$Z^\prime$-portal right-handed neutrino dark matter in the minimal
  U(1)$_X$ extended Standard Model
  We consider a concise dark matter (DM) scenario in the context of a
non-exotic U(1) extension of the Standard Model (SM), where a new U(1)$_X$
gauge symmetry is introduced along with three generation of right-handed
neutrinos (RHNs) and an SM gauge singlet Higgs field. The model is a
generalization of the minimal gauged U(1)$_{B-L}$ (baryon number minus lepton
number) extension of the SM, in which the extra U(1)$_X$ gauge symmetry is
expressed as a linear combination of the SM U(1)$_Y$ and U(1)$_{B-L}$ gauge
symmetries. We introduce a $Z_2$-parity and assign an odd-parity only for one
RHN among all particles, so that this $Z_2$-odd RHN plays a role of DM. The
so-called minimal seesaw mechanism is implemented in this model with only two
$Z_2$-even RHNs. In this context, we investigate physics of the RHN DM,
focusing on the case that this DM particle communicates with the SM particles
through the U(1)$_X$ gauge boson ($Z^\prime$ boson). This "$Z^\prime$-portal
RHN DM" scenario is controlled by only three free parameters: the U(1)$_X$
gauge coupling ($\alpha_X$), the $Z^\prime$ boson mass ($m_{Z^\prime}$), and
the U(1)$_X$ charge of the SM Higgs doublet ($x_H$). We consider various
phenomenological constraints to identify a phenomenologically viable parameter
space. The most important constraints are the observed DM relic abundance and
the latest LHC Run-2 results on the search for a narrow resonance with the
di-lepton final state. We find that these are complementary with each other and
narrow the allowed parameter region, leading to the lower mass bound of
$m_{Z^\prime} \gtrsim 2.7$ TeV.
