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arXiv 2509.08042hep-thhep-ph
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利用轴子-光子耦合检验杂化弦理论

Testing the Heterotic String with the Axion-Photon Coupling

Mario Reig, Timo Weigand

备注：
49 pages, 3 figures; v2: matches the accepted version in JHEP
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AI总结：

本文研究杂化弦理论的非标准嵌入漏洞，通过轴子-光子耦合分析类轴子粒子特性，得到耦合-质量比上限，发现违反该界限的轴子与多数唯象可行弦模型不相容。

摘要：

在QCD线以上发现类轴子粒子将排除大统一理论，包括标准模型嵌入单个$E_8$因子或$SO(32)$的微扰杂化弦。本文研究这一观测结果的可能漏洞，即通过$E_8\times E_8$杂化弦的紧致化，将标准模型非标准嵌入10维规范群。若电磁学嵌入两个$E_8$因子，轴子可通过反常与光子耦合而不与QCD耦合。我们得到这些类轴子粒子的耦合-质量比$g_{aγ}/m_a$的上限，其为超对称破缺尺度和统一规范耦合的函数。为与低能下测得的规范耦合及弱混合角$\text{sin}^2θ_w$兼容，具有非标准$U(1)_Y$嵌入的唯象可行模型需要来自弦态和/或中等能标带电物质的显著单圈阈值修正。我们研究这些效应如何修改$g_{aγ}/m_a$的树级上限，并表明在微扰区它们会降低 leading order 估计。远高于QCD线的类轴子粒子仅在微扰理论失效的特定模型中可能存在。主要结论是：发现违反本文所得界限的轴子将与大量其他唯象可行的弦模型不相容，包括微扰杂化$SO(32)$和$E_8\times E_8$弦、I型弦及某些杂化M理论。最后简要讨论了小规范瞬子和世界面瞬子在使部分类轴子粒子变重并具有宇宙学相关性方面的作用。

英文摘要：

The discovery of an axion-like particle above the QCD line would rule out Grand Unified Theories, including the perturbative heterotic string with the Standard Model embedded in a single $E_8$ factor or $SO(32)$. In this work we study a possible loophole to this observation, given by compactifications of the $E_8\times E_8$ heterotic string with a non-standard embedding of the Standard Model into the 10-dimensional gauge group. If electromagnetism is embedded into both $E_8$ factors, axions can couple to photons via the anomaly without coupling to QCD. We obtain upper bounds to the coupling-to-mass ratio $g_{aγ}/m_a$ for these axion-like particles as a function of the supersymmetry breaking scale and the unified gauge coupling. To be compatible with the measured gauge couplings and the weak mixing angle $\sin^2θ_w$ at low-energies, phenomenologically viable models with non-standard $U(1)_Y$ embedding require sizeable one-loop threshold corrections from string states and/or charged matter at intermediate energy scales. We study how these effects modify the tree-level upper bounds to $g_{aγ}/m_a$ and show that, in the perturbative regime, they reduce the leading order estimates. Axion-like particles far above the QCD line are only possible in certain models where perturbation theory is lost. The main conclusion is that the discovery of an axion violating the bounds found in this work would be incompatible with large classes of otherwise phenomenologically viable string models, including the perturbative heterotic $SO(32)$ and $E_8\times E_8$ string, the type-I string, and certain heterotic M-theories. The role of small gauge instantons and worldsheet instantons in making some of the axion-like particles heavy and cosmologically relevant is briefly discussed.

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