KMT-2021-BLG-0322: Severe degeneracy between triple-lens and higher-order binary-lens interpretations

We investigate the microlensing event KMT-2021-BLG-0322, for which light curve exhibits three distinctive sets of caustic-crossing features. It is found that overall features are approximately described by a binary-lens (2L1S) model, but model leaves substantial residuals. test various interpretations with aim explaining find residuals can be explained either considering nonrectilinear lens-source motion caused microlens-parallax and lens-orbital effects or adding low-mass companion to binary lens (3L1S model). The degeneracy between higher-order 2L1S 3L1S very severe, making it difficult single out correct solution based on photometric data. This was known before two previous events (MACHO-97-BLG-41 OGLE-2013-BLG-0723), led false detections planets in systems, thus identification KMT-2021-BLG-0322 illustrates not only common also emphasizing need check both deviations from models. From Bayesian analysis conducted measured lensing observables timescale, angular Einstein radius, microlens parallax, estimated components have masses $(M_1, M_2) =(0.62^{+0.25}_{-0.26}~M_\odot,0.07^{+0.03}_{-0.03}~M_\odot)$, solutions, mass tertiary component according $M_3=6.40^{+2.64}_{-2.78}~M_{\rm J}$.