Supertwistors, massive superparticles and k-symmetry

We consider a D=4 two-twistor lagrangian for a massive particle that incorporates the mass-shell condition in an algebraic way, and extend it to a two-supertwistor model with N=2 supersymmetry and central charge identified with the mass. In the purely supertwistorial picture the two D=4 supertwistors are coupled through a Wess-Zumino term in their fermionic sector. We demonstrate how the kappa-gauge symmetry appears in the purely supertwistorial formulation and reduces by half the fermionic degrees of freedom of the two supertwistors; a formulation of the model in terms of kappa-invariant degrees of freedom is also obtained. We show that the kappa-invariant supertwistor coordinates can be obtained by dimensional (D=6 -> D=4) reduction from a D=6 supertwistor. We derive as well by 6 -> 4 reduction the N=2, D=4 massive superparticle model with Wess-Zumino term introduced in 1982. Finally, we comment on general superparticle models constructed with more than two supertwistors.