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| Format: | Preprint |
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2025
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| Online Access: | https://arxiv.org/abs/2507.09436 |
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| _version_ | 1866917082581434368 |
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| author | Balytskyi, Yaroslav |
| author_facet | Balytskyi, Yaroslav |
| contents | 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, $(2.55\pm0.22)\times10^{-4}$, dominated by MAMI photoproduction data. We show that this $\approx 5.5\, σ$ discrepancy can be resolved by the new leptophobic, nucleon-triggered vector particle $V_{\mathcal B}$ with \(1.5~\text{GeV}\lesssim m_{V_{\!\mathcal B}}\lesssim5~\text{GeV}\), coupled via the effective operator \(\bigl(\bar N N\bigr)\,\widetilde V_{\mathcal{B}}^{μν}F_{μν}P\). This interaction modifies the \(η^{(\prime)}\!\to\!π^{0}\left(η\right)γγ\) decay rates \textit{only} in the processes involving an external nucleon current, \(γp\!\to\!η^{(\prime)}p\) and \(π^-p\!\to\!η^{(\prime)}n\), but leaves purely leptonic production channels, such as \(e^+e^-\to ϕ\toη^{(\prime)}γ\) at KLOE and \(e^+e^-\to J/ψ\toγη^{(\prime)}\) at BESIII, Standard-Model-like. The same mechanism predicts a \(\approx 10\%\) nucleon-triggered enhancement of the \(η^\prime\!\to\!π^{0}γγ\) decay rate and a negligible shift for \(η^\prime\!\to\!ηγγ\), together with an \(\rm{A}^{2}\)-scaling boost if produced on heavy nuclei instead of protons. $V_{\!\mathcal B}$ can be searched directly in $2\!\to\!3$ photoproduction, for example, $γp\!\to\!V_{\!\mathcal B}\,π^{0} p$. An integrated experimental program that compares $η^{(\prime)}\!\to\!π^{0}(η)γγ$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_09436 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Production-Dependent Interpretation of the KLOE--MAMI $η\!\to\!π^{0}γγ$ Tension via the Nucleon-Triggered Leptophobic Vector $V_{\!\mathcal B}$ Balytskyi, Yaroslav High Energy Physics - Phenomenology 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, $(2.55\pm0.22)\times10^{-4}$, dominated by MAMI photoproduction data. We show that this $\approx 5.5\, σ$ discrepancy can be resolved by the new leptophobic, nucleon-triggered vector particle $V_{\mathcal B}$ with \(1.5~\text{GeV}\lesssim m_{V_{\!\mathcal B}}\lesssim5~\text{GeV}\), coupled via the effective operator \(\bigl(\bar N N\bigr)\,\widetilde V_{\mathcal{B}}^{μν}F_{μν}P\). This interaction modifies the \(η^{(\prime)}\!\to\!π^{0}\left(η\right)γγ\) decay rates \textit{only} in the processes involving an external nucleon current, \(γp\!\to\!η^{(\prime)}p\) and \(π^-p\!\to\!η^{(\prime)}n\), but leaves purely leptonic production channels, such as \(e^+e^-\to ϕ\toη^{(\prime)}γ\) at KLOE and \(e^+e^-\to J/ψ\toγη^{(\prime)}\) at BESIII, Standard-Model-like. The same mechanism predicts a \(\approx 10\%\) nucleon-triggered enhancement of the \(η^\prime\!\to\!π^{0}γγ\) decay rate and a negligible shift for \(η^\prime\!\to\!ηγγ\), together with an \(\rm{A}^{2}\)-scaling boost if produced on heavy nuclei instead of protons. $V_{\!\mathcal B}$ can be searched directly in $2\!\to\!3$ photoproduction, for example, $γp\!\to\!V_{\!\mathcal B}\,π^{0} p$. An integrated experimental program that compares $η^{(\prime)}\!\to\!π^{0}(η)γγ$ 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. |
| title | Production-Dependent Interpretation of the KLOE--MAMI $η\!\to\!π^{0}γγ$ Tension via the Nucleon-Triggered Leptophobic Vector $V_{\!\mathcal B}$ |
| topic | High Energy Physics - Phenomenology |
| url | https://arxiv.org/abs/2507.09436 |