  We study the potential to probe the origin of neutrino masses, by searching
for long-lived right-handed neutrinos~(RHNs) $N$ in the $B-L$ gauge model.
Despite the small active-sterile mixing $|V_{\ell N}|^2$, RHNs are produced
abundantly via SM and exotic Higgs production, as long as the Higgs mixing is
sufficiently large. We reinterpret a search for displaced showers in the CMS
muon system and we find that it is sensitive to parameter space at and below
the canonical seesaw floor, $|V_{\ell N}|^2 \approx 10^{-12}$ ($\ell = e$,
$\tau$) for $m_N \approx 40$ GeV. With existing data constraining such a
well-motivated scenario of neutrino mass generation, we determine the projected
sensitivity at the HL-LHC, motivating dedicated searches for long-lived RHNs
with decay lengths $\approx 10$ m.
