Speaker
Description
We investigate the prospects for improving the heavy neutral lepton (HNL) searches
at the far-forward LHC experiments FASER and FASERν, as well as their proposed HL-LHC
successors, FASER2 and FASERν2, using new HNL production mechanisms. We consider sce-
narios 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−3Lτ ,
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 FASERν having sensitivity to HNLs and FASER2 and FASERν2
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 gov-
erns their visible decays, opening complementary opportunities to explore HNLs at the forward
physics facilities.