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Axions in String Theory $-$ Slaying the Hydra of Dark Radiation
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abstract
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 K\"ahler 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.
Forward citations
Cited by 6 Pith papers
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Gravitational Waves from Multiple Cosmic Superstrings and the Overshoot Problem
A string-theory model with three cosmic superstring species solves the modulus overshoot problem via gravitational-wave friction and predicts a high-frequency multi-peaked stochastic gravitational-wave spectrum.
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Testing F-theory GUTs with the Axiverse
In F-theory GUTs, non-universal ALPs induced by hypercharge flux satisfy g_aγ/m_a well below the QCD axion prediction when gauge couplings unify near the string scale.
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The String Theory Photoverse
Massless string-theory hidden photons acquire dimension-six dipole couplings to SM fermions with suppression scale Λ = αM_s, converting dipole measurements into constraints on the string scale.
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Constraining F-theory Model Building with QCD Axions
F-theory models with the Standard Model spectrum are constrained by QCD axion physics, yielding typical detectable axion masses around 10^{-9} eV and decay constants around 10^{15} GeV in allowed regions.
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Constraining F-theory Model Building with QCD Axions
QCD axions constrain F-theory base threefolds to have rigid or flux-rigidified divisors, yielding typical axion masses around 10^{-9} eV and decay constants near 10^{15} GeV in allowed regions.
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Axions at the meV Crossroads: Theory, Cosmology, Astrophysics, and Experiments
The meV axion window is presented as a coherent, cross-validated search program in which string theory, stellar cooling, dark matter, and new detector concepts converge on the same mass range.
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