New bound on the vectorial axion-down-strange coupling from $K^+ \to \pi^+ \nu \bar \nu$ data

We analyze publicly available $K^+ \to \pi^+ \nu \bar{\nu}$ data collected by NA62 from 2016 to 2022 to constrain the fundamental vectorial coupling of the QCD axion to the down and strange quarks or, equivalently, the Peccei-Quinn scale $f_a$ rescaled by this coupling, obtaining $|(F_V)_{23}| > 1.1 \times 10^{12}$ GeV.