Probing Leptophobic Dark Sector with a Pseudoscalar Portal in the NA64 Experiment at CERN

We consider the possibility of discovering a light pseudoscalar particle a, which could the axion or an axion-like particle (ALP), interacting mainly with quarks using the electron and photon scattering reaction chain e + Z → e + γ + Z; γ + Z → a + Z on nuclei in the NA64 experiment at the CERN SPS. New bounds on the coupling strengths of the axion/ALP with quarks and of pseudoscalar mesons P = π, η, η' with dark fermions are obtained by using existing upper bounds on invisible decay modes of Ps including those recently derived by NA64. We also study a scenario when the a plays the role of a messenger in the communication between our world and the dark sector.