Through astronomical and cosmological observations, scientists have observed that the majority of the energy and matter of our universe is dark- not seen- and does not interact strongly, nor electromagnetically with regular matter. The Planck satellite results suggest that approximately 68 % of the overall budget is dark energy which led to the observation of the acceleration of the expansion of the universe. Dark matter comprises of another 27 % which is still undetected and whose presence would help understand significant observations of the galaxies.

Dark matter interacts gravitationally with baryonic matter. However, any additional existing interactions are very weak, having very small cross sections. The standard model of particle physics does not take into account dark matter. Hence, there is a need to understand physics beyond this standard model to understand the observations. This makes the direct detection and following characterization of dark matter particles one of the biggest experimental challenges of modern particle and astro-particle physics.

DARWIN (Dark Matter WIMP search with liquid xenon) is an Astroparticle observatory targeting Dark Matter in various form (WIMP, ALP, Axions, …), Solar and Supernovae neutrinos and very rare (and beyond the standard model) processes such as neutrinoless double beta decays (0νββ).

DARWIN consists of an ultra-low-background TPC (2.6 m diameter, 2.6 m high) immersed in a multi ton high purity Liquid Xenon target (with a proposed overall LXe mass of 50 t, 40 t in the instrumented region) kept at about 173 K and continuously cleaned in order to remove electronegative impurities and radioactive contaminate that would contribute to the overall background budget of the experiment. 

The titanium based cryostat containing the TPC is in turn immersed in a liquid scintillator vessel optimized to detect neutron interactions and eventually housed in a PMT instrumented water tank acting as veto by detecting the Cherenkov light emitted by passing through muon.

In order to pursue its ambitious Science program, DARWIN is required to cope with challenging technological requirements. The unprecedented detection target volume poses severe constraints on cryogenics, cleanliness, custom manufacturing and photodetection. 

The DARWIN photosensing system needs to be carefully designed and must meet the most stringent requirements in terms of radioactivity, power consumption, reliability and long term operations in a cryogenic environment. The detectors must not generate light, not interfere with the operations of other devices in the array, must be suitable for large area coverage at high fill factor. The key parameters of the candidate photosensor are: high photodetection efficiency (≳35% at 178 nm), low dark count rate (DCR) at operating conditions (less than 0.1 Hz/mm²), low afterpulsing and cross-talking, adequate gain (~10⁶). The use of traditional wavelength shifter compounds (TPB) to shift VUV wavelengths to longer ones are in general not recommended given the distinctive dilution capability of LXe. 

At the present, the candidate photosensors are being evaluated and NYUAD holds the responsibility of leading the Working Group 5 (Light and Charge readout) that focuses at identifying the most suitable device for instrumenting the TPC.

For a more comprehensive power point presentation on DARWIN, please click here.