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Extended Supersymmetric BMS$_3$ algebras and Their Free Field Realisations

We study $N=(2,4,8)$ supersymmetric extensions of the three dimensional BMS algebra (BMS$_3$) with most generic possible central extensions. We find that $N$-extended supersymmetric BMS$_3$ algebras can be derived by a suitable contraction of two copies of the extended superconformal algebras. Extended algebras from all the consistent contractions are obtained by scaling left-moving and right-moving supersymmetry generators symmetrically, while Virasoro and R-symmetry generators are scaled asymmetrically. On the way, we find that the BMS/GCA correspondence does not in general hold for supersymmetric systems. Using the $β$-$γ$ and the ${\mathfrak b}$-${\mathfrak c}$ systems, we construct free field realisations of all the extended super-BMS$_3$ algebras.