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N=8 superconformal mechanics

We construct new models of N=8 superconformal mechanics associated with the off-shell N=8, d=1 supermultiplets (3,8,5) and (5,8,3). These two multiplets are derived as N=8 Goldstone superfields and correspond to nonlinear realizations of the N=8, d=1 superconformal group OSp(4^*|4) in its supercosets OSp(4^*|4)/U(1)_R x SO(5) and OSp(4^*|4)/SU(2)_R x SO(4), respectively. The irreducibility constraints for these superfields automatically follow from appropriate superconformal covariant conditions on the Cartan superforms. The N=8 superconformal transformations of the superspace coordinates and the Goldstone superfields are explicitly given. Interestingly, each N=8 supermultiplet admits two different off-shell N=4 decompositions, with different N=4 superconformal subgroups SU(1,1|2) and OSp(4^*|2) of OSp(4^*|4) being manifest as superconformal symmetries of the corresponding N=4, d=1 superspaces. We present the actions for all such N=4 splittings of the N=8 multiplets considered.