Neutrino Physics at Future Colliders

This is a brief review of the collider phenomenology of neutrino physics. Current and future colliders provide an ideal testing ground for (sub)TeV-scale neutrino mass models, as they can directly probe the messenger particles, which could be either new fermions, scalars, or gauge bosons, associated with neutrino mass generation. Moreover, the recent observation of TeV-scale neutrinos produced at the LHC offers new ways to test the limits of the Standard Model and beyond.

💡 Research Summary

The manuscript “Neutrino Physics at Future Colliders” provides a concise yet comprehensive review of how present and upcoming high‑energy colliders can serve as powerful probes of the mechanisms that generate neutrino masses. It begins by reminding the reader that the Standard Model (SM) predicts massless neutrinos, whereas the observation of neutrino oscillations unequivocally demands physics beyond the SM (BSM). The author emphasizes that collider experiments, traditionally focused on missing‑energy signatures, have recently entered a new era with the first detection of collider‑produced neutrinos by the forward experiments FASER ν and SND@LHC. The forthcoming Forward Physics Facility (FPF) will dramatically increase the flux of TeV‑scale neutrinos, enabling precise measurements of neutrino–nucleus cross sections, forward hadron production, and tests of lepton‑flavour universality.

The core of the paper is devoted to the various theoretical frameworks that embed the dimension‑5 Weinberg operator (LLHH/Λ) and their associated messenger particles. Three tree‑level realizations—type‑I, type‑II, and type‑III seesaws—are discussed. In type‑I, SM‑singlet right‑handed neutrinos (often called heavy neutral leptons, HNLs, or sterile neutrinos) are produced via charged‑current (pp → W → ℓ N) or neutral‑current (pp → Z → ν N) processes. The classic “smoking‑gun” for lepton‑number violation (LNV) in this context is the Keung‑Senjanović process pp → N ℓ → ℓ ℓ jj with no missing transverse energy. However, when the active‑sterile mixing VℓN is large enough to yield observable production rates, the HNLs behave as pseudo‑Dirac particles and the LNV amplitude is suppressed by the tiny light‑neutrino mass. The author highlights two exceptions that can revive LNV signals: (i) resonant enhancement when the mass splitting ΔmN matches the decay width ΓN, and (ii) observable HNL–anti‑HNL oscillations in long‑lived particle searches.

Type‑II seesaw introduces an SU(2)L triplet scalar ΔL = (Δ++, Δ+, Δ0). The doubly‑charged component can be pair‑produced via γ/Z exchange (pp → γ/Z → Δ++ Δ–). Its decay into same‑sign dileptons (Δ++ → ℓ+ℓ+) or same‑sign W bosons (Δ++ → W+W+) provides a clean LNV signature. Current LHC searches have set lower bounds around 800 GeV on the Δ++ mass. In left‑right symmetric models (LRSM), an SU(2)R triplet ΔR and a heavy right‑handed gauge boson WR appear. WR can decay to ℓ NR, and ΔR++ can decay analogously to ΔL++. The neutral component ΔR0 can be light and hadrophobic, leading to displaced‑vertex signatures without LNV.

The paper then turns to sterile neutrinos (heavy neutral leptons) in a model‑independent, bottom‑up approach. It presents the current exclusion regions in the (mN, |VℓN|²) plane, separating constraints from astrophysics/cosmology (sub‑MeV), beam‑dump and meson‑decay experiments (MeV–GeV), collider searches (GeV–TeV), and electroweak precision observables (>TeV). Figure 1 illustrates the electron‑flavour mixing case, showing that neutrinoless double‑beta decay (0νββ) provides the strongest bound for Majorana HNLs, though it can be relaxed by specific mass hierarchies or additional dark interactions. Collider limits from prompt and displaced vertex searches at the LHC already probe |VℓN|² down to 10⁻⁶ for masses up to a few TeV. Future lepton colliders operating at the Z‑pole (FCC‑ee, CEPC) will be sensitive to mixing as low as 10⁻¹², essentially covering the entire parameter space predicted by the canonical seesaw.

Beyond the minimal mixing‑induced production, the author discusses scenarios with extra gauge interactions that lift the suppression. In U(1)′ models, a Z′ boson couples directly to sterile neutrinos, allowing pp/e⁺e⁻ → Z′ → NN production independent of VℓN. Similarly, in left‑right models, WR‑mediated production pp → WR → ℓ NR can dominate. Beam polarization at future e⁺e⁻ colliders could disentangle left‑ and right‑handed contributions. These extensions naturally require three sterile states to cancel anomalies, providing a theoretical motivation for their existence.

Radiative neutrino‑mass models are also covered. The one‑loop Zee model, the two‑loop Zee‑Babu model, and the three‑loop KNT model generate the Weinberg operator only at loop level, involving new charged scalars and/or fermions with masses typically above 100 GeV. These particles can be produced at the LHC and future colliders, leading to distinctive signatures such as same‑sign dileptons, multi‑lepton final states, and lepton‑flavour violation. The author notes that the doubly‑charged scalar of the Zee‑Babu model is already constrained to be heavier than ~800 GeV, but future high‑luminosity runs could extend the reach substantially.

In the concluding section, the manuscript stresses the complementarity between collider searches and intensity‑frontier experiments (oscillation, 0νββ, cosmology). Colliders uniquely test the messenger sector directly, offering the possibility of observing LNV at the production level, while intensity experiments probe the low‑energy effective operators. The synergy of high‑energy colliders (including future 100 TeV pp machines, muon colliders, and high‑luminosity e⁺e⁻ factories) with forward detectors (FPF) forms a comprehensive program capable of exploring the full (sub) TeV landscape of neutrino‑mass generation mechanisms. The paper thus serves as a roadmap for both theorists and experimentalists aiming to uncover the origin of neutrino masses in the next decade.