The two dimensional Heisenberg antiferromagnet on a square lattice is one of the most studied model in quantum magnetism . This is in part due to its intrinsic interest but also to the fact that both the cuprates and the pnictides superconductors belong to this class of model. In the fully quantum case with S=1/2,an outstanding theoretical and experimental challenge is to understand how the excitations evolve as a function of applied magnetic field. Specifically, we wish to investigate how the spinon continuum evolves into a novel mode for finite magnetic fields and to compare our experimental results with state-of-the-art theoretical models.