The world’s most powerful dark matter detector delivers its first results

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LUX-ZEPLIN (LZ) refers to a physics experiment that brought 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 discover dark matter. This reagent, which is now considered 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 roughly 27% of the energy density of our universe. So it will be more abundant than the ordinary substance. Its existence is mainly manifested by gravitational effects within galaxies or galaxy clusters. It is also the origin of the fluctuations observed in the cosmic microwave background radiation. However, despite decades of research, dark matter particles remain elusive, and thus their nature remains mysterious.

One thing is for sure, they interact very little with ordinary matter, which is why they are so difficult to detect. However, scientists are trying to build increasingly sensitive detectors. The LZ detector combines the best performing technologies from two previous experiments: LUX (big xenon underground) and ZEPLIN (Relative luminescence of the region in a liquid noble gas). These were not able to highlight dark matter, but their extreme sensitivity made it possible to rule out several hypotheses. The LZ is designed to improve this sensitivity even further, by a factor of 50 or more.

Detector isolated from any parasitic radiation

Lawrence Berkeley National Laboratory leads the LUX-ZEPLIN trial. After several years of design and construction, the team conducted a series of tests for more than three months: they now claim this detector is the most sensitive in the world. If dark matter really consists of weakly interacting massive particles (WIMPs) – as the theory suggests – then LZ will likely make its first detection in the coming years. ” The LZ team now has the most ambitious tool to make it happen said Natalie Blanc Delabruille, director of the physics department at Berkeley Lab.

A schematic diagram of the LZ detector showing the central xenon detector inside the titanium quencher, and the external detector’s acrylic tanks containing liquid flash, all inside a tank of high purity water. © Imperial College London

The facility consists of two interlocking titanium tanks, filled with 10 tons of ultra-pure liquid xenon and observed through two arrays of photomultiplier tubes (PMTs) – highly sensitive photon detectors. The tanks are further submerged in a bowl of pure water and located deep in the earth, in order to keep them from cosmic radiation, or any other “parasitic” radiation that might mask dark matter signals. Even if dark matter particles interact very little with baryonic matter, it is assumed that the probability of an interaction is not zero: it may be due to the collision of a xenon atom particle.

However, liquid xenon emits a flash of light when it hits a particle – a flash of light that can be instantly recorded by PMTs. WIMP should theoretically produce the same effect: the researchers would then record the first signal due to this shimmering photon. In addition, the affected atom, in turn, must collide with neighboring atoms, ejecting electrons as they pass. These electrons are then directed to the surface of the liquid by an electric field; When they reach the surface where they will encounter a thin layer of gaseous xenon, they will produce a new flash.

Best hope for detecting dark matter

Thus each collision will cause two consecutive optical signals, which are detected by the ultra-sensitive PMTs located throughout the detector; Analysis of their properties will allow characterization of the reaction, in particular its precise location and the type of particle involved.

During the three months of testing, the team was unable to gather enough data to detect dark matter. However, these preliminary experiments make it possible to confirm with certainty that this device is the most sensitive of all. Now it’s ready to go and it’s quite possible that the coming months or years will bring the first evidence of this exotic material. ” We plan to collect about 20 times as much data in the coming years, so we’re just getting started. There is a lot of science to do which is very exciting said Hugh Lippincott, a spokesperson for LZ Collaboration.

This is the first victory for the scientists who worked on this project: the installation is very complicated and they now notice that all of these components are working fine. ” Given that we only started this a few months ago and during the COVID-19 restrictions, it’s impressive that we’ve already had such significant results. says Aaron Manalisay of Berkeley Lab, LZ’s physics coordinator.

After confirming that the LZ and its systems are working properly, the team is looking to begin large-scale observations, with the hope that a dark matter particle will collide with a xenon atom soon. And maybe we can finally solve the mystery of the “missing matter” of the universe.

Source: LUX-ZEPLIN collaboration

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