ShivaSankar K. A., Souvik Das, Arindam Das, Sanjoy Mandal
We study two general U ( 1 ) extensions of the Standard Model (SM) those generate tiny neutrino masses via the seesaw mechanism after general U ( 1 ) breaking. These models predict a new neutral gauge boson ( Z ′ ) and right-handed neutrinos (RHNs), the latter introduced for anomaly cancellation and neutrino mass generation. In both scenarios, left- and right-handed fermions couple differently to the Z ′ , and RHNs mix with light neutrinos, enabling variety of decay modes. Focusing on the high-luminosity LHC (HL-LHC) and the future FASER2 experiment, we explore RHN pair production from Z ′ decays in two cases; (i) long-lived Z ′ decays to visible modes and long-lived RHNs, and (ii) short-lived Z ′ decays to long-lived RHNs, which further decay visibly inside FASER2. We estimate projected limits on the general U ( 1 ) gauge coupling, Z ′ mass, RHN mass, and light-heavy neutrino mixing for various U ( 1 ) charge assignments, and compare them with current experimental bounds.