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On October 31, the online edition of "New Scientist" reported that the Large Underground Xenon (LUX) experiment, conducted at the Sanford Underground Research Center in the U.S., has released its initial findings. The results indicate that the experiment has ruled out a significant range of possible masses for Weakly Interacting Massive Particles (WIMPs), which were once considered strong candidates for dark matter. This means that the search for dark matter remains ongoing, and the LUX experiment did not confirm any previous claims from other studies.
Since the 1930s, scientists have been puzzled by the existence of dark matter, an invisible substance that makes up most of the universe’s mass. Despite extensive research, it remains one of the greatest mysteries in modern physics. Currently, two main approaches are used to detect dark matter: sending instruments into space or conducting experiments deep underground. Underground labs are ideal because they shield detectors from cosmic rays, which can interfere with sensitive measurements.
However, many underground experiments have reported conflicting results about dark matter particles. Earlier this year, "Nature" magazine highlighted the potential of the LUX experiment at the Sanford Underground Research Center in South Dakota to either support or refute previous findings. It is expected to help scientists gain deeper insights into the nature of the early universe.
The LUX experiment uses a 300-kilogram tank of liquid xenon as its detection tool. Xenon is a heavy, cold element, three times denser than water. The lab is located nearly 1,500 meters below the surface, within a former gold mine, and will eventually be more than 2.4 kilometers deep. The experiment was designed to test a highly anticipated dark matter signal.
This signal came from the Super-CDMS experiment in Minnesota, which recently detected three possible dark matter events that were much lighter than previously thought. While the probability of these being WIMPs was 99.81%, it fell short of the 5-sigma threshold required for a discovery. Still, it was seen as one of the most promising leads. However, after three months of searching, the LUX team announced their first results: while the detector performed as expected, no clear evidence of dark matter was found. Most of the signals observed were likely background noise.
Richard Gaitskell, a member of the LUX team, expressed concerns that lightweight WIMP models may not hold up. However, he emphasized that the experiment still has a bright future, as it can search for a broader range of dark matter candidates, possibly uncovering heavier forms of the elusive substance. Another researcher, Dan McKinsey, described LUX as a powerful tool capable of scanning a wide variety of dark matter possibilities.
Today, multiple dark matter experiments are underway globally, including the Alpha Magnetic Spectrometer (AMS) project, Italy’s XENON1T detector, and the Super-CDMS. Recently, Ding Yizhong's team announced the first results from the AMS, which collected over 400,000 positrons with an error margin of just 1%. The data suggests that the positron ratio in cosmic rays aligns with predictions from dark matter theories, offering another intriguing clue in the ongoing quest to understand the universe’s hidden component.
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