We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of Standard Model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the μm scale up to the Big Bang Nucleosynthesis limit of ~ 10^7m. Neutral LLPs with lifetimes above ~ 100m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. We study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC, assuming backgrounds can be rejected as expected. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector. .

中文翻译：

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能量前沿的长寿命粒子：MATHUSLA 物理案例

我们在标准模型 (SM) 扩展的综合调查中研究了 LHC 长寿命粒子 (LLP) 信号的理论动机。LLP 是对各种理论的普遍预测，这些理论解决了未解决的基本谜团，例如自然性、暗物质、重子发生和中微子质量，并代表了超越 SM (BSM) 的物理学的自然和通用可能性。在大多数情况下，LLP 寿命可以被视为从 μm 尺度到大爆炸核合成极限 ~ 10^7m 的自由参数。寿命超过 ~ 100m 的中性 LLP 尤其难以探测，因为 LHC 主探测器的灵敏度受到具有挑战性的背景、触发器和小接受度的限制。MATHUSLA 是在 ATLAS 或 CMS 附近的最小仪器、大容量表面探测器的提议。它将通过在低背景环境中重建位移顶点 (DV) 来搜索 HL-LHC 碰撞中产生的中性 LLP，在长寿命状态下将主要探测器的灵敏度扩展几个数量级。我们研究了 HL-LHC 中类似 MATHUSLA 的探测器提供的 LLP 物理机会，假设可以按预期拒绝背景。我们开发了一种独立于模型的方法来描述 MATHUSLA 对 BSM LLP 信号的敏感性，并将其与 ATLAS 或 CMS 上的 DV 和缺失能量搜索进行比较。然后，我们相当详细地探讨了 LLP 的 BSM 动机，展示了大量新的敏感性研究。虽然我们的讨论特别针对 MATHUSLA 的长寿命制度，但这项调查总体上强调了 LHC 中各种 LLP 搜索计划的重要性。通过将这些结果综合到对 LLP 搜索的自上而下和自下而上的动机的一般讨论中，我们的目标是展示用于构建 MATHUSLA 探测器的物理案例的卓越强度和广度。.