Repels dark matter from other dark matter

Dark matter: the hunt for the invisible

If cosmologists are right, then there is a new form of matter in space that is six times more common than what we know. It is invisible and is therefore called dark matter. Postulated for the first time eighty years ago, it has not yet been directly verified. With experiments like CRESST and XENON100, researchers want to solve the cosmic puzzle in the coming years.

There is hardly any other astrophysical topic that scientists are currently discussing as intensely as dark matter. This was particularly evident in autumn 2011, when physicists from all over the world came together in Munich to discuss the latest results. Three research groups presented measurement results that were compatible with the detection of the mysterious dark matter particles - but contradicted each other. In addition, two other groups had not found anything and explicitly excluded the positive results.

"But it could be that the particles have unusual properties, so that they are noticeable in some detectors but not in others," says Franz Pröbst, who conducts the CRESST experiment at the Max Planck Institute for Physics in Munich.Cryogenic Rare Event Search with Superconducting Thermometers) directs. It is one of those detectors that could have detected dark matter.

The experiment CRESST

Manfred Lindner agrees in principle, but he thinks it is more plausible that CRESST sees some new kind of disruptive effect. The international experiment XENON100, whose contribution Lindner leads at the Max Planck Institute for Nuclear Physics, did not find any events from possible dark matter particles with a significantly higher sensitivity. The heated discussion about "dirty" instruments, sensitivity limits and interference effects is taking place at the very highest level. CRESST and XENON100 are among the most sensitive and purest experiments on earth. “Our detector is probably the cleanest place in the universe,” says Max Planck Director Lindner. In order to understand the requirements placed on the technology, it is necessary to look back briefly into history.

In 1933, the Swiss astronomer Fritz Zwicky, who emigrated to the USA, observed several galaxy clusters. He found that the individual Milky Way systems move so fast that their combined gravity is insufficient to hold the clusters together. He concluded from this that there must be a large amount of invisible matter that expresses itself only through its gravity. Zwicky created the term dark matter.

However, the researcher's observations fell into oblivion and were not revived until the 1970s. At that time, astronomers discovered that spiral galaxies like our Milky Way rotate so fast that they would be torn apart by centrifugal force, were it not for the additional gravity of dark matter.

Unknown elementary particles

Today most astroparticle physicists are convinced that the invisible substance consists of a new kind of elementary particle. These are said to have clustered together to form huge clouds that surround the galaxies and are widely distributed in the galaxy clusters. There are a number of other astrophysical indications of their existence, such as the images of gravitational lenses. In addition, according to the widespread opinion, the dark matter particles ensure that normal matter was able to condense into stars and galaxies relatively quickly after the Big Bang. All clues consistently point in the same direction - and without this mysterious agent and glue we would not exist at all.

Some properties of the invisible particles can be derived from previous observations and theoretical arguments. Accordingly, the most plausible candidates have a mass roughly equivalent to that of atoms. They are electrically neutral and have almost no interaction with normal matter. In other words: you traverse all bodies in the universe almost unhindered. It is because of these properties that they got their name Weakly Interacting Massive Particles (weakly interacting massive particles). Significantly, the acronym WIMP means something like weakling in English.

Dark matter in blue

If you summarize everything that you think you know today, around 100,000 of these particles race on earth every second through an area the size of a thumbnail - even through our bodies without us noticing the slightest thing. That sounds fantastic, but it's not that unusual at all. In the same time, about 65 billion neutrinos, which are created in the interior of the sun, penetrate the same area; and these ghost particles can already be made visible through rare collisions with matter.

All currently working experiments assume that dark matter particles can also collide with normal atoms, albeit at an extremely low rate. But she should reveal that. The unequivocal discovery of WIMPs would be a sensational confirmation of the new worldview and would likely be rewarded with a Nobel Prize. The recipe is similar for all measuring devices: Take a suitable detector material and wait for the very rare event that a WIMP collides with an atomic nucleus in it - and generates a brief flash of light. In addition, electrons are released because the hit atom collides with other atoms and the outer electrons can detach from the atom. If the material is a crystal, the impact energy is transferred to the crystal lattice and the detector heats up. So there are three measurands: light, free charges and temperature. However, none of the current detectors can measure all three quantities at the same time, only two at a time. This has an impact on the current discussion of the results.

Franz Pröbst has been working on the CRESST experiment for 15 years, in which physicists from the Technical University of Munich as well as from Tübingen and Oxford are involved. The heart of the system is formed by calcium tungstate crystals, each four centimeters in height and diameter. Although several million WIMPs should pass through one of these detectors every second, Pröbst does not expect more than one collision per month. When such an event occurs, a very weak flash of light is released and the crystal heats up by a few millionths of a degree. How do you measure that?

Sensitive experiments

“When we started the experiment back then, there was no measurement technology at all,” recalls Pröbst. Together with Wolfgang Seidel and Leo Stodolsky, the physicist developed the cryodetectors used today. They got this name because they work at an extremely low temperature of about a hundredth of a degree above absolute zero (minus 273.15 degrees Celsius). A thin tungsten film is applied to one side of the crystal, which serves as a sensitive thermometer. The temperature is precisely regulated in such a way that the film is in a transition state between normal and superconductivity. Even with the slightest warming, the electrical resistance increases so much that it can be measured.

This can be illustrated with a sensitive seesaw that is currently in equilibrium. Even the smallest weight can make one side sink down. Keeping the temperature stabilization accurate to within a millionth of a degree is an extreme challenge. Each tungsten film has its own transition temperature, which must be set individually. "The calibration of the experiment takes months," reports Pröbst's colleague Michael Kiefer from his own painful experience.

In the clean room

With CRESST, the physicists can also detect the flash of light that a WIMP is supposed to produce when it collides with an atom in the crystal. To do this, the light is directed onto another low-temperature sensor using mirrors that completely surround the crystal. It heats up, from which the researchers finally derive the light energy.

It took years of work to get these detectors working. But there is another problem: Nature has many other sources available that generate signals in the crystals similar to those of the WIMPs. A first step was to set up CRESST in the Gran Sasso underground laboratory. Under 1400 meters of rock in the Abruzzo region, it is largely protected from particles of cosmic rays that patter incessantly from space into the earth's atmosphere.

The greatest enemy, however, is natural radioactivity in the form of tiny traces of unstable isotopes. Among other things, radioactive radon isotopes have a disruptive effect, which occur everywhere as a result of the decay of uranium - incidentally also in the indoor air of a residential building. The atomic nuclei, electrons, neutrons and gamma rays released during radioactive decay can penetrate the detector and produce a signal similar to that of a WIMP.

In order to protect the crystal detectors from radioactive interference, they are made of high-purity materials. And they are surrounded by several jackets made of polyethylene, lead and copper, weighing a total of 44 tons. Nevertheless, a small smudge effect remains. “We now only measure an event every hundred seconds,” says Kiefer. They are the disturbing underground.

If so, then the particles have a mass roughly equivalent to that of a carbon atom. The probability that it is a random, statistical fluctuation is 1 in 100,000. "But it could still be an unknown interference background," says Pröbst. The current aim is to reduce this underground to a tenth through further shielding measures. In addition, the researchers doubled the number of crystals to 18.

Race to discover dark matter

The experiment is currently being set up in the Gran Sasso laboratory and will restart in early 2013. If the protective measures are effective, it should be clear after a further two years of data collection whether CRESST has really detected WIMPs or not. The race to discover dark matter, in which around a dozen groups are participating worldwide, is therefore in full swing. A solution to the riddle seems within reach for the first time. But at the moment the results are still contradicting itself. Two groups in the USA and Italy have also announced a positive result - but in a different mass range than CRESST. All of them contradict the XENON100 experiment, which is also working in the Gran Sasso laboratory, and an American experiment. With XENON100, 162 kilograms of liquid xenon are used as the detector material. When a WIMP collides with an atom in it, it generates a flash of light that sensitive photo detectors register. In addition, electrons are released that are drawn to the surface by an externally applied electric field and measured there.

In this experiment, too, radioactive interference, especially from radon and krypton, is the greatest enemy. The researchers get their xenon as pure as possible from a few producers worldwide. The substance is then further cleaned with extreme effort. “The liquid contains as few impurities as a cubic kilometer of pure water that you cough into,” explains Manfred Lindner. The Heidelberg Max Planck Institute for Nuclear Physics brings decades of experience in neutrino research to this cooperation. The GALLEX solar neutrino experiment was once about the detection of a few germanium atoms generated by neutrinos in a large amount of gallium.

System for gas analysis

“Today we can track down one krypton atom among more than a trillion xenon atoms,” says Hardy Simgen, who knows this, probably the world's most sensitive gas analysis system inside out. “An average of 8,500 radioactive decays take place in the human body every second. We detect only a few decays per year in a hundred kilograms of material. ”All materials that are used for the experiment go through this facility beforehand. Recently there was a problem with new light sensors for the follow-up project with a ton of xenon. Compared to the enormous purity requirements, they were too radioactive, which would completely destroy the measurement. Now, in collaboration with the manufacturing company, the physicists have selected purer materials to meet the requirements.

The xenon is in a constant cleaning cycle - a type of dialysis, which repeatedly removes penetrating radon. In this area, the Heidelberg researchers bring extensive experience from the solar neutrino experiment. With a mobile radon extraction system, they test the cleaning efficiency under realistic conditions.

And with a trick, the scientists have succeeded in making XENON100 by far the most sensitive of all WIMP experiments at the moment: Because the contaminants penetrate the xenon from the outer walls, the physicists only select events that occur inside the detector . You only use the central, particularly clean third of the entire volume for the WIMP search. In this way, the number of disruptive events could be reduced to a minimum. Therefore their result - namely not a significant WIMP event - appears very convincing. If CRESST had really detected dark matter particles, XENON100 would have had to measure more than a hundred events.

Doubts about the validity of the law of gravitation?

Manfred Lindner draws the conclusion from the current situation: One solution would be that two of the three experiments measure a misunderstood background, while the other actually sees WIMPs. However, these would then have to have very unusual properties so that they remain invisible to the XENON100. This cannot be completely ruled out, because CRESST, for example, measures the flash of light and the heat generated by a WIMP when it collides with an atom in the detector. XENON100, on the other hand, measures the flash of light and the generated ionization rate. "The other solution with less specific assumptions is that none of the signals seen so far originate from WIMPs," says Lindner.

The development continues with the XENON experiment. An extension to one ton of xenon is in preparation. The larger the detector, the more WIMP events can take place in it. However, the problem of contamination over the larger surface also increases - and this increases the requirements for the purity of the detector materials and the xenon. This is where the Heidelberg cleaners can play out all their know-how. At the end of 2014, the system should start up with a sensitivity increased by a further factor 100 and deliver an initial result in 2016.

If the dark particles cannot be found, the most plausible explanation of dark matter in the form of WIMPs becomes tight. One would then have to think more seriously about other particles. If dark matter does not exist after all, one could doubt the validity of the laws of gravity. Alternatives have existed for a long time, but they cannot explain all astrophysical phenomena for which dark matter is postulated in a uniformly consistent manner. In addition, Einstein's theory of gravity would then have to be modified in a way that, to put it carefully, appears very little motivated.