The recent KLOE measurement \( {\textrm{BR}}_{\textrm{KLOE}}^{\eta \to \pi 0\gamma \gamma}=\left(0.98\pm 0.{11}_{\textrm{stat}}\pm 0.{14}_{\textrm{syst}}\right)\times 1{0}^{-4} \) is less than half the current world average, (2.55 ± 0.22) × 10−4, dominated by MAMI photoproduction data. We show that this ≈ 5.5 σ discrepancy can be resolved by the new leptophobic, nucleon-triggered vector particle \( {V}_{\mathcal{B}} \) with 1.5 GeV ≲ \( {m}_{V_{\mathcal{B}}} \) ≲ 5 GeV, coupled via the effective operator \( \left(\overline{N}N\right){\overset{\sim }{V}}_{\mathcal{B}}^{\mu \nu}{F}_{\mu \nu}P \) . This interaction modifies the η(′) → π0 (η)γγ decay rates only in the processes involving an external nucleon current, γp → η(′)p and π−p → η(′)n, but leaves purely leptonic production channels, such as e+e− → ϕ → η(′)γ at KLOE and e+e− → J/ψ → γη(′) at BESIII, Standard-Model-like. The same mechanism predicts a ≈ 10% nucleon-triggered enhancement of the η′ → π0γγ decay rate and a negligible shift for η′ → ηγγ, together with an A2-scaling boost if produced on heavy nuclei instead of protons. \( {V}_{\mathcal{B}} \) can be searched directly in 2 → 3 photoproduction, for example, \( \gamma p\to {V}_{\mathcal{B}}{\pi}^0p \) . An integrated experimental program that compares η(′) → π0(η)γγ in the presence of external nucleon currents with purely leptonic production, and conducts direct photoproduction searches for a GeV-scale vector, can decisively confirm or exclude our nucleon-rescaled, leptophobic-vector interpretation of the KLOE-MAMI discrepancy.
