In 2016, the ATOMKI collaboration announced [PRL 116, 042501 (2016)] observing an unexpected enhancement of the $e^+-e^-$ pair production signal in one of the $^8$Be nuclear transitions induced by an incident proton beam on a $^7$Li target. Many beyond-standard-model physics explanations have subsequently been proposed. One popular theory is that the anomaly is caused by the creation of a protophobic vector boson ($X$) with a mass around 17 MeV [e.g., PRL 117, 071803 (2016)] in the nuclear transition. We study this hypothesis by deriving an isospin relation between photon and $X$ couplings to nucleons. This allows us to find simple relations between protophobic $X$-production cross sections and those for measured photon production. The net result is that $X$ production is dominated by a direct $E1^X$ transition without going through any nuclear resonance (i.e. Bremsstrahlung radiation) with a smooth energy dependence that occurs for all proton beam energies above threshold. This contradicts the experimental observations and invalidates the protophobic vector boson explanation.

中文翻译：

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疏质矢量玻色子能解释 ATOMKI 异常吗？

2016 年，ATOMKI 合作宣布 [PRL 116, 042501 (2016)] 在由入射质子引起的 $^8$Be 核跃迁之一中观察到 $e^+-e^-$ 对产生信号的意外增强瞄准$ ^ 7 $ Li目标。随后提出了许多超出标准模型的物理解释。一种流行的理论是，异常是由在核跃迁中产生质量约为 17 MeV [例如，PRL 117, 071803 (2016)] 的原疏矢量玻色子 ($X$) 引起的。我们通过推导出光子和与核子的 $X$ 耦合之间的同位旋关系来研究这个假设。这使我们能够找到原始的 $X$ 产生横截面与测量光子产生横截面之间的简单关系。最终结果是，$X$ 的产生由直接的 $E1^X$ 跃迁控制，没有经过任何核共振（即轫致辐射），并且对于高于阈值的所有质子束能量发生平滑的能量依赖性。这与实验观察相矛盾并使原疏矢量玻色子解释无效。