OSp supergroup manifolds, superparticles, and supertwistors

We construct simple twister-like actions describing superparticles propagating on a coset superspace OSp(1\4)/SO(1,3) (containing the D=4 anti-de Sitter space as a bosonic subspace), on a supergroup manifold OSp(1\4) and, generically, on OSp(1\2n). Making two different contractions of the superparticle model on the OSp(1\4) supermanifold we get massless superparticles in Minkowski superspace without and with tensorial central charges. Using a suitable parametrization of OSp(1\2n) we obtain even Sp(2n)-valued Cartan forms which are quadratic in Grassmann coordinates of OSp(1\2n). This result may simplify the structure of brane actions in super-anti-de Sitter (AdS) backgrounds. For instance, the twister-like actions constructed with the use of the even OSp(1\2n) Cartan forms as supervielbeins are quadratic in fermionic variables. We also show that the free bosonic twister particle action describes massless particles propagating in arbitrary space-times with a conformally flat metric, in particular, in Minkowski space and AdS space. Applications of these results to the theory of higher spin fields and to superbranes in AdS superbackgrounds are mentioned.