Abstract
The QCD axion solving the strong CP problem may originate from antisymmetric tensor gauge fields in compactified string theory, with a decay constant around the GUT scale. Such possibility appears to be ruled out now by the detection of tensor modes by BICEP2 and the PLANCK constraints on isocurvature density perturbations. A more interesting and still viable possibility is that the string theoretic QCD axion is charged under an anomalous U(1)_A gauge symmetry. In such case, the axion decay constant can be much lower than the GUT scale if moduli are stabilized near the point of vanishing Fayet-Illiopoulos term, and U(1)_A-charged matter fields get a vacuum value far below the GUT scale due to a tachyonic SUSY breaking scalar mass. We examine the symmetry breaking pattern of such models during the inflationary epoch with the Hubble expansion rate 10^{14} GeV, and identify the range of the QCD axion decay constant, as well as the corresponding relic axion abundance, consistent with known cosmological constraints. In addition to the case that the PQ symmetry is restored during inflation, there are other viable scenarios, including that the PQ symmetry is broken during inflation at high scales around 10^{16}-10^{17} GeV due to a large Hubble-induced tachyonic scalar mass from the U(1)_A D-term, while the present axion scale is in the range 10^{9}-5\times 10^{13} GeV, where the present value larger than 10^{12} GeV requires a fine-tuning of the axion misalignment angle. We also discuss the implications of our results for the size of SUSY breaking soft masses.
Highlights
JHEP07(2014)092 where the factor 8π2 comes from the convention for the axion decay constant, and MP l ≃ 2.4 × 1018 GeV is the reduced Planck scale
An interesting generalization of this scheme, involving an anomalous U(1)A gauge symmetry with a nonzero U(1)A-SU(3)c-SU(3)c anomaly cancelled by the 4-dimensional Green-Schwarz (GS) mechanism [38], has been discussed before for the purpose of having an intermediate scale QCD axion even when the compactification scale is comparable to the Planck scale [24, 39,40,41,42]
We have examined the cosmological constraints on string theoretic QCD axion in the light of the recent PLANCK and BICEP2 results
Summary
String theory contains a variety of higher-dimensional antisymmetric p-form gauge fields Cp, together with the associated gauge symmetry, under which. An attractive scheme to realize such possibility is that the stringy axion θst is charged under an anomalous U(1)A gauge symmetry, and its modulus partner τ is stabilized at a value near the point of vanishing FI term.5 Such scheme can be realized in many string compactification models, including the Type II string models with D-branes and the heterotic string models with U(1) Yang-Mills bundles on Calabi-Yau manifold. The physical QCD axion is mostly θst which originates from antisymmetric tensor gauge fields, and its decay constant reads fa = Axion cosmology in this case is essentially the same as in the case without anomalous U(1)A symmetry, and the model is in conflict with the inflation scale HI ≃ 1014 GeV.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
