Orateur
Description
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 $|(F_{V})_{23}| > 1.1 \times 10^{12}~\mathrm{GeV}$. Under the complementary assumption that axion production is dominated by weak amplitudes, we derive a model-independent bound on $f_a$, namely $f_a > 1.0 \times 10^5~\mathrm{GeV}$. We also discuss the potential of applying the same approach to $K^+ \to \pi^+ \pi^0 \nu \bar{\nu}$ data, from which we estimate the bound $|\ (F_{A})_{23}| > 1.0 \times 10^8~\mathrm{GeV}$. These constraints represent the strongest existing bounds inferred from controlled experimental setups.
