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

Abstract: The recent KLOE measurement $\mathrm{BR}^{\eta\rightarrow\pi^{0}\gamma\gamma}_{\text{KLOE}} = (0.98\pm0.11_{\text{stat}}\pm0.14_{\text{syst}})\times10^{-4}$ 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 $\approx 5.5\, \sigma$ 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 $\bigl(\bar N N\bigr)\,\widetilde V_{\mathcal{B}}^{\mu\nu}F_{\mu\nu}P$ that modifies the $\eta^{(\prime)}!\to!\pi^{0}\left(\eta\right)\gamma\gamma$ decay rate only in the processes involving an external nucleon current ($\gamma p!\to!\eta^{(\prime)}p$, $\pi^-p!\to!\eta^{(\prime)}n$), but leaves purely leptonic production channels, such as $\phi\to\eta\gamma$ at KLOE and $J/\psi\to\gamma\eta$ at BESIII, Standard-Model-like. The same mechanism predicts a $\approx 10\%$ nucleon-triggered enhancement of the $\eta'!\to!\pi^{0}\gamma\gamma$ decay rate and a negligible shift for $\eta'!\to!\eta\gamma\gamma$, together with an $\rm{A}^{2}$-scaling boost if photoproduction is performed on heavy nuclei instead of protons. An integrated experimental programme that compares $\eta^{(\prime)} \to \eta(\pi^{0}) \gamma\gamma$ 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.