A note on ${\cal N}\ge 6$ Superconformal Quantum Field Theories in three dimensions

Based on the structure of the three-dimensional superconformal algebra we show that every irreducible ${\mathcal N}=6$ three-dimensional superconformal theory containes exactly one conserved U(1)-symmetry current in the stress tensor supermultiplet and that superconformal symmetry of every ${\mathcal N}=7$ superconformal theory is in fact enhanced to ${\mathcal N}=8$. Moreover, an irreducible ${\cal N}=8$ superconformal theory does not have any global symmetries. The first observation explains why all known examples of ${\mathcal N}=6$ superconformal theories have a global abelian symmetry.