WIMPs!

There is a wide range of astronomical evidence that the visible stars and gas in all galaxies — including our own — are immersed in a much larger cloud of non luminous matter, typically containing much greater (by orders of magnitude) amounts of mass. The existence of this “dark matter” is consistent with evidence from large-scale galaxy surveys and cosmic microwave background measurements, which indicate that the majority of matter in the universe is non-baryonic. The nature of this non-baryonic component is still totally unknown, and the resolution of the “dark matter puzzle” is of fundamental importance to cosmology, astrophysics, and elementary particle physics. One leading explanation, motivated by supersymmetry theory, is that dark matter is comprised of as-yet-undiscovered Weakly Interacting Massive Particles (WIMPs) formed in the early universe and subsequently gravitationally clustered in association with baryonic matter.

Figure 1: Schematic representation of rotating disc galaxies in the distant Universe and the present day. Observations with ESO's Very Large Telescope suggest that such massive star-forming disc galaxies in the early Universe were less influenced by dark matter. As a result the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe. Their rotations curves, rather than being flat, drop with increasing radius.

Direct detection

According to the WIMP standard scenario, galaxies like ours are submersed in a “halo” filled with dark matter, in thermal equilibrium with the standard luminous matter that we observe everyday with telescopes and anthennas. As we rotate together with the Sun, around the center of our Galaxy, we are hit by an apparent “wind” of WIMPs, too fleebly interacting with standard matter to be sensed or seen. In principle, WIMPs could be detected in terrestrial experiments through their collisions with ordinary nuclei, giving observable low-energy (below 100 keV) nuclear recoils. The predicted collision rates are extremely small and require ultra-low background detectors with large (1–100 tonnes) target masses, located in deep underground sites to eliminate neutron background coming from cosmic ray muons.

Noble elements like argon are ideal targets to investigate WIMPs: stable, chemically inert, still gets into an excited state whenever some particle scatters on either is nucleus (Nuclear Recoil) or kicks an electron (Electron Recoil). 

De-Excitation light is released, together with ionization electrons and heat. While we do not look at the latter, DarkSide detectors are designed to measure both the “scintillation” and the ionization signal, as widely explained with DarkSide-50 experiment. 

Why argon?

We need a target that is stable, transparent to its own scintillation light, and relatively easy to extract and purify from radioactive contaminants. These impurities—emitting electrons, alpha particles, gamma rays, or neutrinos—could otherwise obscure the rare and still-undiscovered WIMP signal we’re searching for.

Argon stands out as the only noble element that enables clear discrimination between nuclear recoils—from neutrons or potential WIMP interactions—and electron recoils, which arise from trace-level radioactivity in the detector materials. By analyzing the shape of the recorded waveforms, or more practically, by measuring the fraction of prompt scintillation light relative to the total pulse, we can distinguish electron recoil backgrounds from nuclear recoils with extraordinary precision: a misclassification occurs only once in ten million events.

This powerful pulse shape discrimination is key to suppressing backgrounds and paves the way for an instrumentally background-free WIMP search in next-generation experiments like DarkSide-20k.

Figure 2: How does the Pulse shape discrimination look like in DarkSide-50 data? This is one of the crucial graphics in our data analysis, showing the amount of recorded scintillation light, S1, evaluated as total photoelectrons recorded by our photosensors, compared with the Pulse-shape discrimination parameter, f90 in DarkSide-50, corresponding to the fraction of the prompt scintillation light. Most of the backgrounds, electron recoils mainly from the beta-decay of the 39-Argon, sits at f90 about 0.3, as only 30 percent of the scintillation light in these events is released in the first 90 ns. On the other hands, WIMPs, giving Nuclear recoils, will have f90 centered at about 0.7, as shown in the violet area. Picture from Phys. Rev. D 98, 102006 (2018).