All systems go for dark matter detector LUX-ZEPLIN

The world’s largest and most sensitive dark matter experiment has come to life and is delivering results, moving a step closer to offering clues about one of the biggest mysteries of the Universe.

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.

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.

Finding direct evidence of dark matter

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. Dark matter is particularly challenging to detect, as it does not emit or absorb light or any other form of radiation.

The LZ detector will try to capture the very rare and very faint interactions between dark matter and its 7-tonne liquid xenon target. To do this, LZ must be carefully and delicately calibrated and any background noise removed so the experiment can be perfectly tuned to observe these interactions.

The international project led by the Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) has announced the detector is running as hoped after years of careful set-up: First dark matter search results from the LUX-ZEPLIN (LZ) experiment.

An unforgiving experiment

Professor Henrique Araújo, the UK lead and co-lead for the development of the LZ Xenon Detector, 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… Well, it looks like we did get it right.'

LZ involves an international team of 250 scientists and engineers from 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 Bristol, Edinburgh, Imperial, Liverpool, Royal Holloway, Sheffield, UCL, STFC’s Rutherford Appleton Laboratory, and the University of Oxford - where the group is led by Professors Hans Kraus and Kimberly Palladino. Physicists at Oxford were involved in the instrumentation, analysis and simulations.

‘Our contributions to LZ draw on our many years of experience delivering high-precision, low-background devices for dark matter detectors,’ comments Professor Kraus. ‘When it came to developing the instrumentation, former graduate students Junsong Lin, Kathryn Boast, FengTing Liao, Theresa Fruth, Cees Carels and Andrew Stevens played a key role in ensuring that, when it started up, the detector would run flawlessly.’

Producing world-leading limits

Professor Palladino continues: ‘Here at Oxford, we are incredibly proud to be part of the LZ collaboration. Amy Cottle led the backgrounds group whose analysis was truly essential for these world-leading limits to be produced. Graduate students Sparshita Dey, Dan Hunt, Josh Green, Niamh Fearon and Aiham Al Musalhi all helped in the collection and analysis of these first data.’

The tests show that LZ already is the world’s most sensitive dark matter detector, with plans to collect about 20 times more data in the coming years. The take home message from this successful startup: ‘We’re ready and everything’s looking good,’ said past LZ Spokesperson Kevin Lesko, currently on sabbatical in Oxford from Lawrence Berkeley National Lab in the US. ‘It’s a complex detector with many parts to it and they are all functioning well within expectations.’
 
After the successful start, full-scale observations can begin with the hope of finding the first direct evidence of dark matter.

The legacy of ZEPLIN

Many UK groups came from the ZEPLIN programme, which developed the liquid xenon technology for dark matter searches at STFC’s Boulby Underground Laboratory. The UK ZEPLIN groups then joined the LUX experiment in the USA in 2012 and started designing LZ around that time.

Professor Araújo said: ‘It’s been a privilege to work on such a fantastic project, full of talented and fun people. The pioneering ZEPLIN programme lives on as the “Z” in “LZ” – but we needed a much larger international team to achieve something on this scale.’

LZ Spokesperson Professor Hugh Lippincott, from the University of California Santa Barbara, echoes the comment from Araújo: ‘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 – hence 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 -100 degree Celsius, 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.

A global endeavour

Many complex systems had to come together for LZ to work, and this first result shows they are performing in harmony seamlessly. STFC’s Dark Matter Group Leader Pawel Majewski said: ‘It is gratifying to see the LZ experiment delivering its first scientific results. For the Rutherford Appleton Laboratory, one of the LZ founding members, it has been a privilege to be part of this wonderful endeavour bringing together scientists and engineers from all over the world. We look forward to seeing more exciting results with an anticipation of a significant discovery awaiting us in the years to come.’