Bordered contact invariants and half Giroux torsion

Abstract

We show that there exist infinitely many closed contact 3-manifolds containing half Giroux torsion along a separating torus whose contact invariants do not vanish. This provides counterexamples to Ghiggini's conjecture and suggests that separating half Giroux torsion may not obstruct symplectic fillability. The main tools are the bordered contact invariants recently developed by the authors and the innermost contact structures on knot complements. We also show that there exists a closed contact 3-manifold containing convex torsion along a separating torus with non-vanishing contact invariant, which implies that $2\pi$ is the minimal amount of twisting necessary to ensure vanishing of the contact invariant.