Search for $\pi^0 \to$ invisible decays with the NA62 experiment

The search for new physics beyond the Standard Model (SM) is one of the most active fields in particle physics. Complementary to direct searches for new processes at high-energy scale, rare or forbidden SM decays are investigated to look for deviations from the predictions. The NA62 experiment at the CERN SPS, designed for the measurement of the ultra-rare decay $K^+ \to \pi^+ \nu \bar\nu$, investigates new-physics contributions in the kaon and pion sector. The highly efficient, hermetic photon-veto system makes NA62 a perfect apparatus for a high-sensitivity search for $\pi^0$ decays into invisible particles. In a fraction of data collected by NA62 in 2017, about $8.4 \times 10^9 \pi^0$ mesons have been tagged from the reconstruction of the second most- abundant $ K^+$ decay, $K^+ \to \pi^+ \pi^0 (\gamma)$. The background rejection power for visible $\pi^0$ decays, ranging from O($10^8$) to O($10^9$), is estimated by the combination of single-photon detection efficiencies measured in data with Monte Carlo simulations of $K^+ \to \pi^+ \pi^0 (\gamma)$ decays, a novel experimental technique with respect to that used in the most sensitive previous experimental result. The analysis is performed with the blind technique for a cut-based signal region. The $\pi^+$ momentum range is optimized to have the best sensitivity for the expected upper limit on the $\pi^0 \to$ invisible branching ratio in absence of a signal. No signal is observed in excess of the expected background fluctuations. The resulting upper limit on the branching ratio, BR($\pi^0 \to$ invisible) $\leq 4.4 \times 10^{-9}$ at 90% confidence level, improves on the previous best limit by a factor of 60. As a by-product of the analysis, the decay of $K^+ \to \pi^+ X$ is investigated, where $X$ is a neutral particle escaping detection with a mass in the range 0.110–0.155 GeV/$c^2$ and rest lifetime greater than 100 ps.