  Galaxy interactions are thought to be one of the main triggers of Active
Galactic Nuclei (AGN), especially at high luminosities, where the accreted gas
mass during the AGN lifetime is substantial. Evidence for a connection between
mergers and AGN, however, remains mixed. Possible triggering mechanisms remain
particularly poorly understood for luminous AGN, which are thought to require
triggering by major mergers, rather than secular processes. We analyse the host
galaxies of a sample of 20 optically and X-ray selected luminous AGN
(log($L_{bol}$ [erg/s]) $>$ 45) at z $\sim$ 0.6 using HST WFC3 data in the
F160W/H band. 15/20 sources have resolved host galaxies. We create a control
sample of mock AGN by matching the AGN host galaxies to a control sample of
non-AGN galaxies. Visual signs of disturbances are found in about 25% of
sources in both the AGN hosts and control galaxies. Using both visual
classification and quantitative morphology measures, we show that the levels of
disturbance are not enhanced when compared to a matched control sample. We find
no signs that major mergers play a dominant role in triggering AGN at high
luminosities, suggesting that minor mergers and secular processes dominate AGN
triggering up to the highest AGN luminosities. The upper limit on the enhanced
fraction of major mergers is $\leqslant$20%. While major mergers might increase
the incidence of (luminous AGN), they are not the prevalent triggering
mechanism in the population of unobscured AGN.
