Dark exploration: Are scientists looking in the wrong direction?

Dark matter is thought to be the most abundant form of matter in the Universe, with up to five times the mass of the normal matter found in stars and galaxies. There is only one problem: Despite 50 years of efforts, scientists have found only indirect evidence that it exists. Now, a new measurement from the most accurate center suggests that scientists may be looking in the wrong direction.

Something is not right in the Universe. The wheels move so fast and spin so fast that they can be explained by the observed matter of the universe and the accepted laws of physics. The most popular explanation for these strange observations is that the Universe is filled with many invisible – and undetectable – types of matter. This matter, commonly called dark matter, can interact gravitationally with ordinary forms of matter, but dark matter neither emits nor absorbs light.

Although many types of dark matter candidates have been proposed and rejected, since the late 1990s the scientific community has established a popular dark matter candidate called the WIMP (Weakly Interacting Massive Particle). If WIMPs are real, the consensus is that they are stable subatomic particles, electrically neutral, with masses of about 100 to 100,000 GeV. (A GeV is a unit of energy related to mass by Einstein’s equation E = mc². For context, a proton has a mass of about 1 GeV.)

According to popular theory, WIMPs are invisible, even though they use gravity. Unlike normal matter, which coalesces to form stars and planets, dark matter WIMPs are organized as giant clouds of “gas” that orbit and penetrate galaxies, including our own Milky Way. . If such clouds exist, dark matter gas can be found in the Sun’s core and form a “WIMP wind” that constantly passes through Earth. Depending on the weight of the WIMP particles, if you raise your fist, there is probably at least one WIMP particle in the book.

Scientists have searched for this WIMP wind without success, despite a series of detectors that are still sensitive. Since the mid-1980s, detectors have increased their sensitivity a million times. One of the current, world-class detectors is called LUX-Zeplin (LZ). It uses 10 tons of xenon water, cooled to about -150°F (-100°C), and is located about 4850 feet underground (1478 meters) in the Sanford area Underground Research Facility (SURF) in an abandoned gold mine.

A series of photomultiplier tubes designed to detect signals from particle interactions occurring in the LZ liquid xenon detector. (Credit: Matthew Kapust/Sanford Underground Research Facility)

This depth protects the loudspeaker from the constant cosmic rays from space, which can mimic dark signals. Only seven of the 10 tons of xenon in the center of the detector are used in the following analyses; cases where signs of three tons of liquid xenon forming a shell around the interior are seen are rejected. This ensures that any potential WIMP tests are properly calibrated.

The LZ experiment waits for a WIMP particle to pass through a detector and hit a xenon atom. The prediction is that the debris from the collision will pass through the equipment and leave a detectable signal. After running for 280 days (out of the expected 1,000), the LZ scientists did not detect a WIMP pass through their detector. They recently reported their results at the TeVPA and LIDINE conferences. A scientific paper is being prepared, although there is some caution because the published results have not been peer-reviewed. (Premature release of measurements is common for large trials such as LZ, and it is rare for peer review to change the measurement as originally reported.)

The measurement published by LZ shows a five-fold improvement in the WIMP mass index and interaction possibilities are excluded. Although the existence of WIMPs remains a possibility, current measurements make their existence unlikely. There is still one type of crowd that LZ has not been able to control. The analysis they reported does not take into account WIMP abundances below 9 GeV.

Considering the mass favored by most WIMP theories (100 – 100,000 GeV), if dark matter turns out to be that light, it is possible that dark matter is not a WIMP at all. LZ continues to analyze its data. The fact that they avoided masses below 9 GeV is not because they have no data down there; rather, it is that the lower weight tests are more difficult and will require more attention before any results are available.

Future, larger, liquid xenon experiments will be able to make even more sensitive measurements, however, there is a gap in how much WIMP experiments can be improved. You see, the Earth is constantly being showered with particles called neutrinos. These neutrinos originate both from nuclear energy in the sun and from cosmic rays from the atmosphere. Actually, the Earth exists in a neutrino cloud.

Unfortunately, neutrino interactions in WIMP detectors look like dark matter interactions, so while dark matter experiments are easy to detect this neutrino cloud, it is impossible to detect dark matter interactions. darkness. It will be like hearing whispers at a rock concert.
An instrument with about 10 times the sensitivity of LZ can detect neutrino fog. It may take 10 to 15 years to develop detectors with such sensitivity. Once achieved, traditional WIMP searches will no longer be possible.

Does this mean that we have reached the end of the road to search for dark matter? Not at all. After all, the unexpected movement of the constellations needs an explanation. Dark matter may be a form of matter that is not a WIMP. In fact, some scientists are focusing their attention on a person called axion. Axions, like WIMPs, have not yet been detected, but the detection of axions requires very different techniques than WIMP particles. The search for something dark will continue.