University of Sheffield: Unlocking a mystery of the Universe: sensitive dark matter detector comes to life
Scientists at the University of Sheffield have partnered in an international research project to search for invisible dark matter particles.
The LUX-ZEPLIN Dark Matter Experiment (LZ), based at the Sanford Underground Research Facility in South Dakota, US, has gathered its first result – showing the experiment is successfully operating as designed, and offering an insight into the dark matter in the Universe.
LZ is intricately and innovatively designed to find direct evidence of dark matter – a mysterious invisible substance thought to make up most of the mass of the Universe.
Professor Vitaly Kudryavtsev, the Principle Investigator at the University of Sheffield, said: ‘This project is really exciting in the world of physics. Dark matter particles are particularly challenging to detect, as they do not emit or absorb light or any other form of radiation.
“The LZ detector aims to capture the very rare and very faint interactions between dark matter particles and xenon atoms in its 7-tonne liquid xenon target. To do this, LZ must be accurately calibrated and any background noise removed so the experiment can be perfectly tuned to observe these interactions.
“The experiment now needs to run for up to 1,000 days to realise its full sensitivity. This initial result is just a fraction of that exposure, which validates the decade-long design and construction effort.”
The international project, led by the Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab), has announced that the detector is running as hoped after years of careful set-up, and the first data are very exciting.
Professor Henrique Araújo, from Imperial College London, is the UK lead and co-lead for the development of the LZ xenon detector. He said: “An experiment of the scale and sensitivity of LZ can be unforgiving: the smallest design flaw may compromise the whole enterprise. And since the LZ cryostat cannot be opened underground, we needed to make sure we got it right the first time, much like if we were to launch LZ into space. It looks like we did get it right.”
The tests show that LZ is the world’s most sensitive dark matter detector, and there are plans to collect about 20 times more data in the coming years.
After the successful start, full-scale observations can begin with the hope of finding the first direct evidence of dark matter.
The research team
The LZ project involves an international team of 250 scientists and engineers from over 35 institutions from the US, UK, Portugal, and South Korea. The UK team, funded by the Science and Technology Facilities Council (STFC), consists of more than 50 people from the universities of Sheffield, Bristol, Edinburgh, Imperial, Liverpool, Oxford, Royal Holloway, and UCL and STFC’s Rutherford Appleton Laboratory.
Professor Araújo said: “It’s been a privilege to work in such a fantastic project, full of talented and fun people.”
Professor Dan Tovey from the University of Sheffield, who is Chair of the LZ-UK Institute Board, commented: “This is pioneering research, and we at Sheffield are proud to be part of this ground-breaking international team.”
He adds: “We are trying to understand what makes up most of the matter in the universe. As a physicist it doesn’t get more exciting than that.
“We look forward to seeing more exciting results, and there is real anticipation of a significant discovery awaiting us in the years to come.”
LZ Spokesperson Professor Hugh Lippincott, from the University of California Santa Barbara (USA), echoes these comments: “Working across such large distances is always challenging, especially during Covid when we were unable to gather in person. We probably had a few too many Zoom calls, but it was great to see how well people were able to work together, with work passing back-and-forth across the ocean at the beginning and end of each day.”
Hunting the dark matter
LZ is the largest and most sensitive experiment searching for dark matter particles, in particular Weakly Interacting Massive Particles (WIMPs).
These theorised elementary particles interact with gravity – which is how we know about the existence of dark matter in the first place – and possibly through a new weak interaction too.
This means WIMPs are expected to collide with ordinary matter – albeit very rarely and very faintly. This is why very quiet and very sensitive particle detectors are needed for WIMP detection.
How does it work?
At the centre of the experiment is a large liquid xenon particle detector maintained at -100oC, surrounded by photo-sensors. If a WIMP interacts with a xenon atom, and produces even a tiny amount of light, the sensors will capture it.
But in order to see these rare interactions, the team had to carefully remove all natural background radiation from the detector materials first.
But this is not enough – which is why LZ is operating around a mile underground. This shields it from cosmic rays, which bombard experiments at the surface of the earth. The detector and its cryostat sit inside a huge water tank to protect the experiment from particles and radiation coming from the laboratory walls.
Finally, the team made sure that the liquid xenon itself is as pure as possible by carefully removing a key contaminant through a complex years-long process.
Many complex systems had to come together for LZ to work, and this first results shows they are performing in harmony seamlessly.
More information about the LZ international collaboration:
LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. LZ is also supported by U.S. National Science Foundation; the Science & Technology Facilities Council of the United Kingdom; the Portuguese Foundation for Science and Technology; the Institute for Basic Science, Korea; and the LZ collaborating institutions. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.
More information on UK deliverables:
UK deliverables to LZ included the ultralow background titanium cryostat, various contributions to the central xenon detector, two calibration sub-systems, one of the two international Data Centres and a significant contribution to the development of software for modelling and data analysis.
A significant fraction of the very large number of radioassays needed to identify the best materials was conducted in the UK (Boulby, UCL).
The Sheffield team has contributed to software development, simulations and data analysis, and played a major part in modelling the backgrounds to a dark matter signal.