Session C03: Hidden Sector Searches at the Sensitivity Frontier

Searches for ultralight dark matter

Analysis of inter- and intra-galactic velocities indicated a mismatch between potential and kinetic energy that was removed by postulating gravitational impact of non-luminous dark matter on luminous matter. Among the various candidates that have been suggested as dark matter constituents are axions – hypothetical ultra-light particles whose existence was postulated to resolve the strong CP problem in quantum chromodynamics. Up to now no interaction other than gravitation between dark and ordinary matter has been found. Recently, it was suggested that axion like dark matter couples to nuclear spins like a classical field by inducing an oscillating electric dipole or magnetic axial moment. In a static magnetic field, this interaction should add to the classical Zeeman splitting. The search for this interaction requires novel detection schemes that are sensitive to slight changes of the precession frequency of noble gas nuclei in ultra-low magnetic fields.   

Light Mediators to Dark Sectors

We offer a theoretical perspective on searches for light hidden sectors, existing anomalies, and new experimental strategies. Low-mass, very weakly coupled hidden sectors may be portals to dark matter that upend the experimental complementarity of the weakly-interacting massive particle paradigm. This scenario offers rich phenomenology and new experimental handles to understand physics beyond the Standard Model.   

NA64 searching for hidden sectors at the CERN SPS

NA64 is a fixed target experiment at the CERN SPS to search for hidden sectors. In this talk, we will present our latest results on the search for a new sub-GeV vector gauge boson ($A'$) mediated dark matter ($\chi$) production. The $A'$, called dark photon, could be generated in the reaction $e^-Z \to e^-ZA’$ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay $A'\to \chi\chi$. The experimental signature of this process would be a clean event with an isolated electron and large missing energy in the detector. This allows us to set new limits on the $\gamma-A'$ mixing strength and constrain models with light thermal dark matter or light scalar, Majorana or pseudo-Dirac thermal dark matter. Preliminary results on the search for the $X\to e^+e^-$ decay of a new light $X$ boson which could explain a recently observed anomaly in the $^8$Be transitions will be also discussed.