弦论中的轴子$-$消灭暗辐射的水螅
Axions in String Theory $-$ Slaying the Hydra of Dark Radiation
摘要：
人们普遍认为，弦论很容易就能在宇宙学所青睐的质量范围内考虑到QCD轴子。所需的小衰减常数，$f_a\ll M_P$，可以通过使用一个大的紧致化体积来实现。这指向了大体积方案，而大体积方案又做出了某些宇宙学预言：首先，闭弦轴子的行为类似于暴胀前方案中的场论轴子，即初始值可以调整，但受到等曲率波动的约束。此外，体积代表了一个长寿命的模量，可能会导致早期的物质为主的阶段。最后，体积模量衰减到它自己的轴子往往会过度产生暗辐射。在本文中，我们的目标是仔细分析宇宙学的研究模型，不仅允许一个QCD轴子，但也包括通货膨胀。一般来说，等曲率波动的限制限制了我们对相对低尺度暴胀的研究，在当前的弦理论背景下，这就指向了凯勒模量暴胀。作为一个新的功能，我们发现，最轻的（体积）模量强烈的希格斯耦合。因此，它很快衰减到SM，从而解决了原来的暗辐射问题。这种衰变比暴胀子的衰变快得多，这意味着再加热是由暴胀子衰变决定的。暴胀子可能会重新引入暗辐射问题，因为它以相等的速率衰变为较轻的模量和它们的轴子。然而，由于它与QCD-介子的混合，暴胀子也有一个直接的衰变率到SM，增强了SM规范玻色子的数量。这导致暗辐射的量与目前的限制一致，但在未来的测量中可能会检测到。
英文摘要：
It is widely believed that string theory easily allows for a QCD axion in the cosmologically favoured mass range. The required small decay constant, $f_a\ll M_P$, can be implemented by using a large compactification volume. This points to the Large Volume Scenario which in turn makes certain cosmological predictions: First, the closed string axion behaves similarly to a field-theoretic axion in the pre-inflationary scenario, i.e. the initial value can be tuned but one is constrained by isocurvature fluctuations. In addition, the volume represents a long-lived modulus that may lead to an early matter-dominated phase. Finally, the decay of the volume modulus to its own axion tends to overproduce dark radiation. In this paper we aim to carefully analyze the cosmology by studying models that not only allow for a QCD axion but also include inflation. Quite generally, limits on isocurvature fluctuations restrict us to relatively low-scale inflation, which in the present stringy context points to Kähler moduli inflation. As a novel feature we find that the lightest (volume) modulus couples strongly to the Higgs. It hence quickly decays to the SM, thus resolving the original dark radiation problem. This decay is much faster than that of the inflaton, implying that reheating is determined by the inflaton decay. The inflaton could potentially reintroduce a dark radiation problem since it decays to lighter moduli and their axions with equal rates. However, due its mixing with the QCD-saxion, the inflaton has also a direct decay rate to the SM, enhanced by the number of SM gauge bosons. This results in an amount of dark radiation that is consistent with present limits but potentially detectable in future measurements.
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