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B→K+ axionlike particles: Effective versus UV-complete models and enhanced two-loop contributions

Xiyuan Gao* and Ulrich Nierste†

  • *Contact author: xiyuan.gao@kit.edu
  • †Contact author: ulrich.nierste@kit.edu
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Phys. Rev. D 112, 055008 – Published 5 September, 2025

DOI: https://doi.org/10.1103/5j2t-2kdf

Abstract

An axionlike particle a (ALP) can explain the excess of B→K+invisible events at Belle II. However, many analyses of ALP scenarios are oversimplified. We revisit the B→Ka transition rate in a popular minimal and UV complete model with two Higgs doublets (2HDM) and a complex singlet [Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model]. To this end we compare our results with previous studies which derived the b¯sa vertex from the b¯sA vertex, where A is the heavy pseudoscalar 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≳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.

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Phys. Rev. D 112, 055008– Published 5 September, 2025
  • Received 26 June 2025
  • Accepted 5 August 2025
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