2025/06/17 by Gao, Xiyuan, Nierste, Ulrich · 1 citation
#FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph)
paper · doi:10.48550/arxiv.2506.14876
An axion-like particle a (ALP) can explain the excess of B→ K + invisible events at Belle-II. However, many analyses of ALP scenarios are over-simplified. We revisit the B→ K a transition rate in a popular minimal and UV complete model with two Higgs doublets (2HDM) and a complex singlet (DFSZ model). To this end we compare our results with previous studies which derived the bsa vertex from the bsA vertex, where A is the heavy pseudo-scalar of the 2HDM, in terms of an a-A mixing angle. We find this approach to work only at the leading one-loop order, while it fails at the two-loop level. Furthermore, while an approximate Z2 symmetry suppresses the leading-order amplitude by a factor of 1/tanβ, which is the ratio of the two vacuum expectation values of the Higgs doublets, we find the two-loop contribution unsuppressed and phenomenologically relevant for tanβ\gtrsim 5. We determine the allowed parameter space and underline the importance of better searches for Υ→ γ+invisible and for a possible excess in B→ Kμ+μ-. We further study the low-energy axion effective theory which leads to a divergent and basis-dependent amplitude. As a conceptual result, we clarify the ambiguities and identify which low-energy framework is consistent with the DFSZ model.