Dark matter particles?

 Dark matter particles?

Photo source: Xenon team

Dark matter is undoubtedly the most mysterious kind of matter in the universe. When studying galaxy clusters and their motions, astronomers noticed that only the gravity of known matter could not constrain their structures. This makes them realize that, in addition to the material forms we are familiar with, there must be some strange substances hidden in the universe that we cant see, and those substances that may belong to the products of the Big Bang are called dark matter. They dont participate in the electromagnetic effect, and they dont react with photons directly, so they cant be directly observed by us.

1400m underground laboratory

In order to search for such particles, physicists in various countries have made many attempts. For example, Chinas Wukong dark matter satellite is looking for abnormal gamma ray signals on the energy band of more than 10 GeV - such signals may come from the collision between wimps. In addition to space probes, another way to find dark matter particles is to build detection devices deep underground. Hundreds or even thousands of thick rock layers can isolate the interference of most cosmic rays and avoid the influence of noise on the experiment as much as possible, while dark matter particles can pass through smoothly.

At 1400 meters underground in Gran Sasso, Italy, there is the most sensitive underground dark matter detector, xenon1t.

Location of Gran Sasso National Laboratory (image source: Wikipedia)

Xenon is xenon (Xe) in chemical elements, and the liquid xenon with rather lazy properties is used in xenon series experiments. As the third generation detector of xenon experiment, xenon 1t is equipped with 3.2 tons of liquid xenon. If wimp enters the device, it will be possible to have an elastic collision with xenon atoms, making xenon have kinetic energy. At this time, the moving xenon atoms collide with other substances and release electrons, which can emit flash. Therefore, the particles colliding with xenon can be inferred by monitoring the specific flash in the device.

Xenon experiment was started in 2006 and experienced two equipment upgrades. But in the search process of more than ten years, the expected signal of xenon team has not appeared. At the same time, no substantial progress has been made in other search experiments around the world. Although it is not enough to sentence wimp to death, these results mean that the possible energy range of wimp is shrinking.

Schematic diagram of experiment process (picture source: ux-zeplin (LZ) collation / slacnarionacceleratorlaboratory)

In the face of the failure of a round of experiments, xenon team continued to carry out more sensitive wimp search experiments, but also had new considerations. Perhaps the real reason why dark matter particles cant be found is that the target of the search is wrong. So they used xenon1t detector to find another dark matter candidate particle, Axion.

From wimp to Axion

The concept of Axion was put forward in 1970s. Its original purpose is to solve the problem of CP (charge parity) asymmetry in strong interaction. In the mid-1980s, theoretical research found that the big bang could produce enough axions to make up dark matter. Since then, axions have become powerful candidates for dark matter.

Compared with wimp, the mass of Axion is much lower, only 1 u03bc EV ~ 1 MeV. Therefore, the detection method suitable for wimp can not be directly copied to the Axion. Because the axions are too light, they hit xenon atoms just like fat mays shaking trees, without any disturbance. However, xenons electrons are suitable for hitting the target - the Axion and the electron can collide elastically, giving the electron kinetic energy, and then generating a detectable flash.

Based on this idea, during 2016-2018, the research team collected the flash in the device on a 24kev energy segment. During more than two years of operation, they saw 285 flashes. However, most of these signals can be explained. The device built in the deep underground can shield most of the interference above the ground, but a small amount of background noise from radioactive lead, krypton and other isotopes can still enter the device and collide with xenon atoms. Fortunately, researchers can distinguish these noise signals. Of the 285 signals, 232 are from background noise. The question is, what are the other 53 signals generated by?

The xenon team offers three possible explanations.

Inside xenon detector (picture source: enricosacchetti / scencesource)

New discovery or pollution signal?

So, is this conclusion strong enough? In physics, only when the confidence reaches 5 u03c3 (in other words, when the probability of signal coming from noise is less than 3.5 million), can it be called discovery. For the explanation of solar Axion, the confidence is 3.5 u03c3. That is to say, the signal comes from noise, that is, the probability of random fluctuation is one in 5000 - although it looks good, its still a long way from declaring new discovery.

What if its not the suns axis? The second possibility proposed by the xenon team is equally exciting: these signals belong to the more magnetic solar neutrinos. In general, solar neutrinos do not generate signals in the device. But if the neutrinos magnetism is stronger than previously expected, it can match the observed signal. If so, what his research team has achieved will be discoveries beyond the standard model! In contrast, the solar neutrino hypothesis has a lower confidence level of 3.2 u03c3, which means that one in 700 signals is likely to come from background noise.

In addition to the two, there is a disappointing possibility that the signal is not a new discovery in physics, but a contamination of the device by radioactive tritium. When the high energy cosmic rays hit the rocks around the device, they will produce neutrons, which will smash the xenon nucleus and produce tritium, which will decay and produce electrons. Before the start of the experiment, the experimenter will try to avoid the pollution as much as possible, but the trace tritium may not be completely removed. The confidence of this hypothesis is the same as that of the solar neutrino hypothesis, which is also 3.2 u03c3. Since xenon1t has stopped running and is ready to upgrade to the next phase of the experiment, scientists can no longer investigate and trace this possibility.

At present, researchers cannot confirm or exclude any of them. However, they still have a chance to find clues from further experiments.

If the signal comes from the sun - whether its the suns Axion or the neutrino - we should be able to see seasonal changes in the signal. This is because in different seasons, the relative position of the sun and the earth will change, and the number of signals received by the instrument should fluctuate. However, the period of this experiment is only two years, and the signal is too weak, so there is no signal of seasonal fluctuation. But as the team upgrades the detector to a more powerful xenont and plans to start a five-year observation experiment, the first exact answer to the dark matter particle may be born here.

Source: global science editor: Zhang Zutao_ NT5054