Speaker
Description
The QCD axion is a compelling mechanism for solving the strong CP problem. Most studies have focused on axion models with a large decay constant, $f_a \gtrsim 10^9,\mathrm{GeV}$. However, recent work has pointed out that viable axion models may also exist in the low-scale regime, $f_a \sim \mathcal{O}(1),\mathrm{GeV}$, corresponding to an axion mass in the MeV range.
In this mass range, the axion can be produced in kaon decays such as $K_L \to \pi^0 \pi^0 a$, followed by the prompt decay $a \to e^+ e^-$. As a result, the final-state signature coincides with that of the rare decay $K_L \to \pi^0 \pi^0 e^+ e^-$. The KTeV experiment has placed a stringent upper bound on this channel, $\mathrm{Br}(K_L \to \pi^0 \pi^0 e^+ e^-) < 6.6 \times 10^{-9}$, which can therefore be directly reinterpreted as a strong constraint on MeV-scale QCD axion models. In our work, we perform a dedicated analysis of this reinterpretation and derive stringent limits on the axion parameter space from kaon decay measurements.
