Revealing the Origin of Neutrino Masses through Displaced Shower Searches in the CMS Muon System

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 |VN|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, |VN|² ≈ 10⁻¹² (= e, τ) for mN ≈ 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 ≈ 10 m.