Sensitive dark matter detector comes to life

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.  

What is LUX-ZEPLIN? 

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.  

The international project led by the Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) has now announced the detector is running as hoped after years of careful set-up. 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. 

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 the universities of Bristol, Edinburgh, Imperial, 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. The UK ZEPLIN groups then joined the LUX experiment in the USA in 2012 and started designing LZ around that time. 

Professor Alex Murphy who leads Edinburgh’s group on the project said:  

“It’s great to see all these years of development, from ZEPLIN through LUX and now LZ come together. It’s been a huge team effort”  

Hunting the dark matter 

LZ is the largest and most sensitive experiment searching for dark matter particles.  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. These first results place strong constraints on one possibility for what dark matter might be, Weakly Interacting Massive Particles (WIMPs). WIMPs would be 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 dark matter particle interacts with a xenon atom, and produces even a tiny amount of light, the sensors will capture it. In order to see these rare interactions, the team have had to carefully remove all natural background radiation from the detector materials first. 

By operating around a mile underground, LZ is shielded from the cosmic rays that bombard experiments located 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. 

Prof Murphy commented: “With LZ running so well, we should be able to really test the WIMP model for dark matter. There are many other possibilities for what dark matter might be too, several of which are a real focus for the Edinburgh team. It’s a really exciting time for the field.” 


Title: LZ Surface Lab  

Credit: Matthew Kapust, Sanford Underground Research Facility