Recherche de désintégrations rares de mésons B dans l'expérience ALEPH au LEP

The work presented in this thesis consists of the analysis of 3.9 millions hadronic decays of the Z^0 recorded by ALEPH detector at LEP with the purpose of searching B mesons rare decays which occur via the b -> u and the b -> (s,d) gamma transitions. The first study concerns the exclusive hadronic charmless decays which represent direct evidence for a non-zero Vub Cabibbo-Kobayashi-Maskawa matrix element. The limits obtained are between 1.2 x 10^{-3} and 2 x 10^{-5} depending of the channel and are often the best present world limits. The second study is a search of exclusive decays corresponding to the b -> (s, d) gamma transition, which is flavour changing neutral current process in the Standard Model with a loop diagram (penguin diagram). Two candidates in the B -> K^{*o} gamma are observed and allow us to give a branching ratio Br (B -> K^{*o} gamma) = (6,8 ^{+6,1}_{-4,1}) x 10^{-5}$. Some limits at 90% level confidence are given for the other channels (B -> rho^{o} gamma, Bs -> phi gamma).