Research: The nature of dark matter is more elusive than ever

The LUX-ZEPLIN (LZ) dark matter experiment, based at the Sanford Underground Research Center in South Dakota, USA, has analyzed extensive data that has provided unprecedented insights into one of the main candidates for dark matter, known as weakly interacting particles. which is also called them, has analyzed. WIMPs

The findings, presented Monday at the 2024 TeV Particle Astrophysics conference in Chicago, Illinois, and the 2024 LIDINE conference in Sao Paulo, Brazil, are about five times more sensitive than previous research and show that WIMPs rarely interact with ordinary matter, making it confirms Dark matter is difficult to track.

The LZ project is led by the Lawrence Berkeley National Laboratory (Berkeley Lab) of the US Department of Energy (DOE). University of Bristol physicists are part of an international effort involving more than 250 researchers from the US, UK, Australia, Portugal, South Korea and Switzerland. It is the world’s largest and most sensitive experiment to search for dark matter particles, especially WIMPs.

Henning Fletcher, professor of physics and principal investigator of the Bristol group, said: “The results are a significant improvement over previous searches for WIMP dark matter. We have seen the range of masses that dark matter can have and the strength of its interaction with normal matter. But so far the task remains elusive, the search for dark matter is definitely a marathon rather than a sprint, and given that LZ still collects roughly three times as much data as used for these results.

LZ found no evidence of WIMPs above 9 GeV/c mass²where 1 GeV/c² It roughly corresponds to the mass of a hydrogen atom.

The experiment now needs to continue for up to 1,000 days to realize its full sensitivity. This preliminary result is just a fraction of the exposure that validates a decade of design and construction efforts.

Supported in the UK by UKRI’s UKRI Science and Technology Facilities Council, LZ is sophisticated and innovatively designed to find direct evidence of dark matter – the mysterious invisible substance thought to make up most of the mass of the universe. Dark matter is particularly challenging to detect because it does not emit or absorb light or any other form of radiation.

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

These theorized fundamental particles interact gravitationally, which confirms the existence of dark matter in the first place, and possibly through a new weak interaction.

This means that WIMPs are expected to collide with ordinary matter – albeit rarely and very faintly. This is why very quiet and very sensitive particle detectors are needed to detect WIMPs.

At the center of the experiment is a large detector of liquid xenon particles, which is kept at about -110^oC, surrounded by photo sensors. If a WIMP interacts with a xenon atom, a small amount of light should be emitted and the sensors will absorb it. But to observe these rare interactions, the team first had to carefully remove as much background radiation as possible from the detector material.

But that’s not enough and explains why the LZ works about a mile underground. This protects it from the cosmic rays that bombard the experiments on Earth’s surface. The detector and its cryostat are located inside a large water tank to protect the experiment from particles and radiation entering the laboratory walls.

Researchers at the University of Bristol are playing a key role in the experiment, with Christopher Wright and Nathan Panifer working 1,500 meters underground in South Dakota as part of their PhDs and involved in setting up and maintaining the Outer Detector (OD). This LZ component is used to suppress signals from neutrons and gamma rays, backgrounds that can mimic dark matter interactions. Based on data collected with OD, Sam Erickson, a senior research associate, led the development of the neutron veto and its performance measurements for the current analysis.

Professor Fletcher added: “For the search for dark matter, it is vital to suppress any sources of background radiation, particularly neutrons and gamma rays. The Veto LZs detectors enable us to rule out such processes and to be sensitive to interactions. It is very rare to find dark matter sensitivity.

Finally, LZ ensured that the liquid xenon itself was as pure as possible by carefully removing a key contaminant through a complex multi-year process. Integrated coordination.