[论文解读] A No-Lose Theorem for Discovering the New Physics of $(g-2)_\mu$ at Muon Colliders

Rodolfo Capdevilla, David Curtin+2 more|arXiv (Cornell University)|Jan 27, 2021

Particle physics theoretical and experimental studies参考文献 83被引用 26

一句话总结

若μ子反常磁矩（g−2)μ异常被证实，该论文建立了一项无损定理：若( g−2)μ异常被证实，则3 TeV μ子对撞机可保证发现所有产生该异常的、弱耦合的、标准模型单重态的新物理态，且质量高于∼10 GeV；而更高能量的对撞机（最高达30 TeV）则可确保发现或通过实证手段证伪自然性满足MFV的模型，其中带电态质量低于∼10 TeV。

ABSTRACT

We perform a model-exhaustive analysis of all possible beyond Standard Model (BSM) solutions to the $(g-2)_\mu$ anomaly to study production of the associated new states at future muon colliders, and formulate a no-lose theorem for the discovery of new physics if the anomaly is confirmed and weakly coupled solutions below the GeV scale are excluded. Our goal is to find the highest possible mass scale of new physics subject only to perturbative unitarity, and optionally the requirements of minimum flavour violation (MFV) and/or naturalness. We prove that a 3 TeV muon collider is guaranteed to discover all BSM scenarios in which $\Delta a_\mu$ is generated by SM singlets with masses above $\sim $ GeV; lighter singlets will be discovered by upcoming low-energy experiments. If new states with electroweak quantum numbers contribute to $(g-2)_\mu$, the minimal requirements of perturbative unitarity guarantee new charged states below $\mathcal{O}(100 { m TeV})$, but this is strongly disfavoured by stringent constraints on charged lepton flavour violating (CLFV) decays. Reasonable BSM theories that satisfy CLFV bounds by obeying Minimal Flavour Violation (MFV) and avoid generating two new hierarchy problems require the existence of at least one new charged state below $\sim 10$ TeV. This strongly motivates the construction of high-energy muon colliders, which are guaranteed to discover new physics: either by producing these new charged states directly, or by setting a strong lower bound on their mass, which would empirically prove that the universe is fine-tuned and violates the assumptions of MFV while somehow not generating large CLFVs. The former case is obviously the desired outcome, but the latter scenario would perhaps teach us even more about the universe by profoundly revising our understanding of naturalness, cosmological vacuum selection, and the SM flavour puzzle.

研究动机与目标

• 确定能生成( g−2)μ异常但不违反微扰幺正性的新物理最高质量尺度。
• 评估未来μ子对撞机在涵盖所有新物理情景下的发现潜力。
• 确立μ子对撞机在( g−2)μ异常被证实时，唯一具备发现或通过实证手段证伪新物理的能力。
• 探讨在高能μ子对撞机中未发现新物理的影响，包括MFV或自然性可能被违反的情况。

提出的方法

• 对所有贡献于( g−2)μ的新物理情景进行模型穷举分析，重点关注标准模型单重态和电弱多重态。
• 应用微扰幺正性约束以限制新物理最大质量尺度，同时考虑MFV和自然性附加约束。
• 利用包含式产生和Bhabha散射信号评估μ子对撞机的发现能力。
• 通过评估标量单重态和电弱情景的紫外完成及Landau极点，检验其一致性。
• 考虑带电轻子味违反（CLFV）约束和MFV假设对可行参数空间的限制。
• 通过结合幺正性、MFV和自然性约束，推导出无损定理，以预测保证发现或证伪。

实验结果

研究问题

• RQ1 在保持微扰幺正性条件下，能生成( g−2)μ异常的新物理最大质量尺度是多少？
• RQ2 3 TeV μ子对撞机是否能保证发现所有质量高于∼10 GeV的标准模型单重态新物理态？
• RQ3 满足MFV且避免产生大CLFV贡献的电弱尺度新物理模型中，新带电态的质量边界是什么？
• RQ4 若在30 TeV μ子对撞机中未观测到直接产生新粒子，这将如何影响我们对自然性和MFV原理的理解？
• RQ5 若未观测到直接产生，μ子对撞机在探测新物理的间接信号方面发挥什么作用？

主要发现

• 具有1 ab−1亮度的3 TeV μ子对撞机可保证发现所有质量高于∼10 GeV、贡献于( g−2)μ的标准模型单重态新物理态。
• 具有0.4 ab−1亮度的215 GeV μ子对撞机可通过包含式产生直接观测到低至2 GeV的单重态，并通过Bhabha散射间接探测所有允许的质量。
• 对于满足MFV且避免大CLFV贡献的电弱尺度情景，新带电态的质量必须低于∼10 TeV，这使得10 TeV μ子对撞机极具吸引力。
• 若在30 TeV μ子对撞机中未发现任何新粒子，这将实证证明宇宙违反MFV或高度精细调参，从而为自然性和味物理提供深刻洞见。
• 即使未发生直接产生，30 TeV μ子对撞机仍可通过幺正性约束探测到µ+µ−→hγ过程中的偏离，从而确认低于∼100 TeV的新物理。
• 唯一能逃避高能μ子对撞机发现的场景是那些违反MFV或需要极端精细调参的场景，因此非发现本身即成为基础物理的发现。