Majorana fermion dark matter in minimally extended left-right symmetric model

We present a minimal extension of the left-right symmetric model based on the gauge group SU(3)(c) x SU(2)(L) x SU(2)(R) x U(1)(B-L) x U(1)(X), in which a vector-like fermion pair (zeta(L) and zeta(R)) charged under the U(1)(B-L) x U(1)(X) symmetry is introduced. Associated with the symmetry breaking of the gauge group SU(2)(R) x U(1)(B-L) x U(1)(X) down to the Standard Model (SM) hypercharge U(1)(Y), Majorana masses for zeta(L,R) are generated and the lightest mass eigenstate plays a role of the dark matter (DM) in our universe by its communication with the SM particles through a new neutral gauge boson "X". We consider various phenomenological constraints of this DM scenario, such as the observed DM relic density, the LHC Run-2 constraints from the search for a narrow resonance, and the perturbativity of the gauge couplings below the Planck scale. Combining all constraints, we identify the allowed parameter region which turns out to be very narrow. A significant portion of the currently allowed parameter region will be tested by the High-Luminosity LHC experiments.