Speaker
Description
Compactification in string theory generally gives rise to a large number of pseudoscalar, Axion Like Particles (ALPs). Remaining agnostic to the exact form of the resulting Lagrangian, we consider an $N \in [2, 30]$ axion model for which ALPs are mass mixed in the interaction basis. As a consequence, and akin to neutrinos, we find the ALPs flavours to oscillate amongst themselves during propagation. Within the interaction basis, coupling to the standard model is present only for one flavour of ALP with the remaining ALPs being "hidden". The capacity for interacting ALPs to oscillate into a hidden sector effectively dilutes their interactions with the standard model, thereby weakening the bounds on their coupling. We have recast existing astrophysical bounds, paying specific attention to those arising from Very High Energy (VHE) Blazar spectra as well as the CAST experiment. Recent observations have shown the intergalactic space to be more transparent to VHE gamma rays than is expected given the Extra-Galactic Background Light (EBL) density. The introduction of an ALP alleviates this discrepancy by providing a mode within which the VHE photons can propagate unimpeded by EBL. We have extended this to the non-trivial case of multiple axions, where the large inter-galactic distances allow ample time for oscillation to occur. The CAST experiment, in contrast, with its limiting propagation distance, provides a stage on which to consider the effect of a misaligned EM/Electronic ALP mixing on axion couplings in a multi axion scenario.
