Abstract : This dissertation presents a new scheme for cooling atoms by laser: the "velocity selective coherent population trapping" or "velocity selective dark states". In the first chapter, we describe our supersonic beam of metastable helium, cooled by liquid helium, on which we have performed our experiments. In the second chapter, devoted to a new "Mechanical Hanle effect", we introduce theoretical tools that will be needed for the complete analysis of our new cooling scheme. We establish the generalised optical Bloch equations which describe the evolution of the density matrix for the quantised internal and external degrees of freedom of the atoms. The introduction of closed familles of coupled states allows us to easily integrate those equations. The effect presented in this chapter is a new manifestation of the Hanle effect in the ground state: the force experienced by an atom interacting with a polarised travelling wave may show up a narrow resonance around zero as a funetion of an external magnetic field. The third chapter presents the "velocity selective coherent population trapping". The basic idea of this cooling scheme is the existence of a state which is a superposition of different Zeeman sublevels in the ground state and which is not coupled to the excited state by the laser. We present a detailed theoretical analysis as well as experimental results: we have been able to accumulate atoms in a velocity class with a width narrower than the one-photon recoil hk/M.