  We report observation of electron helical dichroism on a material with chiral
structure. In analogy with circular dichroism, a common technique for molecular
structural fingerprinting, we use a nanofabricated forked diffraction grating
to prepare electron vortex beams with opposite orbital angular momenta incident
upon metal nanoparticle clusters and post-select for a zero-orbital angular
momentum final state. We observe a difference in the differential scattering
probability for orbital angular momentum transfer from vortices with opposite
handedness incident on chiral aluminum nanoparticle clusters at 3.5 $\pm$ 0.8
eV. We suggest that the observed electron helical dichroism is due to
excitation of surface plasmon vortices. Electron helical dichroism enables
chirality measurement with unprecedented spatial resolution over a broad range
of energies.
