The LUX-ZEPLIN Dark Matter Experiment (LZ) is delivering its first results, moving closer to unlocking one of the biggest mysteries of the Universe.

Based about a mile underground in South Dakota, USA, the highly sensitive experiment has taken nearly a decade to set up.

Dark matter – thought to make up around 85% of the matter of the Universe – is particularly challenging to detect, as it does not emit or absorb light or any other form of known radiation.

Since LZ’s 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. Professor Henrique Araújo

So far, none has been directly detected, though physicists know it must exist in some form because of its gravitational effects on the behaviour of galaxies and other astrophysical phenomena.

The LZ detector, which has significant Imperial involvement, will try to capture the very rare and very faint interactions between dark matter particles and xenon atoms in its seven tonnes of liquid xenon. 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.

Today, 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 work to set it up, and is already ahead in the hunt for dark matter interactions.

It 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.

How does it work?

At the centre of the experiment is a large liquid xenon particle detector maintained at -100^oC by a cryostat chamber, surrounded by photo-sensors. If a dark matter particle 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 of gas chromatography.

Hunting dark matter

LZ is based at the Sanford Underground Research Facility in South Dakota, US. Imperial’s Professor Henrique Araújo is the UK lead and he co-led 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 LZ’s 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.”

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. After the successful start, full-scale observations can begin with the hope of finding the first direct evidence of dark matter.

LZ is searching for hypothetical dark matter particles called 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.

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.

“We look forward to seeing more exciting results with an anticipation of a significant discovery awaiting us in the years to come.”

The legacy of ZEPLIN

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 Imperial and the universities of Bristol, Edinburgh, Liverpool, Oxford, Royal Holloway, Sheffield and UCL and STFC’s Rutherford Appleton Laboratory.

Many of these UK groups came from the ZEPLIN programme, which developed the liquid xenon technology for dark matter searches at STFC’s Boulby Underground Laboratory – one of the very first liquid xenon prototype detectors operated at Imperial from the late 1990s. 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 in 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 – hence the birth of LUX-ZEPLIN.”