Dark-Sector Search via Pion-Produced η and η^{'} Mesons Decaying Invisibly in the NA64h Detector

Area of Science:

• Particle Physics
• Experimental Physics
• Cosmology

Background:

• Dark sectors are hypothetical extensions of the Standard Model.
• Pseudoscalar mesons (eta and eta-prime) can potentially decay into invisible particles, offering a unique signature for dark sector searches.
• Previous searches have placed limits on these invisible decays, but new experimental techniques are needed to improve sensitivity.

Purpose of the Study:

• To present the first results from a proof-of-concept search for dark sectors using invisible decays of eta and eta-prime mesons.
• To establish stringent limits on the branching ratios of these invisible decays.
• To demonstrate the potential of a novel experimental technique for future dark sector searches.

Main Methods:

• Utilized the NA64h experiment at the CERN Super Proton Synchrotron (SPS).
• Employed a novel technique using the charge-exchange reaction of 50 GeV negative pions on an active target to produce neutral eta and eta-prime mesons.
• Searched for events with a complete disappearance of beam energy, indicative of invisible meson decays.

Main Results:

• No evidence for eta or eta-prime invisible decays was observed with 2.9x10^9 pions on target.
• Set a stringent upper limit on the branching ratio Br(eta-prime -> invisible) < 2.1x10^-4, improving the existing bound by a factor of approximately 3.
• Established an upper limit on the branching ratio Br(eta -> invisible) < 1.1x10^-4, comparable to the current limit.

Conclusions:

• The novel technique using invisible meson decays shows great potential for dark sector physics.
• The results provide crucial guidance for enhancing future searches for invisible neutral meson decays.
• This experiment marks a significant step towards probing dark sector candidates through rare meson decay channels.