New R-axion model evades cosmological and astrophysical constraints

A recent theoretical study proposes a new model for the R-axion, a hypothetical particle associated with an R-symmetry, which allows for the relaxation of stringent constraints on its decay scale, $f_R$. Traditionally, the Dine-Festuccia-Komargodski (DFK) bound implies that $f_R$ must be comparable to the Planck scale, $M_{\rm Pl}$, for a nearly Minkowski vacuum. However, researchers demonstrate that this inference can be avoided in an effective field theory construction. The team achieves this relaxation by tuning the scalar potential near zero via a mixed F- and D-term uplift, leading to a metastable vacuum. In this scenario, the validity of the effective field theory and the metastability of the small $f_R$ vacuum generically imply a relaxed lower bound for $f_R$, approximately $f_R \gtrsim \sqrt{m_{3/2}M_{\rm Pl}}$. This approach allows the intermediate R-axion to circumvent previous objections. Furthermore, the study highlights that if the R-symmetry has a QCD anomaly, this R-axion could potentially play the role of the QCD axion. A crucial aspect is that with TeV-scale supersymmetry, a value of $f_R \sim 10^{11}$ GeV is obtained. This range not only evades certain astrophysical and cosmological axion constraints but notably lies in the window for which the observed dark matter abundance can be reproduced by the R-axion via the misalignment mechanism. This model offers a new avenue for exploring the nature of dark matter and extensions to the Standard Model.