A bstract The recent KLOE measurement BR₊₋₎₄^ 0 = (0. 98 0. 11ₒₓ₀ₓ 0. 14ₒₘₒₓ) 10^-4 BR KLOE η → π 0 γγ = 0. 98 ± 0. 11 stat ± 0. 14 syst × 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 V₁ V B with 1. 5 GeV ≲ mₕ_₁ m V B ≲ 5 GeV, coupled via the effective operator (NN) {V}₁^ F P N ¯ N V ~ B μν F μν P. This interaction modifies the η (′) → π 0 (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′ → π 0 γγ decay rate and a negligible shift for η′ → ηγγ, together with an A 2 -scaling boost if produced on heavy nuclei instead of protons. V₁ V B can be searched directly in 2 → 3 photoproduction, for example,
Yaroslav Balytskyi (Wed,) studied this question.