Using a large data sample of p{bar p} collisions at {radical}s = 1.96 TeV recorded by the CDF and D0 detectors operating at Fermilab Tevatron, the orbitally excited B mesons B{sub 1} and B*{sub 2} are observed directly as two separate states in fully reconstructed decays to B{sup (*)}{pi}. The masses and the mass differences between the states are presented. In the charm system, the orbitally excited D mesons D{sub 1} and D*{sub 2} are observed in fully reconstructed decays to D{sup (*)}{pi}. Measurements of the product branching rates of semileptonic decays B {yields} {bar D}**{mu}{sup +}{nu}{sub {mu}}X and their ratio are presented. They also present the observation of the semileptonic decays B{sub s} {yields} {mu}{nu}D**{sub s} where the excited D**{sub s} states are reconstructed in the mode D**{sub s} {yields} D*K{sub S}{sup 0}.

This thesis presents an analysis of excited states of B0, B+ and B0 s mesons, decaying to B mesons while emitting a pion or kaon. They are reconstructed from their decay products and a selection is performed to discard wrongly reconstructed B(s) mesons with the multivariate analysis software NeuroBayes, as described in chapter 5. In the training process, the sPlot method and measured and simulated data are used. Chapter 6 describes how the properties of excited B(s) are determined by an unbinned maximum likelihood t to their mass spectra. The systematic uncertainties determined in this analysis are described in chapter 7. The results of this thesis are presented in chapter 8 and a conclusion is given in chapter 9. The results shown in this thesis have been published before in [1].