The hunt for dark matter

Bottom-up view from the inside of the Large underground xenon experiment with dark matter

Sorry for poor physicists looking for dark matter - an exotic substance, of which about a quarter of all matter in space consists, interacting with the rest of the Universe only through gravity and weak interaction. And a week does not pass without a new hint of dark matter teasing the physicists, having arisen on the border of statistical error, and then disappearing, breaking their hopes.

To search for dark matter put a huge number of experiments, a whole letter soup of abbreviations, and each uses its own technique and technology. So physicists have to look for something, the exact properties of which are unknown to them. The problem is that although possible hints of dark matter were found in several experiments, they do not agree with each other. If you put the results of different experiments in different colors on one chart, it will look like abstract art.

6 years ago, Juan Kolar of the University of Chicago was full of hopes for the early detection of dark matter. But each subsequent result seemed to point in a new direction. It is not surprising that he begins his report, slightly paraphrasing the “Greater Lebowski”: “We are nihilists, we do not believe in anything.”
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“For the past few years, it seems that we have been chasing our own tail,” Kolar said in an interview.

The good news is that maybe something is slandering again. Physicists see signs in the sky and deep under the earth, and look for other signs in the Large Hadron Collider, which also participates in the hunt for dark matter. The whisper of dark matter becomes louder, and several signals seem to be beginning to converge. The bad news is that these hints are still not consistent, and each of them is too unreliable, as Kathryn Zurek of the University of Michigan says. Many physicists are skeptical that signs of dark matter can be found at all. Some people are generally addicted to nihilism, like Kolar, who said: “It’s hard not to be a nihilist, considering how events develop.”

Mysterious matter

The usual visible matter - planets, stars, galaxies, everything else - is only 4.9% of all that is in the universe. Most of it, 68.3%, consists of dark energy responsible for the accelerating expansion of space. The rest - 26.8% - consists of dark matter.

If physicists do not know exactly what dark matter is, then they are sure of its existence. The concept originated in 1933, when Fritz Zwicky analyzed the speeds of galaxies in one cluster and came to the conclusion that the gravitational attraction exerted by visible matter cannot keep galaxies moving at high speeds from fleeing from the cluster. Decades later, Vera Rubin and Kent Ford found another piece of evidence of "dark matter" Zwicky, watching the stars revolve on the edge of galaxies. The stars had to move more slowly, the farther they are from the center of galaxies, just as the outer planets of our solar system move more slowly around the sun. Instead, the external stars moved as fast as the stars that were closer to the center, but the galaxies did not disintegrate. Something supplemented gravitational attraction.

Dark matter was not the only explanation. Perhaps it was necessary to correct the Einstein model of gravity. Many alternative models have been proposed, such as MOND (modified Newtonian dynamics). Rubin herself once tended to this, and said in an interview with New Scientist in 2005 , that "this was a more attractive option than the Universe, filled with a new type of subnuclear particles."


The total mass of the galaxies of the Bullet cluster is much smaller than the mass of two cluster clouds consisting of hot X-ray gas (marked in red). Blue areas, even more massive than all the galaxies and clouds together, show the distribution of dark matter

But nature is our aesthetic preferences. In 2006, the striking image of the Bullet Cluster (1E 0657-56) put an end to this question. It was visible two clusters of galaxies, passing through each other, and their gases, colliding, created a shock wave in the form of a bullet. The results of the analysis turned out to be surprising: hot gas (ordinary matter) accumulated in more dense formations in the center, where a collision took place, and on the other hand, something that could only be dark matter accumulated. In the collision of clusters, dark matter has passed through, because it very rarely interacts with ordinary matter.

“I think that at this stage we can be confident in the existence of dark matter,” says Dan Hooper, a physicist at the University of Chicago. "As far as I know, no modified theory of gravity explains this."

One leading candidate for dark matter particles is a class of weakly interacting massive particles, WIMP, similar to another subatomic particle, the neutrino, which also rarely interacts with other matter. After the discovery of the Higgs boson , one era of particle physics ended, and public attention moved to a new major discovery. Cosmologist Michael Turner from the University of Chicago said that he considers this decade a decade of WIMP .

Signal / noise

Most theorists initially tended to the variant with heavy WIMP, and believed that dark matter consists of particles with a mass of about 100 GeV. The masses of subatomic particles are measured in units of mass-energy, electron volts. For example, the mass of a proton is 1 GeV. But the latest evidence seems to support the version of light particles, in which their mass is in the range from 7 to 10 GeV. Because of this, registering them directly is difficult, since many experiments rely on measuring the recoil of the nucleus.

Such experiments are usually carried out deep underground - in order to better filter out cosmic rays, which can easily be confused with dark matter signals. They involved a detector with a carefully selected target material, for example, germanium or silicon crystals, or liquid xenon. Then physicists are waiting for rare cases of collision of dark matter particles and the nuclei of atoms of the target material. This should lead to the appearance of flashes of light, and if they are bright enough, they will be recorded by the detector.

This means that in order to detect a particle of dark matter, it must carry enough energy in order to produce a signal in a collision with the core that exceeds the threshold of sensitivity of the detector. And lightweight WIMPs are less likely to do this. Neil Weiner of New York University says that the difference in WIMP scenarios is the same as the collision between two bowling balls and a ping-pong ball with a bowling ball. “A kinetic heavy particle is much easier to transfer such energy than a light one,” he says.

How do physicists search for dark matter? Look at the bursts in the collected data detectors. The strength of a signal is determined by the number of standard statistical deviations, or sigma, from the expected background value. This metric is often compared to a coin that falls several times in a row. The result of three sigmas is already a serious hint, equivalent to a coin falling out by one side nine times in a row.

Many of these signals weaken or disappear, moving into the category of statistically less important with the advent of new data. The gold standard of discovery is five sigma , the equivalent of a 21 decay in a row. If several people at the same time throw up coins, and everyone falls heads several times in a row - or several experiments find a signal of three sigma in one mass gap - even an unlikely result becomes possible.

Some of the hints of dark matter are in the tricky area of ​​2.8 sigma. “All these promising results could be rejected in a week,” said Matthew Buckley of the National Accelerator Laboratory. Enrico Fermi (Fermilab). “But such things always begin with hints.” When you collect more data, the hint becomes statistically more significant. ”

Background noise complicates the task. “You are looking for a signal.” “Background” is all the rest that reminds your signal and makes it difficult to find it, ”wrote Matthew Strausler, a physicist at Rutgers University, on a blog in July 2011 . Later he added: “If you do not take into account the small background, it usually comes out in the form of additional low-energy collisions that will very much resemble light WIMPs. In other words, light dark matter looks just like an erroneous signal. ”

Strasler compared the task with an attempt to find a group of people in a room filled with people . If your friends wear the same bright red jackets, and all the rest wear clothes of other colors, it will be easy to find the signal. If other people wear bright red jackets too, random strangers will hide the signal. Imagine that you incorrectly estimated the number of people in red jackets, or even that you are color blind. In any of these cases, you will make the wrong conclusion: that you have found your friends, when in fact the signal will be a random gathering of strangers.

Evidence for today

Despite these challenges, various experiments have led to some promising, though contradictory, results. More than ten years ago, the DAMA / LIBRA experiment (search for dark matter using a potassium iodide detector with the addition of thallium), located in the depths of the Gran Sasso d'Italia mountain in central Italy, found small fluctuations in the number of collisions per year. A group of scientists claimed to have discovered a dark matter particle in the form of light WIMPs weighing about 10 GeV.


DAMA / LIBRA

Other physicists expressed serious doubts. Although DAMA / LIBRA did have a signal, it could be evidence of something else. The fact that in another experiment, XENON10 , located in the depths of the same mountain, did not manage to detect a signal in the same energy gap, did not help either. The same thing happened with the CDMSII experiment in a deep mine in Sudan, Minnesota. Both recent experiments were sensitive enough to detect a signal of such energy, if the result of DAMA / LIBRA really belonged to dark energy.

Another experiment, CRESST , fixed the signal. But it did not fully correspond to the signal from DAMA / LIBRA, and its analysis could not take into account all possible background noises that could emulate the desired signal. In addition, DAMA / LIBRA irritated scientists by refusing to share the data with the public so that others could study them.

When discussing the differences between experiments, passions often boil. "It happens that you make a report about dark matter, and everything ends in a fight," says Buckley.

But the result of the Italian group of scientists was fairly stable. Kolar, along with other ardent critics, decided to prove the fallacy of the discoveries of DAMA / LIBRA by organizing his experiment, called CoGeNT . In 2011, this plan collapsed, as a preliminary analysis of CoGeNT data confirmed the results.

“We built CoGeNT with the intention of exposing DAMA, and now we are suddenly stuck in the same parameter space,” says Kolar. However, due to the fire in the mine of Sudan, in which the experiment took place, the initial discoveries were obtained from data covering a period of only 15 months. And they show another signal at 2.8 sigma. Now the Kolara team is analyzing the data obtained for all three and a half years of the experiment, which should amplify this signal - if it is real.


CoGeNT experiment

Doubts have not gone away. The results from CDMSII show three events from the same 10 GeV region. Two years before, two events similar to dark matter were recorded on CDMSII, but after careful analysis they were dropped. This time, “we had three clear events,” says Zürek.

“If someone saw dark matter, it would look that way,” she says. But due to the fact that they are still at the turn of 2.8 sigma, "no one will believe that these three events occurred because of dark matter until someone else sees it." The latest evidence has already prompted physicists with XENON10 to reconsider their analysis, and conclude that they mistakenly rejected allusions to light WIMPs found on DAMA / LIBRA.

Suddenly, the light variant of WIMP turns out to be at least plausible, and is supported by Hooper's analysis of the gamma rays emitted from the center of our Milky Way, showing hints of a dark matter signal corresponding to the 10 GeV variant.

But this is not the only option. WIMP without interesting dynamics - whatever the mass they are - just the simplest version of dark matter. There may be several types of dark matter particles, with different types of interactions through the dark forces that make up the whole "dark sector" of the Universe, which theorists are just beginning to explore. Weiner believes that dark-power models are “the most straightforward way to explain some of these anomalies,” but warns that the pilot demonstration is still far away. Zürek agrees: “In principle, we can write down as many theories as we like, but nature will need to choose only one,” she says.

When will we know if all these hints are real? Maybe in the course of a year, maybe it will have to wait much longer. However, physicists who are trying to find dark matter may soon stumble upon more pragmatic limitations: budget cuts. For searches important variety of experiments. “Since we don’t know for which particle physics dark matter interacts with normal, several different experiments minimize the chances of missing dark matter because of the wrong choice, and if something is found out in several experiments, it will be possible to discard theoretical models much faster”, said buckley However, all experiments are required to report the results to the US Department of Energy, and only 2-3 of them will be able to survive.

“The department is putting things in order,” says Kolar. - Diversity is good, but the amount of money is limited. If the detectors under construction will not bring results, it will be very difficult to find the motivation to continue. ”

Translator's note; since the writing of the original article:

• The CRESST detector was updated in 2015, increasing the sensitivity 100 times so that it is now able to detect dark matter particles with a mass approximately equal to the proton mass. He is being replaced by the European Underground Rare Event Calorimeter Array experiment (EURECA).
• CDMSII detector replaced with next generation SuperCDMS detector
• The results of the CoGeNT experiment were processed and concluded that the received signals, taken as WIMP, were unaccounted for background noise.
• The XENON10 detector in 2016 replaced the more sensitive XENON1T, increasing the sensitivity 100 times.
• To reproduce the results of a DAMA / LIBRA sensor in Australia, an underground Stawell Underground Physics Laboratory (SUPL) sensor is being built.
• As of February 2017, not a single convincing evidence was obtained for the detection of dark matter particles.