Groundbreaking research
Dark matter detectives: The hunt for the missing mass of the universe
October 30, 2019
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Credit: Zachary Kenny
Not all detectives wear trench coats and carry a notepad and magnifying glass to solve mysteries.
Gilles Gerbier is using a helium-filled copper sphere, containing a tiny ball at the centre attached to a rod, to search for an elusive signal from an enigmatic, invisible particle that might rule the universe.
Wolfgang Rau is using detectors made of germanium and silicon crystals, cooled close to absolute zero, to detect tiny increases in temperature that may indicate a rare, very weak interaction with this elusive particle, which has yet to be found.
Tony Noble is using the world鈥檚 most sophisticated bubble chamber, filled with a superheated, fluorocarbon fluid, to look for a bubble formation pattern that signifies fluorine interacting with this extraordinary particle 鈥 one that doesn鈥檛 shine like regular matter but is the most abundant form of matter in the universe.
These three Queen鈥檚 particle astrophysics researchers are detectives and leaders in the international hunt for dark matter, which makes up about 85 per cent of matter in the universe, although no one knows what dark matter particles look like or their physical properties. Gerbier, Rau and Noble are each directing or playing key roles in large collaborative teams of Canadian and international researchers conducting three competing, but complementary experiments that use different tools to seek, find and ultimately understand the nature of dark matter.
Mystery of the missing mass
Dark matter is one of the great mysteries of physics and the cosmos. The first evidence for dark matter emerged in the 1930s from calculations by astronomers that clusters of galaxies in our universe are moving so fast that the gravitational pull generated by their observable matter could not possibly hold them together. Astronomers then found that stars also move much faster around galactic centres than expected given their observable mass.
On a cosmic scale, unlocking the secrets of dark matter particles will undoubtedly change the Standard Model of particle physics used by scientists to explain how the universe works.
The hunt for dark matter is a quest to find the missing mass of the universe. 鈥淲e鈥檙e searching for the dominant matter in the universe. Without that matter, galaxies would fall apart, and the world would not be conducive to sustaining life as we understand it. If we want to understand why we鈥檙e here or why the universe works the way it does, dark matter is a fundamental component,鈥 says Rau, leader of the Canadian research teams in the international SuperCDMS (Cryogenic Dark Matter Search) collaboration.
Queen鈥檚 researchers are at the forefront of a global search for the most promising candidate for a dark matter particle, called a Weakly Interacting Massive Particle (WIMP). They are attempting to distinguish it from other weakly interacting particles, like neutrinos, which have very little mass, and do not explain the missing mass of dark matter. Queen鈥檚, through its collaborations with SNOLAB and the , has the largest university research group investigating dark matter in Canada and globally, with about 40 research scientists, postdoctoral fellows and students working in the area. Their findings to date in setting new detection limits to narrow the search for dark matter follow many cutting-edge discoveries in particle astrophysics, including the ground-breaking research at SNOLAB by Queen鈥檚 Professor Emeritus Arthur McDonald, co-recipient of the 2015 Nobel Prize in Physics, demonstrating that neutrinos have mass.
New tools to probe unexplored subatomic space
In recent years, Canada and Queen鈥檚 researchers have leapt to centre stage in the global hunt for dark matter by moving forward with plans to build and launch three next-generation detectors, which will operate 6,800 feet (2,100 metres) underground at SNOLAB, outside Sudbury, Ontario. SNOLAB is the world鈥檚 deepest clean lab, and its great depth provides excellent shielding from background cosmic rays that interfere with dark matter signal detection.
Credit: Zachary Kenny
The three science sleuths will be using new tools with much greater detection capabilities and sensitivities to probe new territory in parameter space that has never been searched before, and pursue the most promising leads to solve the case of the missing mass in the universe.
Gerbier, a world-leading astrophysicist from France known for pioneering new techniques aimed at detecting dark matter, joined Queen鈥檚 as the prestigious Canada Excellence Research Chair (CERC) in Particle Astrophysics in 2014. He鈥檚 leading the NEWS-G (New Experiments with Spheres-Gas) project, an entirely new type of spherical gaseous detector he and a French colleague developed to be sensitive at the unexplored lower end of the particle mass range, comparable in mass to a proton. (The mass of a proton is about one atomic mass unit, or a mass in energy units of about 938.27 million electron volts.) It began operating mid-2019. 鈥淥ur aim with NEWS-G is to explore new territory by looking at lower mass ranges than other experiments,鈥 says Gerbier, who has built a large international collaboration with European, U.S. and Canadian researchers, based at Queen鈥檚.
Gerbier and Rau are also working together prepare for the next generation of the SuperCDMS experiment, which is operated by a collaboration involving more than 100 researchers from the US, Canada, Europe and Asia and is moving from an underground lab in Minnesota to the much deeper SNOLAB facility. It鈥檚 scheduled to launch in 2020. 鈥淲hat鈥檚 exciting is that we鈥檙e building a new generation of cryogenic detectors optimized to detect low-mass dark particles, with lower background and much better sensitivity for dark matter interactions than our previous detectors,鈥 says Rau.
This picture is looking into the top of the shielding tank of the PICO-60 dark matter detector, part of a series of detectors built by the PICO collaboration. The detector is surrounded by a tank filled with water to stop neutrons from interacting inside the detector. The water is temperature controlled because the experiment is very sensitive to temperature fluctuations. Credit: SNOLAB
Tony Noble is leading the PICO 500 project, a new, much larger bubble chamber detector that will begin running at SNOLAB in 2020. The PICO collaboration, which includes researchers from 17 institutions in Canada, the US, Europe and India, leads the world in setting new limits and narrowing the search for spin-dependent dark matter particle interactions.
PICO 500 will push the limits for detection of spin-dependent interactions with its much greater sensitivity and enhanced discrimination capabilities. 鈥淲ith this suite of three large international experiments, we鈥檙e trying to cover a wider range of possibilities to detect dark matter particles and their interactions,鈥 explains Noble, also director of the Arthur B. McDonald Canadian Astroparticle Physics Research Institute (McDonald Institute). 鈥淥ur experiments each have a sweet spot where we鈥檙e most sensitive. If one type of experiment detects a signal that appears to be dark matter, we鈥檒l need to confirm the signal is truly dark matter with a different experiment.鈥
Unraveling the universe
Dark matter is the elusive, cosmic glue that holds everything in the universe together. These three detectives are devising and deploying super-sensitive detection tools deep underground to crack the code of dark matter鈥檚 distinctive particle signature. Their advanced detection tools 鈥 ultra high-tech versions of Sherlock Holmes鈥 magnifying glass 鈥 may also lead to practical commercial spinoff applications. Gerbier鈥檚 gaseous spherical detector, for example, is being developed to measure specific types of radiation levels at nuclear facilities more cheaply and safely than existing devices.
On a cosmic scale, unlocking the secrets of dark matter particles will undoubtedly change the Standard Model of particle physics used by scientists to explain how the universe works. Discovering the unknown properties of the missing mass could also provide exciting new clues to help predict the size, shape and fate of the universe in the future.
The PICO experiment is filled with superheated liquid that will boil at the slightest nudge. Tony Noble and Gevy Cao listen for bubbles that form as a particle passes through the chamber. Credit: Zachary Kenny
PICO probes for dark matter with a spin
The new PICO 500 detector at SNOLAB will be the world鈥檚 largest bubble chamber and have the expanded sensitivity to search for dark matter interactions in territory that has never been explored before. 鈥淲e don鈥檛 know the fundamental properties of dark matter. One important possibility we鈥檙e exploring is that dark matter will have spin and have interactions with other matter that has spin,鈥 explains Tony Noble, leader of the international PICO 500 project.
The PICO detector is designed to detect possible interactions between dark matter particles and the target fluid, octafluoropropane (C3F8), which is ideally suited for the detection of spin-dependent matter because the nature of the fluorine in the fluid leads to a relatively large interaction probability. The detector maintains the target fluid in a superheated state and the energy deposited by a subatomic particle causes the fluid to boil and form a bubble in the chamber. Cameras and acoustic sensors around the chamber observe and listen to the sounds of bubbles forming and popping to help researchers identify the nature of the particles causing the bubbles to form, and to distinguish between possible dark matter interactions and background sources.
鈥淲ith PICO 500, we鈥檙e thousands of times better in the sensitivity of detection than when we started, and we can run the experiments free of backgrounds that could mimic a dark matter signal,鈥 says Noble.
The gas inside NEWS-G is sensitive to the smallest disturbance. Gilles Gerbier and Marie Vidal watch as electrons are released from the gas by a passing particle and collected by the electrode at the centre of the sphere.
Gerbier's gas spheres look lower for elusive dark matter signals
The aim of the NEWS-G project led by Gilles Gerbier is to search for dark matter particle candidates in low mass regions not yet accessible by existing experiments. 鈥淎s an experimentalist, it鈥檚 exciting to develop new tools to see new areas of parameter space. Nearly all the experiments have looked for mass ranges that are multiples of the mass of a proton and we鈥檝e now eliminated those ranges. NEWS-G uses a novel approach for detecting WIMPs that pushes the limits much lower and is sensitive to masses in the range of a proton. We can go even lower with NEWS-G than SuperCDMS by a factor of 10,鈥 explains Gerbier, noting the two detectors are complementary in searching for spin-independent dark matter interactions in different low-mass ranges.
The NEWS-G detector is a large copper gaseous sphere, containing a tiny ball attached to a rod that will be encased in a large lead and polyethylene shield deep underground at SNOLAB. The sphere is filled with a light gas, such as helium, with desirable properties, and contains various sensitive and precisely positioned electrodes, set to high voltage relative to the spherical shell, which can detect interactions between any possible dark matter particles and the nuclei of the gas atoms. Indeed, after they have been hit by a dark matter particle, nuclei recoil inside the gas and liberate electrons from atomic shells of the gas. These free electrons then drift towards the electrodes and induce an electric pulse, under the action of the electric field between the electrodes and the spherical shell. 鈥淎 key advantage is we can also use other light gases, such as hydrogen and neon, and vary the pressure of the gas, to increase the sensitivity and help discriminate between possible dark matter and background interactions,鈥 says Gerbier.
Eleanor Fascione and Wolfgang Rau wait for elusive particles to make their way into the SuperCDMS experiment where they are detected through tiny changes in conductivity in one of the world's coldest and most sensitive crystal detectors.
Super-cool crystals to detect rare dark matter collisions
The SuperCDMS Collaboration is upgrading and enhancing a proven technology that鈥檚 been a world leader in narrowing the search for spin-independent dark matter interactions over the past decades. Wolfgang Rau and his colleagues at Queen鈥檚 are playing a key role in testing and installing the new SuperCDMS cryogenic detector system at SNOLAB.
This next-generation experiment is made of stacks of crystalline germanium detectors, shaped like oversized hockey pucks, and cooled to temperatures very close to absolute zero. The sensors are cooled to such low temperatures to allow detection of very small energies deposited by the collisions of possible dark matter particles with the semiconductor crystals. If a possible dark matter particle hits a germanium or silicon atom inside these crystals, it will cause the crystal lattice to vibrate, which corresponds to a tiny temperature increase. Sophisticated electronics and superconducting thin films are used to measure and analyze the particle interactions, and to help distinguish between background and a potential signal from dark matter particles.
The new detectors will have greatly improved capabilities to hunt for possible dark matter particles with masses less than 10 times the mass of a proton. 鈥淥ur new detectors have the right technology to search for very light dark matter particles. If we can detect WIMPs, we鈥檒l solve an 80-year-old dark matter mystery and make a first big experimental step towards developing a new theory of particle physics,鈥 says Wolfgang Rau, leader of the Canadian research teams in the SuperCDMS collaboration.
