A search for the production of a single top quark in association with invisible
particles is performed using proton-proton collision data collected with the CMS detector
at the LHC at √s = 13TeV, corresponding to an integrated luminosity of 138fb−1. In
this search, a flavor-changing neutral current produces a single top quark or antiquark
and an invisible state nonresonantly. The invisible state consists of a hypothetical spin-1
particle acting as a new mediator and decaying to two spin-1/2 dark matter candidates. The
analysis searches for events in which the top quark or antiquark decays hadronically. No
significant excess of events compatible with that signature is observed. Exclusion limits at
95% confidence level are placed on the masses of the spin-1 mediator and the dark matter
candidates, and are compared to constraints from the dark matter relic density measurements.
In a vector (axial-vector) coupling scenario, masses of the spin-1 mediator are excluded up
to 1.85 (1.85)TeV with an expectation of 2.0 (2.0)TeV, whereas masses of the dark matter
candidates are excluded up to 0.75 (0.55)TeV with an expectation of 0.85 (0.65)TeV
