ACT-DR5 Sunyaev-Zel'dovich clusters: Weak lensing mass calibration with KiDS

We present Weak Gravitational Lensing measurements of a sample of 157 clusters within the Kilo Degree Survey (KiDS), detected with a > 5 sigma thermal Sunyaev-Zel'dovich (SZ) signal by the Atacama Cosmology Telescope (ACT). Using a halo-model approach, we constrained the average total cluster mass, M-WL, accounting for the ACT cluster selection function of the full sample. We find that the SZ cluster mass estimate M-SZ, which was calibrated using X-ray observations, is biased with M-SZ/M-WL = (1 - b(SZ)) = 0.65 +/- 0.05. Separating the sample into six mass bins, we find no evidence of a strong mass dependency for the mass bias, (1 - b(SZ)). Adopting this ACT-KiDS SZ mass calibration would bring the Planck SZ cluster count into agreement with the counts expected from the Planck cosmic microwave background Lambda CDM cosmological model, although it should be noted that the cluster sample considered in this work has a lower average mass M-SZ,M- uncor = 3.64 x 10(14) M-circle dot compared to the Planck cluster sample which has an average mass in the range M-SZ,M- uncor = (5.5 - 8.5)x10(14) M-circle dot, depending on the sub-sample used.