发表机构
Department of Physics and Astronomy, Wayne State University（韦恩州立大学物理与天文学系）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
针对η→π⁰γγ衰变的核子触发矢量玻色子V_B模型，量化了MAMI与JEF能区的预测差异，提出运动学禁戒尾部作为判别依据，还关联了γd反应的角分布偏差。
AI 中文摘要
核子触发矢量玻色子V_B情景的提出，源于KLOE合作组测量的η→π⁰γγ衰变分支比BR^{η→π⁰γγ}_{KLOE}=(0.98±0.11_{stat}±0.14_{syst})×10^{-4}，与以MAMI光生数据为主的当前世界平均值(2.55±0.22)×10^{-4}之间存在统计显著的偏差。该模型中，新相互作用由外部核子激活，导致偏离标准模型预测，却不影响KLOE、BESIII等轻子测量结果。我们量化了残留的运动学与依赖于截断的效应，因MAMI与JEF类实验的产生能区不同，其预测结果接近但不相同。该模型最干净的判别依据是γγ和π⁰γ谱中运动学禁戒的尾部，针对η→π⁰γγ衰变，我们预测在1200个事例样本中，根据选择窗的不同，该区域会产生4至23个事例，这些事例与反冲质子的运动学相关；在MAMI能区，相对于在壳假设的极角可达≈-5°或更小，而在JEF能区仅为≈-0.2°。对于相同的选择窗，JEF类有效分支比超出MAMI类预测不到1%，而拓宽选择窗会使两者预测均增加约4%。两个能区均使重建质量的平均值产生类似的上移，⟨M_{π⁰γγ}⟩-m_η≈2至7 MeV。最后，我们确定了V_B诱导的代表性拓扑结构，其可能对近期报道的γd→π⁰ηd和γd→π⁰π⁰d角分布的偏差有贡献，定量研究留待未来工作。
英文摘要
The nucleon-triggered vector boson $V_{\mathcal B}$ is a hypothetical particle motivated by a statistically significant discrepancy between the KLOE measurement, $\mathrm{BR}^{η\rightarrowπ^{0}γγ}_{\text{KLOE}} = \left(0.98\pm0.11_{\text{stat}}\pm0.14_{\text{syst}}\right)\times10^{-4}$, and the current world average, $\left(2.55\pm0.22\right)\times10^{-4}$, dominated by MAMI photoproduction data. This hypothetical new interaction is activated by external nucleons, causing deviations from the Standard Model predictions, while leaving leptonic measurements, such as KLOE and BESIII, unaffected. We embed this mechanism in representative MAMI- and JEF-like photoproduction settings, deriving distinctive and directly testable kinematic and cut-dependent signatures to quantify small differences between the two regimes. One such signature is a small predicted population of events beyond the nominal on-shell boundaries of the $γγ$ and $π^0γ$ spectra, containing 4--23 events in a sample of 1200, depending on the selection window. These events correlate with the recoil proton kinematics, and the polar angle relative to the on-shell hypothesis can reach $\lesssim -5^\circ$ at MAMI energies, but only $\approx -0.2^\circ$ at JEF. For identical selection windows, the JEF-like effective branching fraction exceeds the MAMI-like prediction by less than $1\%$, while widening the window increases both predictions by up to $\sim4\%$. Both energy regimes produce a similar upward shift of the mean reconstructed mass, $\langle M_{π^0γγ}\rangle-m_η\simeq +2$--$7~\mathrm{MeV}$. Finally, we identify representative $V_{\mathcal B}$-induced topologies that could contribute to recently reported discrepancies in the angular distributions of $γd\toπ^0ηd$ and $γd\toπ^0π^0 d$, leaving their quantitative investigation to future work.
