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

Yu. M. Andreev, A. Antonov, M. A. Ayala Torres +54

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar $\eta$ and ${\eta }^{\prime }$ mesons in the NA64h experiment at the CERN SPS. Our n…