Search for $\pi^0$ decays to invisible particles

TL;DR

The NA62 experiment searched for invisible decays of the neutral pion and set a new, much stricter upper limit on its branching ratio, constraining models involving new particles like axion-like particles or dark scalars.

Contribution

This study provides the most stringent upper limit to date on the $^0$ decay to invisible particles, improving previous bounds by a factor of 60 and exploring new parameter space.

Findings

No excess signal observed beyond background expectations.

Set an upper limit of 4.4 x 10^{-9} on the branching ratio.

Constrained models involving axion-like particles and dark scalars.

Abstract

The NA62 experiment at the CERN SPS reports a study of a sample of $4 \times10^{9}$ tagged $\pi^0$ mesons from $K^+ \to \pi^+ \pi^0 (\gamma)$, searching for the decay of the $\pi^0$ to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of $4.4 \times10^{-9}$ is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model-independent upper limit on the branching ratio for the decay $K^+ \to \pi^+ X$, where $X$ is a particle escaping detection with mass in the range 0.110-0.155 GeV$/c^2$ and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming $X$ to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson.