Heavy Long-lived Dark Vector Via a Gluonic Portal

We study a dark gauge boson $Z'$ that exclusively couples to the QCD gluons through higher dimensional operators. These operators are generated from integrating out of heavy ultraviolet resonances carrying both QCD and dark gauge charges. With $SU(3)_C$ gauge invariance, charge and parity symmetries preserved, we find that the leading effective operators are restricted to have the form of $Z'GGG$ and $Z'Z'GG$ at dimension-eight, which can naturally render the $Z^\prime$ particle long-lived, and serve as a viable dark matter candidate. We investigate the phenomenology of these operators with both collider experiments and cosmological observation, without and with the assumption that this dark gauge boson plays the role of the dominant dark matter component. For an unstable $Z'$, we show that depending on its lifetime, it can be probed by various observables up to ultraviolet physics scale around $10^9$ GeV. For $Z'$ being dark matter, we find that $m_{Z^\prime} \gtrsim 1$ TeV is consistent with the thermal freeze-out scenario. In contrast, in the freeze-in scenario, the extremely small couplings leave the relevant parameter space largely unconstrained by current experiments.