Probing Heavy Neutral Leptons through New Gauge Interactions at the Forward Physics Facilities

Abstract

We investigate the prospects for improving the heavy neutral lepton (HNL) searches at the far-forward LHC experiments FASER and FASER$\nu$, as well as their proposed HL-LHC successors, FASER2 and FASER$\nu$2, using new HNL production mechanisms. We consider scenarios in which HNLs couple directly to a new $U(1)$ gauge boson and decay into visible Standard Model final states through mixing with active neutrinos. Focusing on the $U(1)_{B-L}$, $U(1)_{B-3L_\tau}$, and $U(1)_B$ gauge extensions as benchmark models, we study HNL production through proton bremsstrahlung and Drell--Yan processes, which probe complementary mediator-mass regimes. We analyze several visible decay channels, including charged leptons, neutral mesons, and other hadrons, and evaluate the projected sensitivities for different neutrino-flavor mixing patterns. We find that these new production mechanisms that differ qualitatively from the conventional picture in which both production and decay are controlled by active-sterile mixing enable the far-forward LHC program to probe previously unexplored regions of HNL parameter space, with the first-ever demonstration of FASER$\nu$ having sensitivity to HNLs and FASER2 and FASER$\nu$2 offering substantially enhanced sensitivity during the HL-LHC era, even reaching the canonical seesaw prediction for all three lepton flavors. Our results demonstrate that new gauge interactions can substantially enhance HNL production independently of the active-sterile mixing that governs their visible decays, opening complementary opportunities to explore HNLs at the forward physics facilities.