Endothermic Z'-Portal Dark Matter: LZ-LHC Complementarity

Motivated by the 2.6σ high-energy recoil event recently reported by the LUX-ZEPLIN (LZ) Collaboration, we consider an endothermic Z-portal Majorana dark matter framework and discuss the complementarity between the LZ event and LHC searches for a Z resonance. As a concrete realization, we consider a gauged U(1) B-L extension of the Standard Model. The phenomenology of the framework is essentially controlled by two free parameters: the U(1) B-L gauge coupling g BL and the Z boson mass m Z. For a fixed m Z, the observed dark matter abundance requires the dark matter mass to be near the Z resonance, m DM about m Z/2, and sets a lower bound on g BL. Complementarily, LHC searches for a Z resonance set an upper bound on g BL. The parameter space allowed by the dark matter abundance and LHC constraints can account for the recent LZ event. The synergy between future Z resonance searches at the High-Luminosity LHC and the LZ experiment may provide a test of this framework.