On dark radiation from string moduli decay to ALPs

We examine the issue of dark radiation (DR) from string moduli decay into axion-like particles (ALPs). In KKLT-type models stabilization, axionlike phases fields are expected to decouple whilst in LVS-type stabilization some can remain light and may constitute radiation. evaluate modulus Minimal Supersymmetric Standard Model (MSSM) for more general compactifications. spite tightening error bars on ΔNeff, we find only mild constraints modulus-ALP couplings due somewhat suppressed branching fraction DR owing large number MSSM modes. anticipate that future CMB experiments with greater precision ΔNeff still turn up evidence if ALP associated lightest field is indeed light.