Abstract
We review searches for closed string axions and axion–like particles (ALPs) in IIB string flux compactifications. For natural values of the background fluxes and TeV scale gravitino mass, the moduli stabilisation mechanism of the LARGE Volume Scenario predicts the existence of a QCD axion candidate with intermediate scale decay constant, fa ~ 109÷12 GeV, associated with the small cycles wrapped by the branes hosting the visible sector, plus a nearly massless and nearly decoupled ALP associated with the LARGE cycle. In setups where the visible sector branes are wrapping more than the minimum number of two intersecting cycles, there are more ALPs which have approximately the same decay constant and coupling to the photon as the QCD axion candidate, but which are exponentially lighter. There are exciting phenomenological opportunities to search for these axions and ALPs in the near future. For fa ~ 1011÷12 GeV, the QCD axion can be the dominant part of dark matter and be detected in haloscopes exploiting microwave cavities. For fa ~ 109÷10 GeV, the additional ALPs could explain astrophysical anomalies and be searched for in the upcoming generation of helioscopes and light–shining–through–a–wall experiments.
Highlights
LARGE Volume Scenario (LVS) which exploits both perturbative and non-perturbative effects to fix Kähler moduli gives rise to an axiverse with and
Visible sector with chirality built by wrapping magnetised D7-branes around rigid but not del Pezzo four cycles; D-term conditions stabilise d combinations of Kähler moduli, leaving . flat directions; latter fixed by pert. corrections
> String phenomenology holds the promise of an axiverse – the QCD axion plus a number of further ultralight axion-like particles, possibly populating each decade of mass down to the Hubble scale
Summary
> Standard Model of Particle Physics: Fundamental description of known matter particles and gauge forces. > Standard Model of Cosmology: only about 5 % of energy content of present universe consists of known particles. > Extensions of the Standard Model of Particle Physics: several good motivated candidates for constituents of dark matter. > Welcome side effect: Axion: candidate for dark matter: created non-thermally via misalignment mechanism in form of coherent oscillations of axion field. > strongly motivated extensions of Standard Model based on string theory: Unification of all forces Quantum gravity. > Spectrum of low-energy effective theory in (3+1)-dimensions is supersymmetric and possibly contains several kinds of very weakly interacting slim particles (WISPs): Axion, ALPs (AxionLike Particles if the compact space comprised of the 6 extra dimensions has certain geometrical and topological properties (Calabi-Yau; several cycles)
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