  We propose a minimal gauged U(1)$_X$ extension of the MSSM with R-parity
conservation. In this model, U(1)$_X$ is a generalization of the well-known
U(1) $B-L$. Apart from the MSSM particle content, the model includes three
right-handed neutrino (RHN) chiral superfields, each carrying a unit U(1)$_X$
charge. In the presence of RHNs, the model is free from all gauge and mixed
gauge-gravitational anomalies. However, there are no U(1)$_X$ Higgs chiral
superfields with U(1)$_X$ charge $\pm2$ involved in the model. Two of the RHN
superfields are assigned an odd R-parity, while the last one ($\Psi$) has an
even parity. The U(1)$_X$ symmetry is radiatively broken by the VEV of the
scalar component of $\Psi$. As a consequence of the absence of U(1)$_X$ Higgs
fields and the novel R-parity assignment, the three light neutrinos consist of
one massless neutrino and two Dirac neutrinos. In the early universe, the
right-handed components of the Dirac neutrinos are in thermal equilibrium with
the SM particles through the U(1)$_X$ gauge ($Z^\prime$) boson. The extra
energy density from the RHNs is constrained to avoid disrupting the success of
BBN, leading to a lower bound on the scale of U(1)$_X$ symmetry breaking. In
our model, a mixture of the U(1)$_X$ gaugino and the fermionic component of
$\Psi$ becomes a new dark matter (DM) candidate if it is the lightest sparticle
mass eigenstate. We examine this DM phenomenology and identify a parameter
region that reproduces the observed DM relic density. Furthermore, we consider
constraints from the search for $Z'$ boson resonance at the LHC. The three
constraints obtained from the success of BBN, the observed DM relic density,
and the $Z^\prime$ resonance search at the LHC complement each other, narrowing
down the allowed parameter region.
