  The mass-constraining variable $M_2$, a $(1+3)$-dimensional natural successor
of extremely popular $M_{T2}$, possesses an array of rich features having the
ability to use on-shell mass constraints in semi-invisible production at a
hadron collider. In this work, we investigate the consequence of applying a
heavy resonance mass-shell constraint in the context of a semi-invisible antler
decay topology produced at the LHC. Our proposed variable, under additional
constraint, develops a new kink solution at the true masses. This enables one
to determine the invisible particle mass simultaneously with the parent
particle mass from these events. We analyze in a way to measure this kink
optimally, exploring the origin and the properties of such interesting
characteristics. We also study the event reconstruction capability inferred
from this new variable and find that the resulting momenta are unique and well
correlated with true invisible particle momenta.
