U-duality from matrix membrane partition function

We analyse super-membrane instantons (fully wrapped supermembranes) by comp uting the partition function of the three-dimensional supersymmetrical U(N) matrix model under periodic boundary conditions. By mapping the model to a cohomological field theory and considering a mass-deformation of the model , we show that the partition function exactly leads to the U-duality invari ant measure factor entering supermembrane instanton sums. On the other hand , a computation based on the quasi-classical assumption gives the non U-dua lity invariant result of the zero-mode analysis by Pioline et al. (JHEP 03 (2001) 036). This is suggestive of the importance of supermembrane self-int eractions and shows a crucial difference from the matrix string case. (C) 2 001 Elsevier Science B.V. All rights reserved.