  The first search exploiting the vector boson fusion process to probe heavy
Majorana neutrinos and the Weinberg operator at the LHC is presented. The
search is performed in the same-sign dimuon final state using a proton-proton
collision data set recorded at $\sqrt{s}$ = 13 TeV, collected with the CMS
detector and corresponding to a total integrated luminosity of 138 fb$^{-1}$.
The results are found to agree with the predictions of the standard model. For
heavy Majorana neutrinos, constraints on the squared mixing element between the
muon and the heavy neutrino are derived in the heavy neutrino mass range 50
GeV-25 TeV; for masses above 650 GeV these are the most stringent constraints
from searches at the LHC to date. A first test of the Weinberg operator at
colliders provides an observed upper limit at 95% confidence level on the
effective $\mu\mu$ Majorana neutrino mass of 10.8 GeV.
