  The study of electroweak boson pair production provides a powerful test of
the spontaneously broken gauge symmetry of the Standard Model (SM) and can be
used to search for new phenomena beyond the SM. Extra neutral vector bosons
$Z'$ decaying to charged gauge vector boson pairs $W^+W^-$ are predicted in
many scenarios of new physics, including models with an extended gauge sector.
The diboson production allows to place stringent constraints on the $Z$-$Z'$
mixing factor $\xi$ and $Z'$ mass, $M_{Z'}$. We present the $Z'$ exclusion
region in the $\xi-M_{Z'}$ plane for the first time by using data comprised of
$pp$ collisions at $\sqrt{s}=13$ TeV and recorded by the ATLAS and CMS
detectors at the CERN LHC, with integrated luminosities of 13.2 and 35.9
fb$^{-1}$, respectively. The exclusion region has been significantly extended
compared to that obtained from the previous analysis performed with Tevatron
data as well as with LHC data collected at 7 and 8 TeV. Also, we found that
these constraints on the $Z$-$Z'$ mixing factor are more severe than those
derived from the global analysis of electroweak data. Further improvement on
the constraining of this mixing can be achieved from the analysis of data to be
collected at higher luminosity.
