This paper studies how a proposed Forward Physics Facility (FPF) placed about 1.5 km down‑stream of the collision point at a 100 TeV Future Circular Collider (FCC‑hh) could detect new particles tied to neutrino mass. The authors focus on models that add a new neutral gauge boson called Z' and right‑handed neutrinos (also called heavy neutral leptons). These new particles can be long lived and travel far from the collision point before decaying. The study asks whether the FPF can see their visible decays.

The researchers work within anomaly‑free chiral U(1) gauge extensions of the Standard Model. In plain terms, they consider simple, well‑behaved models that predict a Z' and three right‑handed neutrinos together with a new scalar field. They simulate several ways these particles could be made at the collider: from decays of light hadrons (mesons), from proton bremsstrahlung (radiation off an incoming proton), and from Z' decays. They consider three complementary scenarios: a long‑lived Z' that decays to visible Standard Model particles, a long‑lived Z' that decays into pairs of heavy neutrinos, and a promptly produced Z' that immediately decays into long‑lived heavy neutrinos.

To estimate what the FPF could see, the paper models two detector concepts. The smaller FPF1 has a 5 m × 5 m × 50 m decay volume, while the much larger FPF2 is 20 m × 20 m × 400 m. Both assume a visible‑energy threshold of 100 GeV, a magnetic spectrometer to track charged particles, veto systems to remove incoming charged backgrounds, a vacuum decay volume to reduce neutrino backgrounds, and an electromagnetic calorimeter to help reconstruct electrons and positrons. The authors fold in realistic detector geometry, decay probabilities, and the visible final states to compute expected event rates and derive projected sensitivities to the heavy‑neutrino mass, the mixing between light and heavy neutrinos (often called active‑sterile mixing), and the Z' mass and gauge coupling.