The most powerful dark matter detector in the world delivers its first results

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LUX-ZEPLIN (LZ) refers to a physics experiment that brings together nearly 250 scientists from 35 institutes in the United States, the United Kingdom, Portugal, and Korea. It is located approximately one mile underground at the Sanford Underground Research Facility in South Dakota. This is a second generation experiment to detect dark matter. This detector, now considered to be the most sensitive in the world, presented its first scientific results a few days ago.

Dark matter is a hypothetical type of matter that would make up almost 27% of the energy density of our universe; it would therefore be much more abundant than ordinary substance. Its existence is essentially manifested by gravitational effects in galaxies or galaxy clusters; it is also the cause of the fluctuations observed in the radiation from the cosmic microwave background. Yet, despite decades of research, dark matter particles remain elusive, and their nature therefore remains a mystery.

One thing is for sure, they interact very little with ordinary substance, which is why they are so hard to detect. Researchers are nonetheless trying to build increasingly sensitive detectors. The LZ detector combines the best technologies from two previous experiments: LUX (Large underground Xenon) and ZEPLIN (ZonEd Proportional scintillation in liquefied natural gas). These were not able to highlight the dark matter, but their extreme sensitivity made it possible to rule out several hypotheses. The LZ was designed to improve this sensitivity even more, by a factor of 50 or more.

A detector isolated from any parasitic radiation

The LUX-ZEPLIN experiment is led by Lawrence Berkeley National Lab. After several years of design and construction, the team engaged in a series of tests for more than three months: they now claim that this detector is the most sensitive in the world. If dark matter actually consists of weakly interacting solid particles (WIMPs) – as the theory suggests – LZ can probably achieve an initial detection in the coming years. ” The LZ team now has the most ambitious instrument at hand to achieve this said Nathalie Palanque-Delabrouille, director of the Berkeley Labs physics department.

Schematic of the LZ detector showing the central xenon detector inside the titanium cryostat, the outer detector acrylic reservoirs containing a liquid scintillator, all in a reservoir of ultrapure water. © Imperial College London

The plant consists of two interlocking titanium tanks, filled with 10 tons of very pure liquid xenon and observed by two arrays of photomultiplier tubes (PMTs) – very sensitive photon detectors. The tanks are also immersed in a tub of purified water and lie deep underground, to protect them from cosmic radiation or any other “parasitic” radiation that can mask signals from dark matter. Although dark matter particles interact very little with baryonic matter, it is assumed that the probability of an interaction is not zero: it may be that a particle collides with a xenon atom.

However, liquid xenon emits a flash of light when struck by a particle – a flash of light that will be immediately detected by PMTs. A WIMP should, in theory, produce the same effect: the researchers would then record a first signal because of this scintillation photon. In addition, the affected atom would again collide with neighboring atoms and emit electrons as it passes. These electrons would then be directed at the surface of the liquid by an electric field; arrived at the surface where they would encounter a thin layer of gaseous xenon, they would produce a new scintillation.

The best hope of discovering dark matter

Thus, each collision will cause two consecutive light signals, detected by the ultra-sensitive PMTs located around the detector; the analysis of their properties will make it possible to characterize the interaction, in particular its exact location and the type of particle involved.

During a three-month test, the team was unable to collect enough data to detect dark matter. Nevertheless, these preliminary experiments make it possible to confirm with certainty that this device is the most sensitive ever built. It is now ready to go, and it is very possible that in the coming months or years it will bring the first evidence of the existence of this exotic material. ” We plan to collect about 20 times more data in the coming years, so we’re just getting started. There is a lot of science to do and it is very exciting said Hugh Lippincott, spokesman for the LZ collaboration.

This is a first victory for the scientists who worked on this project: the installation is very complex and they now notice that all these components work perfectly. ” Given that we first started it a few months ago, and under the COVID-19 limits, it’s impressive that we already have such significant results. says Aaron Manalaysay of Berkeley Lab, LZ Physics Coordinator.

After confirming that LZ and its systems are working properly, the team looks forward to launching large-scale observations in the hope that a dark matter particle will very soon collide with a xenon atom. And maybe we will finally solve the mystery of the universe’s “missing matter”.

Source: LUX-ZEPLIN collaboration

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