Story of m=0 spin

One of the most astonishing prediction of quantum mechanics is the π-phase shift of fermions by 2π rotations. On the other hand, bosons seem to be trivial under three-dimensional rotations since they return exactly the same states by 2π rotations. Here, we report on an experiment on a non-trivial π-phase shift of a seemingly trivial m=0 spin state of boson, which is associated with π rotations instead of 2π rotations. (author)