DEAP-3600
DEAP-3600
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(Left): DEAP-3600 team members standing in front of the water tank containing the detector in SNOLAB
(Right): Zoomed-in photo of the poster depicting the detector on the water tank
Image credit: Catherine Bina
DEAP-3600 (DEAP) is single-phase liquid argon scintillation counter, shown in the photos below, designed to search for Weakly Interacting Massive Particles (WIMPs), a class of dark matter candidates with masses on the scale of typical nuclei, up to about 100 times heavier that scatter on target nuclei inside the detector, producing a signal typically below ~100 keV, with spin-independent elastic scattering cross sections far weaker than that of any other known particle. DEAP-3600 used a 3.3 tonne volume of liquid argon (LAr), contained in a 50 cm-thick acrylic vessel, within a muon veto water tank, at SNOLAB, a laboratory 2 km underground in an active nickel mine in Sudbury, Ontario, Canada. Due to its large size and extremely low backgrounds, DEAP-3600 demonstrated the ability to scale LAr dark matter detectors to large target masses and achieved leading constraints for the WIMP interaction cross section on the argon nucleus. This sensitivity was due in large part to the pulse shape discrimination technique that DEAP-3600 was able to deploy to use the scintillation time profile to separate electronic and nuclear recoils, allowing us to decrease electronic recoils backgrounds by 8-10 orders of magnitude -- the strongest demonstration of this technique in LAr to-date, and a key ingredient to LAr's effectiveness in rare event searches. In addition to our WIMP search, we pioneered several new physics analyses, including searches for more exotic dark matter candidates. The UCR group has played a particularly leading role in DEAP-3600 analyses, including in the construction of the background model, the WIMP search in our first-year dataset, and in developing and carrying out additional physics analyses. The current work of the UCR group largely focuses on carrying out these physics analyses and developing the techniques needed to carry them out.
For more information, see the collaboration website, here: link
And you can learn about SNOLAB here: link
Left: A picture of the DEAP-3600 acrylic vessel under construction at SNOLAB. It has an inner 5 cm-thick acrylic shell with 45 cm-thick acrylic light guides coming out of it, which PMT photosensors attach to. The acrylic and layers of plastic that were later inserted between the light guides provide thermal insulation and neutron shielding, so that the PMTs can operate at warmer temperatures and the LAr can be shielded from neutrons, which may produce fake WIMP signals
Middle: DEAP-3600 at a later stage of construction, hanging inside the water tank that forms the muon veto. PMTs have been aded to the light guides, and copper collars are around them
Right: DEAP-3600 upon completion of its construction: the PMTs are now enclosed in a stainless steel vessel and another set of PMTs are mounted on the outside of it. These PMTs view the water, which the surround tank was later filled with, allowing them to detect Cherenkov radiation from cosmic-ray muons approaching the detector, which may otherwise produce WIMP-like backgrounds
WIMP search results from our first 235 live days of data collection.
The curve shown in red represents the highest WIMP nucleon cross section that is consistent with the zero-backgrounds observed in our WIMP search region of interest. These results were a significant accomplishment in achieving ultra low backgrounds in a tonne-scale dark matter detector. If dark matter particles have a mass in the range shown here, then the cross section for scattering on a nucleon must be above the illustrated curves in order to be consistent with direct detection constraints.
[DEAP Collaboration. "Search for dark matter with a 231-day exposure of liquid argon using DEAP-3600 at SNOLAB". Phys. Rev. D 100, 022004 (2019)]
Recent astronomical observations indicate that the halo of stars making up the Milky Way may be more complex than previously thought, and astrophysical simulations indicate that these stellar substructures might correspond to similar substructures in the local dark matter population. Additionally, the standard WIMP paradigm makes several assumptions about how WIMPs couple to nucleons, in order to calculate the relationship between the cross section for scattering on a single nucleon and the full nucleus. In particular, we often assume that dark matter couples equally to all nucleons in a way that does not depend on any of their properties or on the dark matter speed or the momentum transferred to the target nucleus. While this scenario is well-motivated, it is far from the only possibility, and other well-motivated couplings may also exist. These possibilities can all be explored within the context of a non-relativistic effective field theory, which allows one to write out all possible couplings and dependencies that might arise in dark matter-nucleus scattering. In addition to interactions that add velocity- or momentum transfer-dependence, dark matter may couple differently to protons and neutrons, potentially causing varying degrees of interference in the scattering physics. In DEAP-3600, we recast our null results from our first year of data collection to place constraints on all possible interactions that might mediate dark matter-argon interactions, considering also scenarios where dark matter couples equally to protons and neutrons (isospin), where opposite couplings to protons and neutrons cause destructive interference (isovector), and where proton and neutron couplings happen to cancel in xenon nuclei, creating a hole in the leading results currently held by xenon detectors (xenon-phobic). We also explored how different hypothetical substructures in the dark matter halo might influence each of these constraints. For more details on the interaction types and the way we modeled these potentially substructures, see the citation below
[DEAP Collaboration. "Constraints on dark matter-nucleon effective couplings in the presence of kinematically distinct halo substructures using the DEAP-3600 detector". Phys. Rev. D 102, 082001 (2020)]
You may have noticed in the previous plots that constraints on dark matter-nucleon scattering cross sections tend to get weaker for heavier dark matter candidates. This is because astrophysical measurements tell us the local density of dark matter, which means the number density is inversely proportional to the dark matter mass. In other words, fewer dark matter particles enter the detector. At some point, the number density becomes so low that no dark matter would have entered an experiment, meaning that they can place no constraints at all on the dark matter's coupling to the Standard Model, and cross sections large enough to permit dark matter to scatter several times within a detector are still allowed. These distinctive signals would also be missed by most standard direct detection dark matter analyses, which focus on single-scatter signals. As a result, we had to devise a dedicated analysis to search for multiply-interacting dark matter particles with extremely high masses, accounting for the fact that the rock above the lab would attenuate dark matter on its way to the detector.
DEAP-3600 performed the first search for ultra-heavy dark matter with masses up to the Planck scale--the rough theoretical upper bound on the mass of simple particle dark matter, according to many of the most straightforward models. Our dedicated search for this unique signal observed no events consistent with our search, but allowed us to place the first direct detection constraints at such high masses using this technique, and has paved the way for future experiments to attempt similar analyses. Shown on the left are two different superheavy dark matter models we considered in this analysis -- see the paper cited below for more details.
[DEAP Collaboration. "First direct detection constraints on Planck-scale mass dark matter with multiple-scatter signatures using the DEAP-3600 detector". Phys. Rev. Lett. 128, 011801 (2022)]
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