  We investigate the pair-production of right-handed neutrinos via the Standard
Model (SM) Higgs boson in a gauged $B-L$ model. The right-handed neutrinos with
a mass of few tens of GeV generating viable light neutrino masses via the
seesaw mechanism naturally exhibit displaced vertices and distinctive
signatures at the LHC and proposed lepton colliders. The production rate of the
right-handed neutrinos depends on the mixing between the SM Higgs and the
exotic Higgs associated with the $B-L$ breaking, whereas their decay length
depends on the active-sterile neutrino mixing. We focus on the displaced
leptonic final states arising from such a process, and analyze the sensitivity
reach of the LHC and proposed lepton colliders in probing the active-sterile
neutrino mixing. We show that mixing to muons as small as $V_{\mu N} \approx
10^{-7}$ can be probed at the LHC with 100 fb$^{-1}$ and at proposed lepton
colliders with 5000 fb$^{-1}$. The future high luminosity run at LHC and the
proposed MATHUSLA detector may further improve this reach by an order of
magnitude.
