LUX-ZEPLIN (LZ) refers to a physics experiment bringing together nearly 250 scientists from 35 institutes in the United States, 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 dark matter detection experiment. This detector, 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 constitute almost 27% of the energy density of our universe; it would therefore be much more abundant than ordinary matter. Its existence is essentially manifested by gravitational effects within galaxies or clusters of galaxies; it is also at the origin of the fluctuations observed in the radiation of the cosmic microwave background. Yet, despite decades of research, dark matter particles remain elusive and their nature therefore remains a mystery.
One thing is certain, they interact very little with ordinary matter, which is why they are so difficult to detect. Scientists are nevertheless trying to build increasingly sensitive detectors. The LZ detector combines the best performing technologies from two previous experiments: LUX (Large Underground Xenon) and ZEPLIN (ZonEd Proportional scintillation in LIquid Noble gas). These were not able to highlight the dark matter, but their extreme sensitivity, however, made it possible to exclude several hypotheses. The LZ was designed to improve this sensitivity even further, by a factor of 50 or more.
A detector isolated from any parasitic radiation
The LUX-ZEPLIN experiment is led by the 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 is indeed made up of weakly interacting massive particles (WIMPs) — as the theory suggests — the LZ could probably achieve a first detection in the coming years. ” The LZ team now has in hand the most ambitious instrument to achieve this said Nathalie Palanque-Delabrouille, director of the Berkeley Lab’s physics division.
The facility consists of two interlocking titanium tanks, filled with 10 tons of very pure liquid xenon and observed by two arrays of photomultiplier tubes (PMTs) — highly sensitive photon detectors. The tanks are further submerged in a vat of purified water and are nestled deep underground, so as to preserve 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 could be that a particle collides with a xenon atom.
However, liquid xenon emits a flash of light when hit by a particle — a flash of light that would be immediately registered by PMTs. A WIMP should in theory produce the same effect: the researchers would then record a first signal due to this scintillation photon. In addition, the impacted atom should in turn collide with neighboring atoms, ejecting electrons as it passes. These electrons would then be directed to 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 detecting dark matter
Each collision will thus cause two successive light signals, detected by the ultra-sensitive PMTs located all 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.
In three months of testing, the team could not gather enough data to detect dark matter. Nevertheless, these preliminary experiments make it possible to affirm with certainty that this device is the most sensitive ever built. It is now ready to go into action and it is quite possible that it will bring in the months or years to come the first proofs 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’s a lot of science to do and it’s very exciting said Hugh Lippincott, spokesperson 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 note that all of these components work perfectly well. ” Considering we only started it a few months ago and during the COVID-19 restrictions, it’s impressive that we already have such significant results. says Aaron Manalaysay of the Berkeley Lab, LZ physics coordinator.
After confirmation that LZ and its systems are working properly, the team is looking forward to starting large-scale observations, in the hope that a dark matter particle will collide with a xenon atom very soon. And maybe we will finally solve the mystery of the “missing matter” of the Universe.