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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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Cosmic superstrings in large volume compactifications: PTAs, LISA and time-varying tension
Cosmic superstrings with a time-varying tension in large-volume string compactifications generically predict a LISA-band spectral drop and a high-frequency f^4 boost, encoding the mass and decay of the volume modulus.
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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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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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String Theory and Grand Unification Suggest a Sub-Microelectronvolt QCD Axion
String-theoretic axions consistent with grand unification and proton decay are predicted to have masses in the 3e-11 to 1e-8 eV window, with at most 47 axions in the Kreuzer-Skarke ensemble.
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Preheating Axions in String Cosmology
Preheating of closed-string axions in fibre inflation can overproduce dark matter, yielding new upper bounds on the axion mass and constraints on model parameters.
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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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