Speaker
Description
Axion-like particles (ALPs) are leading dark matter candidates originally motivated by the strong CP problem and also arise in theories of string compactifications. We present a sensitive probe for ALPs as ultra-light dark matter - the birefringence in the cosmic microwave background (CMB). Birefringence arises from the oscillating ALP field's inhomogeneity and is also relevant for laboratory axion searches.
New constraints on the ALP-photon coupling are derived by theoretical treatment of oscillating ALPs across recombination and local observation along with comparison to observations. Our results give orders of magnitude improvement over prior constraints (CAST 2017), with even further prospects for upcoming cosmological birefringence observations. These limits, in hitherto unconstrained regions of the coupling vs. ALP mass parameter-space, are independent of assumed magnetic fields and relatively robust to ALP dark matter fraction.
Furthermore, recent tentative hints of a detection of cosmic birefringence in the CMB would imply the presence of parity-violating ALPs at specific combinations of ultra-light mass and ALP-photon coupling.
Summary
Our new theoretical analysis of CMB birefringence improves ALP-photon coupling constraints by several orders. It also has exciting implications in the context of recent cosmic parity violation hints (Planck 18 data).
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