In this letter we point out that the Belle II experiment has a unique sensitivity to visibly decaying long-lived dark photons. Concentrating on the signatures with a single high-energy photon in association with a displaced pair of charged particles, we find that Belle II will be able to probe large regions of parameter space that cannot be covered by any other running or proposed experimental facility. While the signature with charged muons or pions in the final state is expected to be background-free after all selections are applied, the case of final state electrons is more involved and requires an in-depth study. We discuss possible ways to further suppress backgrounds and the corresponding experimental prospects.

在这封信中，我们指出 Belle  II 实验对明显衰减的长寿命暗光子具有独特的敏感性。专注于单个高能光子与一对位移的带电粒子相关的特征，我们发现 Belle  II 将能够探测任何其他正在运行或提议的实验设施无法覆盖的参数空间的大区域。虽然在应用所有选择后，最终状态的带电 μ 子或π介子的签名预计将是无背景的，但最终状态电子的情况更复杂，需要深入研究。我们讨论了进一步抑制背景的可能方法和相应的实验前景。