New particles ϕ in the MeV-GeV range produced at colliders and escaping detection can be searched for at operating b- and τ-factories such as Belle II. A typical search topology involves pair-produced τs (or mesons), one of which decaying to visibles plus the ϕ, and the other providing a tag. One crucial impediment of these searches is the limited ability to reconstruct the parents' separate boosts. This is the case in the ‘typical’ topology where both decay branches include escaping particles. We observe that such topology lends itself to the use of kinematic variables such as $M_2$, designed for pairwise decays to visibles plus escaping particles, and endowed with a built-in (‘MAOS’) way to efficiently guess the parents' separate boosts. Starting from this observation, we construct several kinematic quantities able to discriminate signal from background, and apply them to a benchmark search, $\tau \to e+\varphi$, where ϕ can be either an axion-like particle or a hidden photon. Our considered variables can be applied to a wider range of topologies than the current reference technique, based on the event thrust, with which they are nearly uncorrelated. Application of our strategy leads to an improvement by a factor close to 3 in the branching-ratio upper limit for $\tau \to e\varphi$, with respect to the currently expected limit, assuming $m_{\varphi }\lesssim 1$ MeV. For example, we anticipate a sensitivity of $1.7\times {10}^{-5}$ with the data collected before the 2022 shutdown.

新的粒子φ在撞机产生，并且可以在操作中搜索逸出检测MeV的-GeV的范围b -和τ -factories如百丽II。典型的搜索拓扑涉及成对产生的τ s（或介子），其中一个衰减到可见光加上ϕ，另一个提供标签。这些搜索的一个关键障碍是重建父母单独提升的能力有限。在“典型”拓扑中就是这种情况，其中两个衰变分支都包含逃逸粒子。我们观察到这种拓扑结构有助于使用运动学变量，例如${米}_2$，专为成对衰减到可见光和逃逸粒子而设计，并具有内置（'MAOS'）方法来有效地猜测父母的单独提升。从这个观察开始，我们构建了几个能够从背景中区分信号的运动量，并将它们应用于基准搜索，$\tau \to \mathrm{电子}+\phi$，其中ϕ可以是轴子状粒子或隐藏光子。我们考虑的变量可以应用于比当前参考技术更广泛的拓扑，基于事件推力，它们几乎不相关。我们的策略的应用导致分支比上限的提高接近 3 倍$\tau \to \mathrm{电子}\phi$，关于当前预期的限制，假设 ${米}_{\phi }\lesssim 1$兆伏。例如，我们预计敏感度为$1.7\times {10}^{-5}$ 使用 2022 年关闭之前收集的数据。