KLOE-MAMI $η\!\to\!π^{0}γγ$ Puzzle Solution via Nucleon-Rescaled Leptophobic Vector from Scalar Portal

The recent KLOE measurement $\mathrm{BR}^{η\rightarrowπ^{0}γγ}_{\text{KLOE}} = (0.98\pm0.11_{\text{stat}}\pm0.14_{\text{syst}})\times10^{-4}$ is less than half the current world average, $(2.55\pm0.22)\times10^{-4}$, which is dominated by MAMI photoproduction data. We show that this $\approx 5.5\, σ$ discrepancy can be resolved by the presence of a new leptophobic vector particle $V_{\mathcal B}$ with $2\ \text{GeV}\lesssim m_{V_{\!\mathcal B}} \lesssim 5\ \text{GeV}$, whose couplings arise through the \rm{GeV}-scale scalar portal. Integrating out the scalar generates the contact operator $\bigl(\bar N N\bigr)\,\widetilde V_{\mathcal{B}}^{μν}F_{μν}P$ that modifies the $η^{(\prime)}\!\to\!π^{0}\left(η\right)γγ$ decay rate only in the processes involving an external nucleon current ($γp\!\to\!η^{(\prime)}p$, $π^-p\!\to\!η^{(\prime)}n$), but leaves purely leptonic production channels, such as $ϕ\toηγ$ at KLOE and $J/ψ\toγη$ at BESIII, Standard-Model-like. The same mechanism predicts a $\approx 10\%$ nucleon-triggered enhancement of the $η'\!\to\!π^{0}γγ$ decay rate and a negligible shift for $η'\!\to\!ηγγ$, together with an $\rm{A}^{2}$-scaling boost if photoproduction is performed on heavy nuclei instead of protons. An integrated experimental programme that compares $η^{(\prime)} \to η(π^{0}) γγ$ decays in the presence of nucleon currents, as opposed to purely leptonic production channels, searches for a narrow nucleon-philic vector in the $2\!-\!5\,\mathrm{GeV}$ mass range, and probes scalar portals above $1\,\mathrm{GeV}$, can decisively confirm or exclude our nucleon-rescaled, leptophobic-vector explanation of the KLOE-MAMI discrepancy.