  The expanding solar wind plasma ubiquitously exhibits anisotropic non-thermal
particle velocity distributions. Typically, proton Velocity Distribution
Functions (VDFs) show the presence of a core and a field-aligned beam. Novel
observations made by Parker Solar Probe (PSP) in the innermost heliosphere have
revealed new complex features in the proton VDFs, namely anisotropic beams that
sometimes experience perpendicular diffusion. This phenomenon gives rise to
VDFs that resemble a "hammerhead". In this study, we use a 2.5D fully kinetic
simulation to investigate the stability of proton VDFs with anisotropic beams
observed by PSP. Our setup consists of a core and an anisotropic beam
populations that drift with respect to each other. This configuration triggers
a proton-beam instability from which nearly parallel fast magnetosonic modes
develop. Our results demonstrate that before this instability reaches
saturation, the waves resonantly interact with the beam protons, causing
significant perpendicular heating at the expense of the parallel temperature.
Furthermore, the proton perpendicular heating induces a hammerhead-like shape
in the resulting VDF. Our results suggest that this mechanism probably
contribute to producing the observed hammerhead distributions.
