A joint explanation of the $B\to \pi K$ puzzle and the $B \to K \nu \bar{\nu}$ excess

Abstract

In light of the recent branching fraction measurement of the $B^{+}\to K^{+} \nu\bar{\nu}$ decay by Belle II and its poor agreement with the SM expectation, we analyze the effects of an axion-like particle (ALP) in $B$ meson decays. We assume a long-lived ALP with a mass of the order of the pion mass that decays to two photons. We focus on a scenario where the ALP decay length is of the order of meters such that the ALP has a non-negligible probability to decay outside the detector volume of Belle II, mimicking the $B^{+}\to K^{+} \nu\bar{\nu}$ signal. Remarkably, such an arrangement is also relevant for the long-standing $B\to \pi K$ puzzle by noting that the measured $B^{0}\to \pi^{0}K^{0}$ and $B^{+}\to \pi^{0}K^{+}$ decays could have a $B^{0}\to a K^{0}$ and $B^{+}\to a K^{+}$ component, respectively. We also argue based on our results that the required ALP-photon effective coupling belongs to a region of parameter space that can be extensively probed in future beam dump experiments like SHiP.