Kaon experiments at CERN: recent results and prospects

TL;DR

Recent CERN kaon decay experiments have set new limits on lepton number violation, searched for sterile neutrinos and scalars, and measured the pion transition form factor, advancing understanding of rare kaon processes.

Contribution

The paper reports new experimental limits on rare kaon decay modes, searches for hypothetical particles, and a first measurement of the pion electromagnetic form factor slope.

Findings

Upper limit on lepton number violating decay $K^ ightarrow o ext{pi}^ ext{mp} ext{mu}^ ext{pm} ext{mu}^ ext{pm}$ is $8.6 imes 10^{-11}$.

Constraints on heavy sterile neutrino and scalar resonance production in kaon decays.

First observation of a non-zero electromagnetic transition form factor slope of the $oldsymbol{ extpi^0}$ in the time-like region.

Abstract

The NA48/2 and NA62-$R_K$ experiments at the CERN SPS collected large samples of charged kaon decays in flight in 2003--07. The data analysis is still on-going (with over 20 publications produced so far); the recent results from these experiments are presented. A new upper limit on the rate of a lepton number violating decay $K^\pm\to\pi^\mp\mu^\pm\mu^\pm$ is reported: $\mathcal{B} < 8.6\times 10^{-11}$ at 90\% CL. Searches for heavy sterile neutrino $N_4$ and neutral scalar resonances ($\chi$) in $K^\pm\to\pi\mu\mu$ decays are reported. Upper limits on the products $\mathcal{B}(K^\pm\to\mu^\pm N_4)\mathcal{B}(N_4\to\pi^\mp\mu^\pm)$ and $\mathcal{B}(K^\pm\to\pi^\pm \chi)\mathcal{B}(\chi\to\mu^+\mu^-)$ are set in the range $10^{-10}$ to $10^{-9}$ for resonance lifetimes up to 100~ps. A preliminary measurement of the electromagnetic transition form factor slope of the $\pi^0$ from $1.05\times 10^6$ fully reconstructed $\pi^0\to\gamma e^+e^-$ decays is presented: the obtained result $a = (3.70 \pm 0.53_\text{stat} \pm 0.36_\text{syst})\times 10^{-2}$ represents the first observation of a non-zero slope in the time-like region of momentum transfer.