New variant of N=4 superconformal mechanics

Proceeding from a nonlinear realization of the most general N=4, d=1 superconformal symmetry, associated with the supergroup D(2,1;alpha), we construct a new model of nonrelativistic N=4 superconformal mechanics. In the bosonic sector it combines the worldline dilaton with the fields parametrizing the R-symmetry coset S^2 ~ SU(2)/U(1). We present invariant off-shell N=4 and N=2 superfield actions for this system and show the existence of an independent N=4 superconformal invariant which extends the dilaton potential. The extended supersymmetry requires this potential to be accompanied by a d=1 WZW term on S^2. We study the classical dynamics of the bosonic action and the geometry of its sigma-model part. It turns out that the relevant target space is a cone over S^2 for any non-zero alpha \neq \pm 1/2. The constructed model is expected to be related to the `relativistic' N=4 mechanics of the AdS_2 times S^2 superparticle via a nonlinear transformation of the fields and the time variable.