Siegel’s superparticle

We compare Siegel’s superparticle with the original one. The former contains an extra fermionic degree of freedom which cannot be gauged away. If one tries to fix all gauges completely, ambiguities appear. In both the light-front and proper-time gauges, a gauge symmetry remains. Becchi-Rouet-Stora-Tyutin quantization, in the light-front gauge, is unambiguous, and yields a physical spectrum which is an irreducible supermultiplet with positive energy. We discuss in detail how the remaining gauge symmetry allows one to evade well-known problems concerning the localization of elementary systems and how the problems reappear when gauge-invariant extensions of the relevant operators are sought. Our attention is restricted to four space-time dimensions throughout.