The Outer Veto for the DarkSide-20k Dark Matter Search

An astronomical body of evidence indicates that 27% of the energy density of the universe is composed of dark matter — a form of matter that cannot be described by the Standard Model of particle physics and only weakly couples to the known particles. Dark Matter has clear gravitational effects evident in astrophysical and cosmological observations.

A promising dark matter candidate is the Weakly Interacting Massive Particle (WIMP), a relic of the early universe with weak couplings to nucleons within atoms. A WIMP halo is expected to surround the entire galaxy including our solar system. The Earth’s motion relative to it creates a flux of WIMPs that may scatter on a target nucleus in the detector, creating a nuclear recoil.

The DarkSide-20k experiment seeks to detect such WIMPs using a 50 tonne active volume of liquid argon (LAr) as part of a "dual-phase time projection chamber (TPC) detector", capable of completely suppressing electromagnetic backgrounds. As a result of these measurements, liquid argon detectors are capable of reaching large, nearly-background-free exposures.

However, neutrons stand out as a remaining prominent background: they are produced in rare nuclear reactions in detector materials by cosmic-ray muons interacting with the detector and its environment. These cosmogenic neutrons are produced with high energies and can penetrate the detector shielding, potentially producing WIMP-like nuclear recoils in the detector.

The work funded by this grant will finalize the design and carry out the implementation of the DarkSide-20k Outer Veto, and ensure the detector can operate free of cosmogenic neutron backgrounds. At the same time, this work will provide valuable hands-on training to students in detector design and instrumentation through prototyping and installation work, as well as in skills related to photoelectronics, data acquisition systems, and simulation and analysis tools needed to inform the detector design.

The work will also develop 3D detector visualization tools for use in both outreach and in algorithm development.

This project supports the work to be carried out by the University of California, Riverside group of the DarkSide-20k experiment, responsible for the design and implementation of the Outer Veto. This work will involve optical and mechanical simulations of the Outer Veto instrumentation to ensure its stability and that it meets the requirements needed to eliminate cosmogenic neutron backgrounds.

Based on the simulation-informed design, a small-scale prototype detector will be built to test the optical and mechanical systems on campus before deploying them in the full detector. Additionally, cost-effective reflectors capable of covering large-area detectors with low photosensor coverage will be tested to ensure that they maintain their reflectance when submerged in liquid argon.

In addition to the instrumentation, the work supported by this grant will carry out the design and implementation of the data acquisition and low-level data processing systems for the Outer Veto. The funded work will also complete the acquisition of the getter for the LAr TPC purification system. The outreach project funded by this experiment will involve programming a 3D headset to display an image of the DarkSide-20k detector and visualize various classes of signals occurring in it.

This tool will be brought to outreach events around the Riverside area, shown to members of the community and at local schools, and it will provide a useful tool for algorithmic development while developing analysis tools for use in the DarkSide-20k dark matter search.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.