New constraints on axion-gauge field dynamics during inflation from
  Planck and BICEP/Keck data sets
  We present new constraints on spectator axion-${\rm U}(1)$ gauge field
interactions during inflation using the latest Planck ($PR4$) and BICEP/Keck
2018 data releases. This model can source tensor perturbations from amplified
gauge field fluctuations, driven by an axion rolling for a few e-folds during
inflation. The gravitational waves sourced in this way have a strongly
scale-dependent (and chiral) spectrum, with potentially visible contributions
to large/intermediate scale $B$-modes of the CMB. We first derive theoretical
bounds on the model imposing validity of the perturbative regime and negligible
backreaction of the gauge field on the background dynamics. Then, we determine
bounds from current CMB observations, adopting a frequentist profile likelihood
approach. We study the behaviour of constraints for typical choices of the
model's parameters, analyzing the impact of different dataset combinations. We
find that observational bounds are competitive with theoretical ones and
together they exclude a significant portion of the model's parameter space. We
argue that the parameter space still remains large and interesting for future
CMB experiments targeting large/intermediate scales $B$-modes.
