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

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar η and η^{'} mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV π^{-} on nuclei of an active target as the source of neutral mesons. The η,η^{'}→invisible events would exhibit themselves via a striking signature-the complete disappearance of the incoming beam energy in the detector.

Laboratory realization of relativistic pair-plasma beams.

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black-hole and neutron-star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with electron-positron pairs. Their role in the dynamics of such environments is in many cases believed to be fundamental, but their behavior differs significantly from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies, which are rather limited.

First Results in the Search for Dark Sectors at NA64 with the CERN SPS High Energy Muon Beam.

We report the first search for dark sectors performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of 160  GeV/c are directed to an active target. The signal signature consists of a single scattered muon with momentum <80  GeV/c in the final state, accompanied by missing energy, i.

Search for Light Dark Matter with NA64 at CERN.

Thermal dark matter models with particle χ masses below the electroweak scale can provide an explanation for the observed relic dark matter density. This would imply the existence of a new feeble interaction between the dark and ordinary matter. We report on a new search for the sub-GeV χ production through the interaction mediated by a new vector boson, called the dark photon A^{'}, in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS.

Search for a New B-L Z^{'} Gauge Boson with the NA64 Experiment at CERN.

A search for a new Z^{'} gauge boson associated with (un)broken B-L symmetry in the keV-GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with 3.22×10^{11} electrons on target collected during 2016-2021 runs, no signal events were found.