Experimental results show that hosing of a long particle bunch in plasma can
be induced by wakefields driven by a short, misaligned preceding bunch. Hosing
develops in the plane of misalignment, self-modulation in the perpendicular
plane, at frequencies close to the plasma electron frequency, and are
reproducible. Development of hosing depends on misalignment direction, its
growth on misalignment extent and on proton bunch charge. Results have the main
characteristics of a theoretical model, are relevant to other plasma-based
accelerators and represent the first characterization of hosing