SHiP is a proposed high-intensity beam dump experiment set to operate at the CERN SPS. It is expected to have an unprecedented sensitivity to a variety of models containing feebly interacting particles, such as Heavy Neutral Leptons (HNLs). Two HNLs or more could successfully explain the observed neutrino masses through the seesaw mechanism. If, in addition, they are quasi-degenerate, they could be responsible for the baryon asymmetry of the Universe. Depending on their mass splitting, HNLs can have very different phenomenologies: they can behave as Majorana fermions -- with lepton number violating (LNV) signatures, such as same-sign dilepton decays -- or as Dirac fermions with only lepton number conserving (LNC) signatures. In this work, we quantitatively demonstrate that LNV processes can be distinguished from LNC ones at SHiP, using only the angular distribution of the HNL decay products. Accounting for spin correlations in the simulation and using boosted decision trees for discrimination, we show that SHiP will be able to distinguish Majorana-like and Dirac-like HNLs in a significant fraction of the currently unconstrained parameter space. If the mass splitting is of order $10^{-6}$ eV, SHiP could even be capable of resolving HNL oscillations, thus providing a direct measurement of the mass splitting. This analysis highlights the potential of SHiP to not only search for feebly interacting particles, but also perform model selection.

中文翻译：

---

SHiP 的 Dirac 与 Majorana HNL（及其振荡）

SHiP 是一个提议的高强度光束转储实验装置，用于在 CERN SPS 上运行。预计它对包含弱相互作用粒子的各种模型具有前所未有的敏感性，例如重中性轻子 (HNL)。两个或更多的 HNL 可以通过跷跷板机制成功解释观察到的中微子质量。此外，如果它们是准简并的，它们可能是宇宙重子不对称的原因。根据它们的质量分裂，HNL 可以具有非常不同的现象：它们可以表现为马约拉纳费米子——具有轻子数违反 (LNV) 特征，例如同号双轻子衰变——或者作为仅具有轻子数守恒 (LNC) 的狄拉克费米子) 签名。在这项工作中，我们定量证明了 LNV 过程可以与 SHiP 的 LNC 过程区分开来，仅使用 HNL 衰减产物的角度分布。考虑到模拟中的自旋相关性并使用增强决策树进行区分，我们表明 SHiP 将能够在当前不受约束的参数空间的很大一部分中区分类 Majorana 和类 Dirac 的 HNL。如果质量分裂为 $10^{-6}$ eV，SHiP 甚至可以解决 HNL 振荡，从而提供质量分裂的直接测量。该分析突出了 SHiP 的潜力，不仅可以搜索微弱的相互作用粒子，还可以执行模型选择。我们表明 SHiP 将能够在当前不受约束的参数空间的很大一部分中区分 Majorana-like 和 Dirac-like HNL。如果质量分裂为 $10^{-6}$ eV，SHiP 甚至可以解决 HNL 振荡，从而提供质量分裂的直接测量。该分析突出了 SHiP 的潜力，不仅可以搜索微弱的相互作用粒子，还可以执行模型选择。我们表明 SHiP 将能够在当前不受约束的参数空间的很大一部分中区分 Majorana-like 和 Dirac-like HNL。如果质量分裂为 $10^{-6}$ eV，SHiP 甚至能够解决 HNL 振荡，从而提供质量分裂的直接测量。该分析突出了 SHiP 的潜力，不仅可以搜索微弱的相互作用粒子，还可以执行模型选择。