Cosmological observations allow to measure the abundance of light relics produced in the early Universe. Most studies focus on the thermal freeze-out scenario, yet light relics produced by freeze-in are generic for models in which new light degrees of freedom do not couple strongly enough to the Standard Model (SM) plasma to allow for full thermalization in the early Universe. In ultraviolet (UV) freeze-in scenarios, rates for light relic production associated with non-renormalizable interactions typical of beyond the SM (BSM) models grow with temperature more quickly than the Hubble rate. Thus, relatively small couplings to the SM can be probed by current and next-generation cosmic microwave background (CMB) experiments. We investigate several representative benchmark BSM models, such as axion-like particles from Primakoff production, massless dark photons and light right-handed neutrinos. We calculate contributions to the effective number of neutrino species, ΔN _eff, in corners of parameter space not previously considered and discuss the sensitivity of CMB experiments compared to other probes. In contrast to freeze-out scenarios, ΔN _eff from UV freeze-in is more dependent on both the specific BSM physics model and the reheating temperature. Depending on the details of the BSM scenario, we find that the sensitivity of next-generation CMB experiments can complement or surpass the current astrophysical, laboratory or collider constraints on the couplings of the SM to the light relic.

中文翻译：

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用当前和下一代 CMB 观测限制光遗迹的紫外线冻结


宇宙学观测可以测量早期宇宙中产生的光遗物的丰度。大多数研究都集中在热冻结情景上，但冻结产生的光遗迹对于新光自由度与标准模型 （SM） 等离子体耦合强度不足以实现早期宇宙完全热化的模型来说是通用的。在紫外线 （UV） 冻结场景中，与 Beyond the SM （BSM） 模型典型的不可重整化相互作用相关的光遗迹产生速率随温度的增长速度比哈勃速率增长得更快。因此，可以通过当前和下一代宇宙微波背景 （CMB） 实验来探测与 SM 的相对较小的耦合。我们研究了几个具有代表性的基准 BSM 模型，例如来自 Primakoff 生产的轴子样粒子、无质量暗光子和轻右旋中微子。我们计算了以前未考虑的参数空间角落中对中微子种类有效数量 ΔN_eff 的贡献，并讨论了与其他探针相比 CMB 实验的灵敏度。与冻结情况相比，UV 冻结的 ΔN_eff 更多地取决于特定的 BSM 物理模型和再加热温度。根据 BSM 情景的细节，我们发现下一代 CMB 实验的灵敏度可以补充或超越当前对 SM 与光遗物耦合的天体物理学、实验室或对撞机限制。