$B^+\to K^+ \nu \bar{\nu}$ Excess and DM semi-annihilation

Abstract

In 2023, Belle II collaboration announced the observarion of the $B^+ \to K^+ \nu\bar{\nu}$ decay channel for the first time. This decay channel provides a clean signal with high precision in theoretical calculation. However, we encounter $2.8\sigma$ deviation from the Standard Model (SM) prediction. To resolve this excess, we study scalar dark matter (DM) model with local discrete $Z_3$ symmetry. Assuming dark $U(1)_X \equiv U(1)_{L_\mu - L_\tau}$ symmetry, this $U(1)_{L_\mu - L_\tau}$ symmetry is spontaneously broken into local discrete $Z_3$ by non-zero vacuum expectation value of dark Higgs boson. Considering dark Higgs mass is $2$GeV, we can explain the recent ${\rm Br} (B^+ \to K^+ \nu\bar{\nu})$ excess reported from Belle II collaboration and relic abundance at the same time.