ALP-Mediated Dark Matter at Low Reheating Temperature and the Belle II Excess

Belle II has reported a $2.7σ$ excess in $B^+\to K^+ν\barν$ over its Standard Model prediction, recently interpreted as arising from an axion-like particle mediating a thermal dark matter candidate. We reassess this interpretation using a new mass-differential measurement of the same data set and adding two visible-decay searches that earlier analyses omitted. The resulting bounds leave the predictions from a standard cosmological history only marginally compatible with the excess, viable in a narrow window of parameter space. Thus, we consider a situation where dark matter freezes out during a low-temperature reheating era, featuring entropy injection diluting the relic abundance. This requires smaller couplings compared to those in standard cosmology in order to match the observed dark matter density, opening a region of parameter space where the compatibility of the model with the Belle II excess is significantly improved.

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Authors

Joel Jones-Pérez; Luca Merlo; Javier Silva-Malpartida

Published

Sep 30, 2026

Paper type

preprint

Source

Arxiv