50 GeV $\pi^-$ in, nothing out: a sensitive probe of invisible $\eta$ and $\eta'$ decays with NA64h

TL;DR

This paper reports the first search for invisible decays of $ ext{eta}$ and $ ext{eta'}$ mesons using a novel technique with the NA64h experiment, setting new limits on their branching ratios and demonstrating the method's potential for dark sector physics.

Contribution

Introduces a new approach using charge-exchange reactions of 50 GeV $ ext{pi}^-$ to probe invisible meson decays, improving existing limits and guiding future dark sector searches.

Findings

No evidence for invisible decays was observed.

Set a new limit on ${ m Br}( ext{eta'} o invisible)$ < 2.1 x 10^{-4}.

Set a limit on ${ m Br}( ext{eta} o invisible)$ < 1.1 x 10^{-4}.

Abstract

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar $\eta$ and $\eta'$ mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV $\pi^-$ on nuclei of an active target as the source of neutral mesons. The $\eta, \eta' \to invisible$ events would exhibit themselves via a striking signature - the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with $2.9\times10^{9}$ pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio ${\rm Br}(\eta' \to invisible) < 2.1 \times 10^{-4}$ improving the current bound by a factor of $\simeq3$. We also set a limit on ${\rm Br}(\eta \to invisible) < 1.1 \times 10^{-4}$ comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.