  We discuss the diboson excess seen by the ATLAS detector around 2 TeV in the
LHC run I at $\sqrt{s}=8$ TeV. We explore the possibility that such an excess
can arise from a $Z'$ boson which acquires mass through a $U(1)_X$ Stueckelberg
extension. The corresponding $Z'$ gauge boson is leptophobic with a mass of
around 2 TeV and has interactions with $SU(2)_L$ Yang-Mills fields and gauge
fields of the hypercharge. The analysis predicts $Z'$ decays into $WW$ and $ZZ$
as well as into $Z\gamma$. Further three-body as well as four-body decays of
the $Z'$ such as $WWZ, WW\gamma, WWZZ$ etc are predicted. In the analysis we
use the helicity formalism which allows us to exhibit the helicity structure of
the $Z'$ decay processes in an transparent manner. In particular, we are able
to show the set of vanishing helicity amplitudes in the decay of the massive
$Z'$ into two vector bosons due to angular momentum conservation with a special
choice of the reference momenta. The residual set of non-vanishing helicity
amplitudes are identified. The parameter space of the model compatible with the
diboson excess seen by the ATLAS experiment at $\sqrt s=8$ TeV is exhibited.
Estimate of the diboson excess expected at $\sqrt s= 13$ TeV with 20 fb$^{-1}$
of integrated luminosity at LHC run II is also given. It is shown that the
$WW$, $ZZ$ and $Z\gamma$ modes are predicted to be in the approximate ratio
$1:\cos^2\theta_W (1+ \alpha \tan^2\theta_W)^2/2: (1-\alpha)^2\sin^2\theta_W/2$
where $\alpha$ is the relative strength of the couplings of hypercharge gauge
fields to the couplings of the Yang-Mills gauge fields. Thus observation of the
$Z\gamma$ mode as well as three-body and four-body decay modes of the $Z'$ will
provide a definite test of the model and of a possible new source of
interaction beyond the standard model.
