23 October 2017
In laboratories, observatories and colliders around the world, the global hunt is on for dark matter. The UK is at the forefront of the search for this elusive substance.
Thought to be an undiscovered type of particle, dark matter makes up ~25% of the universe – the remaining 75% consists of 70% dark energy and 5% visible matter. Dark matter is powerful, it’s ubiquitous, and it holds the key to a fuller understanding of the universe. Scientists hunt dark matter in three main ways: direct detection, indirect detection and collider searches.
Many experts believe that dark matter is composed of one or more types of exotic particle, frequently referred to as weakly interacting massive particles (WIMPs). Direct detection refers to the hunt for these particles coming from space. Because these elusive particles hardly ever interact with our matter, researchers hope to see their interactions with the atoms contained in massive detectors, deep underground where they are shielded from cosmic rays.
Indirect detection studies explore separate cosmic phenomena that could be linked to the existence of dark matter. Both cosmic and gamma rays may well contain distinctive signatures caused by the annihilation of dark matter particles deep in space; if we can detect these signatures, we’ll have indirect evidence for the existence of dark matter.
Finally, collider experiments seek to uncover the existence of dark matter by producing their own dark matter particles in the laboratory through very energetic collisions. If we can both detect suitable particles from space and understand their properties by creating them in the lab, we’ll have a convincing understanding of dark matter and how it explains what we observe in the universe.
A huge variety of dark matter research is taking place all around the world and the UK has long been a leader in this work. In the late 1980s, UK astronomers Professors Carlos Frenk, Simon White and George Efstathiou, with a US colleague, published a pioneering group of papers on the role of cold dark matter in the formation of the structure of the universe.
This is no different now. With a proud legacy in the field and collaborations with experts around the globe, UK research is world-leading in both its scope and achievements. Here we consider a handful of collaborations that demonstrate the UK’s vital contribution to the international hunt for dark matter.
The LUX-Zeplin experiment is set up to detect WIMPs. Located at the Sanford Underground Research Facility (SURF) in South Dakota, construction of the LZ experiment is expected to complete in April 2020. Once operational, LZ will become the world’s most sensitive WIMP-detecting dark matter experiment.
LZ was born out of an international collaboration that brought together 30 institutes around the world, including the UK, US, Portugal and Russia. The UK arm of the collaboration includes approximately 50 researchers from nine UK institutes and is funded by the Science and Technology Facilities Council.
This pioneering project combines expertise from two of the most significant dark matter experiments to date: the original LUX experiment at SURF and the UK’s Zeplin programme. UK groups were, and still are, very active in LUX and contributed heavily to the world-leading results over the last 4 years. Meanwhile, Zeplin was a series of pioneering experiments that took place at Boulby Underground Laboratory in Cleveland, UK, informing future techniques for dark matter detection.
(Credit: X-ray: NASA/CXC/M.Markevitch et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al. Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.)
The UK’s Boulby Laboratory has an unrivalled legacy in the field of dark matter direct detection, and it continues to press forward with state-of-the-art technologies.
Zeplin was one of the UK’s pioneering dark matter projects, and the first to use a dual-phase xenon target in the search for dark matter. Using xenon as a target material in dark matter detectors makes them more sensitive, and this technique that now dominates the field. Zeplin concluded in 2011, but the expertise has now been transported over to LZ for the next generation of dark matter studies.
The laboratory currently hosts the DRIFT (Directional Recoil Identification from Tracks) programme: a collaboration between the UK and partners in the US. DRIFT focusses on the development and testing of detectors designed to identify the direction of WIMPs.
Located in Ontario, Canada, the SNOLAB underground laboratory is home to multiple pioneering dark matter experiments in which the UK has a considerable stake.
DEAP-3600 is one of the world’s most advanced direct detectors. The DEAP collaboration involves more than 65 researchers from ten institutions in the UK, Canada and Mexico.
The UK developed DEAP-3600’s calibration system, with STFC’s Particle Physics Department playing a pivotal role. The system has already begun operations, and first results are expected in February 2018.
Additionally, the SuperCDMS (Cryogenic Dark Matter Search) SNOLAB collaboration is set to become one of the world’s foremost dark matter experiments for low-mass WIMPs. Expected to come online in the early 2020s, SuperCDMS SNOLAB will use low-temperature solid-state detectors to search for WIMPs.
This collaboration brings together 20 different institutions from Europe and North America, with the UK contribution led by Durham University, whose researchers have supported the development of SuperCDMS by helping to refine experimental setup and data analysis.
Whilst many dark matter experiments are currently focused on direct detection of galactic WIMPs, particle colliders also have a vital role to play in the hunt for this elusive substance. It is possible to ‘make’ potential dark matter in colliders, and this approach to finding dark matter has recently grown in prominence as the technology behind the technique has developed.
One theory which is popular with theorists is Supersymmetry (SUSY) – this could provide a natural explanation of what the dark matter particles are; and it is hoped that these as-yet hypothetical particles could be produced in proton collisions in the LHC.
The Large Hadron Collider is home to two huge detectors currently engaged in the search for dark matter: CMS and ATLAS., both of which are strongly supported by STFC. The CMS collaboration is made up of approximately 200 institutes in 40 countries. The UK played a leading role in the design and construction of the CMS calorimeters, which are essential for looking for the “missing energy” which characterises SUSY particles.
Meanwhile, ATLAS involves a further 182 institutions from 38 countries, 10% of which are UK groups. The UK built and operates several subsystems including the trigger system, which identifies collision events that might contain dark matter particles.
The collider-based and direct detection-based approaches to dark matter complement one another perfectly. Together, these communities can study the problem from different angles, providing a more comprehensive approach to dark matter detection.
The search for dark matter is a joint effort by cosmologists, astrophysicists, theorists, astronomers and particle physicists – all disciplines in which the UK is a world leader.
STFC works closely with the Dark Matter UK Consortium (DM-UK) of research scientists to promote communication and collaboration within the research community and to spearhead campaigns that engage the public with dark matter and the unseen universe. There are approximately 20 institutions in the UK hunting for dark matter and more than 100 scientists are part of DM-UK, which brings together the UK’s dark matter research community to share research, foster collaboration and provide a forum for training the next generation of researchers.
The UK is at the forefront of efforts to find dark matter, collaborating with researchers from all over the world. Dark matter may be elusive, but with the weight of UK research behind it, dark matter is running out of places to hide.