Hidden quantum-mechanical supersymmetry in extra dimensions
  We study higher dimensional field theories with extra dimensions from a 4d spectrum point of view. It is shown that 4d mass spectra of spinor, gauge and gravity field theories are governed by quantum-mechanical supersymmetry. The 4d massless modes turn out to correspond to zero energy vacuum states of the supersymmetry. Allowed boundary conditions on extra dimensions compatible with the supersymmetry are found to be severely restricted. 
Makoto Sakamoto
hep-th