TWISTOR-LIKE D=10 SUPERPARTICLE ACTION WITH MANIFEST N=8 WORLD-LINE SUPERSYMMETRY

We propose a new formulation of the D = 10 Brink-Schwarz superparticle which is manifestly invariant under both the target-space super-Poincare group and the world-line local N = 8 super-conformal group. This twistorlike construction naturally involves the sphere S8 as a coset space of the D = 10 Lorentz group. The action contains only a finite set of auxiliary fields, but they appear in unusual trilinear combinations. The origin of the on-shell D = 10 fermionic kappa-symmetry of the standard Brink-Schwarz formulation is explained. The coupling to a D = 10 super-Maxwell background requires a new mechanism, in which the electric charge appears only on shell as an integration constant.