parameters of these decays, ( gamma gamma partial width), are extracted using a maximum likekihood analysis. Theoretical models of charmonium explain correctly the properties of the charmonium, including the results presented in this thesis. 57 refs. (ERA citation 13:016730)