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Dark Matter

25 Nov 2020

Dark Matter

Precise cosmological measurements coupled with astronomical evidence has allowed us to piece together a model of the Universe that explains much of what we observe. This model is known as the ΛCDM model and tells us that we live in an inflationary universe that is made up of contributions from 'Dark Energy' (68%), responsible for the accelerating expansion of the Universe, baryonic 'normal' matter (5%), and a 'Dark Matter' component, which makes up the remaining 27%. However, the nature of the dark elements remains unknown and consequently, fundamental questions still cloud the model. From both astrophysical and particle physics considerations, stable and heavy Weakly Interacting Massive Particle (WIMP) candidates that arise naturally from extensions to the Standard Model of particle physics are particularly compelling.

The ΛCDM model is built combining a wide range of astronomical and cosmological data that includes mapping fluctuations in the Cosmic Microwave Background, surveys of galaxy clusters, and measurements of distant supernovae. The best fit to this data suggests most of the Universe is "dark" - in the form of 'Dark energy' that causes the rate of expansion of the Universe to accelerate, and 'Dark Matter' accounting for 85% of all matter. Although Dark Matter cannot be seen directly, its gravitational influence is obvious. The arcs and rings seen in images of galaxies are formed by light emitted by distant sources being bent due to the gravity from an invisible body of Dark Matter between us and the galaxies.

WIMPs, interacting with regular matter only very rarely, would satisfy the requisite abundance today, be stable on the timescales of the Universe, and would have been non-relativistic at the time of decoupling - providing the gravitational seeds for large scale structure formation. Its presence today acts as the glue holding galaxies such as our own together. Confirmation of the WIMP hypothesis would cast light not only on Beyond Standard Model physics and extrapolation to GUT scales, but would considerably advance our understanding of the matter composition and evolution of the Universe. For these reasons, the discovery of Dark Matter has been identified as among the most important scientific missions of the 21st century.

Three avenues are presently pursued with the aim to identify Dark Matter: indirect, accelerator, and direct searches. Indirect methods deploy space-based and terrestrial instruments to search for signals from decay products following WIMP annihilations in regions of high density. Accelerator searches such as at the LHC seek to achieve sufficient collision energies to be able to produce WIMPs and observe missing reconstructed energy in the final states to infer their mass. However, neither of these techniques is by itself capable of providing a robust statement on the nature of Dark Matter. Expected annihilation signatures from indirect searches are easily mimicked by standard sources, and accelerator searches cannot address stability of the inferred particles and conclude that they represent galactic WIMPs. Instead, it is direct observation of WIMPs within our own Milky Way that is recognised as the definitive channel for detection. Internationally, meeting this challenge and detecting galactic WIMPs is recognised as one of the highest priorities in science, featuring often above all else in roadmaps and research recommendations worldwide.

Direct Dark Matter Searches

Galactic rotation curves suggest the Dark Matter forms an extended halo around galaxies such that our solar system, moving through this halo, is exposed to an apparent flux of WIMPs. These WIMPs may be directly detected by recording the energy deposition made by a recoiling nucleus following a WIMP scatter. However, given the kinematics, the recoil would generate energies of only a few keV. Moreover, the event rate is dictated by the coupling of a Dark Matter particle through the Weak force, resulting in extremely rare signals. Experiments must, therefore, operate detectors with very low thresholds, very low intrinsic and external radiological background, located in deep underground sites shielded from cosmic rays. Despite these challenges, several technologies have been developed and deployed successfully over the past 30 years in attempts to be the first to discover WIMPs. Where no signal has been detected, constraints on the WIMP-nucleon interaction cross-section have been derived. As yet, no unambiguous discovery of Dark Matter has been made.

Dark Matter is thought to exist as an extended halo encompassing the Milky Way. If WIMPs are the Dark Matter, then we should be able to detect their presence with terrestrial detectors when they scatter off regular nuclei. The difficulties are the very small detectable signatures (of the order of a few keV) and the rarity of the collisions (a few per year per kg of target mass). Many technologies are being developed to exploit the scintillation light, heat, and/or charge that is liberated following a recoil, but all experiments must necessarily have extremely low thresholds, be made from radiologically clean materials to suppress background rates, be located in deep underground sites to shield from cosmic rays, and be able to discriminate between Standard Model backgrounds and elusive WIMPs.

Here at UCL we work on the leading experiments in the search for Dark Matter. These experiments use liquified noble gas targets, operated as two-phase time projection chambers. Such technology has rapidly accelerated the race for Dark Matter detection over recent years, with xenon detectors in particular dominating the field and emerging as the most promising detector technology for a first detection. The LUX detector, which contained 350 kg of ultra-pure liquid xenon, operated underground at the Sanford Underground Research Facility (SURF) in S. Dakota, USA, and set a series of world-leading results in the search for WIMPs between 2013 and 2017, culminating in a final exposure of 332 days to reach a minimum WIMP-nucleon spin-independent cross section of 1.1x10-46 cm2. LUX also pioneered novel calibration techniques that have allowed liquid xenon detectors to extend their reach to low-mass WIMPs. UCL made significant contributions to LUX, particularly with development of primary pulse and signal selection, key idenitification and classification algorithms, and final WIMP detection efficiencies that fed into the world-leading constraints.

Noble gas time projection chambers use scintillation light (S1) and ionisation (S2) to identify particle interactions. Recording these two signals from any scatter allows the vertex position to be accurately reconstructed via the relative timing of the signals and their distribution within the detector. Additionally, the ratio of the signals identifies the interacting particle - helping to reject non-WIMP background events and identify the Dark Matter. The LUX detector (shown above) uses liquid xenon as a target, as will LUX-ZEPLIN (LZ), building on technology that has proven remarkably successful.

LUX-ZEPLIN (LZ) brings together the LUX project and ZEPLIN programme that pioneered the use of xenon time projection chamber technology through a series of xenon detectors at the Boulby Underground Laboratory. LZ will be a mammoth detector containing 10 tonnes of liquid xenon, with 7 tonnes in the active volume, and will deliver unparalleled search capability for Dark Matter. It will be housed using the same infrastructure as LUX at SURF, and will have the sensitivity to sweep virtually all the theoretically favoured parameter space for WIMPs. LZ construction has been completed and it is being commissioned presently. UCL hold co-responsibilty for the crucial Backgrounds and Screening Work Package, responsible for ensuring the experiment is constructed meeting its low-background requirements for the detection of WIMP Dark Matter, and for construction of the high-precison Background Model against which any possible signal will be evaluated. We utilise cutting edge radio-purity assay facilities to screen all potential construction materials for trace amounts of radio-impurities, with mass spectrometry at our dedicated ICP-MS laboratory at UCL, radon emanation measurements at the Mullard Space Science Laboratory and UCL, and gamma spectroscopy with the BUGS instruments at the Boulby Underground Laboratory. UCL also hold major responsibilities in commissioning the experiment, preparing for data taking, developing statistical tools, Monte Carlo simulations, and analyses to search for the first evidence of Dark Matter and Beyond the Standard Model physics. LZ will begin science operations in 2021.

Overview of the LZ experiment showing the main components. LZ will be the most sensitive Dark Matter experiment ever constructed.

We also conduct leading R&D to address challenges for the next generation of Dark Matter detector and multi-tonne noble liquid observatories. These planned future experiments will have unprecedented sensitivity to definitively test the standard WIMP hypothesis across remaining accessible parameter space, search for previously inaccessible but theoretically well-motivated alternative (non-WIMP) thermal relic Dark Matter candidates, and probe Beyond the Standard Model physics that no other techniques can reach. As part of the Xenon Futures programme we are pioneering the development of world-leading low-background and radiopure capabilities neccessary for the construction of such 'Generation-3' (G3) experiments.

Finally, we are exploring the use of novel quantum technologies to broaden the search for Dark Matter. Using opto-mechanically levitated nano-spheres and superconducting wires we are working with colleagues in UCL's AMOPP group and the Cosmoparticle Initiative to develop the technologies to search for ultra-light Dark Matter candidates, complementing the large underground searches for higher mass particle Dark Matter with LZ and future G3.

For more information on the direct Dark Matter research at UCL contact Prof. Chamkaur Ghag.