“New probes of axions in the sky”
An axion, a pseudo-scalar field with a discrete shift symmetry, could have a wide range of phenomenological and cosmological applications. Though the axion scenario was proposed more than 40 years ago, it continues to serve as one of the most motivated hypothetical feebly-coupled particles beyond the Standard Model of particle physics, with heightening interests in all kinds of search strategies. In this talk, I will discuss two new possible astrophysical/cosmic probes of axions. The first one relies on neutron stars (NS’s). NS’s with high nucleon densities could be powerful factories for axions, in the presence of axion couplings to nucleons. The axions produced could subsequently convert into hard X-rays in the strong magnetic fields around the NS’s, if axions also couple to photons.
I will show that beyond the overall X-ray flux from rotating NS’s which has been studied intensively in the literature, the pulsation structures in both the intensity and polarization of X-rays from NS’s could provide us additional valuable information about axions and their couplings. The second one is a whole new suite of cosmological observables for axion isocurvature, which could help test the presence of axions, as well as its coupling to the inflaton and other heavy spectator fields during inflation such as the radial mode of the Peccei-Quinn field. They include correlated clock signals in the curvature and isocurvature spectra, and mixed cosmological-collider non-Gaussianities involving both curvature and isocurvature fluctuations with shapes and running unconstrained by the current data analyses.
Host: Witold Skiba